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DOI: 10.1055/a-2179-1338
Chemical Synthesis of Substituted Naphthalene Derivatives: A Review
The authors are highly thankful to Institute of Eminence, Banaras Hindu University (IoE BHU), funded by the University Grants Commission, for the seed grant and DST-SERB for EEQ/2021/000553 grant.
Abstract
This review outlines progress in the synthesis of substituted naphthalene derivatives. Naphthalene and its derivatives exhibit various biological activities such as anti-inflammatory, anticancer, antiviral, antitubercular, antimicrobial, antihypertensive, antidiabetic, etc. Several strategies have been developed for the construction of naphthalene derivatives, primarily focused on metal-catalyzed reactions (palladium, copper, zinc, rhodium, platinum, nickel, etc.,) and Lewis acid catalyzed transformations. This review discusses the preparations of naphthalene derivatives using various salts such as gallium chlorides, gold chlorides, gold bromides, various gold complexes as well as Brønsted acids like triflic acid and trifluoroacetic acid, and Lewis acids such as boron trifluoride etherate. Additionally, miscellaneous types of reactions are explored involving both metal and Lewis acids. The transformational approaches covered in this review include cycloadditions, carboannulations, benzannulations, electroannulations, rearrangements, and cross-dehydrogenative coupling reactions. Overall this review provides a comprehensive and up-to-date account of the current state of preparations of substituted naphthalenes, highlighting their medicinal and industrial importance.
1 Introduction
1.1 Medicinal Importance of Naphthalenes
2 Synthesis of Substituted Naphthalenes
2.1 Metal-Catalyzed Reactions
2.1.1 Palladium-Catalyzed Reactions
2.1.2 Copper-Catalyzed Reactions
2.1.3 Zinc-Catalyzed Reactions
2.1.4 Iron-Catalyzed Reactions
2.1.5 Rhodium-Catalyzed Reactions
2.1.6 Platinum-Catalyzed Reactions
2.1.7 Nickel-Catalyzed Reactions
2.1.8 Other Metal-Catalyzed Reactions
3 Lewis Acid Catalyzed Reactions
4 Miscellaneous Reactions
5 Conclusion
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Biographical Sketches


Mittali Maheshwari (Research Scholar) under the supervision of Dr. Nazar Hussain at Banaras Hindu University (BHU). She completed her master’s degree in 2019 from Subodh College, University of Rajasthan, India. She cleared CSIR-NET with AIR-50 in 2019 and also GATE in 2019 with AIR-1163, Recently she has been working on C-glycosides which have numerous medicinal applications in nature.


Nazar Hussain obtained his Ph.D. in Chemical Sciences under the guidance of Dr. Debaraj Mukherjee from CSIR-Indian Institute of Integrative Medicine, Jammu, India in 2019. His Ph.D. research focused on the synthesis of 2-C-branched sugars and their transformation into chiral aromatic building blocks. Subsequently, after obtaining a doctorate, he worked as a postdoctoral research associate at Armand Frappier Research Institute-INRS, Canada with Prof Charles Gauthier. Currently, he is an Assistant Professor in the Department of Medicinal Chemistry, Institute of Medical Sciences, Banaras Hindu University-Varanasi. His research interest is in the synthesis of C-glycosides, Pd-catalyzed reactions, total synthesis of natural products, and medicinal chemistry.
Introduction


‘Polycyclic aromatic hydrocarbons’ (PAHs)[1] are aromatic hydrocarbons with two or more merged aromatic rings (Figure [1]). These compounds are broadly classified into two important groups: planar and bowl-shaped PAHs.[2] [3] The planar PAHs include naphthalene (1), phenanthrene (2), anthracene (3), pyrene (4), and others. The bowl-shaped PAHs consists of hemifullerene/corannulenes 5 and 6. These compounds exhibit intriguing electrochemical and optical properties, making them valuable in materials chemistry.[6] [7] They find application in various fields such as light-emitting diodes,[4] [5] semiconductors in organic field-effect transistors,[6] [7] and solar cells.[8] [9] Naphthalene (1) is particularly significant as it serves as a fundamental scaffold and is found in numerous natural products.[10] [11] Naphthalene is an organic hydrocarbon that is white-colored and consist of fused aromatic benzene rings with the chemical formula C10H8, it exists in solid and crystalline form and is characterized by a distinct mothball odor. Naphthalene (1) occurs naturally in very low concentration, approximately 0.08 ppm by mass.[12] The extraction of naphthalene (1) involves distillation of coal tar. In 1826, Michael Faraday provided the chemical formula of naphthalene (1). Emil Erlenmeyer[13] proposed the structure of naphthalene (1) which later confirmed by Carl Gräbe.[14] The synthesis of naphthalene (1) was achieved by R. D. Haworth in 1932, starting from benzene as a substrate, and involves various steps (Scheme [1]).[15]


The molecular geometry of naphthalene is similar to that of benzene, with a planar structure. However, the C–C bonds are not equal in length.[16] Naphthalene (1) is stabilized by resonance (Figure [2]) and possesses a resonance energy of 61 kcal/mol. The hydrogen atoms in naphthalene (1) are categorized into two sets of equivalent hydrogens: α-hydrogens at positions 1, 4, 5, and 8 and β-hydrogens at positions at 2, 3, 6, and 7.[17] The synthesis of polycyclic aromatic compounds, such as naphthalene (1), is of significant interest due to their wide range of applications.


Traditionally the electrophilic aromatic substitution reaction has been employed to synthesize disubstituted naphthalenes. The regioselectivity of the reaction depends on the functional group attached to the naphthalene 1 (Scheme [2]). When an electron-releasing group is attached to naphthalene 1c electrophilic aromatic substitution occurs predominantly at C4 giving 1d, on the other hand, if an electron-withdrawing group is attached to 1c, functionalization takes place at the C5 or C8 positions giving 1e.[18] [19]


Medicinal Importance of Naphthalenes
Naphthalene derivatives exhibit a wide range of antagonistic and therapeutic activities (Figure [3]).[17] These activities include antiviral 17, 18,[20] antimicrobial 11–13,[21] [22] antitubercular 19, 20,[23] anti-inflammatory 14–16,[24] anticancer 8–10,[25] [26] [27] antipsychotic,[28] antihypertensive,[29] antidiabetic 21, 22,[30] antidepressant 29, 30,[31] antineurodegenerative 23–25,[32] [33] [34] and anticonvulsant 26–28.[35] The most reactive metabolites of naphthalenes are epoxides and naphthoquinones, which contribute to their cytotoxic nature by forming covalent bonds with the cysteine amino acids of cellular proteins. Several drugs like podophyllotoxin, rifampicin, patentiflorin A, justiprocumin A and B, tolnaftate, nafcillin, propranolol, naphyrone, and others, contain naphthalene cores within their chemical structures.
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# 2
Synthesis of Substituted Naphthalenes
2.1Metal-Catalyzed Reactions
There are various approaches in which transition metals have been utilized in the synthesis of naphthalene (1) and its derivatives. In this section, we describe the utilization of different metals in their synthesis.
2.1.1Palladium-Catalyzed Reactions
In 2002, Huang and Larock synthesized substituted naphthalenes using a palladium-catalyzed carboannulation approach (Scheme [3]).[36] Utilizing internal alkynes and o-allylaryl halides 31 in the presence of palladium acetate, triphenylphosphine, and triethylamine in DMF at 80 °C gave naphthalenes 32 in 60–88% yield. The substrate scope was explored by employing various internal alkynes and o-allylaryl halides 31 giving, for example, naphthalenes 33–38. Unsymmetrical alkynes gave mixtures of regioisomers. Internal alkynes bearing electron-rich substituents gave lower yields. Mechanistically (Scheme [4]), oxidative addition of the Pd(0) species to aryl halide 31a, resulting in aryl-Pd complex 31b. After that, 31b addition on c-c triple bond takes place, followed by the intramolecular cis attack on c-c double bond, yielding alkyl-Pd complex 31d. Finally, β-hydride elimination occurs, resulting in the formation of substituted naphthalene 33 by the isomerization of intermediate 31e.
Wang and Burton, in 2006, investigated the impact of the ester group in enyne substrates (Scheme [5]).[37] These enynes were synthesized through the Sonogashira reaction of α-bromocinnamates 40, prepared by the Wittig reaction, with terminal alkynes. Subsequently, the enynes underwent cyclization using DBU as a base to give substituted naphthalenes 41. Starting from 2-iodobiphenyl followed by Sonogashira reaction with terminal alkynes to give 2-alk-1-ynylbiphenyls and subsequent cyclization with DBU gave 9-alkylphenanthrenes (not shown). The substituted naphthalene-2-carboxylates, such as 42–46, were obtained in moderate to good yields. Interestingly, the substitution by electron-withdrawing or electron-releasing groups on the benzene ring of the α-bromocinnamate had minimal effect on the reaction yield. However, α-bromo-3-nitrocinnamate required a lower temperature in compared to other substituted aromatic derivatives for the synthesis of 43. Furthermore, α-bromocinnamaldehyde, containing an aldehyde group instead of an ester, underwent the same reaction sequence with 4-phenylbut-1-yne to give 4-phenethyl-2-naphthaldehyde (47) in moderate yield.[37]










Wu et al., in 2008, reported a one-pot synthesis of substituted naphthalenes 50 from arenes 48 and alkynes 49 using catalytic palladium acetate with silver acetate in acetonitrile at 110 °C (Scheme [6]).[38] Using 1,3- and 1,4-disubstituted benzenes containing bulky substituents, such as isopropyl and tert-butyl, gave lower yields of the corresponding products compared to 1,3- and 1,4-dimethylbenzenes. Benzene and toluene gave the corresponding products in lower yields and with lower purity. However, the reaction of p-xylene with diphenylacetylene gave 51 as a pure product in good yields. Mechanistically (Scheme [7]) starting from p-xylene 48a and diphenylacetylene (39), electrophilic palladation of p-xylene 48a leads to the formation of arylpalladium acetate 48b. Subsequently, the syn-addition of the Ar–Pd bond results in the generation of the vinylpalladium species 48c, followed by the formation of aryl-butadienyl palladium intermediate 48d. The intramolecular electrophilic palladation of 48d leads to palladabenzocycloheptatriene 48f. Finally, reductive elimination occurs to give naphthalene 51.


In 2010, Wu et al. reported the direct construction of substituted naphthalenes through the reaction of N-acetanilides 59 and diarylacetylenes in the presence of palladium acetate as a catalyst, 4-toluenesulfonic acid as an additive, and K2S2O8 as an oxidant in toluene to give substituted naphthalenes 60 in excellent yields (Scheme [8]).[39] The reaction exhibited chemoselectivity and regioselectivity to give naphthalenes such as 61–66. In terms of scope, no products were afforded when monoaryl-, alkyl-, and dialkyl-substituted alkynes were used as substrates and electron-deficient diarylacetylenes exhibited higher reactivity than those of electron-rich diarylacetylenes. Acetanilides substituted with electron-donating groups, such as methyl, were converted smoothly into the corresponding products, but the naphthalene products were not observed for acetanilides containing an electron-withdrawing group.


In Loh and Feng reported the bisolefination of the C≡C bond in a synthesis of 1-vinylnaphthalenes 69 (Scheme [9]).[40] For example, reaction of 1-ethynyl-2-vinylbenzene 67 and butyl acrylate, styrene, or vinyl sulfone 68 in the presence of 5 mol% PdCl2 as catalyst and molecular oxygen as the sole oxidant in DMSO at 110 °C gave 1-vinylnaphthalenes 69. Various functional groups were tolerated with good reactivity under these reaction conditions and the 1-vinylnaphthalene products, for example 70–75, were obtained in good to excellent yields. A plausible mechanistic route for the synthesis was proposed.




In 2012 Shimizu et al. reported the use of double cross-coupling reactions to synthesize polysubstituted naphthalenes 78 and fused phenanthrenes (Scheme [10]).[41] Coupling of 1,2-diborylalkenes 76 and 1-bromo-2-(2-bromovinyl)benzenes 77 under palladium catalysis in tetrahydrofuran at 80 °C for 48 h gave naphthalenes 78 in good to excellent yields. The proposed mechanism begins with the oxidative addition of palladium(0) to the C–Br bond of alkenyl 77, forming an alkenylpalladium bromide complex. Subsequently, this complex undergoes transmetalation with the diboryl reagents 76 facilitated by hydroxide ions, leading to the generation of a bis(alkenyl)palladium complex. Reductive elimination then occurs, resulting in the formation of the initial coupling product that repeats these steps to ultimately give the annulated product 78.
Gandeepan and Cheng, in 2013 presented a methodology that showcased the advantages of using oxygen as the sole oxidant in the Pd(OAc)2/Cu(OAc)2/TFA-catalyzed reaction of 1,3-diarylpropenes 85 with acetylenes to give 2-(arylmethyl)naphthalenes 86 (Scheme [11]).[42] The reaction proceeds through the ortho-selective activation of the C–H bond of via π-chelation. This strategy was explored using different diarylacetylenes and 1,3-diarylpropenes 85 to synthesize a library of substituted naphthalenes 86. Notably, when the reaction was conducted with 1,3-diphenylpropane as the substrate, no product formation occurred due to the absence of allylic double bonds in the substrate. The proposed mechanism (Scheme [12]), commences with the formation of a cationic palladium moiety, exhibiting electrophilic behavior in the presence of TFA. The coordination of Pd(II) with the ipso and ortho positions of the substrate leads to intermediate 85b. Cyclometalation then occurs, resulting in the formation of the Pd(II) σ-aryl complex 85c, which subsequently attacks the C≡C of diphenylacetylene (39), generating an intermediate vinyl palladium 85d. Intramolecular allylic double bond cyclization then takes place in intermediate 85d. The formation of alkyl palladium species 85e follows, which ultimately undergoes β-hydride elimination to yield 87.[42]


In 2014, Reddy et al. employed Morita–Baylis–Hillman (MBH) acetates of acetylenic aldehydes 93 and boronic acids 94 for the synthesis of substituted naphthalenes 95 (Scheme [13]).[43] This method serves as a general strategy for the synthesis of substituted naphthalenes and aromatic and heteroaromatic compounds. The (4+2) benzannulation reaction between boronic acids 94 and acetylenic aldehydes 93 occurs in the presence of palladium acetate as a catalyst and DBU as a base at r.t. Various substituted boronic acids with various functional groups, such as OMe, CN, CF3, etc., were utilized to evaluate scope of the protocol. The strategy was successfully applied to the synthesis of phenanthrene derivative 100 through the reaction of naphthylboronic acids. The mechanism of the reaction involves the initial formation of a π-allylpalladium intermediate; deprotonation occurs through the addition of the base (DBU), followed by a 6-exo-dig carbo cyclization. Finally, a 1,7-hydrogen shift takes place, leading to the synthesis of naphthalenes 95 with aromatization.




In 2018, Wei et al. proposed an efficient protocol for the synthesis of substituted naphthalenes 103 by the annulation reaction between 1-bromo-2-vinylbenzene derivatives 102 and alkynes using palladium acetate as a catalyst with cesium acetate in 1,4-dioxane at 110 °C for 3 h (Scheme [14]).[44] The scope of the reaction was investigated by using various substituted alkynes with both electron-withdrawing and electron-releasing groups and excellent yields were obtained. A gram-scale synthesis was also examined and exhibited excellent yields on up to a 2-g scale. The proposed mechanism begins with oxidative addition of the substrate, followed by the migration of a 1,4-palladium to form a vinylpalladium intermediate. The insertion of an internal alkyne and subsequent cyclization leads to the formation of 103.




In 2019, Zhang et al. prepared naphthalenes 112 by the reaction of 2-bromobenzaldehydes 110 and N-sulfonylhydrazones 111 in the presence of a palladium catalyst, triphenylphosphine, and potassium carbonate in MeCN at 60 °C (Scheme [15]).[45] Various substituents were utilized on both aldehydes and hydrazones to give naphthalenes 112 in moderate to good yields. Mechanistically, the key step is enabled by a palladium-carbene migratory insertion. After that, a sequence of reversible allylic alkylation and intramolecular condensation takes place to give the substituted naphthalene derivatives 112.
In 2019, Xu, Loh, and co-workers reported a direct and atom-economic methodology for the regioselective construction of naphthalene derivatives (Scheme [16]). The key step involves a cascade cross-coupling reaction between o-(alk-1-ynyl)styrenes 119 and an allylic ketone 120 in the presence of catalytic palladium acetate and copper chloride in a mixture of DMSO and water at 80 °C for 12 h under an oxygen atmosphere to give 4-(1-naphthyl)but-2-ones 121. o-(Alk-1-ynyl)styrenes substituted with various electron-rich and electron-deficient groups gave 4-(1-naphthyl)but-2-ones 121 in good to excellent yields. The use of o-(alk-1-ynyl)styrenes 119 with electron-withdrawing groups present at ortho position resulted in lower yields compared to electron-donating groups. The proposed mechanism involves coordination of palladium with the C=C bond of allylic alcohol 120 leading to the formation of a π-complex. This is followed by carbopalladation and insertion of the alkene, resulting in the formation of 121.[46]


In 2020, Yin and co-workers reported a one-pot synthesis of diamino-substituted naphthalenes 131 in a domino process that includes the formation of 2 C–N bonds, 3 C–C bonds, and one aromatic ring (Scheme [17]).[47] The reaction of aryl halides 128, 1,4-bis(trimethylsilyl)butadiyne (129), and amine substrates 130 in DMF at 80 °C under the cooperative catalysis of copper triflate, Cu(xantphos)I, and palladium acetate gave 1-aryl-2,4-diaminonaphthalenes. Aryl halides substituted with electron-withdrawing groups such as NO2, CN, gave products 134–137 in moderate 36–63% yield. Using 1-bromo-4-methoxybenzene as the substrate gave the corresponding naphthalene product in very low yield, and it could not be isolated. This is attributed to the presence of an electron-donating group, which is unfavorable for nucleophilic attack.[47] A proposed mechanism for the reaction involves a double Sonogashira coupling using palladium acetate and Cu(xantphos)I, followed by intermolecular hydroamination with the assistance of copper triflate and Cu(xantphos)I. Finally, a benzannulation step leads to the formation of 131.


In 2020, Yu, Yu, and Huang reported the synthesis of 1,3-bis(aminomethyl)-2-vinylnaphthalenes 140 (Scheme [18]).[48] The reaction involves the cyclization of enyne-tethered allylic alcohols 138 and amines 139 using a palladium complex as the catalyst in tetrahydrofuran at 80 °C for 12 hours. Under the optimized reaction conditions, the both electron-withdrawing and electron-releasing groups on the enyne-tethered allylic alcohols 138 were capable of participating in the reaction, resulting in good to moderate yields of products 140.




He, Fan, and co-workers, in 2020, reported the PdCl2-catalyzed, TBHP-promoted, and toluene-mediated dehydrogenation/[4+2] cycloaddition of saturated cyclic amines with 2-alkynylbenzaldehydes to give 1,2,3,4-tetrahydrobenzo[g]quinolines and 2,3-dihydro-1H-benzo[f]indoles 149 (Scheme [19]).[49] In contrast, using DMSO/water as the solvent gave a dehydrogenation–intermolecular condensation–C–N bond cleavage–intramolecular condensation pathway to give 2-(ω-nitroalkyl)naphthalenes (not shown). The reaction of o-alkynylbenzaldehyde 147 and cyclic amines 148 in the presence of palladium chloride as a catalyst with tert-butyl hydroperoxide as a promoter in toluene at 80 °C for 8 h gave 1,2,3,4-tetrahydrobenzo[g]quinolines 149 in 41–81% yield and 2,3-dihydro-1H-benzo[f]indoles in 28–46% yield. An electron-releasing group on the benzaldehyde ring gives better results in comparison with an electron-withdrawing groups.[49]
In 2021, Ess, Michaelis, and co-workers reported the synthesis of substituted naphthalenes (Scheme [20]).[50] The reaction of aryl iodides 156 with 2 molecules of ketone 157 in the presence of bimetallic palladium complex and silver triflate in 1,4-dioxane at 80 °C for 1–2 h gave 1,3-disubstituted naphthalenes 158. The reaction was initiated by the formation of a palladium complex with a bidentate 2-phosphinoimidazolium. The substrate scope was examined for both aryl halide and ketone moieties and the naphthalenes 158 were obtained in good yields. o-, m-, and p-substituted aryl iodides gave a single regioisomer of the product in excellent yields. Electron-withdrawing and electron-releasing groups attached to the ketonic substrate gave the products in good to excellent yields, but o-substituted acetophenones gave a lower yield of products in comparison to m-substituted acetophenones.


# 2.1.2
Copper-Catalyzed Reactions
In 2010, Xu et al. developed a simple and efficient methodology for the synthesis of functionalized naphthols 167 (Scheme [21]).[51] The cascade reaction of various 2-halophenyl ketones 165 with 1,3-dicarbonyls, α-cyano ketones, or malononitrile 166 in the presence of copper chloride as a catalyst and cesium carbonate as a base in DMF under nitrogen gave 1,2,3,4-tetrasubstituted naphthalenes 167 in moderate to good yields. 2-Bromophenyl derivatives 165 gave the corresponding products at room temperature, while the use of methyl 3-(2-chlorophenyl)-3-oxopropanoate provide good results at 60 °C.
Sun et al., in 2011, used readily available 1,4-diarylbuta-1,3-diynes and cyclic amines for the synthesis of amino-substituted naphthalene derivatives (Scheme [22]).[52] For example, the reaction of 1,4-diphenylbuta-1,3-diyne (174) with pyrrolidine (175) in the presence of copper chloride as a catalyst at 80 °C gave 1-phenyl-2,4-dipyrrolidinonaphthalene (176). Various 1,4-diarylbuta-1,3-diynes containing electron-donating and electron-withdrawing groups were utilized and the cyclic amines included pyrrolidine, piperidine, and morpholine. Mechanistically (Scheme [23]), intermolecular hydroamination of 1,4-diphenylbuta-1,3-diyne (174) with a cyclic amine gives enamine intermediate 174a. Next, copper chloride coordinates with the enamine intermediate 174a, coordination Cu with the C≡C bond and insertion into the ortho C–H bond generates an intermediate which then reacts with another molecule of amine, forming a copper–nitrogen complex (Cu-NR2) 174b. This step involves an intramolecular amination process. Finally, reductive elimination occurs gives the product 176.[52]






In 2012, Chen et al. developed an efficient procedure to synthesize amine-substituted naphthalenes 185 (Scheme [24]).[53] The coupling of 1-aryl-2-haloacetylenes 183 and amines 184 in the presence of a copper complex as a catalyst in diethylamine as a solvent at 80 °C for 6 h gave amine-substituted naphthalenes 185 in 43–99% yield. The scope and utility of this methodology were explored; 1-aryl-2-haloacetylenes containing an electron-rich substituent on the benzene ring gave the corresponding products in excellent yields. The use of 1-bromo-2-phenylacetylene gave 186 in a good 88% yield while 1-iodo-2-phenylacetylene gave 186 in a lower 62% yield. meta-Substituted 1-bromo-2-arylacetylenes gave a regiospecific product, the 6-substituted 1,3-diamino-2-arylnaphthalene, but ortho substitution did not lead to 1,3-diaminonaphthalenes and gave 2-substituted 1,1-ethylenediamine derivatives in good yields instead.


Also in 2012, Liu et al. synthesized polycyclic arenes, such as naphthalenes, by the reaction of 2-alk-1-enylphenyl ketones 192 and aldehydes catalyzed by Cu(OTf)2 (Scheme [25]).[54] The use of 2-(2-methylprop-1-enyl)phenyl ketones (R2 = Me) gave 1-substituted 3-methylnaphthalenes in 60–80% yield while 2-(2-phenylprop-1-enyl)phenyl ketones (R2 = Ph) gave 1-substituted 3-phenylnaphthalenes in 80–90% yield.


In 2013, Reddy et al. reported a novel and efficient one-pot cyclization for the synthesis of substituted naphthalenes (Scheme [26]).[55] The cascade reaction of 4-(2-bromophenyl)but-2-enoates 200 in the presence of CuCN in DMF at 150 °C for 12 h gave 1-aminonaphthalene-2-carboxylates 201 in excellent yields. Substitution of 4-(2-bromophenyl)but-2-enoates by fluoro, nitro, and methoxy groups gave the corresponding products, e.g. 202–206, in 73–91% yields.


The mechanistic study (Scheme [27]) was supported by deuterium labeling experiments. Hydrolysis of enol ether 200a by using 2 N DCl followed by Wittig reaction gives a mixture of 200b and 200c in 3:2 ratio. CuCN reacts with 200b/200c in the presence of dimethylformamide to give deuterated complex 201a in 73% yield. Using a 1H NMR study, it was found that the benzylic deuterium peak was not present hence 201b was not formed which confirms that isomerization occurs through an intermediate 200d. The proposed mechanism from 200c is thus through isomerization of the C=C bond, intramolecular C–C bond cyclization, and aromatization in a single step resulting in the formation of products.[55]




In 2015, Lehnherr et al. synthesized polyheterohalogenated naphthalenes in good to excellent yields by using a benzannulation reaction of 1-aryl-2-haloacetylenes 207 and 2-(phenylethynyl)benzaldehydes 208 in the presence of copper triflate with trifluoroacetic acid in DCE at 100 °C (Scheme [28]).[56] 2-(Phenylethynyl)benzaldehydes 208 with electron-deficient groups in the benzaldehyde ring react with 1-aryl-2-haloacetylenes 207 to give the corresponding polyheterohalogenated naphthalene products in good yields. 1-Aryl-2-haloacetylenes 207 substituted with an ester, halide, nitrile, and methyl group gave polyheterohalogenated naphthalene products 201–212, 214 in excellent yields compared to electron-rich substituent 4-methoxyphenyl that gave 213 in only 34% yield due to its lower thermal stability.
In 2016, Zhang et al. reported the copper-catalyzed aminobenzannulation of 2-alky-1-ynylphenyl ketones 216 and primary/secondary amines using Cu(OAc)2·H2O (5 mol%) as a catalyst in toluene at 130 °C for 12 h to give naphthalen-1-amines 217 in good to excellent yields (Scheme [29]).[57] Various arylamines substituted in the para-substitution with electron-withdrawing or electron-rich groups gave excellent yields of 217. The reaction of 2-(phenylethynyl)acetophenone with aliphatic amines gave products such as 221 and 222, while arylamines gave product such as 218–220 in good to excellent yields. The reaction of 2-ethynylacetophenone with aniline gave 1-anilinonaphthalene in only 56% yield.


In 2018, Shi and co-workers synthesized cyclopenta[b]naphthalene derivatives by the coupling reaction of diazo compounds 225 with terminal yne-alkylidenecyclopropanes 224 using a copper complex (Cu(MeCN)4BF4) as the catalyst in triethylamine and chloroform at 25 °C for 20 h (Scheme [30]).[58] A wide scope of substrates gave the corresponding products in good to excellent yields. The coupling of 224 where the aromatic ring bears electron-withdrawing or electron-releasing groups gave the cyclopenta[b]naphthalenes in fair yields only. Mechanistically (Scheme [31]), the reaction initiates with the formation of copper acetylide 224a from yne-ACP 224, which reacts with diazo compound 225 to give copper carbene complex 224b. Migratory insertion reaction of 224b followed by 6π-electrocyclization gives product 226.




In 2019, Su et al. prepared substituted naphthylamines that play an important role in various biological and chemical processes (Scheme [32]).[59] Cyclization reaction of various 2-bromoacetophenone 234, amides 235, and terminal alkynes 233 in the presence of copper iodide as a catalyst with sodium hydroxide as a base in water at 120 °C gave naphthalen-1-amines 236 in up to 95% yield. The reaction of N-methylformamide with phenylacetylene and 2-bromo-4-methoxypropiophenone gave 6-methoxy-N,2-dimethyl-3-phenylnaphthalen-1-amine (241) in only 65% yield, while the reaction of N,N-dimethylformamide, phenylacetylene and benzophenone gave 2,3-diphenyl-substituted naphthalen-1-amine 242 in trace amounts due to steric hindrance.


# 2.1.3
Zinc-Catalyzed Reactions
In 2011, Fang et al. reported the synthesis of selected naphthalene derivatives by a [4+2] benzannulation reaction (Scheme [33]).[60] Reaction of 2-ethynylbenzaldehydes 147 and alkynes in the presence of zinc chloride and DCE at 80 °C gave naphthalenes 243 in moderate to good yields. The reaction of 2-ethynylbenzaldehyde with 1-phenyl-2-(phenylthio)acetylene gave 2-phenyl-3-(phenylthio)naphthalene (245) in 84% yield. The reaction of 2-ethynylbenzaldehyde with dimethyl acetylenedicarboxylate gave dimethyl naphthalene-2,3-dicarboxylate (249) in trace amounts only. A mechanism was proposed involving coordination of zinc chloride to the alkyne C≡C bond, which increases the electrophilicity of the alkyne making it susceptible to nucleophilic attack, attack of the electron-deficient alkyne by carbonyl oxygen of the benzaldehyde, Diels–Alder reaction, and bond rearrangement.


In 2014, Sakthivel and Srinivasan prepared 1-aminonaphthalene-2-carboxylates using the Blaise reaction of 2-ethynylbenzonitriles 250 with bromoacetates (Scheme [34]).[61] Blaise reaction of 2-ethynylbenzonitriles 250 with bromoacetates in the presence of a Zn complex in 1,4-dioxane as solvent followed by 6-endo-dig carbannulation gave 1-aminonaphthalene-2-carboxylates 251 in moderate to good yields with high chemo- and regioselectivity. 2-(Arylethynyl)benzonitriles 250 with the aryl group as an electron-donating group phenyl gave products 251 in moderate to good yields; reaction of does not occur with 2-ethynylbenzonitrile due to hydrogen abstraction by the Reformatsky reagent.


In 2014, He et al. developed a synthesis of 1-aminonaphthalene-2-carboxylates 251 (Scheme [35]) and arylnaphthalene lactone lignans 268 and 269 (Scheme [36]).[62] The tandem reaction of 2-ethynylbenzonitriles 250 and the Reformatsky reagent ethyl bromoacetate (258) in the presence of zinc as a catalyst in tetrahydrofuran at 80 °C for 30 min gave 1-aminonaphthalene-2-carboxylates 251 in good to excellent yields. The best yields were obtained using 4-chloro- or 4-fluoro-2-(4-methoxyphenyl- or 4-tolylethynyl)benzonitriles 250, for example giving 260–262.




Using 2-(3-hydroxyprop-1-ynyl)benzonitriles to replace 250 and an Reformatsky reagent, formed in situ from ethyl bromoacetate, gave 1-amino-3-(hydroxymethyl)naphthalen-2-carboxylates via the Blaise reaction that simultaneously underwent lactonization to give a range of substituted 9-aminonaphtho[2,3-c]furan-1(3H)-ones. This was applied in the synthesis of chinensin (268) and taiwanin (269), plant-derived natural products and members arylnaphthalene lactone family (Scheme [36]). Chinensin (268) and taiwanin (269) were synthesized from a common intermediate 265 that was prepared from the of 2-(3-hydroxypropyl-1-ynyl)-4,5-(methylenedioxy)benzonitrile (263) with the Reformatsky reagent from ethyl bromoacetate in the presence of zinc to give 9-amino-6,7-(methylenedioxy)naphtho[2,3-c]furan-1(3H)-one (264) which was further reacted with HCl and NaNO2 followed by KI to give 9-iodo-6,7-(methylenedioxy)naphtho[2,3-c]furan-1(3H)-one (265). Suzuki coupling of 265 with 3,4-(methylenedioxy)phenylboronic acid (267) gave taiwanin (269). Coupling of 265 with 3,4-dimethoxyphenylboronic acid (266) gave chinensin (268).[62]


# 2.1.4
Iron-Catalyzed Reactions
In 2012, Liu et al. synthesized polysubstituted naphthalene derivatives by using a novel iron-catalyzed tandem cross-dehydrogenative coupling reaction (CDC) (Scheme [37]).[63] The benzannulation reaction of styrenes 271 and 1,2-diarylpropenes 270 using FeCl3 as the catalyst with DDQ in nitromethane at 50 °C gave 2,4-diaryl-1-methylnaphthalenes 272 in good to excellent yields.
Also in 2012, Bu et al. developed a versatile and novel procedure for the synthesis of substituted naphthalenes and phenanthrenes (Scheme [38]).[64] The annulation of arylacetaldehydes 279 with alkynes 38a using FeCl3 as a catalyst in DCE at room temperature for 2 h gave mono-, di-, and polysubstituted naphthalenes in moderate to good yields with excellent regioselectivity. The use of arylacetaldehydes containing an electron-donating or -neutral group in the benzene ring gave the corresponding products in moderate to good yields, but arylacetaldehydes containing a strong electron withdrawing group did not react. The reaction of silyl-substituted alkynes gave unprecedented complete Me3SiOH elimination selectivity.


In 2013, Zhu et al. reported a practical and efficient method to construct naphthalene derivatives (Scheme [39]).[65] The benzannulation reaction of various 2-(2-oxoethyl)benzaldehydes 287 and alkynes in the presence of FeCl3 as a catalyst in DCE at 60 °C for 1–2 h gave 2-substituted naphthalenes 243 in good to excellent yields. Substitution on the arylacetylene by either electron-withdrawing or electron-donating groups and the use of arylacetylenes, arylalkynes, alkynes, and bromoacetylenes had little effect on the yield. When 2-(2-oxoethyl) benzaldehyde was substituted by electron-donating groups gave lower yields compared to those containing electron-withdrawing groups. Mechanistically (Scheme [40]), the reaction is initiated by the formation of dienol 287a via the enolization process. This dienol results in the formation of hemiketal 287b through an intramolecular nucleophilic attack. Reaction of 287b with an alkyne gives bridged intermediate 287c that undergoes aromatization to give 243.[65]




In the synthesis of naphthalene and isoquinoline derivatives described by Zhang et al. in 2018 (Scheme [41]), a benzannulation reaction occurs between 2-(2-oxoethyl)phenyl ketones 294 and alkynes 38a with FeCl3 as the catalyst in DCE at 80 °C to give naphthalenes 295 in 30–90% yield. Terminal alkynes reacted efficiently in the reaction, but the use of 4-methoxyphenylacetylene gave no product due to its sensitivity to oxidation by FeCl3; 3,5-bis(trifluoromethyl)phenylacetylene, an electron-deficient compound, gave the corresponding product 299 in lower yield (30%). Interestingly, when zinc chloride was used as a catalyst instead of iron(III) chloride, the yield of naphthalene product increased to 63–94%. The proposed mechanism of the reaction involves the enolization of the diketone, followed by intramolecular cyclization to form a hemiketal intermediate. This intermediate then undergoes dehydroxylation, resulting in a dienone species. A Diels–Alder reaction between the dienone and the alkyne leads to the formation of the naphthalene product.[66]


# 2.1.5
Rhodium-Catalyzed Reactions
In 2009, Satoh, Miura, and co-workers reported the synthesis of highly substituted naphthalene and anthracene derivatives (Scheme [42]).[67] The coupling reaction of alkynes and arylboronic acids 302 using a rhodium complex catalyst with copper acetate/air oxidant in DMF at 100 °C for 2 h gave 1,2,3,4-tetrasubstituted naphthalenes in good yields. Arylboronic acids substituted with a chloro, bromo, CF3, methyl, and methoxy group reacted with diphenylacetylene to give 304–308 in 72–89% yield; and arylboronic acids containing electron-deficient groups are less reactive than electron-rich groups. Various diarylacetylenes and 1-phenylprop-1-yne were successfully utilized, but oct-4-yne gave 1,2,3,4-tetrapropylnaphthalene in only 10% yield.


Satoh, Miura, and co-workers, in 2010, reported an efficient synthesis of multisubstituted naphthalene derivatives (Scheme [43]).[68] The cross-cyclodimerization of aliphatic alkynes and diarylacetylenes 310 in the presence of a rhodium complex and triphenylphosphine and 20 mol% of a Brønsted acid in xylene for 6 h gave 1,2,3-trisubstituted naphthalenes 311. Various substituted diarylacetylenes 310 were utilized; when the substituent was an electron-withdrawing group or electron-releasing group the products, 1,2,3,7-tetrasubstituted naphthalenes were obtained in good to excellent yields. The use of diarylacetylenes with an acid-labile substituent, such as CN, Me, and CO2Me gave the corresponding products 314, 315, and 317 in 46–89% yield. The reaction of diphenylacetylene with the unsymmetrical 1-phenylhex-1-yne gave 2-butyl-1,3-diphenylnaphthalene (313) in 63% yield while the reaction of activated alkynes, such as 3-phenylprop-1-yn-1-ol and ethyl phenylpropynoate with diarylacetylenes was unsuccessful.


In 2012, Qian et al. prepared 1,2,3-triarylnaphthalenes 318 by the dimerization of diarylacetylenes 310 in the presence of Wilkinson’s catalyst, with silver fluoride as an oxidant, and a Grignard reagent as an additive in toluene at 110 °C for 12 h under nitrogen (Scheme [44]).[69] Various substituted diarylacetylenes 310 gave 1,2,3-triarylnaphthalenes 318 in moderate to excellent yields, but the regioselectivity was poor. The use of bis(4-methoxyphenyl)acetylene gave 7-methoxy-1,2,3-tris(4-methoxyphenyl)naphthalene (321) in 63% yield, while bis(3,5-dimethylphenyl)acetylene gave the corresponding product 322 in only 45% yield.


In 2016, Zhu and co-worker utilized directed C–H functionalization in the coupling of enaminones with alkynes to give substituted naphthalenes (Scheme [45]).[70] The coupling reaction of enaminones 324 and alkynes in the presence of a rhodium catalyst with various additives (to increase the yield of the product) in DCE at 80 °C for 6–12 h gave 2-hydroxynaphthalene-2-carbaldehydes 325 in good yields (75–85%) when the enone contained electron-donating groups; lower yields (50–70%) were obtained than when they contained electron-withdrawing groups.


In 2017, Li and co-workers reported an effective and direct synthesis of naphthols by C–H activation of sulfoxonium ylides 333 (Scheme [46]).[71] Reaction of sulfoxonium ylides 333 and alkynes in the presence of a rhodium complex and zinc acetate in DCE at 80 °C for 16 h under a nitrogen atmosphere gave 1-naphthols 334 (32 examples) in up to 99% yield. Sulfoxonium ylides 333 substituted with an electron-withdrawing or -donating group in the para position of the phenyl ring gave the corresponding 6-substituted 1-naphthols in 86–95% yield with the exception of the para-nitro group which gave 337 in only 37% yield. ortho-Substituted sulfoxonium ylides 333 gave 8-substituted 1-naphthols in 34–96% yield, showing the tolerance of the methodology to steric hindrance. The alkyne was a symmetrical diarylacetylene with substituents in the ortho-, meta- and para-positions, dihetarylacetylenes, 1-arylalk-1-ynes and 3-arylpropynoates.


Mechanistically (Scheme [47]), the reaction starts with the Rh(III)-catalyzed C–H activation of the sulfoxonium ylide 333 and complexation of Rh with diphenylacetylene to produce a 5-membered rhodacyclic intermediate 333a. Migratory insertion of 333a gives seven-membered intermediate 333b. Then O-to-C bond tautomerization gives intermediate 333c that further undergoes α-elimination to give α-oxo carbenoid species 333d. The Rh-alkenyl bond readily inserts into the resultant carbenoid to produce a Rh(III) alkyl species 333e; protonolysis of the Rh–C bond by HX gives product 334.[71]


In 2019, Chen et al. reported a cascade reaction for the formation of functionalized naphthalenes 344 (Scheme [48]).[72] The cascade reaction of α-diazocarbonyl compounds 343 with sulfoxonium ylides 333 using a rhodium complex/AgSbF6 catalyst in 2,2,2-trifluoroethanol at 100 °C for 24 h gave 2-(dimethyl(oxo)-λ6-sulfanylidene)naphthalen-1(2H)-ones 344 in up to 58% yield. ortho- and para-Substitution of electron-rich and electron-deficient groups in the phenyl ring of 333 gave the corresponding products in ~50% yield, while meta-substituted chloro and bromo groups gave mixtures of regioisomers. The reaction was carried out on a gram scale with 6 mmol of sulfoxonium ylide and 5 mmol of α-diazocarbonyl substituent to give the naphthalene in 42% yield.


Also in 2019, Hanchate et al. reported the synthesis of furanone-fused naphthols 352 in up to 75% yield by using the domino reaction of sulfoxonium ylide 333 and 4-hydroxyalk-2-ynoates such as 351 using a rhodium complex as catalyst, silver salt as an activator, chloroacetic acid as an additive, and ethyl acetate as a solvent at 80 °C under argon for 16 h (Scheme [49]).[73] The substrate scope was examined, the reaction of 4-methylphenyl sulfoxonium ylide under the optimized conditions gave 353 in 58% yield. A 4-methoxyphenyl sulfoxonium ylide gave the corresponding product in 62% yield while the 3-methoxyphenyl sulfoxonium ylide gave 358 in only 39% yield. An electron-withdrawing group in the phenyl ring of the sulfoxonium ylide, such as cyano and nitro, gave 356 and 357 in 61% and 67% yield, respectively. Mechanistically, the reaction is initiated by the activation of the catalyst by using AgNTf2 and chloroacetic acid which bonds to the oxygen of 333 and activates the C–H bond, which is followed by alkyne insertion and leads to the formation of a 7-membered rhodacycle that on elimination eliminates DMSO; with the reductive elimination the formation of product and regeneration of catalyst takes place.




# 2.1.6
Platinum-Catalyzed Reactions
In 2004, Kusama et al. reported the synthesis of 1-acyl-4-alkoxynaphthalenes 362 (X = O) or 1-acyl-4-(alkylsulfanyl)naphthalenes 362 (X = S) by the reaction of o-alkynylbenzoates 359 or -benzothioates 360 and vinyl ethers 361 catalyzed by platinum chloride in toluene at room temperature (Scheme [50]).[74] Various o-alkynylbenzoates 359 as the alkyl and phenyl esters gave 1-acyl-4-alkoxynaphthalenes such as 363–366 and 1-acyl-4-phenoxynaphthalene 368 while o-alkynylbenzothioates 360 as the alkyl ester gave 1-acyl-4-(alkylsulfanyl)naphthalenes such as 367. However, the use of o-alkynylbenzamides was unsuccessful. The proposed mechanism of the reaction involves a (3+3) cycloaddition reaction of carbonyl ylides, followed by a 1,2-alkyl migration. Finally, with a C–H insertion step, the formation of naphthalene derivatives takes place.
An efficient method for the synthesis of functionalized naphthalenes 370 was reported by Kang et al. in 2012 (Scheme [51]).[75] Intramolecular hydroarylation of arylenynes 369 using PtCl4 (5 mol%) as a catalyst in toluene at 110 °C for 10 min gave 1-arylnaphthalene-3-carboxylates in up to 87% yield. Ethyl 3-aryl-2-ethynylacrylates 369 with diverse substitution on the aryl group gave the products in good to excellent yields. The ethynyl group of ethyl 3-aryl-2-ethynylacrylates 369 was substituted with various groups including alkyl, vinyl, and aryl.


To study the mechanism of the reaction, firstly incorporation of deuterium was carried out by using toluene/D2O and gave 70% yield with 57% D-incorporation, which shows that the reaction proceeds via activation of ethyl 3-aryl-2-ethynylacrylates 369 with the platinum catalyst. As shown in Scheme [52], activation of ethyl 3-aryl-2-ethynylacrylate 369 by the platinum catalyst followed by 6-endo cyclization gives zwitterionic intermediate arylplatinum 369b. Finally, aromatization of 369b gives naphthalene 370.[75]






# 2.1.7
Nickel-Catalyzed Reactions
In 2005, Hsieh and Cheng reported a novel strategy to generate naphthalene derivatives (Scheme [53]).[76] Cocyclotrimerization of benzyne scaffold 377 and diynes 378 in the presence of zinc powder CsF and NiBr2(dppe) in acetonitrile as a solvent at 80 °C for 12 h gave naphthalene derivatives 379 in moderate to good yields. Interestingly, the reaction of hex-5-ynenitrile (386) with benzyne precursor 377 gave two products: phenanthrene (387) and naphthalene derivative 388 in a 56:11 molar ratio with a combined yield of 67% (Scheme [54]). Mechanistically, the reduction of the Ni(II) species to a Ni(0) species by zinc metal then coordination of nickel metal to both C≡C bonds of the diyne, cyclometalation produces a nickelacyclopentadiene intermediate. Then benzyne coordination and then insertion into the Ni(II)–carbon bond of the nickelacyclopentadiene intermediate gives a nickelacycloheptatriene, finally reductive elimination results in the formation of naphthalenes 379.[76]
In 2008, Hsieh and Cheng successfully extended their formation of naphthalene derivatives (Scheme [55]).[77] The (2+2+2) cycloaddition reaction of acetylenes and o-dihaloarenes 389 using a nickel complex as a catalyst in acetonitrile at 100 °C gave naphthalenes 390 in moderate to excellent yields with good tolerance of functional groups. The reaction of 1,2-diiodobenzene with dipropyl-, bis(methoxymethyl)-, bis(alkoxycarbonyl)-, and diarylacetylenes gave cycloaddition products 390 in moderate to excellent yields, while substituted diiodobenzenes bearing dimethyl, dimethoxy, and dioxole moieties on the aryl ring also underwent cyclization with alkynes; 1,2,3,4-tetrafluoro-5,6-diiodobenzene containing four electron-withdrawing fluoro groups also underwent the nickel-catalyzed cycloaddition reaction in lower yield. In addition, 1,2-dibromoarenes reacted successfully with acetylenes to give naphthalenes 390 in moderate to good yields (51–85%). They also examined the nickel-catalyzed reaction of diiodobenzenes with various diynes to furnish the corresponding naphthalenes.[77]


# 2.1.8
Other Metal-Catalyzed Reactions
In 2007, Kuninobu et al. reported the reaction of aldehyde 398 and aromatic ketimines 397 with dienes 399 catalyzed by a rhenium complex in toluene at 115 °C followed by treatment with sulfuric acid/acetic acid at r.t. to give naphthalenes 400 in good yields (Scheme [56]).[78] Using benzaldehydes substituted with a electron-releasing group gave naphthalenes 400 in lower yields (66–72%) compared to an electron-withdrawing groups (88%). A reaction with an aliphatic aldehyde gave the corresponding product in only 35% yield. Mechanistically, this reaction proceeds via C–H bond activation, insertion of an aldehyde, intramolecular nucleophilic cyclization, reductive elimination, elimination of aniline, a Diels–Alder reaction, and successive dehydration.


In 2008, Liu and co-workers reported an alternate pathway to produce substituted naphthalenes (Scheme [57]).[79] The reaction of inden-1-one with trimethylsilyldiazomethane gave cyclopropane-fused indenols 407 that underwent rearrangement in the presence catalytic europium triflate in DCE at 80 °C for 3 h to give 1-arylnaphthalenes 408 in 42–85% yield. The scope of the reaction included substitution by aryl groups such as 4- and 2-substituted and 2,6-disubstituted phenyl groups and 1-naphthyl.


In 2021, Maestri and co-workers used inexpensive and abundant manganese dioxide in a synthesis of tricyclic naphthalenes (Scheme [58]).[80] The cyclization of enynes 415 in the presence of manganese dioxide in dichloromethane at room temperature overnight gave naphthalene-1-carbaldehydes 416 in good yields. The scope of the reaction was examined, using a 4-chloro-substituted styrene derivative gave 420 in 54% yield, using a 3-methoxystyrene gave two isomers in 28% and 11% yield, and using a 3-(trifluoromethyl)styrene gave 418 in low yield (17%). The alkyne tether could be an ether (X = O) or a tosylamine (X = NTs), for example 421 was formed in 48% yield.


A gold complex was utilized by Malhotra et al. in 2013 for the synthesis of dihydronaphthalenes 424 (Scheme [59]). The annulation reaction of 2-alkynylbenzaldehyde 147 with acyclic or cyclic vinyl ethers, such as ethyl vinyl ether (423), using a gold complex as a catalyst in dichloromethane at room temperature for 30 min gave naphthalene-1-carbaldehydes 424 in moderate yields.[81]


#
#
# 3
Lewis Acid Catalyzed Reactions
In 2002, Viswanathan et al. reported a new protocol for the synthesis of naphthalenes 432 in 40–70% yield by the coupling of aromatic alkynes 430 and phenylacetaldehydes 431 in the presence of gallium trichloride (5.5 mol%) as catalyst in DCM at reflux for 20 h (Scheme [60]). Various aromatic alkynes were reacted with different phenylacetaldehydes to produce naphthalene derivatives in good to excellent yields.[82]


They next prepared naphthalenes 436 in 40–67% yield by the cross-coupling of epoxides 435 with alkynes in the presence of gallium trichloride in chloroform under reflux (Scheme [61]). The yields of naphthalenes 436 using stilbene oxides were generally better compared to those using styrene oxides.[83]


In 2002, Yamamoto, Asao, and co-workers used a novel gold-catalyzed strategy for the synthesis of 1-acylnaphthalenes 443 (Scheme [62]).[84] The benzannulation reaction of 2-ethynylbenzaldehydes 147 and alkynes using AuCl3 as a catalyst in DCE at 80 °C for 1.5 h gave 1-acylnaphthalenes 443 in moderate to good yields. The use of alkynes substituted with an electron-withdrawing group gave 1-acylnaphthalenes 443 in good yields; using internal alkynes gave moderate yields. The mechanism of the reaction was supported by a deuterium study. Mechanistically (Scheme [63]), the gold chloride complexes to the C≡C bond of the 2-ethynylbenzaldehyde 147, which increases the electrophilic nature of the alkyne. The successful attack of nucleophilic carbonyl oxygen on the electrophilic alkyne gives complex 147c, which undergoes a (4+2) cycloaddition reaction to give 147d and aromatization/elimination to give naphthalene derivatives 443.[84]




Next, in 2003, Yamamoto, Asao, and co-workers prepared 1-acylnaphthalenes and decarbonylated naphthalenes (Scheme [64]).[85] The [4+2] cycloaddition reaction of 2-ethynylphenyl ketones or 2-ethynylbenzaldehydes 147 with alkynes in the presence of gold chloride with DCE as solvent at 80 °C gave naphthyl ketones such as 450 in high yields. Performing the same reaction with copper triflate as the Lewis acid at 100 °C resulted in the formation of decarbonylated naphthalenes in good yields (not shown). Various substituted alkynes utilized as shown for the formation of 451–456. Mechanistically, increased electrophilicity of the alkynes by coordination of the gold catalyst with C≡C bond, followed by the nucleophilic attack of the oxygen atom of the carbonyl group, and finally Diels–Alder reaction gives the 2-ethynylphenyl ketones or 2-ethynylbenzaldehydes.




In 2004, Asao, Yamamoto, and co-workers reported a new methodology for preparing naphthalene derivatives (Scheme [65]).[86] The [4+2] cycloaddition of 2-alkynylbenzaldehyde 147 with enolizable ketones using gold bromide as the catalyst in 1,4-dioxane at 100 °C for 3 h gave 1-acylnaphthalenes 443 in good yields. Variously substituted enynes 147 and ketones were utilized and showed good tolerance towards the aryl group (446), alkyl group (457), alkoxy group (460), and other functional groups. Mechanistically, formation of a coordination bond between the gold catalyst and the C≡C bond of 147 occurs. The coordination enhances the electrophilic nature of the alkyne, facilitating the attack of the nucleophile oxygen atom of the carbonyl group. Subsequently, a reverse electron demand-type Diels–Alder reaction occurs followed by the dehydration to give 1-acylnaphthalenes.
In 2006, Shibata et al. reported the cycloisomerization of 1-ethynyl-2-vinylbenzenes 462 in the presence of gold chloride and silver triflate in dichloromethane at room temperature to give substituted naphthalenes 463 in 71–93% yield (Scheme [66]).[87] Cyclization of the 6-endo-dig-type proceeded dominantly to give 1,3-di- and 1,2,3-trisubstituted naphthalenes; the yields remained high irrespective of counteranions of the Ag salt. Various 1-ethynyl-1-vinylbenzenes were utilized, including phenyl-substituted enynes, to give the 6-endo-dig-type product 463 in high yields. When oxygen-substituted enynes were employed, a small amount of the 5-exo-dig-type product was formed. The use of 1-ethynyl- or 1-(iodoethynyl)-2-(1-methylvinyl)benzene gave predominantly the 5-exo-dig-type product.


Gevorgyan and co-workers, in 2008, reported a gold-catalyzed cascade reaction for the synthesis of unsymmetrically substituted naphthalenes (Scheme [67]).[88] The benzannulation reaction was initiated by the cycloisomerization of propargylic esters 470 in the presence of gold triflate with DCE as the solvent at room temperature. The cascade reaction involves 1,3- and 1,2-migration of two different groups. Various substrates exhibiting 1,2 migration underwent smooth reactions and gave naphthalenes 471 in good yields. Both terminal as well as substituted acetylenes gave naphthalenes in good yields.
In 2009, Balamurugan and Gudla reported an electrophilic addition reaction for the synthesis of substituted naphthalenes 479 (Scheme [68]).[89] Starting from arylacetaldehydes 478 and alkynes in the presence of gold chloride/silver complex as a catalyst in dichloromethane as solvent gave naphthalenes 479 in good yields. Mechanistically (Scheme [69]), the coordination of the oxygen atom of the carbonyl group with the gold catalyst leads to 478a, then electrophilic attack on the alkyne leads to the formation of a C–C bond at the β-carbon of the alkyne and generating the vinyl carbocation 478c. Subsequently, an aromatic electrophilic reaction takes place, ultimately providing naphthalene 479 through aromatization.






In 2011, Youn and co-workers reported the synthesis of polysubstituted naphthalenes (Scheme [70]) through a cyclization reaction. For example, the reaction of 3-(2-substituted-phenyl)-3-oxopropanoates 486 using 5 mol% of Ph3PAuCl/AgOTf (1:1) as the catalyst in DCE at 100 °C for 1.5 h gave naphthalenes 487/488 in up to 95% combined yield. 3-(2-Vinylphenyl)-3-oxopropanoates 486 with electron-donating or electron-withdrawing groups in the phenyl ring gave naphthalenes in moderate to excellent yields and the reaction was also applicable to 2-alkenylphenyl ketones and 2-alkenylbenzaldehydes.[90]




Also in 2011, Gudla and Balamurugan reported an intramolecular approach for the synthesis of arylnaphthalenes 498 (Scheme [71]).[91] Starting from ω-ethynyl benzyl ketones 497 using AuCl3/AgSbF6 in dichloromethane solvent at r.t. for 2 h gave carbo- and heterocycle-[b]fused naphthalenes 498 in good to excellent yields by initial intramolecular benzannulation of 497 followed by electrophilic attack of C=O on the alkyne. The ω-arylethynyl benzyl ketones 497 could be substituted by various groups in the benzyl group, such as F (504), Cl (502), and in the aryl group, such as OMe (503). Variation of the tether to the ethynyl group gave naphthalenes with fused tetrahydropyran (500), and tetrahydrofuran (499) rings. The naphthalene products were further employed in the synthesis of lactones natural products such as taiwanin C, retrojusticidin B, and justicidin E.[91]
In 2012, Song et al. reported the synthesis of oxanorbornenes and naphthalenes by efficient and selective cascade reactions (Scheme [72]).[92] The reaction of hydroxy enynes 505 in the presence of a gold complex/AgSbF6 in 1,4-dioxane at 100 °C gave naphthalenes 506 in moderate to good yields, while the use of the gold complex without AgSbF6 gave oxanorbornenes. Hydroxy enynes bearing an electron-withdrawing group gave higher yields of 506 compared to those with an electron-donating group.


Gudla and Balamurugan, in 2013, successfully synthesized 1-arylnaphthalenes 514 (Scheme [73]).[93] The rearrangement of epoxides 513 using a gold complex catalyst, followed by electrophilic attack of an alkyne in dichloromethane under refluxing conditions for 4–7 h gave 1-arylnaphthalenes 514. Substituted aryl epoxides 513 and 1-arylalk-1-ynes gave the corresponding arylnaphthalenes 514 in moderate to good yields. Substitution by various groups was tolerated, such as Cl (515, 518), F (517), and Br (517), while the use of the SiMe3 group lead to desilylated product.


In 2014, Liu et al. introduced a novel strategy for the synthesis of naphthalene derivatives (Scheme [74]).[94] The cascade cyclization of 1,5-enynes 513 using a gold complex catalyst in methanol at 80 °C gave various 1-alkoxynaphthalenes 514 in good yields. Substitution of the phenyl ring by an electron-donating group gave 1-alkoxynaphthalenes in good yields, but substitution by an electron-withdrawing group was only partially tolerated. Specifically, only the chlorine-substituted 1,5-enyne gave the corresponding product 516 in 75% yield; fluorine and nitro groups did not give the desired reaction. Mechanistically (Scheme [75]), initial alkoxylation of the alkyne in 513a through electrophilic activation gives dienol ether metal species 513b. Subsequently, the gold catalyst bonds to the enol ether, resulting in tautomerization and subsequent elimination of oxonium and methanol to give product 514a.[94]




In 2015, Yan et al. presented an efficient method for the synthesis of hydronaphthalene derivatives and cinnoline derivatives (Scheme [76]).[95] The Diels–Alder reaction of diyne esters 521 and substituted alkenes 522 in the presence of a gold complex catalyst in DCE at room temperature for 24 h gave hexahydronaphthalenes 523 in good yields. Notably, alkenes bearing electron-deficient substituents, such as tetracyanoethylene, gave the corresponding products, such as 524 and 528, in good yields.




In 2003, Barluenga and co-workers developed a regioselective synthesis of substituted naphthalenes in moderate to good yields (Scheme [77]).[96] The reaction of 2-alk-1-ynylbenzaldehydes 530 and alkynes in the presence of IPy2BF4 in dichloromethane at 0 °C to room temperature gave 1-iodonaphthalenes 531, while reaction with alkenes gave 1-acylnaphthalenes 443. For the formation of 1-iodonaphthalenes 531 or 1-acylnaphthalenes 443, the acetylene or ethene could be mono- or disubstituted with aryl, alkyl, and ester groups. Mechanistically, for the formation of 1-iodonaphthalenes 531, iodonium ion bonds to the alkyne, resulting in the formation of a benzo[c]pyridinium cation; subsequently, a cycloaddition reaction occurs, leading to the synthesis of 531.
In 2010, Abbastabar Ahangar et al. utilized the reaction of benzaldehydes, 2-naphthol, and urea in the presence of perchloric acid with silica with no solvent at 150 °C for the synthesis of 1,3-oxazino-fused naphthalenes 533 (Scheme [78]).[97] Using benzaldehydes substituted with electron-withdrawing or electron-releasing groups gave naphthalenes 533 in good to excellent yields. Mechanistically, the formation of an ortho-quinone methide or acylimine intermediate is followed by the addition of 2-naphthol and finally cyclization to give 533.


In 2011, Takai and co-workers reported the synthesis of 1-(4-halophenyl)naphthalenes 543 that were used in the Sonogashira cross-coupling of meso-tetraethynylporphyrins to give meso-tetrakis(arylethynyl)porphyrins (Scheme [79]).[98] The reaction of aromatic ketamine 540, 4-halobenzaldehyde 541, and N-methylmaleimide or alkene 542 in the presence of a rhenium complex catalyst in toluene at 115 °C for 48 h gave 1-(1-halophenyl)naphthalenes 543 in good to excellent yields. Alkenes N-methylmaleimide, dialkyl ethylenedicarboxylates and octene gave naphthalenes 544–549 under the optimized conditions and no significant steric and electronic effects were observed for the olefin substituents with the aryl halides remaining intact.


In 2013, Wei and co-workers reported the benzannulation reaction of enamines 550 and alkynes in the presence of iodosylbenzene and BF3·Et2O in TFE at 0 °C to r.t. to give a series of 1-aminonaphthalenes 551 (Scheme [80]).[99] Both electron-withdrawing and electron-releasing groups on enamines 550 and alkynes resulted in smooth reaction to give 1-aminonaphthalenes 551 in moderate to good yields. Mechanistically, depolarization of iodosylbenzene and BF3·Et2O results in reaction with the enamines and then attack of the nucleophile and subsequent electrophilic attack gives 551.


In 2013, Xiang, Wang, and co-workers reported the annulation reaction of arylacetylenes 559 and arylacetaldehydes 558 using the inexpensive catalyst BF3 ·Et2O in DCE at 80 °C for 15 h to give 1-arylnaphthalenes 560 (Scheme [81]).[100] The reaction proceeded with various substituted phenylacetylenes 559 and phenylacetaldehydes 558, resulting 1-arylnaphthalenes 560 in moderate to excellent. To investigate the synthetic applicability of this process, the reaction was scaled up to 10 mmol of starting materials, leading to the synthesis of the 1.32 g of a naphthalene derivative in 65% yield.


In 2014, Vishwakarma, Bharate, and co-workers used the reaction of electron-rich 1-styryl-2-methoxybenzenes 567 mediated by TFA catalyst at 80 °C for 30 min to give 2-phenylnaphthalenes 568 in good to excellent yields (Scheme [82]). Various substituted 1-styryl-2-methoxybenzenes 567, such as fluoro (569), chloro (570, 573), and methyl (571, 574) were utilized.[101]


Manojveer and Balamurugan, in 2014, prepared 1-acylnaphthalenes 576 in 57–90% yields by the domino reaction of 1-acyl-2-[(het)arylethynyl]benzenes with trimethyl orthoformate and catalytic trifluoromethanesulfonic acid in nitromethane at room temperature for 30 to 60 min (Scheme [83]).[102] This domino reaction proceeds via in situ incorporation of an acetal followed by intramolecular heteroalkyne metathesis/annulation in an o-alkynylacetophenone derivative. To understand the mechanism of the reaction, a deuterium experiment (Scheme [84]) was conducted. In the experiment, fully deuterated trimethyl orthoformate (DC(OCD3)3) was used in place of trimethyl orthoformate under the optimized conditions that showed deuterium incorporation at C2 of the 1-acylnaphthalene 583.[102]




Manojveer and Balamurugan next, in 2015, reported that the reaction between 2-(arylethynyl)benzaldehydes 584 and cycloalkanones 585 in the presence of triflic acid and trimethyl orthoformate in acetonitrile at room temperature gave 1-acylnaphthalenes 586 in moderate to good yields (Scheme [85]).[103] 2-(Phenylethynyl)benzaldehydes 584 reacted with cyclohexanones to give products with methoxy 589, fluoride 588, chloride 587, and hydroxy 591 groups. However, TBDMS protection was found to be unsuitable under the reaction conditions. The reaction of 2-(arylethynyl)benzaldehydes 584 and acyclic ketones require modification of the conditions to give 1-acyl-2,3-disubstituted naphthalenes 594 in good yields (Scheme [86]). The use of fluoro (596), methoxy (597), and hydroxy (599) substitution was successful. The mechanism for the formation of 1-acylnaphthalenes is by the formation of an oxocarbenium ion, followed by (2+2) cycloaddition reaction to form 586 or 594.




In 2017, Hu, Xu, and co-workers utilized the cascade reaction of ketonic ester 601 in the presence of 3,4,5-trimethylphenol (602) with hafnium triflate catalyst under nitrogen at 0 °C to r.t. to obtain naphthalene-2-carboxylates 603 (Scheme [87]).[104] The ketonic ester 601 substituted with an electron-donating group and fused rings gave 603 in good yields, but 601 substituted with an electron-withdrawing group gave low yields or no product formation.


Mutoh, Saito and co-workers, in 2018, reported the cycloisomerization and 1,2-carbon migration of 2-alk-1-ynylstyrenes 610 using a ruthenium complex with NaBArF4·2H2O in xylene at 130 °C for 1 h to give a range of 1,2-di- and 1,4,7-trisubstituted naphthalenes 611 in moderate to good yields (Scheme [88]).[105] Notably, 2-(arylethynyl)styrenes with an ester, methoxy, or alkyl group in the aryl substituent successfully gave 612–614 and 616 and 2-(benzoylethynyl)styrene gave 617 under these conditions; an ester group in the styrene benzene ring, however, gave 615 in low yield (43%).


In 2019, Chen et al. reported the (2+2+2) benzannulation of phthalic acid 618 and two alkynes using a ruthenium complex catalyst in γ-valerolactone (GVL)/DMF at 100 °C for 12–20 h to give multisubstituted 1-naphthoic acids 619 in 73–90% yield (Scheme [89]). The reaction tolerated substitution in the para position of 618, such as halide (623) and alkyl (621), and strong electron-withdrawing groups, such as CF3 (625) and NO2 (624). Mechanistically, activation of the C3–H bond of the phthalic acid 618 by the active ruthenium catalyst. Insertion of alkyne takes place, followed by decarboxylation and addition of second alkynes. Finally reductive elimination occurs, leading to the formation of 619.[106]




In 2019, Chen and co-workers reported the annulation of 1-(substituted ethynyl)-2-ethynylbenzenes 626 with 1 equivalent of triflic acid in chloroform at 50 °C to give 1-(triflyloxy)naphthalenes 627 in moderate to good yields (Scheme [90]).[107] The use of 1-(arylethynyl)-2-ethynylbenzenes with various substitution on the aryl and benzene ring were successful, but 1,2-bis(arylethynyl)- or 1,2-dialk-1-ynylbenzenes were unsuccessful. Mechanistically, initially activation of C-C triple bond occurs by using triflic acid, resulting in vinyl cation. This vinyl cation undergoes, followed by intramolecular annulation, results the naphthalene cation. Finally, an intermolecular nucleophilic attack of the OTf anion removed in 1-(triflyloxy)naphthalene 627.




In 2020, Balamurugan and co-workers used diversity-oriented method to prepare benzo-fused fluorenes, fluorenones, and naphthyl ketones. The synthesis of benzo[a]fluorenes 635 (Scheme [91]) and naphthyl ketones (not shown) was achieved selectively using TfOH and AgBF4, respectively, via in situ formed acetals. Aryl-fused 1,6-diyn-3-ones undergo triflic acid mediated intramolecular cyclization, leading to benzo[b]fluorenone derivatives via a radical intermediate (not shown).[108] The cycloisomerization of aryl-fused 1,6-diyn-3-ones 634 in the presence of trimethyl orthoformate and triflic acid in dichloromethane under nitrogen at r.t. for 3 h gives benzo[a]fluorenes 635 in moderate to good yields. However, the use of a 1,7-diarylhept-4-en-1,6-diyn-3-one gave a complex mixture. The mechanism for the formation of benzo[a]fluorenes 635 is shown in Scheme [92].[108] Initially, trimethyl orthoformate and TfOH prompt the conversion of compound 634 into acetal 634a. Subsequently, under the influence of a potent acid, one -OMe group undergoes protonation, leading to the expulsion of MeOH and the creation of the oxocarbenium ion intermediate 634c from intermediate 634b. At this juncture, an intramolecular cyclization of the alkyne transpires on the proton-activated alkyne, as depicted in intermediate 634c. This process may be facilitated by the electron-withdrawing nature of the oxocarbenium ion. Consequently, the resulting intermediate 634d undergoes aromatic electrophilic substitution on the vinyl carbocation to give rise to 634e. Ultimately, the benzo[a]fluorene derivative 635 materializes through the isomerization of intermediate 634e, followed by quenching with methanol.
In 2021, Saikia and co-workers used a simplified methodology to synthesize substituted 9H-dibenzo[3,4:6,7]cyclohepta[1,2-a]naphthalenes 644 in up to 85% yields from phenylacetaldehydes 643 and 2-alk-1-ynylbenzyl alcohols 642 in the presence of Lewis acid BF3·OEt2 in toluene at 100 °C (Scheme [93]).[109] Using a 2-alk-1-ynylbenzyl alcohol 642 substituted with an electron-donating group gave the products, e.g. 647 and 648, in moderate yields. However, when a moderate electron-withdrawing group, such as Cl, was present, the yield was low and strong electron-withdrawing groups did not yield any product. The mechanism proceeds with the activation of the aldehyde in the presence of Lewis acid followed by double Friedel–Crafts reaction to synthesize the desired product.


Also in 2021, Wang, Hu, and co-workers reported the aldol and Friedel–Crafts reactions between 1,2-diarylethanone 651 and acetophenone 652 in the presence of triflic acid catalyst in toluene at 80 °C for 4 h to give 1,3-disubstituted naphthalenes 653 (Scheme [94]).[110] Ketones and arylacetaldehyde substituted with NO2 (654), CF3 (655), Me (659), OMe (658), and Br (656) all gave 1,3-disubstituted naphthalenes in good to excellent yield. Mechanistically, initial formation of an enol intermediate in the presence of triflic acid is followed by aldol reaction with the α,β-unsaturated ketone, resulting in the formation of a stilbene intermediate. Finally, a Friedel–Crafts reaction takes place to form a 1,3-disubstituted naphthalene.


# 4
Miscellaneous Reactions


In 2006, Wang et al. used the double nucleophilic substitution reaction of 1,4-dilithio-1,3-dienes 661 and hexafluorobenzene in the presence or absence of DME at r.t. for 3–24 h to prepare variously substituted fluorinated naphthalene derivatives 662 in moderate to good yields (Scheme [95]).[111] When 1,4-dilithio-1,3-dienes were treated with hexachlorobenzene, chloropentafluorobenzene, or bromopentafluorobenzene, chlorine–lithium or bromine–lithium exchange reactions, instead of nucleophilic substitution, took place.
In 2010, Xu and co-workers utilized the domino reaction of 2-(2-oxoethyl)benzaldehydes 669 and nitroalkenes 670 with pyrrolidine as a catalyst and DMAP as an additive in CH2Cl2/water (1:1) at 0 °C to r.t. for 4 h to obtain 3-nitronaphthalene-1-carbaldehydes 671 in 45–65% yield (Scheme [96]).[112] When electron-deficient groups were attached to the aryl group on the nitroalkenes, the reaction proceeded more rapidly compared to electron-releasing groups. Mechanistically, formation of an enamine occurs by reaction of the dialdehyde with the pyrrolidine catalyst and then Michael reaction of the enamine and the nitroalkene generates a zwitterion intermediate. This is followed by a Henry reaction, and finally, dehydration and oxidation steps to give the 3-nitronaphthalene-1-carbaldehyde.


Also in 2010, Zhou and co-workers reported an allene-mediated electrocyclization method for the synthesis of polyfunctionalized benzenes and naphthalenes (Scheme [97]).[113] Electrocyclization of 3-{2-[3-(phenylthio)prop-1-ynyl)phenyl}acrylates or -acrylamides 678 in the presence of triethylamine or DBU with acetonitrile under nitrogen at 60 °C for 12 h gave 3-[(phenylthio)methyl]naphthalene-2-carboxylates or -carboxamides 679 in moderate to good yields. The electron-withdrawing group attached to 678 increases the acidic nature of the CH2 group adjacent to the sulfur atom. This acidity enhancement promotes the propargyl-allenyl isomerization, which, in turn, facilitates the six π-electrocyclization process. This improves the smoothness of the reaction and increases the overall yield of naphthalene.
In 2012, Wang et al. reported an unexpected rearrangement in the reaction of 1,2-bis(bromomethyl)benzenes 686 with 1,3-dicarbonyl compounds 687 using Cs2CO3 (2 mmol) in DMSO solvent at 80 °C for 2 h to give 2,3-disubstituted naphthalenes 688 in good yields (Scheme [98]).[114] Mechanistically (Scheme [99]), alkylation of the 1,3-dicarbonyl compound by the 1,2-bis(bromomethyl)benzene gives intermediate tautomers 686a and 686b . The enolic forms results in competition between the seven-membered ring 686e, through O-alkylation, and the five-membered ring 686c, by C-alkylation. Due to the stability of the seven-membered ring, it is the major product. Rearrangement of 686e leads to naphthalene 686f and aromatization in the presence of Cs2CO3 gives naphthalene 688.[114]






Also in 2012, Liang and co-workers used the cascade reaction of 1-[2-(4-methoxyphenylethynyl)phenyl]-3-phenylprop-2-yn-1-ols 695 using iodine monochloride in propanol at r.t. to give 1,3-diiodo-2-phenylnaphthalenes 696 (Scheme [100]).[115] In terms of substrate scope, substitution at R3 of 695 by an electron-withdrawing or -donating group gives 696 in good to excellent yields but the reaction give very low yields when substitution at R4 is present (Cl or Me).


In 2015, Ponra et al. reported a metal-free synthesis of polysubstituted naphthalenes 704 (Scheme [101]). The benzannulation reaction of phenylacetaldehyde 703 and 1-arylalk-1-yne 704 using HNTf2 as a catalyst in DCE at r.t. gave polysubstituted naphthalenes 704 in 41–78% yield with broad substrate scope. The reaction was also tested on a gram scale to obtained the naphthalene product in 68% yield.[116]


In 2016, Yanai et al. developed a two-step process to construct tetrasubstituted naphthalenes 715 (Scheme [102]).[117] In the first step, the reaction of coumarins 711 and silyl acetal 712 in the presence of zwitterion 713 in DCE at 0 °C–r.t. gave silyl ethers 714. In the second step, desilylation of the alkyne silylated ethers 714 with TBAF followed by aldol condensation in tetrahydrofuran at 70 °C for 1–3 h resulted 715 in excellent yields for both steps. The acetylene silylated ethers tolerated various functional groups such as thienyl, acetyloxy, and MOM groups. Mechanistically, fluoride-induced ring-opening reaction of the silyl ether 714 is followed by proton transfer and then intramolecular Michael reaction or 6π-electrocyclization, and finally aromatization to yield 715.


Also in 2016, Chang and Cheng utilized a novel route using the Diels–Alder reaction of β-keto sulfones 723 and styryl bromides 722 with potassium carbonate and acetone to give 724 that on reduction with sodium borohydride followed by treatment with BF3·OEt2 (reflux, 10 h or r.t. 5 h) gives 1-aryl-4-(bromomethyl)naphthalenes 726 or 1-aryl-4-(bromomethylene)-2-sulfonyltetralins 725 (Scheme [103]).[118] Substitution by both electron-withdrawing and -donating groups was tolerated and gave naphthalenes 726 in moderate to good yields. The mechanism of the reaction involves SN2 annulation of the reduced intermediate 724, followed by deprotonation and aromatization, resulting in the formation of 726.


Wu and co-workers, in 2016, reported the transition-metal-free preparation of functionalized naphthalenes (Scheme [104]).[119] The (2+2+2) reaction of alkynoates 735, benzyne precursor 734, and ketones 733 in the presence of cesium fluoride in acetonitrile at 80 °C gave naphthalene-1-carboxylates 736. The substrate scope of the ketones 733, which included cyclohexane-1,3-diones, was explored and gave the naphthalenes 736 in good yields (74–88%). Various alkynoates 735, such as diethyl acetylenedicarboxylate, gave naphthalenes 736 in moderate to good yields (56–89%), and various substituted benzynes gave naphthalenes 736 in moderate to good yields (59–89%). Mechanistically (Scheme [105]), reaction of 733a with CsF give the anion 733b that reacts with DMAD 735a to give vinylic anion 733c. Reaction of 2-(trimethylsilyl)phenyl triflate 734 with CsF gives benzyne 734a that reacts with 733c to give intermediate 733d that is aromatized to give naphthalene 736a.




In 2017, Wang and Zheng used the photocatalyzed(4+2) annulation reaction of alkynes and 1-aminobenzocyclobutenes 744 using an iridium complex in toluene for the formation of naphthalenes 745 in 40–65% yield (Scheme [106]).[120] The reaction utilized various arylacetylenes and unsymmetrical internal alkynes and substitution by more labile groups in 744, like OBn (746), OMe (750) was tolerated.


In 2018, Singh et al. reported a one-pot, transition-metal-free protocol for the domino Michael/SNAr reaction of alk-1-ynyl phenyl ketone 752 and nitromethane using cesium carbonate in DMF at 100 °C for 12 h to give to 4-nitro-1-naphthols 753 in moderate to excellent yields (Scheme [107]).[121] Nitromethane is utilized as a one carbon carbanion source that is incorporated into a variety of ynones and ends up as an aromatic nitro substituent. The ynones display wide substrate scope and functional group tolerance and substituted nitromethanes as well as with alicyclic o-haloynones have been successfully used. Mechanistically, addition of nitromethane to the ynone results in an intermediate that undergoes tautomerization to generate an enone, which undergoes carbanion formation followed by SNAr reaction that displaces the aryl bromide. Finally, aromatization forms the 4-nitro-1-naphthol 753.[121] This reaction was used in the total synthesis of the polycyclic alkaloid macarpine.


Also in 2018, Cao et al. used a cascade radical addition reaction of arylacetylene 761 with 2-vinylaniline 760 using catalytic tetrabutylammonium iodide with isoamyl nitrite in benzotrifluoride at 70 °C for 24 h to give 1,3-diarylnaphthalenes 762 in 15–61% yield (Scheme [108]). The substrate scope included substitution by electron-deficient and electron-rich groups on the 2-vinylaniline 760 and arylacetylene 761.[122]


Shu, Wu, and co-workers, in 2018, reported the benzannulation reaction of benzynes (from precursor 734), alkynoates, and cyanomethyl ketones 769 using cesium fluoride and cesium carbonate in acetonitrile at 80 °C for 15 h to give polysubstituted naphthalenes 770 in 42–82% yield (Scheme [109]).[123] The substrate scope included electron-rich, electron-deficient, and electron-neutral groups on cyanoacetophenones 769 and substituted 2-(trimethylsilyl)phenyl triflates. Dibenzoylmethane was also used instead of cyanomethyl ketones 769 and gave naphthalenes 48–84% yield.


In 2018, Mukherjee and co-workers synthesized naphthalenes 780 by the Diels–Alder reaction between arynes (from precursor 734) and substituted glycals 779 followed by π-annulation (Scheme [110]).[124] The reaction of arynes (from precursor 734) and substituted glycals 779 in the presence of KF with 18-crown-6 as an additive in MeCN at r.t. for 10 h resulted in complete conversion to give naphthalenes 780 in 47–89% yield. Various glycal-based donors were utilized particularly focusing on donors with electron-withdrawing groups (EWGs) such as esters. The presence of EWGs increased the possibility of ring opening to give meta-disubstituted naphthalenes 780. The reaction was also applied to chiral phenanthrenes by using a glycal donor with a 1,2-naphthyne precursor. Mechanistically (Scheme [111]), cycloaddition reaction between the benzyne source 734 and the diene 779a gives an allylic carbanion 779c that undergoes alkoxide elimination followed by protonation leading to annulation and π-extension to give 780a. Overall, this methodology provides an efficient and practical approach to the synthesis of naphthalene derivatives, allowing for the incorporation of different substituents and even chiral centers. The mild reaction conditions and high yields make it a valuable tool in synthetic organic chemistry.[124]




In 2019, Ma et al. reported an electrochemical synthesis of naphthalene derivatives (Scheme [112]).[125] The (4+2)-benzannulation reaction of styrenes 791 and 792 using a carbon felt anode and a Ni plate cathode in CH3CN/THF with an electrolyte solution of Bu4NBF4 at 25 °C gave disubstituted naphthalenes 793. Styrenes 791 containing a primary alkyl group in the α-position (R2) gave the corresponding naphthalenes in high yields, but tertiary alkyl groups gave naphthalenes in lower yields.


Also in 2019, Feng et al. reported the reaction of allenyl 2-ethynylphenyl ketone 800 with tert-butyl nitrite (801) in DCE at 50 °C to give cyclobutanol-fused 2-nitro-1-naphthols 802 (Scheme [113]).[126] In terms of substrate scope, substituents (R1) at the benzene ring, like chloro (804) and methoxy (805) gave the products in moderate to good yields. Using aryl-substituted acetylenes (R2 = Ar), like chloro (806) and (807), gives the products in moderate to good yields. However, when a cyclopropyl group was attached phenyl ring bonded with alkyne, the reaction became more complicated, likely leading to lower yields or undesired side products. The reaction was tunable and starting from the same substrate but using tert-butyl nitrite/TEMPO gave nitrocyclobutane-fused naphthalene-1,2-diones.


In 2021, He, Gao, and co-workers reported the tandem Ti(O-iPr)4-promoted photoenolization Diels–Alder reaction and aromatization of 2-methylbenzaldehydes 809 and sterically hindered cyclohexenones 810 to give cyclohexane-fused naphthols 811 in up to 68% yield (Scheme [114]).[127] The photoenolization Diels–Alder reaction of 2-methylbenzaldehydes 809 and sterically hindered cyclohexenones 810 in the presence of Ti(O-iPr)4 in 1,4-dioxane was followed by oxidation with 2-iodoxybenzoic acid in the presence of DMSO and aromatization employing DDQ in toluene. Under these conditions, different substrate scopes for the formation of 812–817 which were obtained in moderate yields. Cyclopentane-fused naphthalenes were also successfully synthesized in up to 77% yield.


In 2022, Chen et al. synthesized aryldihydronaphthalenes 819 (Scheme [115]).[128] Intramolecular cyclization of acyl enynes 818 in the presence of 2,6-di-tert-butyl-4-methylphenol (BHT) in 1,4-dioxane at 80 °C for 32 h gave aryldihydronaphthalenes 819 in good to excellent yields. The substrate scope included the halo group or electron-releasing groups The synthetic importance of the reaction was further demonstrated by performing the reaction on a gram scale, which resulted in the corresponding product in 82% yield. Mechanistically intramolecular dihydro-Diels–Alder reaction of styrene-ynes leads to aryldihydronaphthalene product.




Also in 2022 Lu, Lei, and co-workers reported an electrochemical reaction for the oxidative annulation of styrenes 826 and 827 in an undivided cell using tris(4-bromophenyl)amine as a mediator, n-Bu4NClO4 as an electrolyte, and a mixed solvent of acetonitrile and hexafluoroisopropanol at r.t. under nitrogen for 4 h to give 1,2-dihydronaphthalenes 828 (Scheme [116]).[129] Styrene 826 containing an α-methyl substituent (R2) gave dihydronaphthalenes such as 829 to 834 in 47–78% yield. Substitution in the benzene ring of the styrene (R1) such as tert-butyl and bromo gave 831 and 833, respectively, in 56% and 47% yield. Styrene 826 containing an α-substituent (R2) such as isopropyl, hexyl, ethyl, etc. did not give the reaction.
Also in 2022, Zou et al. reported the synthesis of naphthalene derivatives using a (4+2) annulation reaction (Scheme [117]).[130] Reaction of styrene oxides 835 with substituted alkynes, shown for diphenylacetylene, in the presence of MeOTf as a catalyst and tosyl chloride in DCE at 110 °C for 24 h gave naphthalenes such as 836 in up to 95% yield. The reaction was also performed on a gram scale and gave 78–82% yield.


Gnanaprakasam and co-workers, in 2022, synthesized naphthols and benzo[e/g]indole derivatives from α- and β-tetralones and alcohols or 2-amino alcohols. For example as shown in Scheme [118], the reaction of β-tetralone (842) and alcohols 843 using sodium hydroxide in toluene at 140 °C for 24 h gave 3-substituted 2-naphthols 844 in good to excellent yields. Alcohols 843 substituted with electron neutral groups such as benzyl alcohol gave the 3-phenyl-2-naphthol 844 85% yield. When the benzyl alcohol 843 had an electron-donating group then 3-phenyl-2-naphthols were obtained in 56–83% yield, while electron-deficient groups, such as chloro (847), bromo (849), and CF3 (848) gave moderate to good yields.[131]
# 5
Conclusion
In summary, this review discusses various chemical methodologies for the synthesis of substituted naphthalene derivatives using transition metal complexes and Lewis acids. Naphthalene synthesis is an active area of research because it shows numerous applications in the pharmaceutical, medicinal, and agrochemical industries. Substituted naphthalenes have been prepared by using various reactions such as cross-coupling reactions, cascade reactions, domino reactions, cycloisomerization reactions, Diels–Alder reactions, etc. These reactions are catalyzed by complexes of different transition metals such as rhenium, palladium, zinc, copper, etc, and different Lewis acids like gold, gallium, and boron complexes and trifluoroacetic acid, etc. Miscellaneous types of reactions involve both metals and a Lewis acid as a catalyst for the synthesis of naphthalene derivatives. While covering this review we observed that transition metals and Lewis acids are generally required to complete the reactions for the preparation of naphthalene derivatives. To overcome these limitations, novel approaches with simple, mild, and greener reagent systems as alternative approaches need to be developed for the construction of substituted naphthalene derivatives.


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Conflict of Interest
The authors declare no conflict of interest.
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Corresponding Author
Publication History
Received: 21 July 2023
Accepted: 20 September 2023
Accepted Manuscript online:
20 September 2023
Article published online:
11 December 2023
© 2023. Thieme. All rights reserved
Georg Thieme Verlag KG
Rüdigerstraße 14, 70469 Stuttgart, Germany
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