Arenophile-Mediated Dearomative Functionalization Strategies

Abstract

The dearomatization of arenes is a fundamental synthetic strategy, providing a direct connection between simple hydrocarbons and valuable, more complex intermediates. While several strategies exist, the functionalization with concurrent introduction of functionality (i.e., dearomative functionalization) is still a largely underdeveloped field. This Synpacts article provides an overview and insights from our recent work in this area using small molecules—arenophiles.

1 Introduction

2 Arenophiles

3 Olefin-Like Dearomative Functionalizations

4 Arenophiles and Transition-Metal Catalysis

5 Applications in Natural Product Synthesis

6 Conclusion

Introduction

Aromatic hydrocarbons (arenes) are some of the most elementary feedstock chemicals, produced annually on a million-metric ton scale. Chemical transformations of these compounds play a fundamental role in everyday life; for example, in the production of polymers, paints, agrochemicals, and pharmaceuticals. An integral part of the arene reaction toolbox is dearomatization:[1] a transformation that converts simple, readily available arenes into more synthetically elaborated, unsaturated intermediates. Several classes of these reactions exist, including the venerable dissolving metal (Birch) reduction,[2] dearomative hydrogenation,[3] oxidative dearomatization of phenols,[4] transition-metal-mediated dearomatizations,[5] arene-alkene meta-photocyclo­addition,[6] and microbial oxidation[7] (Scheme [1]). Historically, development of these processes has had an immense ­impact on the science of synthesis, providing novel disconnections and rapid access to numerous natural products and compounds of interest, as exemplified with syntheses of morphine[8] (1), oseltamivir[9] (Tamiflu™) (2), and jesterone[10] (3).

While established dearomatization methods are exceptionally powerful, they generally do not result in introduction of additional functionality. Therefore, most dearomatized products have to be subjected to further manipulations to reach the desired level of complexity. To date, only stoichiometric reactions of transition-metal complexes based on Os, Ru, Re, Cr, or Mn can enable more elaborate dearomative functionalizations;[11] however, the cost and toxicity of these metals have been major obstacles for more widespread use of these methods.

Arenophiles

Arene-based cycloaddition reactions are an important group of dearomatization strategies.[12] While aromatic compounds are well known for their inertness, they become remarkably reactive upon photoexcitation and can even undergo cycloaddition with alkenes[6] (arene-alkene meta-photo­cycloaddition, see Scheme [1]). This pronounced reactivity is attributed to a relatively high π,π*-singlet state of the aromatic nucleus which is accessed using a high-energy UV light. Although meta-photocycloaddition is well documented and has found several synthetic applications in organic synthesis, the orthogonal ortho- and particularly the para-photocycloadditions are rarer and have not received as much attention.

In 1984, Sheridan reported a unique visible-light-mediated transformation between naphthalene (4) and N-methyl-1,2,4-triazoline-3,5-dione (MTAD) (5),[13] delivering naphthalene-MTAD para-cycloadduct 6 (Scheme [2]). The product 6 was obtained in a ca. modest 40% isolated yield, likely due to its labile nature as the reported half-life was 12 hours at room temperature. A few years later, Sheridan described a similar transformation, this time using benzene (7) as a substrate.[14] The corresponding adduct 8 was thermally labile and could be detected and elucidated only using low-temperature NMR, with a reported half-life of 1 hour at 0 °C. Although the exceptional, superdienophile reactivity of MTAD in thermal [4+2] cycloaddition is well precedented,[15] including for polynuclear arenes such as anthracene, this report established the first highly efficient and selective para-cycloaddition on simpler starting arenes such as ­benzene and naphthalene.

On the basis of several mechanistic studies,[16] Sheridan postulated that the photoaddition of MTAD to naphthalene and phenanthrene proceeds via both singlet and triplet MTAD states and that it is most likely a concerted process. While application of the Rehm–Weller equation has shown that electron transfer from naphthalene to ¹MTAD* is exergonic, these studies have also revealed that electron transfer between benzene and ¹MTAD* is endergonic, suggesting that a different mechanism is operating for this mononuclear arene. In addition to Sheridan’s studies, Breton’s group explored the reversibility of cycloadduct formation between naphthalene derivatives and MTAD.[17] To the best of our knowledge, no other studies have been reported on this photochemical transformation or any other reactions involving the corresponding MTAD-arene cycloadducts.

We recognized that this para-dearomative cycloaddition process could be synthetically very useful, providing orthogonal periselectivity to arene-alkene cycloaddition under milder visible-light-mediated conditions. Therefore, we decided to perform a broader search for photoactivable 2π-components that could react with arenes in a para-fashion, defined as arenophiles, in analogy with thermal cyclo­addition processes. Based on Sheridan’s seminal investigations,[16] arenophiles likely react with arenes through the formation and collapse of two different types of exciplexes: photo-induced electron-transfer (I) or charge-transfer (II) exciplex between the arene and the excited arenophile (Scheme [3]). This notion served as a fundamental electronic requirement, as both the HOMO and LUMO energies of the arenophile have to be within the range of the HOMO of the arene for electron transfer to occur. In this context, we evaluated the feasibility of potential arenophiles for photocyclo­addition chemistry using computational analysis with benzene (HOMO = –9.9 eV) and naphthalene (HOMO = –8.4 eV) as standards. In addition to MTAD (5), a number of other 1,2,4-triazoline-3,5-diones 9, 11, and 12, and other symmetric, electron-deficient, cyclic (Z)-diazo-containing compounds, such as 10 and 13, were found to have HOMO–LUMO gaps within the range of benzene (dotted red line). Indeed, upon their synthesis and evaluation, we have found that compounds 9, 5 and 11 reacted with benzene and naphthalene, and that compounds 10, 12, and 13 underwent significant visible-light-induced decomposition. Since arenophile 11 is labile—it could be prepared and used only in situ—and arenophile 9 requires a rather expensive synthetic precursor [3,5-bis(trifluoromethyl)phenyl isocyanate], we decided to continue our studies with MTAD (5), which could be routinely prepared on a decagram scale.

Several aspects regarding para-cycloaddition of MTAD with arenes are noteworthy: (1) simply using commercial-grade, visible light diodes resulted in complete reaction in two to three hours on multi-millimole scale. In addition to ¹H NMR analysis, completion of the reaction was clearly indicated by bleaching of the characteristic magenta color of MTAD. (2) The photocycloaddition reaction proceeded successfully in dichloromethane, ethyl acetate, propionitrile, or acetone as solvent, and was found to be relatively insensitive to moisture and oxygen. (3) The benzene-MTAD cyclo­adduct proved to be stable to temperatures up to –50 °C. However, we observed slow retrocycloaddition between –50 °C and –30 °C, and rapid cycloreversion to benzene and MTAD was seen above –20 °C. Cycloreversion resulted in quantitative formation of benzene and MTAD (>98% by ¹H NMR analysis with an internal standard).

Olefin-Like Dearomative Functionalizations

Arenophiles offer a conceptually distinct approach toward dearomative functionalization, as the corresponding para-cycloadducts could undergo a plethora of functionalization methods using olefin or transition-metal-catalyzed transformations. Specifically, by performing chemoselective alkene reactions on arene-arenophile cycloadducts and subsequent cycloreversion of the arenophile moiety, a formal dearomative functionalization of a single π-bond within the aromatic starting material could be achieved (Scheme [4]). Moreover, the arenophile could undergo fragmentation to deliver functionalized cyclohexane-1,4-diamine derivatives. Consequently, the proposed synthetic strategy bridges the gap between the limited dearomative toolbox and the vast area of olefin functionalization chemistry.

Our initial foray into this area was in the development of a dearomative cis-dihydroxylation strategy,[18] a chemical equivalent of microbial arene oxidation (Scheme [5]). Thus, under optimized conditions, using MTAD (5) as an arenophile and modified Upjohn dihydroxylation conditions,[19] a series of benzene derivatives underwent dihydroxylation (Scheme [5, a]). The addition of pTsNH₂ was needed to accelerate osmate ester hydrolysis, a relatively slow process, especially at low temperatures. While free bicyclic diols could successfully undergo cycloreversion, the corresponding dihydrodiols proved rather unstable to purification and storage; therefore, they were protected as acetonides. The use of hydrazine or KOH, followed by oxidation of the resulting hydrazine to a diazene, and instantaneous expulsion of nitrogen, generated dihydrodiol derivatives 14–23. In addition to benzene (14), alkyl (15), silyl (16), and substituents incorporating alcohols (17 and 18), acetals (19 and 20), acid (21), and halogens (22 and 23) were tolerated. The mildness of this method permits the use of bromobenzene, with product 23 constituting the first example of chemical-based dearomatization of this feedstock compound.

On the other hand, dihydroxylated cycloadducts can undergo two-step arenophile fragmentation to deliver tetrafunctionalized diaminohydrodiols (24–32). Accordingly, one-pot hydrolysis of urazole and protection of the cyclic hydrazine with benzoyl chloride, and subsequent single-electron reduction of the N–N bond using SmI₂ gave the desired products. Again, a range of functionality was tolerated, including silyl (26), hydroxy (27–29), and halogen (31 and 32) substituents. Importantly, all dihydrodiol and diaminohydrodiol products were obtained as a single constitutional isomer and diastereoisomer.

In addition to mononuclear arenes, higher-order aromatic compounds proved viable substrates for this dearomative protocol (Scheme [5, b]). Thus, a two-step dearomative diaminodihydroxylation strategy, involving Sharpless modification of Upjohn dihydroxylation[20] and one-pot urazole fragmentation using hydrazine and subsequent hydrogenolysis with Raney-Ni, furnished a set of highly functionalized small molecules (33–42). This approach displayed high site-selectivity for terminal, non-substituted aromatic rings, and tolerated halogen (34–37), amine (37), and benzylic acetal (38) functionality. Moreover, the naphthalene moiety reacted preferentially in the presence of phenyl or 2-pyridinyl systems (39 and 40), likely because of the lower ionization potential, corresponding to the ease of exciplex formation. Finally, even trinuclear acridine and phenanthrene furnished the desired products (41 and 42) as a single constitutional isomer and diastereoisomer.

Next, by applying the arenophile/olefin functionalization/arenophile manipulation blueprint (Scheme [4]), we were able to perform a formal, chemoselective reduction of a single double bond, delivering cyclohexadienes and 1,4-diaminocyclohexene derivatives (Scheme [6, a]).[21] The most suitable reducing agent was found to be diimide,[22] well known for selective reduction biased on alkene substitution patterns. Accordingly, MTAD-arene cycloadducts underwent reduction in situ by employing diimide precursor potassium azodicarboxylate and acetic acid. A variety of cyclohexa-1,3-dienes (43–55) were obtained, all complementary to those accessed by Birch reduction. Perhaps most notable are products containing benzylic heteroatom functionality (49–53), as such arenes could not be used in dissolving metal reductions. Additionally, 2-phenylbenzoic acid gave diene product 55, showcasing the orthogonal chemoselectivity of arenophile-mediated dearomatization to dissolving metal reduction. Finally, the reduced bicycloadducts were also subjected to arenophile fragmentation to furnish a variety of functionalized syn-1,4-diaminocyclohex-2-enes (56–64).

Importantly, this dearomative strategy was also successfully applied to polynuclear arenes and heteroarenes (Scheme [6, b]). For example, in addition to naphthalene and derivatives (65–67), phenanthrene (68), 3-bromoquinoline (69), quinazine (70), acridine (71), and benzo[h]quinoline (72), all underwent a formal dearomative bis-1,4-hydroamination. While arenophile fragmentation in these cases provided the corresponding 1,4-diamines, it could also be additionally altered to install other functionality (Scheme [6, c]). For instance, naphthalene-derived reduced cycloadduct 73 could undergo urazole cycloreversion and subsequent cycloaddition with oxygen or nitrosobenzene, to furnish γ-hydroxyketone 74 after Kornblum–DeLaMare rearrangement[23] or 1,4-hydroxyamine 75 after N–O bond cleavage.

Arenophiles and Transition-Metal Catalysis

While arenophile-based chemistry enables olefin functionalization chemistry to be applied on arenes, it also opens a new avenue to explore these cycloadducts through transition-metal-catalyzed processes. Since arene-arenophile para-cycloadducts are strained molecules characterized by bis-allylic bridgehead positions bearing electron-deficient heteroatoms, they could be competent substrates for oxidative addition with low-valent metals. Thus, a suitable nucleophilic ring-opening reaction would not only capture the latent topography of the heterobicyclic framework, but also install new functionalities depending on the nature of the nucleophile and transition-metal catalyst. Along these lines, we have recently reported a catalytic dearomative carboamination strategy for benzene (7),[24] using a nickel catalyst and aryl Grignard as a nucleophile (Scheme [7, a]). Thus, exposure of the MTAD-benzene cyclo­adduct to [Ni(cod)₂], dppf (77) and arylmagnesium bromide delivered the desired product 76 in 74% yield, whilst the use of (R,R_p )-iPr-Phosferrox (78) as the ligand provided a 75% yield and 98:2 er.

This dearomative elaboration is mechanistically very intriguing and unusual (Scheme [7, b]). Although more detailed studies are required, the reaction likely starts with π-coordination of the substrate to the electron-rich Ni(0) complex that is anti to the arenophile (species I). Oxidative addition then affords intermediate II, followed by transmetalation with the Grignard reagent to form species III. This symmetric η⁵-Ni(II) complex undergoes regioselective reductive elimination to deliver diene complex IV, which, after decomplexation, yields the product 76 and regenerates the Ni catalyst. Importantly, the exclusive selectivity of nucleo­philic addition to the terminal position of the cyclohexa­dienyl complex can be rationalized by the well-established Davies–Mingos–Green rules.[25] Enantiodiscrimination according to this proposed mechanism involves the differentiation of the enantiotopic termini of the η⁵-system III (positions 2 vs 6). Alternatively, the Ni catalyst could also undergo oxidative addition into the bis-allylic C–N bond followed by outer-sphere nucleophilic attack from the face opposite the metal. This dearomatization reaction served as an entry point to the synthesis of pancratistatins (vide infra).

To learn more about the general applicability and reactivity of arenophile-arene cycloadducts with transition metals, we also initiated further investigations with other metals and nucleophiles. Accordingly, we found that palladium readily engaged cycloadducts in nucleophilic allylic substitution reactions with lithium enolates, delivering syn-1,4-carboaminated products (Scheme [8, a]).[26] Since this dearomative strategy proceeds through π-allyl Pd species V, the resulting products are complementary to those obtained through ring opening of more strained azabicycles, which was pioneered by Lautens.[27] Thus, under optimal reaction conditions, using Pd(dba)₂ and dppb (2.5/3.0 mol%) as catalyst and Li-enolates prepared from ketones or esters and LDA, a range of carbonyl compounds reacted successfully (Scheme [8, b]). In addition to aryl ketones (79–82) bearing various levels of substitution, aliphatic, acyclic (83–85) and cyclic (86–89) ketones also performed well. Moreover, a representative set of esters (90–93), and even glycine derivative (94) gave the desired products in synthetically useful yields. Importantly, in all cases with naphthalene (4) as a substrate, the products were obtained as a single constitutional isomer. Finally, this dearomative strategy could be conducted in an asymmetric fashion and several of these compounds were prepared in high optically purity using SEGPHOS- or Phosferrox-type ligands.

Equally broad was the scope of arenes (Scheme [8, c]). Symmetrically-substituted (95–97) and other naphthalene derivatives (98–103), as well as polynuclear phenanthrene (104) were good substrates for Pd-catalyzed dearomative carboamination. Constitutional isomers were obtained in all cases where cycloadducts were unsymmetrical, with moderate selectivity of ring opening observed for 1-substituted naphthalene derivatives (98–102). Moreover, in addition to polynuclear arenes, benzene (7) also reacted with enolates derived from ketones or esters (Scheme [8, d]) (105–108).

Finally, the synthetic utility of dearomative bis-1,4-carboamination was showcased using naphthalene-derived product 109, which could be prepared on a gram scale in 82% yield (Scheme [8, e]). Thus, the urazole moiety (NR_U) could be completely removed (109 → 110) using hydrogen and Pd/C, oxidized to a ketone (111, 112, 114) with household bleach or tBuOCl, or converted into an amine (113) using Adams’ conditions.[28] Ultimately, these results demonstrate the applicability of arenophile chemistry to rapidly access functionalized small molecules that would be otherwise challenging to prepare.

Applications in Natural Product Synthesis

Dearomative functionalization strategies with arenophiles offer a unique way forward for rapid and controlled formation of molecular complexity. Thus, we have recently demonstrated the application of arenophile-based dearomative functionalizations in the synthesis of several aminocyclitol natural products (Scheme [9]). Specifically, our dearomative dihydroxylation strategy was applied in the synthesis of conduramine A (117),[18] MK7607 (120),[18] lycoricidine (130),[29] and narciclasine (133).[29] Using benzene (7) as a starting material, conduramine A (117) was prepared in 5 steps through MTAD-mediated dearomative dihydroxylation with subsequent cycloreversion and nitroso-Diels–Alder reaction. The resulting bicycle 116 was converted into the natural product after reductive N–O bond cleavage and acetonide deprotection. Similarly, by applying the di­hydroxylation sequence, benzyl acetate (118) was converted into the corresponding dihydrodiol derivative 18, which after a protection, dihydroxylation, and deprotection sequence gave MK7607 (120).

Dearomative dihydroxylation of bromobenzene (121) also served as an entry point into the synthesis of the Amaryllidaceae alkaloids lycoricidine (130) and narciclasine (133). Since Upjohn-type dihydroxylation proceeded with low conversions, a Narasaka–Sharpless modification[30] was used. This protocol permitted higher turnovers at lower temperatures and is based on the utilization of the arylboronic acid (122) under aprotic conditions to hydrolyze the intermediate osmate ester and release of osmate species back into the catalytic cycle. The resulting product 123 contains both the vinyl bromide and arylboronate moiety and is therefore suitable for formal transpositive Suzuki coupling. Thus, exposure of this compound to catalytic amounts of Pd(dppf)Cl₂ and base delivered the desired biphenyl dihydrodiol precursor 124. Protection of the diol (124 → 125) and cycloreversion under standard conditions gave biphenyl dihydrodiol 126 that was converted into the fully decorated aminocyclitol core via nitroso-Diels–Alder reaction and reductive N–O bond cleavage/lactam formation (126 → 128). A global deprotection furnished lycoricidine (128 → 129 → 130).

Moreover, we were able to establish a direct connection between lycoricidine (130) and its hydroxylated congener narciclasine (133) by demonstrating a direct sp² C–H hydroxylation. Thus, after silyl protection of lycoricidine intermediate 128, the lactam 131 was subjected to Uchiyama’s directed cupration/oxidation protocol[31] to provide, after in situ acetylation, fully protected narciclasine derivative 132. Again, a global deprotection of alcohols, amide, and phenol furnished natural product 133.

Finally, arenophile-based dearomatization was also applied in synthesis of (+)-7-deoxypancratistatin and (+)-pancratistatin (138 and 139) (Scheme [10]).[24] Application of Ni-catalyzed dearomative carboamination of benzene with aryl Grignard 134 (see Scheme [7]) served as a key strategy, which provided diene 135, amenable to further olefin functionalizations. Thus, formal dihydroxylation, proceeding through epoxidation and hydrolytic opening of the allylic epoxide, provided diol 136. Next, Upjohn dihydroxylation and reductive cleavage of the urazole moiety to amine 137 gave the fully decorated, hexasubstituted aminocyclitol core of pancratistatin. This intermediate was obtained as a single stereoisomer in four steps from benzene and could be prepared on several grams. Construction of the lactam was needed to complete the synthesis of 7-deoxypancratistatin (138), which was accomplished via a two-step sequence encompassing electrophilic aromatic bromination and intramolecular Co-catalyzed carbamoylation under UV-light conditions.[32] Direct C-7 hydroxylation of 7-deoxypancratistatin into pancratistatin (139) was realized by sequential persilylation of the tetraol with HMDS and Uchiyama’s cupration/oxidation, which after acidic workup gave natural product 139.

Conclusion

Dearomative cycloaddition reactions of arenes have played an important role in synthetic organic chemistry. In this Synpacts article we have presented our recent work in the area of para-cycloadditions with arenophiles and the numerous opportunities that this chemistry offers. In addition to directly translating olefin-like functionalizations into the area of dearomatizations, arenophile-arene cyclo­adducts are also amenable to transition-metal catalysis, providing unsaturated products that could be challenging to access by any other means. Finally, dearomatizations with arenophiles provide unique and enabling retrosynthetic disconnections, where aromatic starting materials could be considered as hypothetical cyclohexatriene derivatives. Application of this dearomative logic has been demonstrated in the synthesis of complex small molecules and natural products.

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  CrossrefSearch in Google Scholar
• 5 Pape AR. Kaliappan KP. Kündig EP. Chem. Rev. 2000; 100: 2917
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• 9 Zutter U. Iding H. Spurr P. Wirz B. J. Org. Chem. 2008; 73: 4895
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  Search in Google Scholar
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