Subscribe to RSS
DOI: 10.1055/a-2124-4037
Metal-Loaded Semiconductor-Photocatalysis of Alcohols for Selective Organic Synthesis: A Personal Account
This work was supported by MEXT/JSPS Grant-in-aid for Transformative Research Areas (A) Green Catalysis Science, Specially Promoted Research, and International Leading Research, KAKENHI (Grant # 23H04904, 23H05404, and 22K21346 to S.S.), and partially by JST CREST (Grant # JPMJCR22L2 to S.S.), MEXT Grant in aid-for transformative research areas (B): Deuterium Science, KAKENHI (Grant # 20H05740, to H.N.), and JSPS KAKENHI (Grant # 26410115 and 23H01958, to H.N.).
Dedicated to Professor Hisashi Yamamoto on the occasion of his 80th birthday
Abstract
In this account, we review our research over the last decade on metal-loaded semiconductor-photocatalyzed organic transformations using alcohols. Different from many reactions using alcohols as mere sacrificial electron donors, our study has demonstrated alcohols as useful organic building blocks incorporated into value-added products. Besides such recollections of previous results, we briefly introduce our ongoing project involving photocatalytic C–C bond-forming reactions via the C–C bond scission of tertiary alcohols.
1 Introduction
2 Dehydroxylative Hydrogenolysis of Allylic Alcohols
3 Acceptorless Dehydrogenation of Activated/Unactivated Alcohols
4 N-Alkylation of Amines using Alcohols as Alkylating Agents
5 Summary and Outlook
#
Key words
semiconductors - photocatalysts - alcohols - dehydroxylative hydrogenolysis - dehydrogenation - N-alkylation - N-methylation - β-scissionBiographical Sketches
Shogo Mori has been an Assistant Professor of Integrated Research Consortium on Chemical Sciences at Nagoya University since 2022. He obtained his B.S. (2017), M.S. (2019) and Ph.D. (2022) under the supervision of Prof. Susumu Saito at Nagoya University. Shogo passed through the Graduate Program of Transformative Chem-Bio Research (GTR) at Nagoya University (2022). His research focuses on semiconductor photocatalysis for sustainable organic synthesis.
Shu Sakurai received his B.S. degree from Nagoya University in 2022. Then, he entered the Graduate School of Science, Nagoya University (supervisor: Prof. Susumu Saito). Shu is interested in the formation of carbon-centered radicals from 3° alcohols under semiconductor-photocatalysis and their application to synthetic organic chemistry.
Hiroshi Naka is currently Associate Professor at Kyoto University. His research group specializes in sustainable catalysis and material design, with an emphasis on the development of deuteration reactions and their applications in organic chemistry. He obtained his B.S. (2003) and M.S. (2005) from the University of Tokyo. He received his Ph.D. (2008) from Nagoya University. His Ph.D. thesis focused on developing chemoselective aluminate bases under the supervision of Prof. Masanobu Uchiyama. As a Ph.D. candidate, he was also a JSPS research fellow at the University of Tokyo in 2005–2006. Hiroshi was a Research Associate in the laboratory of Prof. Yoshinori Kondo at Tohoku University (2006–2008). From 2008–2020 he was an Assistant Professor in the laboratory of Prof. Ryoji Noyori at Nagoya University, where he developed transition-metal catalysis for selective hydration and heterogeneous photocatalysis for alcohol transformations in collaboration with Prof. Ryoji Noyori and Prof. Susumu Saito. Since 2020, he has been at the Graduate School of Pharmaceutical Science, Kyoto University. From 2023 he has also served as the director of the Deuterium Science Research Unit at Center for the Promotion of Interdisciplinary Education and Research, Kyoto University.
Susumu Saito, born in 1969 at Okayama, Japan, obtained his doctoral degree of Engineering (1998) from Nagoya University (NU). He attended Prof. E. Jacobsen’s group at Harvard, USA, as Visiting Researcher (1994). His first academic position was Assistant Professor at Graduate School of Engineering, NU (1995), and he became Associate Professor of Institute for Advanced Research (IAR), NU (2002), and Full Professor and IAR Fellow, NU since 2015. He is an Asian Core Program Lecturer awarded from China (2009) and from Taiwan (2009), a recipient of Nagase Research Award (2018) and SSOCJ Tosoh Award for Environment and Energy (2019); PI of CREST project, JST since 2022. Saito’s current interest is broad organic chemistry/catalysis and one is focusing on the development of new concepts and catalytic methodology for organic synthesis using alcohols, carboxylic acid derivatives, CO2, H2, and H2O as key ingredients under thermal, light, electric, and chemical energy. Pursuing catalyses include hydrogenation of inert carbonyl compounds such as carboxylic acids and unactivated amides; chemical upcycling of polymers; dehydrative C–C, C–N, C–O coupling, CO2 transformation, and artificial photosynthesis directing toward organic synthesis (APOS).
Introduction
Semiconductor photocatalysts (SPs) have drawn significant attention in a wide range of green science and sustainable technology.[1] This is because photo-irradiated SPs can simultaneously and continuously generate pairs of holes (h +s) and electrons (e –s), that can promote attractive reactions via both oxidation and reduction processes under ambient conditions in one-pot operations. Since the properties of SPs, including the wavelength of absorbed light and the charge-carrier lifetime, can be altered drastically by loading metal-nanoparticle (NP) cocatalysts on the surfaces[2] or by doping impurity elements in the lattices,[3] SPs have infinite potential for organic synthesis. In fact, a wide variety of SPs have been designed, prepared, and utilized for certain important environmental issues since the 1970s when the Honda–Fujishima effect was coined.[4] Representative examples include water splitting (water oxidation coupled with proton reduction),[5] [6] CO2-to-fuel conversion (water oxidation coupled with CO2 reduction),[7] [8] or organic pollutant degradation in which active oxygen species (•OH, O2 •–) are supposed to be generated through water oxidation and/or O2 reduction.[9] [10] Some developed green technologies seem close to being implemented in practice [CO2 reduction with the solar-to-formate energy conversion efficiency of 7.2% (0.1 m2 irradiation area);[11] water splitting with the solar-to-hydrogen energy conversion efficiency of 0.76% (9 m2 irradiation area)[12]]. Although the application of the SPs in synthetic organic chemistry was falling behind before 2000, several research groups, including the groups of Saito,[13] Yoshida,[14] Pieber,[15] König,[16] Walton,[17] Scaiano,[18] and other researchers,[19] [20] [21] reported their remarkable works on organic transformations using SPs during the past decade.
We have used SPs mainly for the useful and practical organic transformations of alcohols since they are widely available and many of them are inexpensive and safe.[22] [23] Looking back to the 1980s, Kagiya and Ohtani reported a series of germinal studies on SPs-induced organic transformations of alcohols. Representative examples were the alkylation of heteroatom nucleophiles by metal-loaded TiO2 (M/TiO2) photocatalysts, where alcohols were used as alkylating agents (in other words, alcohols were incorporated into product structures).[24] [25] Although their studies promised further potential of SPs for useful organic transformations of alcohols, the synthetic scope of organic substrates in light of both reactivity and selectivity has yet to be demonstrated. In most of the relevant contributions, alcohols have merely been used as sacrificial electron donors to promote desired reductive transformations, such as hydrogenation of unsaturated bonds, and dehalogenation or reductive coupling of organic halides.[26] [27] To open up the new potential of alcohols as carbon resources in photocatalysis, we have been focused on the synthetically useful transformations of alcohols by SPs beyond the mere use of alcohols as sacrificial agents.
In this account, we overview our research on ‘metal-loaded semiconductor-photocatalysis of alcohols for useful and practical organic synthesis’ (Figure [1]).
Our related publications can be categorized into different reaction types; dehydroxylative hydrogenolysis of allylic alcohols (Section 2), acceptorless dehydrogenation of activated/unactivated primary (1°) and secondary (2°) alcohols (Section 3), and N-alkylation of amines using alcohols as alkylating agents (Section 4). Lastly, we summarize our past achievements and also briefly introduce the next direction of our research: SPs-induced C–C bond-formation reactions through β-scission of tertiary (3°) alcohols (Section 5). We hope that this account helps readers understand the gist of the concept behind the semiconductor photocatalysis, use our synthetic methods for practical applications, and also spur further innovation in semiconductor photocatalysis for selective organic synthesis.
# 2
Dehydroxylative Hydrogenolysis of Allylic Alcohols
In the early 2010s, we started studying the reductive transformation of allylic alcohols into alkenes, so-called dehydroxylative hydrogenolysis, using Pd/TiO2 under near-UV/Vis light irradiation (Xe lamp, λ >365 nm) with a catalytic amount of an acid additive.[28] [29] [30] In general, alcohols are likely to be oxidized by the h +s of TiO2.[26] [27] Allylic alcohols were seemingly reduced by the e –s of TiO2 when methanol was used as a solvent, which would function as a dominant sacrificial electron and hydrogen atom donor.
Initially, we reported the Pd/TiO2-photocatalized dehydroxylative hydrogenolysis of the simplest and biomass-derived allylic alcohol, allyl alcohol (1) to propylene (2), which is a useful industrial carbon resource for the production of plastics (polypropylene) and other high-value-added chemicals (Figure [2]A).[28] One of the most attractive features of this system is the high selectivity to retain olefinic groups (Figure [2]B).
The desirable dehydroxylative hydrogenolysis of 1 to 2 proceeded exclusively against hydrogenation of the C=C double bond of 2 to give propane (3) and H2 evolution, while 1 remained in the reaction mixture within 9 h. Side reactions started to be observed after the complete consumption of 1 in 12 h. Since the hydrogenation of the πC=C bond should be thermodynamically and kinetically more favorable than the hydrogenolysis of the σ C–O bond, the unexpected selectivity to give 2 seemed to originate from a spatial separation between the πC=C bond and the Pd/TiO2 surface, which is explained later in detail (Figure [3]C).
In a similar photocatalytic system using Pd/TiO2, a variety of allylic alcohols underwent dehydroxylative hydrogenolysis over olefin reduction.[29] [30] Cinnamyl alcohol [(E)-4] selectively gave trans-β-methyl styrene [(E)-5]. cis-β-Methyl styrene [(Z)-5], 3-phenyl-1-propene (6) and propylbenzene (7), which could be formed via isomerization or over-reduction, were hardly observed as side products (Figure [3]A). The dehydroxylative hydrogenolysis of other allylic alcohols proceeded efficiently and selectively, in which a C=C double bond incorporated separately from the allylic position in carbon chains remained unreacted (Figure [3]B). The regioselectivity of the reduced carbon was controlled and thus predictable, where a sterically less congested carbon is preferred for accepting a hydrogen atom. A β-substituted allylic alcohol 8 gave an internal alkene product 9 selectively against a terminal alkene 10. Geraniol (11), with an E-configuration, and nerol (14), with a Z-configuration, were converted stereospecifically into internal alkenes (E)-12 and (Z)-12, respectively. When γ-substituted allylic alcohols were used as starting materials, hydrogen atoms were preferentially introduced to the less hindered α-position of the allylic alcohols 11 and 14, and the γ-carbon of 15. Ultimately, from 11, 14, and 15, internal alkenes (E)-12 and (Z)-12 were obtained selectively against the terminal alkene 13. The C=C double bonds of the allylic alcohols and those distal from the reduced carbon are essentially not reduced and tolerated the Pd/TiO2-photocatalytic conditions (11, 14, and 15). Additionally, the synthetic shortcut to a natural product, (S)-(+)-lavandulol (17) from a non-symmetrical diol 16, was enabled through the selective dehydroxylative hydrogenolysis at the carbon of the allylic alcohol against the homoallylic alcohol moiety. Allylic alcohols (2 mmol) underwent the photocatalytic hydrogenolysis smoothly, and the Pd/TiO2 photocatalyst could be reused for five runs without significant loss of catalytic activity. These results indicate that the Pd/TiO2 photocatalyst is robust and that the present method has great potential for larger scale reactions.
We proposed a possible mechanism (Figure [3]C) in which TiO2 generates 2e – and 2h + upon its photoexcitation. Methanol undergoes two-electron oxidation by the 2h+ delivered from TiO2, generating two protons and CH2=O. One of the protons activates the OH group of the allylic alcohol by protonation. The other is reduced on a Pd NP by the 2e –, also delivered from TiO2, to a formally written hydride (H– = H+ + 2e –) species. Nucleophilic substitution at the α- or γ-carbons of the activated allylic alcohol by the H– species gives the corresponding alkene and H2O. Allylic alcohols and methanol are likely to adopt a self-assembled alignment on the catalyst surface upon interacting with their OH groups, where the alkenyl carbon chains form upright structures over the solid surface so that the C=C double bonds could remain unreacted because of their spatial separation from the catalytic surface.
To further probe the applicability of this system, 3-phenylprop-2-yn-1-ol (18), containing a C≡C triple bond, was tested (Figure [3]D). At the early stage of the reaction within 5 h, the C≡C triple bond was selectively hydrogenated to an allylic alcohol containing a C=C double bond with a Z-configuration [(Z)-4)]. In 12.5 h, the dehydroxylative hydrogenolysis of (Z)-4 took place, giving alkenes (Z)-5 and (E)-5 predominantly against hydrogenation of the C=C double bonds. After the complete consumption of the unsaturated alcohols 18 and (Z)-4, (Z)-5 was further reduced to 7 (20 h). In other words, the Pd/TiO2-photocatalyzed hydrogenation of the C≡C triple bond did occur but that of the C=C double bonds was inhibited in the presence of the propargylic and/or allylic alcohols, where these alcohols would negatively affect the inherent hydrogenation ability of Pd/TiO2, as Pb(OAc)2, CaCO3, and quinoline poisoned similarly the catalytic surface of the Lindlar’s heterogeneous Pd catalyst.
We developed the chemoselective dehydroxylative hydrogenolysis of allylic alcohols with promising scalability, which could provide industrially important compounds and naturally occurring products. Other researchers reported similar systems using Pd/SPs for dehydroxylative hydrogenolysis of benzylic alcohols, where the transformation of allylic alcohols was not tested.[31] [32] [33]
# 3
Acceptorless Dehydrogenation of Activated/Unactivated Alcohols
In interdisciplinary collaboration research between Kudo’s inorganic/photochemistry group (Tokyo University of Science) and our synthetic organic chemistry group, we learned basic skills to prepare Kudo’s rhodium-doped and ruthenium-loaded strontium titanate (Ru/SrTiO3:Rh) originally designed for photocatalytic water splitting, and tried to apply the method to useful organic synthesis. Finally, we enabled redox-selective conversion of 1° and 2° alcohols into aldehydes and ketones, respectively, along with H2 evolution in a stoichiometric molar ratio of 1:1. The acceptorless dehydrogenation was promoted by Kudo’s Ru/SrTiO3:Rh catalyst in a biphasic system of toluene and aqueous H3PO4 under visible-light irradiation (Xe lamp, λ >420 nm) (Figure [4]A).[34] [35] [36] The Ru/SrTiO3:Rh powder was efficiently dispersed over the interface between the toluene–water biphasic layers (Figure [4]A), where the hydrophilic hydroxy groups of the alcohols could approach the Ru/SrTiO3:Rh surface while other hydrophobic functionalities could not. That might be why almost no over-oxidation of hydrophobic benzaldehydes to carboxylic acids was observed (20a–h), and that reactive but hydrophobic bromo, acetyl, nitro, alkenyl, and alkynyl groups could survive in both photo-oxidative and photo-reductive conditions (20c–g). Stereochemistry around a C=C double bond of an allylic substrate was preserved (20i). Secondary alcohols were converted into the corresponding ketones 20j and 20k.
In addition to the synthetic value discussed above, energetically notable benefits should be stressed here. The transformation of 1° or 2° alcohols into aldehydes or ketones with H2 evolution was found to be thermodynamically uphill (endergonic, Gibbs free energy changes: ΔG° >0) according to our DFT calculations (Figure [4]A). Moreover, the Ru/SrTiO3:Rh-promoted alcohol dehydrogenation could be run under sunlight irradiation (Figure [4]B). These results suggest that solar energy could be converted into chemical energy through organic transformation. This study provided us the inspiration for our ongoing project ‘artificial photosynthesis directing toward organic synthesis’ using water as the key reagent.[37]
While the success in Ru/SrTiO3:Rh-photocatalysis convinced us of the versatility of SPs in organic synthesis, the dehydrogenation protocol still had several drawbacks: (i) A simple 1° aliphatic alcohol 19l was transformed into the corresponding aldehyde in relatively low yield (20l, Figure [4]C). This could partially be attributed to the over-oxidation of 20l to the carboxylic acid 21l. (ii) The apparent quantum efficiency at the wavelength of 420 nm was determined to be <1%. (iii) UV-light irradiation aiming at a further acceleration of the reaction resulted in over-oxidation to carboxylic acid 21a and other side reactions that afforded 22a and 23a (Figure [4]D). (iv) The scalability was unclear.
Therefore, we next sought to establish an efficient and scalable method for the acceptorless dehydrogenation of 1° aliphatic alcohols. After screening different M/TiO2 photocatalysts, we found that the drawbacks of Ru/SrTiO3:Rh mentioned above could be overcome by using Au/TiO2 in ethyl acetate under UV/Vis light irradiation (Xe lamp, λ = 300–470 nm) at 45 °C (Figure [5]A).[38] A 1° aliphatic alcohol (2 mmol) was smoothly and cleanly transformed into the aldehyde (20m, Figure [5]B). The apparent quantum yield was 20% at a wavelength of 365 nm. Potentially reactive and/or bulky functionalities tolerated the Au/TiO2-photocatalytic conditions reasonably (20n–p).
While there are several other reports on SPs-promoted acceptorless dehydrogenation of alcohols, the generality of alcohols, the chemoselectivity, and the scalability have rarely been discussed.[39] [40] [41]
# 4
N-Alkylation of Amines using Alcohols as Alkylating Agents
The success in the alcohol dehydrogenation using Au/TiO2 spurred our research on the synthesis of more useful and complex organic compounds. Inspired by the borrowing hydrogen methodology successful for N-alkylation of amines using homogeneous transition-metal catalysts,[42] [43] we envisioned that the transformation could also be induced through three sequential steps by M/TiO2 (Figure [6]): (i) 2h +-promoted dehydrogenation of a 1° or 2° alcohol to an aldehyde or a ketone on the TiO2 surface; (ii) condensation of an amine and the formed carbonyl compound giving an imine or iminium ion intermediate; and (iii) (nH+ + 2e –)-promoted hydrogenation of the intermediate on the loaded metal-NP cocatalyst to give an alkylated amine.
Over the last decade, we have developed several M/TiO2-photocatalytic methods for the efficient and scalable N-alkylation of amines using alcohols as alkylating agents, which provides useful and complex organic molecules, including compounds of pharmaceutical importance. The attractive features of our methods include mild reaction conditions (ambient temperature) and high atom-economy (H2O as the sole byproduct).
We mainly focused on the N-methylation of amines using methanol as a methylating agent for three reasons: (i) Among N-alkylation of amines, N-methylation using methanol is one of the most demanded reactions from biology and pharmaceutical chemistry.[44] (ii) The use of methanol is challenging in the thermally activated and transition-metal-catalyzed borrowing hydrogen methodology.[42] [43] (iii) We were aware that methanol would be reactive with the h + of TiO2 according to our previous study using the Pd/TiO2-photocatalytic system.[28] [29] [30]
After the screening of SPs including Au/TiO2 and Pd/TiO2 using N-allyl-N-benzyl amine (24a) as a model substrate, we unexpectedly found that Ag/TiO2 allowed chemoselective, efficient, and scalable N-methylation of 24a to give 25a using methanol under UV/Vis light irradiation (Xe lamp, λ = 300–470 nm) at room temperature (Figure [7]A).[45] Gram-scale synthesis of 25a·HCl was also successful (1.82 g, 92%). A series of 2° amines smoothly gave the corresponding 3° N-methylamines in good to excellent yields [25b–i(·HCl), Figure [7]B]. This system was also effective for N,N-dimethylation of a variety of 1° amines [25j–n(·HCl)]. Since the reactions proceeded cleanly, most of the formed 3° amines could be isolated as HCl salt precipitates after simple treatment with an ether solution of hydrochloric acid without column chromatography. A wide range of functional groups tolerated the photocatalytic conditions, such as allyl, benzyl, formyl, and tert-butoxycarbonyl (Boc) groups at nitrogen atoms, and also an alkenyl group and a ketone moiety. Note that selective N-methylation of amines containing these reactive functional groups is often difficult in transition-metal-catalysis under relatively harsh reaction conditions under which they frequently deteriorate.[42] [43] Stereochemistry at a carbon α to a nitrogen atom was well preserved (25j·HCl). The Ag/TiO2-photocatalytic system could be extended to N-methylation of highly hydrophilic substrates such as aqueous ammonia and l-Pro in a water/methanol mixed-solvent system, allowing the clean formation of trimethylamine hydrochloride (25o•HCl) and N-methyl-l-Pro (25p) (Figure [7]C).
Reactions using CH3OH, CH3OD, CD3OH, or CD3OD selectively gave 25a-d 0, 25a-d 1, 25a-d 2, or 25a-d 3, respectively (Figures [7]A and 7D). These results indicate that two hydrogen atoms of the methyl group of the product originate from the methyl group of methanol and that the rest of the hydrogen atoms is from the protic hydroxy group of methanol.
Later, we found that Pd/TiO2 promoted N-methylation of furfuryl amine (24q) more smoothly in methanol than Ag/TiO2 (Figure [8]A).[46] Other N-methylated amines bearing heterocycle structural motifs (indole, imidazole, pyridine, pyrimidine, thiophene, etc.) could also be synthesized efficiently by using the Pd/TiO2 photocatalytic system (25r–y, Figure [8]B).
The N-methylation using methanol, N-ethylation using ethanol, and N-deuterioalkylation using deuterated alcohols were also applied to the synthesis of pharmaceutical compounds such as loxapine (25z), (deuterio)venlafaxine [25aa (sub-gram scale) and 25aa-d 6], (deuterio)butenafine (25ab and 25ab-d 3), octacaine (25ac), and alverine-d 5 (25ad-d 5). It should be noted that adiphenine could be synthesized through a cascade involving in situ Cbz-deprotection followed by N,N-diethylation with ethanol (25ae). Furthermore, late-stage multiple N-methylation of colistin (a cationic lipopeptide antibiotic used for the treatment of gram-negative bacterial infection) was successful, as suggested by LC/ESI-MS analysis. The N-alkylation protocol using alcohols can be coupled with Pd/TiO2-photocatalyzed self-condensation of 1° amines to 2° amines. A 2° amine generated in situ from 3-phenylpronane amine could be ethylated with ethanol over Pd/TiO2 under light irradiation, leading to 25ad.[47]
Although Au/TiO2 showed the highest photocatalytic ability for dehydrogenation of 1° aliphatic alcohols among other M/TiO2 tested (Figure [5]),[38] Au/TiO2 was not the first choice in the N-alkylation of several amines (Figure [7]A, Figure [8]A, and Figure [9]A), since the hydrogenation of the imine or iminium ion intermediates, which is supposed to be the rate-determining step in the M/TiO2 photocatalytic systems,[33] [48] would especially be slow with Au/TiO2 (Figure [6]). Therefore, we tested several combinations of Au/TiO2 and other M/TiO2 to accelerate overall orthogonal steps all the way to N-alkylation (Figure [9]A).
As a consequence, we established a rapid N-alkylation method using a combination of Au/TiO2 and Cu/TiO2, compared with individual performances of Ag/TiO2, Pd/TiO2, Au/TiO2, and Cu/TiO2.[49] A wide variety of N-alkylamines could be efficiently synthesized by using the Au/TiO2–Cu/TiO2 mixed photocatalytic system (Figure [9]B).
The synergistic effect of the two photocatalysts was undoubtedly confirmed by control experiments where each photocatalyst was separately used for the synthesis of different N,N-dimethylamines including 25ar. N,N-Diethylation and N,N-dipropylation of a 1° amine were also successful when ethanol and 1-propanol were used as solvents (25ag and 25ah). Interestingly, monoalkylation was enabled when relatively small amounts of alcohols were used in organic solvents (26ag–al). The monoalkylated products could subsequently undergo N-alkylation using different alcohols to give non-symmetrical 3° alkyl amines. The Au/TiO2–Cu/TiO2 mixed photocatalytic system accommodated the presence of pharmaceutically important functionalities (25am–ao). Several pharmaceutical compounds could be synthesized by the methylation using methanol, including rivastigmine [(S)-25af] and Me-desloratadine (25ap). When CD3OD was used, N-deuteriomethylation occurred smoothly, providing deuterated pharmaceutical molecules such as imipramine-d 3 (25aq-d 3) and 25af-d 6 (1.0 g, 91%). The heterogeneous SPs could easily be recovered after the reaction and reused for 10 cycles with a trivial decrease in reactivity. Scanning transmission electron microscope-energy dispersive spectroscopy (STEM-EDS) analysis of a recovered powder after a photocatalytic reaction indicated the formation of an alloy of heterobimetallic NPs of Cu and Au deployed on the TiO2 surface.
As the culmination of our continuous study of this subject, we recently reported an M/TiO2-photocatalyzed N-methylation protocol using methanol, which was further applied to 20 different α-amino acid motifs found in the protein in the human body under near-UV light irradiation (LED, λ = 365 nm) (M = Ag or Pd) (Figure [10]).[50] Without protection, Gly, l-Ala, l-Val, l-Leu, l-Phe,l-Met, l-Ile, l-Arg, and GlyGly were smoothly N,N-dimethylated at the α-NH2 group (25as–ay, 25bb, and 25bc). A 1 mmol-scale reaction of l-Val was also successful (25au, 92%). N-Monomethylation of l-Pro (25p) and N,N,N′,N′-tetramethylation of l-Lys (25ba) proceeded well. In the case of l-His·HCl·H2O, the imidazole moiety might be protonated, and the N,N-dimethylation at the α-NH2 group proceeded efficiently and selectively (25az·HCl). When appropriately protected at reactive functionalities including OH and NH2 groups of the side chains, l-Ser, l-Thr, l-Tyr, l-Asp·HCl, l-Glu·HCl, l-Asn, and l-Gln were converted into the corresponding α-N-methylated products in moderate to good yields (25bd–bf, 25bh–bk). In accordance with our previous study,[46] O-Me-l-Trp containing a heterocycle structure underwent N,N-dimethylation at the α-NH2 in the presence of Pd/TiO2, which is more reactive for the reduction of the intermediary imine and iminium ion than Ag/TiO2 (25bg, see also Figure [6]). In contrast, a reaction of O-Me-l-Trp·HCl using Ag/TiO2 gave a six-membered ring product, which would be formed by an intramolecular reaction of the nucleophilic indole ring with protonated, electrophilic iminium ion intermediate (25bg′). l-Cys underwent a similar cyclization reaction through the intramolecular nucleophilic attack of the SH group (25bl′).
Most of the N-methylated α-amino acid products were isolated through simple filtration and concentration procedures, without tedious purification processes such as an aqueous workup and column chromatography since the reactions proceeded cleanly, generating H2O as the sole byproduct (few exceptions: 25bg, 25bg′, 25bj, and 25bk). The retention of the chirality was confirmed by polarimetric analysis, chiral gas chromatography-mass spectrometry analysis, and/or nuclear magnetic resonance spectroscopy.
The present protocol with the advantages mentioned above would provide future opportunities to synthesize biochemically important compounds. For instance, proteins containing N-methylated α-amino acid residues have great potential for biochemical application in proteomics[51] and epigenetics.[52] The structures of 25au, 25av, 25aw, 25ay, and 25bg are found in naturally occurring compounds with potent bioactivity. Compound 25bl′ has an N-methyl N,S-acetal structure available in the Kent ligation.[53]
Throughout our continuous work on the M/TiO2-photocatalyzed N-alkylation of amines using alcohols as alkylating agents, we have enabled a scalable synthesis of many complex molecules including pharmaceuticals by well-elaborated catalytic systems. Compared with other related works,[33] , [54] [55] [56] [57] we believe that our longstanding contribution to synthetic photocatalysis on the surfaces of SPs underpins the rethinking of, and sets new horizons for, selective organic synthesis and scalable production of fine- and platform chemicals.
# 5
Summary and Outlook
For more than a decade, we have developed semiconductor photocatalysis utilizing methanol and 1°/2° alcohols for useful organic synthesis beyond the simple use of alcohols as sacrificial proton/electron donors (Sections 2–4, Figure [11]). In addition, Arisawa and some of the authors demonstrated that Pt/TiO2 can promote the self-condensation of benzylic alcohols to give dibenzylic ethers under visible-light irradiation.[58] Considering our continuous study of this chemistry, we are now convinced that semiconductor photocatalysis will have a greater potential for making further innovations in several untapped organic transformations of alcohols.
To step forward, we are now interested in another reaction channel of alcohols on the TiO2 photocatalytic surface: 3° alcohols undergo β-scission after being adsorbed on the TiO2 surface, giving carbon-centered radicals and ketones, both of which would also be adsorbed and stabilized on the surface (Figure [11] and Figure [12]).[59]
Although this adsorptive interaction was experimentally confirmed by detecting a carbon-centered radical itself or products/intermediates derived from the carbon-centered radical, together with ketone detection by mass-spectrometric analysis,[59] [60] it has not been extended to useful organic synthesis. We consider that this photoinduced β-scission is attractive from the following three aspects: (i) It would expand the synthetic utility of 3° alcohols as limiting reagents in organic synthesis; (ii) Judicious choice of symmetrical/non-symmetrical 3° alcohols would enable the selective formation of targeted carbon-centered radicals (Figure [13]); and (iii) Organometallic reagents (R–Ms), which have been used as conventional two-electron nucleophiles, would be available as precursors of carbon-centered radicals (Figure [14]).
(i) Considering that the photocatalytic β-scission of 3° alcohols on the TiO2 surface is initiated by their adsorption, we could assume that the alcohols can be used as limiting reagents for C–C bond formation, as carboxylic acids undergo photocatalytic decarboxylative carbon-centered radical formation upon adsorption on the TiO2 surface, followed by C–C bond formation.[17] [61]
(ii) The β-scission of 3° alcohols on the photo-irradiated TiO2 surface can provide unstable carbon-centered radicals that are otherwise difficult to generate. For instance, Heiz and co-workers revealed that methyl radical could be generated from a symmetrical 3° alcohol, tert-butanol (27) using illuminated TiO2 (Figure [13]A).[60] The methyl radical is useful for the homologation of a carbon chain of synthetic importance.[44] [62] The authors also reported the exclusive formation of ethyl and propyl radicals against methyl radical from a non-symmetrical 3° alcohol, 3-methyl-3-hexanol (28, Figure 13B).[59]
The selectivity as to which alkyl radical (carbon-centered radical) would be favorably generated is affected by both thermodynamic and kinetic parameters: by the degree of stabilization of the alkyl radical and the ketone by-product on the surface; and also by how small the activation energy of the transition state where the carbon chain to be released from the 3° alcohol pre-interacts with the surface.[59] Deep consideration of the thermodynamics and kinetics may enable the selective formation of the targeted carbon-centered radical.
(iii) Many R–Ms are unable to undergo addition reactions to C=C double bonds (Figure [14]). In contrast, they can react smoothly with electrophilic C=O double bonds of ketones to give 3° alcohols. The β-scission of 3° alcohols on the TiO2 photocatalytic surface can release carbon-centered radicals (R•), which can undergo radical addition to the C=C double bonds. In other words, the TiO2 photocatalytic surface would provide a platform for switching from closed-shell organometallic species (two-electron species) to open-shell carbon-centered radicals (one-electron species) through the formation of 3° alcohols.
Based on the wisdom obtained by our experiments and a plausible mechanism proposed by Heiz and co-workers (Figure [15]),[60] we expect that critical parameters for applying the β-scission to useful organic synthesis may be (a) concentrations of bridge-bond oxygen (BBO)-vacancy on the TiO2 surface, (b) reaction temperatures during photo-irradiation, (c) loaded metal-NP cocatalysts on the TiO2 surface, and (d) coverage extents of H2O molecules on the TiO2 surface.
(a) BBO-vacancy is one of the most common defects of TiO2, at which 3° alcohols undergo dissociative adsorption so that the 3° alkoxy groups and the hydrogen atom are separately deployed on TiO2 (Figure [15]). The concentration of BBO-vacancy of TiO2 is relevant to the photocatalytic viability to promote the β-scission.[60]
However, it is known that thermally and photocatalytically induced dehydration of 27 giving isobutene (29) is also promoted at the BBO-vacancy.[60] [63] Therefore, a suitable concentration of BBO-vacancy on the TiO2 surface would be important for efficient and selective β-scission. Altering the method of calcination of TiO2 is the most reliable approach to controlling the concentration of BBO-vacancy.[64]
(b) After the β-scission of 3° alcohols, residual ketones at the BBO-vacancy have a detrimental influence on the photocatalytic ability of TiO2 (Figure [15]).[60] Reaction temperatures are critical for the thermal desorption of the ketones from the BBO-vacancy. However, it is known that the dehydration of 27 is promoted at high reaction temperatures.[60] Therefore, an appropriate reaction temperature would enable efficient and selective β-scission.
(c) We have frequently observed the positive effects of loaded metal-NP cocatalysts on the photocatalytic ability in organic reactions (Sections 2–4). According to the literature, loaded metal-NP cocatalysts can promote organic transformations by stabilizing organic radical species and/or by activating organic substrates.[65] [66] Therefore, metal-loading on TiO2 is expected to be a promising approach to promote selective β-scission.
(d) H2O molecules affect the adsorption of alcohols and ketones on the TiO2 surface.[67] [68] Sugimoto and co-workers recently reported that methyl radicals would be stabilized on an SP surface, which would also be affected by H2O molecules adsorbed on the surface, based on their theoretical calculations.[69] Thus, the water content of the reaction mixture should be carefully optimized[37] for efficient and selective β-scission.
Studies of C–C bond-formation reactions through the β-scission of 3° alcohols on the TiO2-photocatalytic surface are now under scrutiny in our group, taking into account the critical parameters discussed above. We envision unique transformations of 3° alcohols through the selective formation and transfer of a carbon-centered radical out of the three alternatives (three different substituents, such as alkyl, aryl, and alkynyl groups, bound to the carbon α to the hydroxy group of non-symmetrical 3° alcohols). The selectivity in the carbon group transfer in the heterogeneous TiO2-photocatalytic system involving several radical species (Figure [15]) should be different from that in the homogeneous catalytic aluminum system to transfer ‘two-electron’ alkynyl groups of propargylic alcohols developed by Ooi and Maruoka.[70] [71] Additionally, this work would provide useful information on the stability and reactivity of the carbon-centered radicals on the photocatalytic surface for developing otherwise unexplored organic reactions under semiconductor photocatalysis.
#
#
Conflict of Interest
The authors declare no conflict of interest.
Acknowledgment
We thank all the co-workers for their significant contributions to the chemistry described in the paper. We thank Profs. R. Noyori (Nagoya U.) and A. Kudo (TUS), as well as the industry members of the Noyori Forum Project for their encouragement and suggestions. We also thank Dr. H. Saito and Prof. T. Sugimoto (IMS) for a fruitful discussion.
-
References
- 1 Hoffmann MR, Martin ST, Choi W, Bahnemann DW. Chem. Rev. 1995; 95: 69
- 2 Wenderich K, Mul G. Chem. Rev. 2016; 116: 14587
- 3 Khlyustova A, Sirotkin N, Kusova T, Kraev A, Titov V, Agafonov A. Mater. Adv. 2020; 1: 1193
- 4 Fujishima A, Honda K. Nature 1972; 238: 37
- 5 Kudo A, Miseki Y. Chem. Soc. Rev. 2009; 38: 253
- 6 Wang Q, Domen K. Chem. Rev. 2020; 120: 919
- 7 Yoshino S, Takayama T, Yamaguchi Y, Iwase A, Kudo A. Acc. Chem. Res. 2022; 55: 966
- 8 Nahar S, Zain MF. M, Kadhum AA. H, Hasan HA, Hasan MR. Materials 2017; 10: 629
- 9 Koe WS, Lee JW, Chong WC, Pang YL, Sim LC. Environ. Sci. Pollut. Res. 2020; 27: 2522
- 10 Hong J, Cho K.-H, Presser V, Su X. Curr. Opin. Green Sustainable Chem. 2022; 36: 100644
- 11 Morikawa T, Sato S, Sekizawa K, Suzuki TM, Arai T. Acc. Chem. Res. 2022; 55: 933
- 12 Nishiyama H, Yamada T, Nakabayashi M, Maehara Y, Yamaguchi M, Kuromiya Y, Nagatsuma Y, Tokudome H, Akiyama S, Watanabe T, Narushima R, Okunaka S, Shibata N, Takata T, Hisatomi T, Domen K. Nature 2021; 598: 304
- 13 Mori S, Saito S. Green Chem. 2021; 23: 3575
- 14 Park S, Jeong J, Fujita K, Yamamoto A, Yoshida H. J. Am. Chem. Soc. 2020; 142: 12708
- 15 Pieber B, Malik JA, Cavedon C, Gisbertz S, Savateev A, Cruz D, Heil T, Zhang G, Seeberger PH. Angew. Chem. Int. Ed. 2019; 58: 9575
- 16 Ghosh I, Khamrai J, Savateev A, Shlapakov N, Antonietti M, König B. Science 2019; 365: 360
- 17 Manley DW, McBurney RT, Miller P, Howe RF, Rhydderch S, Walton JC. J. Am. Chem. Soc. 2012; 134: 13580
- 18 Pitre SP, Scaiano JC, Yoon TP. ACS Catal. 2017; 7: 6440
- 19 Gisbertz S, Pieber B. ChemPhotoChem 2020; 4: 456
- 20 Riente P, Noël T. Catal. Sci. Technol. 2019; 9: 5186
- 21 Parrino F, Bellardita M, García-López EI, Marcì G, Loddo V, Palmisano L. ACS Catal. 2018; 8: 11191
- 22 Henkel T, Brunne RM, Müller H, Reichel F. Angew. Chem. Int. Ed. 1999; 38: 643
- 23 Ertl P, Schuhmann T. J. Nat. Prod. 2019; 82: 1258
- 24 Ohtani B, Osaki H, Nishimoto S, Kagiya T. J. Am. Chem. Soc. 1986; 108: 308
- 25 Ohtani B, Kakimoto M, Nishimoto S, Kagiya T. J. Photochem. Photobiol., A 1993; 70: 265
- 26 Stroyuk OL, Kuchmy SY. Theor. Exp. Chem. 2020; 56: 143
- 27 Ma D, Zhai S, Wang Y, Liu A, Chen C. Molecules 2019; 24: 330
- 28 Caner J, Liu Z, Takada Y, Kudo A, Naka H, Saito S. Catal. Sci. Technol. 2014; 4: 4093
- 29 Takada Y, Caner J, Kaliyamoorthy S, Naka H, Saito S. Chem. Eur. J. 2017; 23: 18025
- 30 Takada Y, Caner J, Naka H, Saito S. Pure Appl. Chem. 2018; 90: 167
- 31 Ruberu TP. A, Nelson NC, Slowing II, Vela J. J. Phys. Chem. Lett. 2012; 3: 2798
- 32 Fu H, Chen H, Gao B, Lu T, Su Y, Zhou L, Liu M, Li H, Yang X. ChemCatChem 2022; 14,: 202200120
- 33 Shiraishi Y, Fujiwara K, Sugano Y, Ichikawa S, Hirai T. ACS Catal. 2013; 3: 312
- 34 Sasaki Y, Nemoto H, Saito K, Kudo A. J. Phys. Chem. C 2009; 113: 17536
- 35 Sasaki Y, Iwase A, Kato H, Kudo A. J. Catal. 2008; 259: 133
- 36 Liu Z, Caner J, Kudo A, Naka H, Saito S. Chem. Eur. J. 2013; 19: 9452
- 37 Yamauchi M, Saito H, Sugimoto T, Mori S, Saito S. Coord. Chem. Rev. 2022; 472: 214773
- 38 Shibata M, Nagata R, Saito S, Naka H. Chem. Lett. 2017; 46: 580
- 39 Qi M.-Y, Conte M, Anpo M, Tang Z.-R, Xu Y.-J. Chem. Rev. 2021; 121: 13051
- 40 Zhao L.-M, Meng Q.-Y, Fan X.-B, Ye C, Li X.-B, Chen B, Ramamurthy V, Tung C.-H, Wu L.-Z. Angew. Chem. Int. Ed. 2017; 56: 3020
- 41 Imamura K, Tsukahara H, Hamamichi K, Seto N, Hashimoto K, Kominami H. Appl. Catal., A 2013; 450: 28
- 42 Reed-Berendt BG, Latham DE, Dambatta MB, Morrill LC. ACS Cent. Sci. 2021; 7: 570
- 43 Kabadwal LM, Bera S, Banerjee D. Org. Chem. Front. 2021; 8: 7077
- 44 Barreiro EJ, Kümmerle AE, Fraga CA. M. Chem. Rev. 2011; 111: 5215
- 45 Tsarev VN, Morioka Y, Caner J, Wang Q, Ushimaru R, Kudo A, Naka H, Saito S. Org. Lett. 2015; 17: 2530
- 46 Wang L.-M, Jenkinson K, Wheatley AE. H, Kuwata K, Saito S, Naka H. ACS Sustainable Chem. Eng. 2018; 6: 15419
- 47 Wang L.-M, Kobayashi K, Arisawa M, Saito S, Naka H. Org. Lett. 2019; 21: 341
- 48 Chen W, Fu X, Liu X, Ye L, Yuan Y. Mol. Catal. 2023; 538: 112993
- 49 Wang L.-M, Morioka Y, Jenkinson K, Wheatley AE. H, Saito S, Naka H. Sci. Rep. 2018; 8: 6931
- 50 Hashiba Y, Mori S, Huang I, Morioka Y, Naka H, Saito S. Asian J. Org. Chem. 2023; e202300230 DOI:
- 51 Yu Q, Shi X, Greer T, Lietz CB, Kent KC, Li L. J. Proteome Res. 2016; 15: 3420
- 52 Kim J, Lee H, Yi S.-J, Kim K. Exp. Mol. Med. 2022; 54: 878
- 53 Dawson PE. Isr. J. Chem. 2011; 51: 862
- 54 Zhang L, Zhang Y, Deng Y, Shi F. Catal. Sci. Technol. 2015; 5: 3226
- 55 Stíbal D, Sá J, van Bokhoven JA. Catal. Sci. Technol. 2013; 3: 94
- 56 Zhang L, Zhang Y, Deng Y, Shi F. RSC Adv. 2015; 5: 14514
- 57 Wang B, Deng Z, Fu X, Xu C, Li Z. Appl. Catal., B 2018; 237: 970
- 58 Wada Y, Akiyama T, Harada K, Honma T, Naka H, Saito S, Arisawa M. RSC Adv. 2021; 11: 22230
- 59 Courtois C, Eder M, Schnabl K, Walenta CA, Tschurl M, Heiz U. Angew. Chem. Int. Ed. 2019; 58: 14255
- 60 Walenta CA, Kollmannsberger SL, Courtois C, Tschurl M, Heiz U. Phys. Chem. Chem. Phys. 2018; 20: 7105
- 61 Zhu Q, Nocera DG. J. Am. Chem. Soc. 2020; 142: 17913
- 62 Aynetdinova D, Callens MC, Hicks HB, Poh CY. X, Shennan BD. A, Boyd AM, Lim ZH, Leitch JA, Dixon DJ. Chem. Soc. Rev. 2021; 50: 5517
- 63 Liu P, Xue G, Lai Y, Li Y, Chen X, Guo Q. J. Phys. Chem. C 2022; 126: 3457
- 64 Bi X, Du G, Kalam A, Sun D, Yu Y, Su Q, Xu B, Al-Sehemi AG. Chem. Eng. Sci. 2021; 234: 116440
- 65 Lee J, Choi W. Environ. Sci. Technol. 2004; 38: 4026
- 66 Wada E, Takeuchi T, Fujimura Y, Tyagi A, Kato T, Yoshida H. Catal. Sci. Technol. 2017; 7: 2457
- 67 Coronado JM, Zorn ME, Tejedor-Tejedor I, Anderson MA. Appl. Catal., B 2003; 43: 329
- 68 Suda Y, Morimoto T, Nagao M. Langmuir 1987; 3: 99
- 69 Sato H, Ishikawa A, Saito H, Higashi T, Takeyasu K, Sugimoto T. Commun. Chem. 2023; 6: 8
- 70 Ooi T, Miura T, Maruoka K. J. Am. Chem. Soc. 1998; 120: 10790
- 71 Ooi T, Miura T, Ohmatsu K, Saito A, Maruoka K. Org. Biomol. Chem. 2004; 2: 3312
Corresponding Authors
Publication History
Received: 21 June 2023
Accepted after revision: 06 July 2023
Accepted Manuscript online:
06 July 2023
Article published online:
25 August 2023
© 2023. Thieme. All rights reserved
Georg Thieme Verlag KG
Rüdigerstraße 14, 70469 Stuttgart, Germany
-
References
- 1 Hoffmann MR, Martin ST, Choi W, Bahnemann DW. Chem. Rev. 1995; 95: 69
- 2 Wenderich K, Mul G. Chem. Rev. 2016; 116: 14587
- 3 Khlyustova A, Sirotkin N, Kusova T, Kraev A, Titov V, Agafonov A. Mater. Adv. 2020; 1: 1193
- 4 Fujishima A, Honda K. Nature 1972; 238: 37
- 5 Kudo A, Miseki Y. Chem. Soc. Rev. 2009; 38: 253
- 6 Wang Q, Domen K. Chem. Rev. 2020; 120: 919
- 7 Yoshino S, Takayama T, Yamaguchi Y, Iwase A, Kudo A. Acc. Chem. Res. 2022; 55: 966
- 8 Nahar S, Zain MF. M, Kadhum AA. H, Hasan HA, Hasan MR. Materials 2017; 10: 629
- 9 Koe WS, Lee JW, Chong WC, Pang YL, Sim LC. Environ. Sci. Pollut. Res. 2020; 27: 2522
- 10 Hong J, Cho K.-H, Presser V, Su X. Curr. Opin. Green Sustainable Chem. 2022; 36: 100644
- 11 Morikawa T, Sato S, Sekizawa K, Suzuki TM, Arai T. Acc. Chem. Res. 2022; 55: 933
- 12 Nishiyama H, Yamada T, Nakabayashi M, Maehara Y, Yamaguchi M, Kuromiya Y, Nagatsuma Y, Tokudome H, Akiyama S, Watanabe T, Narushima R, Okunaka S, Shibata N, Takata T, Hisatomi T, Domen K. Nature 2021; 598: 304
- 13 Mori S, Saito S. Green Chem. 2021; 23: 3575
- 14 Park S, Jeong J, Fujita K, Yamamoto A, Yoshida H. J. Am. Chem. Soc. 2020; 142: 12708
- 15 Pieber B, Malik JA, Cavedon C, Gisbertz S, Savateev A, Cruz D, Heil T, Zhang G, Seeberger PH. Angew. Chem. Int. Ed. 2019; 58: 9575
- 16 Ghosh I, Khamrai J, Savateev A, Shlapakov N, Antonietti M, König B. Science 2019; 365: 360
- 17 Manley DW, McBurney RT, Miller P, Howe RF, Rhydderch S, Walton JC. J. Am. Chem. Soc. 2012; 134: 13580
- 18 Pitre SP, Scaiano JC, Yoon TP. ACS Catal. 2017; 7: 6440
- 19 Gisbertz S, Pieber B. ChemPhotoChem 2020; 4: 456
- 20 Riente P, Noël T. Catal. Sci. Technol. 2019; 9: 5186
- 21 Parrino F, Bellardita M, García-López EI, Marcì G, Loddo V, Palmisano L. ACS Catal. 2018; 8: 11191
- 22 Henkel T, Brunne RM, Müller H, Reichel F. Angew. Chem. Int. Ed. 1999; 38: 643
- 23 Ertl P, Schuhmann T. J. Nat. Prod. 2019; 82: 1258
- 24 Ohtani B, Osaki H, Nishimoto S, Kagiya T. J. Am. Chem. Soc. 1986; 108: 308
- 25 Ohtani B, Kakimoto M, Nishimoto S, Kagiya T. J. Photochem. Photobiol., A 1993; 70: 265
- 26 Stroyuk OL, Kuchmy SY. Theor. Exp. Chem. 2020; 56: 143
- 27 Ma D, Zhai S, Wang Y, Liu A, Chen C. Molecules 2019; 24: 330
- 28 Caner J, Liu Z, Takada Y, Kudo A, Naka H, Saito S. Catal. Sci. Technol. 2014; 4: 4093
- 29 Takada Y, Caner J, Kaliyamoorthy S, Naka H, Saito S. Chem. Eur. J. 2017; 23: 18025
- 30 Takada Y, Caner J, Naka H, Saito S. Pure Appl. Chem. 2018; 90: 167
- 31 Ruberu TP. A, Nelson NC, Slowing II, Vela J. J. Phys. Chem. Lett. 2012; 3: 2798
- 32 Fu H, Chen H, Gao B, Lu T, Su Y, Zhou L, Liu M, Li H, Yang X. ChemCatChem 2022; 14,: 202200120
- 33 Shiraishi Y, Fujiwara K, Sugano Y, Ichikawa S, Hirai T. ACS Catal. 2013; 3: 312
- 34 Sasaki Y, Nemoto H, Saito K, Kudo A. J. Phys. Chem. C 2009; 113: 17536
- 35 Sasaki Y, Iwase A, Kato H, Kudo A. J. Catal. 2008; 259: 133
- 36 Liu Z, Caner J, Kudo A, Naka H, Saito S. Chem. Eur. J. 2013; 19: 9452
- 37 Yamauchi M, Saito H, Sugimoto T, Mori S, Saito S. Coord. Chem. Rev. 2022; 472: 214773
- 38 Shibata M, Nagata R, Saito S, Naka H. Chem. Lett. 2017; 46: 580
- 39 Qi M.-Y, Conte M, Anpo M, Tang Z.-R, Xu Y.-J. Chem. Rev. 2021; 121: 13051
- 40 Zhao L.-M, Meng Q.-Y, Fan X.-B, Ye C, Li X.-B, Chen B, Ramamurthy V, Tung C.-H, Wu L.-Z. Angew. Chem. Int. Ed. 2017; 56: 3020
- 41 Imamura K, Tsukahara H, Hamamichi K, Seto N, Hashimoto K, Kominami H. Appl. Catal., A 2013; 450: 28
- 42 Reed-Berendt BG, Latham DE, Dambatta MB, Morrill LC. ACS Cent. Sci. 2021; 7: 570
- 43 Kabadwal LM, Bera S, Banerjee D. Org. Chem. Front. 2021; 8: 7077
- 44 Barreiro EJ, Kümmerle AE, Fraga CA. M. Chem. Rev. 2011; 111: 5215
- 45 Tsarev VN, Morioka Y, Caner J, Wang Q, Ushimaru R, Kudo A, Naka H, Saito S. Org. Lett. 2015; 17: 2530
- 46 Wang L.-M, Jenkinson K, Wheatley AE. H, Kuwata K, Saito S, Naka H. ACS Sustainable Chem. Eng. 2018; 6: 15419
- 47 Wang L.-M, Kobayashi K, Arisawa M, Saito S, Naka H. Org. Lett. 2019; 21: 341
- 48 Chen W, Fu X, Liu X, Ye L, Yuan Y. Mol. Catal. 2023; 538: 112993
- 49 Wang L.-M, Morioka Y, Jenkinson K, Wheatley AE. H, Saito S, Naka H. Sci. Rep. 2018; 8: 6931
- 50 Hashiba Y, Mori S, Huang I, Morioka Y, Naka H, Saito S. Asian J. Org. Chem. 2023; e202300230 DOI:
- 51 Yu Q, Shi X, Greer T, Lietz CB, Kent KC, Li L. J. Proteome Res. 2016; 15: 3420
- 52 Kim J, Lee H, Yi S.-J, Kim K. Exp. Mol. Med. 2022; 54: 878
- 53 Dawson PE. Isr. J. Chem. 2011; 51: 862
- 54 Zhang L, Zhang Y, Deng Y, Shi F. Catal. Sci. Technol. 2015; 5: 3226
- 55 Stíbal D, Sá J, van Bokhoven JA. Catal. Sci. Technol. 2013; 3: 94
- 56 Zhang L, Zhang Y, Deng Y, Shi F. RSC Adv. 2015; 5: 14514
- 57 Wang B, Deng Z, Fu X, Xu C, Li Z. Appl. Catal., B 2018; 237: 970
- 58 Wada Y, Akiyama T, Harada K, Honma T, Naka H, Saito S, Arisawa M. RSC Adv. 2021; 11: 22230
- 59 Courtois C, Eder M, Schnabl K, Walenta CA, Tschurl M, Heiz U. Angew. Chem. Int. Ed. 2019; 58: 14255
- 60 Walenta CA, Kollmannsberger SL, Courtois C, Tschurl M, Heiz U. Phys. Chem. Chem. Phys. 2018; 20: 7105
- 61 Zhu Q, Nocera DG. J. Am. Chem. Soc. 2020; 142: 17913
- 62 Aynetdinova D, Callens MC, Hicks HB, Poh CY. X, Shennan BD. A, Boyd AM, Lim ZH, Leitch JA, Dixon DJ. Chem. Soc. Rev. 2021; 50: 5517
- 63 Liu P, Xue G, Lai Y, Li Y, Chen X, Guo Q. J. Phys. Chem. C 2022; 126: 3457
- 64 Bi X, Du G, Kalam A, Sun D, Yu Y, Su Q, Xu B, Al-Sehemi AG. Chem. Eng. Sci. 2021; 234: 116440
- 65 Lee J, Choi W. Environ. Sci. Technol. 2004; 38: 4026
- 66 Wada E, Takeuchi T, Fujimura Y, Tyagi A, Kato T, Yoshida H. Catal. Sci. Technol. 2017; 7: 2457
- 67 Coronado JM, Zorn ME, Tejedor-Tejedor I, Anderson MA. Appl. Catal., B 2003; 43: 329
- 68 Suda Y, Morimoto T, Nagao M. Langmuir 1987; 3: 99
- 69 Sato H, Ishikawa A, Saito H, Higashi T, Takeyasu K, Sugimoto T. Commun. Chem. 2023; 6: 8
- 70 Ooi T, Miura T, Maruoka K. J. Am. Chem. Soc. 1998; 120: 10790
- 71 Ooi T, Miura T, Ohmatsu K, Saito A, Maruoka K. Org. Biomol. Chem. 2004; 2: 3312