Applications of Photoredox Catalysis for the Radical-Induced Cleavage of C–C Bonds

Abstract

Selective cleavage of C–C bonds forms one of the greatest challenges in current organic chemistry, due to the relative strength of these bonds. However, such transformations are an invaluable instrument to break down and construct new carbon–carbon bonds. To achieve this, photochemistry can be used as a tool to generate radicals and induce the cleavage of these bonds due to their high reactivity. This review examines some of the most influential contributions in this field since 2010.

1 Introduction

2 C–C Bond Cleavage

2.1 Homogeneous Catalyst

2.1.1 N-Centered Radical

2.2.2 O-Centered Radical

2.2 Heterogeneous Catalyst

3 C=C Bond Cleavage

3.1 Homogeneous Catalyst

3.2 Heterogeneous Catalyst

4 C≡C Bond Cleavage

4.1 Homogeneous Catalyst

4.2 Heterogeneous Catalyst

5 Conclusion

Introduction

Second to hydrogen, carbon is the most abundant element present in organic structures which is reflected by the fact that such molecules largely consist of carbon–carbon bonds. Unfortunately, due to their covalent and unpolarized nature, cleavage of these bonds has proven to be challenging. Therefore, over the last few decades, much effort has been put into the development of new procedures to achieve this transformation.[1]

Traditionally, similar to the activation of C–H bonds, it was proposed to activate C–C bonds via the insertion of a transition metal.[2] Thermodynamically, however, M–H bonds are generally stronger than M–C bonds.[3] Therefore, one can easily see that the activation of a C–C bond via metal insertion is only thermodynamically feasible when the bond dissociation energy (BDE) of two M–C bonds exceeds that of the C–C bond. One way to manage this is to select the metal catalyst based on the M–C bond strength. For example, M–C_aryl bonds are relatively strong in the cases of Rh and Ir, sometimes even stronger than M–H bonds.[4] Another way to facilitate the metal insertion is to introduce strain in the substrate, thereby allowing for strain relief in the product and transition state as a kinetic and thermodynamic driving force in the C–C activation via metal insertion.[5] For unstrained C–C bonds, methods such as chelation assistance and decarbonylation can also be used.[6]

Besides the activation via metal insertion, another powerful tool for C–C bond cleavage is found in radical chemistry, which has seen major developments over the years.[7] An attractive method to access these reactive radical species is via photoredox chemistry, where light irradiation is used as the energy source to trigger single-electron transfer (SET) processes.[8] Because photoredox chemistry allows for remarkable selectivity and functional group tolerance under mild reaction conditions, it has sparked an enormous attraction in the organic chemistry community as the increasing need for sustainable approaches increases.[9]

This short review focuses on the application of photoredox chemistry towards the cleavage of C–C, C=C, and C≡C bonds, with the emphasis on providing a broad overview on recent literature reports, more so than on the fundamental theory behind the chemical transformation. To the best of our knowledge, there are no previous reviews that discuss all three types of carbon–carbon bonds. This short review categorizes reports based on the bond cleaved (C–C, C=C, and C≡C) and then on the type of catalyst, with a further classification in one case to maintain a clear overview.

C–C Bond Cleavage

The C–C single bond is evidently more challenging to cleave than double or triple bonds, due to the lack of π-electrons. Therefore, such cleavage reactions are currently only available for activated C–C bonds, with an adjacent heteroatom. This results in the majority of the procedures utilizing a heteroatom radical to initiate a β-scission.

Homogeneous Catalyst

N-Centered Radical

One of the first procedures for the photocatalytic cleavage of C–C single bonds was published in 2012 by Li, Wang and co-workers (Scheme [1]).[10] They generated two reactive species, namely an iminium ion II and α-amino radical III, via the single-electron oxidation of N,N,N′,N′-tetramethylethylenediamine (TMEDA; 2) using a homogeneous ruthenium complex. Both species could engage in transformations individually, as depicted in Scheme [1]. Upon excitation of the photocatalyst using visible light, the excited state readily oxidizes the diamine substrate 2 through a SET event, yielding the reduced Ru(I) photocatalyst, which is oxidized back to the active Ru(II) species by molecular oxygen, and the radical cation of TMEDA I. A reorganization of the electron density on this radical cation results in cleavage of the central C–C single bond, providing the iminium II and α-amino radical species III. This concept was proven by performing an aza-Henry reaction with the in situ generated iminium ion II to give N,N-dimethyl-2-nitroalkanamines 3 in 63–91% yields (11 examples). Additionally, a single example of a reaction was given in which olefin 4 underwent a polymerization after a Giese-type radical addition of α-amino radical III.

In 2018, Xiao and co-workers successfully performed a Cu-catalyzed three-component coupling involving an oxime ester, styrene, and boronic acid (Scheme [2]).[11] In this case, the oxime ester 6 undergoes SET by the copper catalyst yielding the N-centered iminyl radical intermediate I that initiates the opening of the strained cyclobutane ring (path a). The resulting γ-cyanoalkyl radical II then undergoes coupling with styrene 7 providing radical intermediate III, which then undergoes oxidative addition to the LCu^IIPh species that is obtained via transmetalation of the boronic acid 8. The coupled product is obtained through rapid reductive elimination of the high-valent Cu(III) complex IV. Remarkably, when performing the reaction in the dark, the yield only decreased from 72% to 50%, suggesting an alternative pathway in the mechanism that circumvents excitation of the Cu(I) catalyst (path b).

In 2019, Xiao and co-workers reported another three-component coupling of an oxime ester, this time with carbon monoxide gas and an amine.[12] In this mechanism, however, after the oxidative addition of the γ-cyanoalkyl radical to the copper center, CO insertion occurs to provide the coupled amide product after reductive elimination.

A further application of oxime esters was described in 2019 by the Wu group, where the iminyl radical intermediate initiates C–C bond cleavage to generate open-shell acyl radicals which are then captured by a variety of Michael acceptors such as styrenes or acrylamides.[13]

The γ-cyanoalkyl radical is generally generated from oxime esters over oximes to avoid competitive O–H bond dissociation.[14] However, Xiang, Chen, Yang, and co-workers generated the key iminyl radical intermediate via scission of the oxime N–O bond.[15] The excited Ir(III) catalyst (E _1/2 = +1.21 V vs. SCE)[16] is reductively quenched by Ph₃P (E _1/2 = +0.98 V vs. SCE)[17] to give Ir(II) and the triphenylphosphine radical cation. In the presence of base, a new O–P bond is formed with the oxime which readily undergoes β-scission to generate the key iminyl radical. A sequential ring-opening through C–C bond cleavage provides the open-shell γ-cyanoalkyl radical that is captured by alkenes to generate a C-centered radical species. After reductive SET by the Ir(II) catalyst, a reduced anionic intermediate is formed which readily abstracts a proton or attacks an electrophile.

O-Centered Radical

In 2013, the Xia group described a procedure for the oxidative cleavage of the C_α–C_β bond of 2,2-disubstituted aldehydes under visible light to give the corresponding ketones.[18] Here, the aldehyde reacts with piperidine to form an enamine in situ which undergoes a single-electron oxidation by the excited Ru(II) photocatalyst to yield the reduced Ru(I) photocatalyst that is converted into the active Ru(II) species via reductive quenching by molecular oxygen. On the other hand, the oxidized enamine radical intermediate reacts with a superoxide radical to form a cyclic dioxetane species which delivers the ketone product through expulsion of N-formylpiperidine. Via this pathway, the Xia group developed a tandem reaction where the oxidative C–C bond cleavage was preceded by an intramolecular Michael addition. Here, piperidine was replaced by diisopropylamine (DIPA) which, besides yielding the reactive enamine species in the second step, also served as a catalyst in the Michael addition.

Hirao, Soo, and co-workers were, in 2015, one of the first to design a procedure for the visible-light-mediated C–C bond cleavage of lignin, and hereby paved the way for others to follow.[19] Prior protocols primarily involved cleavage of the C–O ether bond using expensive Ir-based photosensitizers and sacrificial reducing agents.[20] They, on the other hand, were able to selectively cleave the C–C bond using an earth-abundant vanadium(V) oxo complex. In 2017, they reported kinetic and density functional theory (DFT) studies where they investigated other vanadium(V) oxo complexes for their potential in lignin C–C bond cleavage and successfully uncovered other catalysts that lowered the activation energy barrier and increased selectivity.[21] Despite the procedure being selective towards C–C bonds over C–O bonds, it appeared that multiple C–C bonds were targeted as various fragments were observed in yields ranging from 6% to 70%.

Trying to improve this selectivity, He, Zhang, and co-workers successfully developed a highly selective C_α–C_β cleavage of lignin model substrates using an Ir(III) catalyst Ir-1 (Scheme [3]).[22] The mechanism involves SET from the aromatic moiety of the substrate to the excited Ir(III) catalyst, providing the radical cation intermediate I. The presence of a base allows for a proton-coupled electron transfer (PCET) event that causes relocation of the free electron towards the alkoxyl oxygen atom. Radical species II is key in the C–C cleavage reaction, as it can initiate a β-scission, a powerful tool for selective bond cleavage. After the β-scission, C-centered anisole radical III is generated which is quenched by thiophenol through a hydrogen atom transfer (HAT) event. The high selectivity of the reaction was reflected in the fact that they obtained the aldehyde 11 and anisole 12 fragments in yields of up to 97% and 96%, respectively.

In 2016, the Zuo group described the C–C bond β-scission of cyclic alcohols, generating a radical species that is trapped by di-tert-butyl azodicarboxylate (DBAD) under visible light (Scheme [4]).[23] Under visible light, the cerium chloride/alcohol complex I is brought to its strongly reducing excited state II (E _1/2 = –2.2 V vs. SCE), readily reducing nitrogen-centered radical intermediate V which generates Ce(IV) species III. The authors suggest that the following β-scission is facilitated due to the oxidizing nature of Ce(IV) species III. This generates a reactive radical intermediate IV that promptly couples with DBAD to generate the nitrogen-centered radical intermediate V, which is reduced by the excited Ce(III)/alcohol complex II to provide the product 14.

The Knowles group successfully eliminated the need for the DBAD radical trap by introducing thiophenol to trigger a HAT event that would quench the carbon-centered radical.[24] Another adjustment they made was replacing CeCl₃ by the Ir(III) complex [Ir(dFCF₃ppy)₂(5,5′-dCF₃bpy)]PF₆ (Ir-1). A major limitation of this reaction was that a strongly electron-donating p-methoxyphenyl (PMP) substituent on the α-carbon atom was mandatory to successfully yield the reaction product. The excited photocatalyst initiates the reaction by abstracting an electron from the PMP unit, which, in the presence of a base, accepts an electron from the proximal oxygen atom, yielding the corresponding alkoxyl radical. Consecutive β-scission and HAT then delivers the desired ketone product while thiophenol is regenerated via an electron transfer from the Ir(II) species followed by a proton transfer from the protonated base. They later improved the reaction conditions as the need for the strongly electron-donating PMP substituent was avoided by selecting a different Brønsted base and HAT reagent.[25]

Instead of trapping the radical with a hydrogen atom as in Knowles’ reaction, Zhu and co-workers showed in 2018 that adding N-bromosuccinimide could deliver the brominated product (Scheme [5]).[26] Besides the conventional PCET mechanism in which the photocatalyst provides the alkoxyl radical I (path a), an alternative pathway is proposed. Here, the key alkoxyl radical intermediate I is obtained through visible-light-mediated homolytic cleavage of the O–I bond in the alcohol-PIDA construction III (path b). However, both pathways involve visible light, as performing the reaction in the dark provided no product.

The use of hypervalent iodine reagents for the β-C–C cleavage of alcohols was inspired by the work of Chen and co-workers, who performed their reaction using a Ru(II) photocatalyst in the presence of a cyclic iodine(III) reagent (Scheme [6]).[27] Acetoxybenziodoxole (BI-OAc) readily generates BI⁺ ions that serve as a Lewis acid to coordinate with the alcoholic oxygen atom to give intermediate I. Homolytic cleavage of this newly formed I–O bond is facilitated via SET by the oxidized ruthenium species, initiating the β-scission that yields the C-centered alkyl radical III which then undergoes radical addition to the alkynyl-bound benziodoxole 18. The procedure proved successful for both strained cyclic as well as linear alcohols and was the first visible-light-mediated­ alcohol oxidation to generate alkoxyl radicals using a cyclic iodine(III) catalyst. However, a minor disadvantage of this procedure was the requirement for a pre-formed alkynyl-BI instead of forming it in situ.

Another procedure that made use of the alkoxyl radical intermediate was the catalytic ring expansion of cyclic alcohols described by the Knowles group (Scheme [7]).[28] In this context, PCET activation of the O–H bond by a Brønsted base and an excited Ir(III) catalyst allows the formation of the key alkoxyl radical species, which then mediates the β-scission of an adjacent C–C bond to deliver a new carbonyl group and a terminal C-centered radical I. This radical is then trapped by introducing an olefinic moiety on the alcoholic carbon atom of the substrate 20, authorizing either an n+1- or n+2-sized ring via an exo- or endo-trig event, respectively.

Wang and co-workers described photoinduced ring-opening of cyclic hemiacetals through β-scission of alkoxyl radicals with Hantzsch ester (HE) as the reductant (Scheme [8]).[29] Upon activation of the hemiacetal with N-hydroxy­phthalimide (NHPI), coordination with HE results in a donor­–acceptor complex I that allows SET reduction of the substrate upon visible-light irradiation. After elimination of the phthalimido moiety, alkoxyl radical III is generated which initiates the crucial β-scission, providing C-centered radical IV that is trapped by alkynyl sulfone 24 give the alkynylated product 25. Additionally, alkenyl sulfones were introduced as radical trap and provided the alkenylated products in moderate 54–78% yields. Note that HE was required in stoichiometric amounts, therefore, this reaction was not of a catalytic nature.

In 2020, the Zuo group described the C–C bond cleavage of both cyclic and acyclic ketones via generation of the alkoxyl radical using a cerium(IV) catalyst and 9,10-diphenylanthracene (DPA) (Scheme [9]).[30] The addition of TiCl₄ as Lewis acid and TMSCN to the ketone substrate delivers the corresponding cyanohydrin I which is then undergoes subsequent coordination with the cerium catalyst and visible-light-mediated Ce–O bond cleavage to yield the O-centered radical species III. β-Scission of III leads to a distal carbon-centered radical IV for which the consecutive reaction pathway depends on the ring size of the substrate. For cyclobutanones or cyclopentanones (n <2), the alkyl radical undergoes addition to the nitrile carbon atom, generating the iminyl intermediate V which readily mediates a cleavage of the adjacent C–C bond, causing a net migration of the cyanide group to form the product 27. For 6-membered, or larger, rings and acyclic ketones, the alkyl radical IV is trapped by diisopropyl azodicarboxylate (DIAD), avoiding migration of the CN moiety.

In 2021, the Knowles group reported polymer degradation of commercial phenoxy resin and high-molecular-weight hydroxylated polyolefin derivatives by the generation of an alkoxyl radical from hydroxyl substituents and cleavage of the adjacent C–C bond.[31] The requirement of photocatalyst, base, and a thiol indicated the presence of a consecutive PCET/β-scission. However, the Knowles group did not perform detailed mechanistic studies to propose a reaction mechanism.

In 2021, the Zuo group further expanded their work from 2016[23] and 2020[30] on the application potential of cerium salts in Ce-mediated photocatalytic C–C bond cleavage (Scheme [10]).[32] In this case, they performed a ring expansion via subsequent β-scission and oxidation of cyclic ketones. Whereas, in 2020, DPA was used to complete the cerium catalytic cycle, they now introduced cyanoanthracene as an organic photocatalyst (OPC) as it apparently enhanced the turnover number (TON), rendering the target macrolactone product 30 within 20 minutes. Due to the Lewis acidic nature of Ce(III), the ketone/lactol equilibrium of 29a shifts towards the lactol upon coordination with the metal salt. The in situ formed cerium(III) lactol complex (E _1/2 = 0.40 V vs. SCE) is readily oxidized by the radical cation of cyanoanthracene (E _1/2 = 1.61 V vs. SCE), to the Ce(IV) state, which provides alkoxyl radical I upon photoirradiation through Ce–O bond cleavage. After the β-scission, C-centered radical II is generated which captures O₂, delivering peroxy radical III that, after subsequent reduction and protonation, is converted into the product 30a after releasing H₂O.

This procedure tolerated a broad range of substrates with over 55 diverse substrates converted into their respective macrolactone products with ring sizes of up to 19. Additionally, the procedure was successfully applied to the concise synthesis of sonnerlactone, metabolite isolated from fungi found in the South China Sea.[33]

A 2021 report by the Mo group involved a novel procedure for the photoinduced radical borylation of hemiacetal derivatives via C–C bond cleavage.[34] Their method provided a facile new synthetic route for boronic esters as invaluable reagents in, for example, cross-coupling reactions.[35] DFT calculations confirmed the proposed mechanism, which is initiated when the substituted hemiacetal substrate 23 interacts with solvated bis(catecholato)diboron [B₂(cat)₂] to form an additive that is activated through energy transfer by Ir(ppy)₃. This photosensitizing process generates a radical species that generates an O-centered hemiacetal radical intermediate to promote ring opening by β-scission. The obtained C-centered radical then adds to boron in the solvated B₂(cat)₂ structure, which then undergoes a B–B bond cleavage to deliver the final boronic ester product. Besides, 5-, 6-, and 7-membered rings, also linear hemiacetals were successfully borylated in yields of up to 81%

Heterogeneous Catalyst

There have been many reports, dating back to 1996, of the use of heterogeneous catalysts in the photocatalytic cleavage of carbon–carbon bonds.[36] However, the majority of these reports focus on the degradation of organic dyes, instead of applications in chemical synthesis.[37] At present, the use of semiconductors in selective C–C bond cleavage is still an enormous hurdle to cross.

In 2017, Guo, Ma, Su, Yang, and co-workers investigated photocatalytic C–C bond cleavage in ethylene glycol (EG) on TiO₂ and hereby studied the effect of deposited metal nanoparticles (NPs).[38] They revealed that pristine TiO₂ was highly capable of converting EG into 2 equivalents of formaldehyde and hydrogen gas since the substrate molecules adsorbed purely on the Ti sites, not on the metal. However, introducing Pt, Pd, or Au NPs appeared to improve hydrogen desorption, which increased formaldehyde formation with Pt performing best. Since oxygen played a major role in the C–C bond cleavage, the risk of overoxidation of the aldehyde product to CO₂ gas was imminent. Also, the presence of water allowed for polymerization of the product, causing the formation of paraformaldehyde. In 2018, Su and co-workers examined the relation between selectivity and the type of noble metal NPs.[39] They showed that Ag and Au promoted the formation of the polymer and suppressed overoxidation by rapidly providing water as adsorbed hydrogen atoms efficiently reacted with superoxide radicals (O₂ ^•–). On the other hand, Pt promoted complete oxidation and inhibited polymerization due to insufficient water caused by an indirect pathway for water formation.

The Wang group published a report on the C–C bond cleavage in lignin models in 2018, using carbon nitrides under visible light.[40] In this study, three different catalysts were used: a carbon nitride material synthesized from melamine (C₃N₄-M), one from urea (C₃N₄-U), and one mesoporous graphitic carbon nitride (mpg-C₃N₄). The objective was to selectively perform the aerobic cleavage over the oxidation of the alcohol. Out of the three carbon nitride catalysts, mpg-C₃N₄ gave the highest conversion (96%), together with the optimal selectivity towards the C–C bond cleavage (91%). Brunauer–Emmett–Teller measurements of the catalysts then revealed the significant structural difference between the three as the specific surface area for C₃N₄-M and C₃N₄-U were 6.6 m² g^–1 and 48.2 m² g^–1, respectively, whereas mpg-C₃N₄ has a surface area of 206.5 m² g^–1, explaining the increased catalytic activity of the latter. A similar trend was observed for the pore volumes, where the values of C₃N₄-M, C₃N₄-U, and mpg-C₃N₄ were determined to be 0.04 cm³ g^–1, 0.15 cm³ g^–1, and 0.42 cm³ g^–1, respectively. The authors proposed that after excitation of the semiconductor, a β-H of the substrate is abstracted at the valence band holes to generate a C-centered radical, which then captures molecular oxygen to form the peroxy radical. The hydroperoxide intermediate formed after HAT, undergoes a rearrangement through a 6-membered transition state to provide both cleavage products by releasing water. Finally, the reaction procedure was applied to 5 substrates, other than the model one, and even though high conversions were achieved (>88%), the selectivity of the C–C bond cleavage towards other scaffolds was not achieved.

C=C Bond Cleavage

Carbon–carbon double bonds are among the most common functional groups present in organic scaffolds and have also exhibited highly versatile applications in the synthesis of organic molecules through transformations such as addition,[41] cycloaddition,[42] metathesis,[43] reduction,[44] oxidation,[45] and polymerization.[46] Among these reactions, the oxidation of C=C bonds is a remarkable tool to obtain carbonyl-containing compounds such as aldehydes, ketones, carboxylic acids, esters, and amides. Traditional alkene oxidation reactions such as ozonolysis[47] and the Lemieux–Johnson oxidation[48] lack selectivity as well requiring harmful oxidants.

By contrast, photocatalysis can provide a solution due to its potential to generate reactive oxygen species (ROS) such as singlet oxygen (¹O₂) and O₂ ^•–.[49] These species can undergo [2+2] cycloadditions with olefins or couplings with oxidized olefins, respectively, to provide a key dioxetane intermediate. This 4-membered ring can then undergo oxidative cleavage to generate two oxidized products containing a carbonyl moiety. As will become clear, the majority of the procedures discussed in this section will strongly rely on the formation of this cyclic intermediate.

Homogeneous Catalyst

In 2002, the Itoh group reported a study of photocatalytic C=C bond cleavage using a heterogeneous catalyst (mesoporous silica FSM-16) and I₂ (see also Section 3.2, Scheme [13]).[50] However, this procedure proved inadequate when using α- or β-substituted styrenes as the substrates. In 2009 they successfully used catalytic CBr₄ for the aerobic photo-oxidative cleavage of the C=C bond of a variety of styrenes to give the respective carboxylic acids.[51] Under irradiation by a Hg lamp, one of the C–Br bonds of the CBr₄ catalyst is homolytically cleaved, generating reactive bromine radicals that readily add to the C=C bond of the styrene. The resulting C-centered radical species then reacts with molecular oxygen gas, after which hydrogen abstraction of a solvent molecule provides a hydroperoxide species. Upon elimination of water, the α-bromo ketone is formed, which is followed by one of two reaction pathways that both deliver the acyl radical. The first possibility is that the radical is formed through a light-induced homolytic C–C bond cleavage. The second route starts by generating a reactive bromine radical after cleavage of the C–Br bond of the substrate. The simultaneously generated, C-centered radical, reacts with molecular oxygen to deliver the corresponding α-hydroperoxy ketone. Subsequent elimination of water then provides the dione which undergoes C–C bond cleavage under UV light irradiation, generating 2 equivalents of acyl radical. Addition of oxygen and hydrogen abstraction from the solvent then delivers the peroxy acid which is efficiently converted into the carboxylic acid under light irradiation. The substrate scope of the reaction showed that electron-deficient alkenes were not very well tolerated (4-nitrostyrene: 16% yield). This was expected since the C-centered radical that was formed upon the addition of Br^•, is strongly destabilized by electron-withdrawing moieties, hereby preferring the unpaired electron to be on the adjacent carbon atom.

In 2014, the Itoh group reported a similar procedure that focused on the photooxidative C=C bond cleavage of stilbenes using I₂ and trifluoroacetic acid instead of CBr₄ for radical generation.[52] In this case the proposed mechanism is slightly different from that in their work on the CBr₄-mediated C=C bond cleavage.[51] Here, an O-centered methoxy and iodide radical add to the C=C bond, after which the C–I bond is cleaved under irradiation by light, generating a C-centered radical intermediate. Subsequent reaction with molecular oxygen and hydrogen abstraction provides the β-methoxy hydroperoxide. Cleavage of the C–C bond by elimination of water under acidic conditions delivers the aldehyde and methyloxonium salt. The latter is converted into the aldehyde product in the presence of methanol and acid.

In 2009, You and co-workers focused on the use of singlet oxygen, more specifically its [2+2] cycloaddition reaction, for the photochemical triggering of drug delivery systems such as liposomes or prodrugs (Scheme [11]).[53] Since UV light cannot penetrate deeper than 1 mm into tissue, only low energy light from the (near-)IR region can be used. For this reason, 5,10,15-triphenyl-20-(4-hydroxyphenyl)-21H,23H-porphyrin (TPP-OH) was selected as a photosensitizer. Without TPP-OH, nearly all substrates demonstrated negligible reactivity (<1%) with molecular oxygen and light irradiation. Unfortunately, only 4 out of 15 substrates exceeded a yield of 75%. One example was 1,2-bis(phenylthio)ethene (31c) which gave the carbonylated product in only 14%. A comparison of 1,2-bis(phenoxy)ethene (31b) (80%) with its thiolated counterpart 31c (14%), showed that the latter reaction was disadvantageous since more side products were formed. However, despite the unfavorable reaction yields, a major advantage of this reaction that visible light of up to 800 nm could be used to successfully perform this C=C bond cleavage.

In 2015, the Yadav group disclosed a procedure to oxidatively cleave C=C bonds through generation of O₂ ^•– with eosin­ Y as a homogeneous photocatalyst.[54] The excited eosin­ Y photocatalyst oxidizes the styrene substrate to a radical cation intermediate, after which the reduction of O₂ to O₂ ^•– closes the catalytic circle and delivers the eosin Y back in its initial state. Then radical-radical coupling generates a dioxetane intermediate that undergoes a cleavage reaction to deliver the aldehyde product with release of formaldehyde.

Wang and co-workers, in 2014, reported the visible-light-induced photocatalytic conversion of enamines to amides.[55] In the presence of a Ru(II) complex, Cs₂CO₃, and oxygen gas, several enamine substrates were successfully converted into their formamide counterparts in 40–89% yields. Additionally, this procedure also tolerated ketone-derived enamines, as the corresponding amides were obtained in 82–99% yields. However, this substrate scope was limited and only consisted of 4 enamines.

Continuing with enamines and their derivatives, the Lee group were able to convert the C=C bond in N-sulfonyl-enamides into a C=O bond, contributing to the challenging selective acylation of azulenes.[56] Traditionally, harmful reagents such as POCl₃ (Vilsmeier–Haack)[57] or AlCl₃ (Friedel–Crafts)[58] were required, which contributed to the low yields, diacylation, and poor functional group tolerance of these reactions.[59] The Lee group reported a synthetic procedure for the formation of azulen-1-yl ketones via oxidative cleavage of the C=C bond in N-sulfonyl-enamides via reaction with Cs₂CO₃, under air and sunlight, and in the absence of a photosensitizer. A mechanism was proposed based on literature reports and experimental observations, which is based on the formation of a dioxetane intermediate that is formed through a cyclization with oxygen.[60] This was suggested after labelling experiments with ¹⁸O₂ delivered the ¹⁸O-inserted ketone product, unlike the reaction with H₂ ¹⁸O. Moreover, the oxidative cleavage did not proceed in the absence of light, indicating that the sunlight was crucial for the reaction. Additionally, the importance of the N–H proton in the amine moiety of the N-sulfonyl-enamide was shown by experiments which showed that a tertiary amine moiety in the enamide did not yield the ketone product. The proposed mechanism involves the formation of an alkene radical cation and O₂ ^•– through photoinduced electron transfer of a contact charge-transfer (CCT) complex between the enamide substrate and O₂. After the cyclization reaction, providing the dioxetane cyclic intermediate, it decomposes to give the ketone product after abstraction of the N–H proton in the N-sulfonyl-enamide. Substrates containing both electron-rich or electron-deficient aromatic moieties were tolerated and gave the corresponding products in 85–91% yields. Even N-mesyl-enamides with thiophen-2-yl and cyclohex-1-enyl groups successfully provided the respective products in 88% and 76% yields. Additionally, N-sulfonyl-enamide was generated in situ through a tandem Cu-catalyzed [3+2] cycloaddition, Rh-catalyzed arylation, photooxygenation, and ring-opening reaction in one-pot under air and sunlight. This will be discussed in more detail in Section 4.1.

Besides enamines, in 2015 Wan, Wen, and co-workers utilized enaminones as substrates in visible-light-induced C=C bond cleavage.[61] Similar to enamines, the C=C bond was replaced with C=O generating 1,2-diones, instead of the monoketones obtained with enamines as substrates.

In 2017, Noël, Wang, and co-workers reported the visible-light-mediated oxidative cleavage of C=C bonds with evidence of the formation of an olefin–disulfide charge-transfer complex (Scheme [12]).[62] Upon looking for a photoinitiated radical that could reversibly add to the C=C bond of the substrate, they imagined that thiyl radicals generated by the photolysis of disulfides could be ideal.[63] However, dissociation of typical aromatic S–S bonds cannot occur under visible light and therefore requires UV irradiation.[64] In 1951, Mantell and co-workers showed that the reaction rate of the radical addition of a thiyl radical to an olefin was significantly higher than the SET oxidation of thiol by itself, owing to the formation of an olefin–thiol charge-transfer complex (CTC).[65] Noël, Wang, and co-workers used this principle and extended it to disulfides. The presence of the CTC was confirmed by examining the upfield shift of the methoxy protons in bis(4-methoxyphenyl) disulfide in the presence of olefin. Higher olefin/disulfide ratios accounted for stronger upfield shifts, indicating an increasing electron density on the disulfide. They also performed control experiments where two different disulfide substrates were mixed under visible light, in the absence of olefin. Since, after 1 h, no mixed disulfide had been formed, it could be concluded that the S–S bond cleavage did not occur under visible light in the absence of olefin. Performing the same reaction with irradiation from a medium-pressure Hg lamp provided product in 9% yield. Regarding the mechanism, the thiyl radical formed through CTC-mediated photolysis of the S–S bond readily adds to the olefin 33, generating a C-centered radical I that captures molecular oxygen to form the peroxyl radical intermediate II. Then, a cyclization reaction occurs to give the dioxetane intermediate III, which decomposes to yield the ketone product 34. All products were obtained in 60–92% yields, with the only exception being (Z)-β-methylstyrene (44%), in which part of the starting material was either epoxidized or isomerized to the (E)-alkene.

The Wan group designed a novel synthesis route to α-keto esters via aerobic C=C bond cleavage using a metal-free photocatalyst.[66] Here, rose bengal is excited under visible-light irradiation, after which the excited species activates oxygen gas providing reactive singlet oxygen.[67] This species then couples to the C=C bond of the enaminone substrate generating a dioxetane intermediate that undergoes a ring-opening reaction triggered by protonation. Subsequent to the ring-opening reaction, a new N–O bond is formed to provide a 4-membered cyclic intermediate which gives a zwitterionic species upon nucleophilic attack of an alcohol. This species then decomposes into the α-keto ester product and an amino alcohol. The conditions tolerated a broad substrate scope in the enaminone substrates and gave the α-keto ester products in 65–84% yields. Additionally, the conversion of natural 16-dehydropregnenolone acetate into its respective α-keto ester was successfully achieved in 45% yield

Heterogeneous Catalyst

In 2002, the Itoh group reported the synthesis of carboxylic acids by C=C bond cleavage in styrenes using I₂ and a mesoporous, recyclable silica (FSM-16) photocatalyst (Scheme [13]).[50] However, their substrate scope proved the limited applicability of the reaction since substrates with di- (or higher) substituted double bonds 35b–d were converted in poor, 12–33% yields. Furthermore, reaction of aliphatic dodec-1-ene under these conditions gave no product formation. However, this reaction provided sufficient foundation for further research by the Itoh group and others,[51] [52] [68] see also Section 3.1.

In 2006, the Hirai group noticed that silica-based materials showed promising results in the photocatalytic oxidation of olefins with oxygen; they compared a range of heterogeneous catalysts.[69] Previously, often photocatalytic systems based on TiO₂ were described, which were operative in the UV range and promoted complete oxidation of olefins to CO₂.[70] During their investigation, the aim of the Hirai group was not the cleavage of the C=C bond, however, their reactions gave the cleaved product. Among the assessed catalyst materials, Cr-SiO₂ prepared by a conventional sol-gel method provided the highest yields for the majority of substrates, reaching up to 89% of the ketone product obtained via C=C bond cleavage. A major setback of this reaction was the limited substrate scope. Additionally, the procedure was only successful for terminal alkenes.

Instead of these inorganic heterogeneous photocatalysts, Zhang and co-workers performed their visible-light-promoted C=C bond cleavage of styrene with conjugated microporous polymers (CMPs) as photocatalysts (Scheme [14]).[71] Three CMPs were compared, which were synthesized from triethynylbenzene and thiophene (BTh), triethynylbenzene and benzothiodiazole (BBT), and triethynylbenzene with 50% thiophene and 50% benzothiodiazole (BThBT). These catalysts were selected based on their different electron affinities, since the thiophene moiety acts as a strong electron donor while the enzothiodiazole has strong electron-accepting properties. During the screening experiments, it became clear that the solvent polarity strongly influenced benzaldehyde formation and the optimal solvent system was determined to be MeCN/H₂O (1:24). Among the catalysts, BBT performed best, achieving a conversion of 91% with 85% of benzaldehyde formed. This was in agreement with the pore volume measurements, where BBT had the highest volume of 0.631 cm³ g^–1. Counterintuitively, Brunauer–Emmett–Teller (BET) measurements showed that the BBT catalyst had the smallest surface­ area of only 129 m² g^–1 compared to BThBT with 445 m² g^–1 and BTh with 806 m² g^–1. The band structures of all three catalysts were determined and revealed that BBT exhibited the highest LUMO level at –0.92 V vs SCE, indicating its strong ability to generate O₂ ^•– from O₂ for which a potential of only –0.57 V vs SCE is required. The formation of this intermediate was crucial in the proposed mechanism where, upon excitation with visible light, substrate 37 is oxidized at the HOMO of the semiconductor, generating a radical cation intermediate. At the LUMO of the photocatalyst, superoxide radicals are generated through reduction of oxygen gas. This species then reacts with the styrene radical and delivers a dioxetane intermediate. 1-Phenylethane-1,2-diol is then formed due to exposure to water, which is eventually cleaved into the aldehyde product 38, releasing formaldehyde as potential side product. Overall, even though all substrates were almost completely converted (>99%), selectivity towards the cleavage reaction product was relatively low (27–65%) and in some cases, the aldehyde was not present.

Staying in the field of polymeric catalysts, the Das group cleaved C=C bonds using polymeric carbon nitrides (PCN) to generate the key superoxide radical species.[72] These catalysts were able to initiate this SET reduction reaction, according to the position of the conduction band at –1.55 V vs. SCE.[73] After the optimization, the reaction conditions were set at 0.25 mmol of substrate, 8 mg of PCN catalyst, and 20 mol% of N-hydroxysuccinimide (NHS) in MeCN in an O₂ atmosphere and under irradiation of blue LED. Two plausible mechanisms were proposed. Both started with the excitation of the semiconductor, which reduces O₂ to O₂ ^•– at the conduction band while generating a radical cationic intermediate through oxidation of the substrate at the valence band. The first reaction pathway involves the traditional cyclization reaction between both radical species, hereby providing the dioxetane intermediate which undergoes a ring-opening reaction to give the two cleavage products. The second pathway, however, includes the beneficial effect of NHS, whereas HAT from NHS to O₂ ^•– delivers the hydroperoxide anion together with an O-centered radical. The latter then adds to the oxidized olefin, generating a cation which readily undergoes nucleophilic attack by the hydroperoxide anion. This hydroperoxide species can be converted into the dioxetane intermediate through expulsion of NHS, opening up the ring-opening pathway that delivers the cleavage products. A broad substrate scope including various 1,1-disubstituted alkenes was provided, which were all well-tolerated in the reaction procedure as they were converted in moderate to excellent 61–90% yields. Additionally, several 1,2-disubstituted alkenes were introduced in the reaction. However, this appeared to be more challenging as the respective cleavage products were only formed in 39–70% yields. They also applied their reaction procedure to several complex scaffolds, all of which were underwent cleavage, delivering the products in 57–71% yields (Scheme [15]). To show the application potential of their methodology, they also successfully performed the C=C bond cleavage under sunlight, instead of the blue LED light source. Here, they were able to convert one gram of starting material into the target cleaved product in 66–71% yields.

In 2020, Natarajan and co-workers performed a comparative study in which they characterized various BiMXO₅ (M = Mg, Cd, Ni, Co, Pb, Ca and X = V, P) materials using powder X-ray diffraction and optical absorption methods.[74] BiCdVO₅ was potentially a promising catalyst due to its advantageous­ chemical stability and accessible band gap (2.45 eV).[75] For this reason, nano and bulk BiCdVO₅ were assessed in the aerobic, oxidative C=C bond cleavage reaction of styrene, which revealed the strong photocatalytic ability of the nanocatalyst in the model reaction (81%). Several other olefins were used as substrates in this reaction, providing the respective target cleavage products in 76–89% yields. The reactivity of the catalyst towards the aerobic oxidation of benzyl alcohols to benzaldehydes was also assessed whereas the majority of the substrates provided the oxidation product in 84–95% yields, thus illustrating the potency of the catalyst.

Another example of an inorganic photocatalyst was used in a contribution by the Li group, who demonstrated the potential of chalcogenide ZnIn₂S₄ in the photocatalytic oxidative cleavage of C=C bonds in olefins under visible light.[76] Their work was inspired by the disulfide-mediated C=C bond cleavage developed by Noël, Wang, and co-workers,[62] in which the cleavage of S–S bonds generated reactive thiyl radicals that could reversibly add onto olefins and resulted in their activation towards oxygen capture and subsequent cycloaddition to provide a dioxetane intermediate (see Section 3.1, Scheme [12]). The Li group chose benzyl mercaptan as an additive as a precursor for the thiyl radical. This thiol appeared to be crucial for their reaction, as no product was obtained in its absence; electron paramagnetic resonance (EPR) spin-trap experiments confirmed the generation of the thiyl radical at the valence band of the semiconductor. A limited scope of 7 olefin substrates gave the corresponding cleavage products in 65–81% yields, thus illustrating the robustness of the reaction. Additionally, under these reaction conditions with the addition of an alcohol and under an air atmosphere, the 7 olefin substrates were converted into their respective acetals in 56–81% yields.

In 2021, Niu and co-workers[77] aimed to design a reaction procedure for oxidative C=C bond cleavage without the need for an initiator such as a disulfide,[62] thiol,[76] or NHS.[72] They were convinced that hydroxyl radicals (^•OH) could be used, which are traditionally generated via electron transfer from adsorbed hydroxyl ions or water to the holes of the semiconductor.[78] However, in this reaction, they proposed that the hydroxyl radicals are obtained via HAT from water to O₂ ^•–, the latter is generated at the conduction band of the tubular carbon nitride (TCN) catalyst. At the valence band, on the other hand, the olefin substrate is supposedly oxidized to deliver a radical cation. Subsequently, when a hydroxyl radical adds onto this intermediate, a cationic intermediate is formed which can undergo a nucleophilic attack from ^–OH. By releasing water, a dioxetane intermediate is formed, which undergoes a cleavage process to yield the oxidized products. This mechanism was preferred over a traditional cycloaddition between the radical cation of the substrate and O₂ ^•–, since isotope-labelling experiments showed that the oxygen atom in the reaction product stemmed from H₂O. The reaction had extensive substrate scope and after formation of the aldehyde product, further oxidative coupling with MeOH afforded methyl esters under photooxidative conditions.[79]

C≡C Bond Cleavage

Currently, there few reports on the visible-light-induced cleavage of C≡C bonds as their BDE is the highest of all the types of carbon–carbon bonds (C≡C BDE = 954 kJ/mol,[79] C=C BDE = 728 kJ/mol,[80] C–C BDE 618 kJ/mol)[81] and their tendency for over-oxidation (Figure [1]).[82] Additionally, the majority of reports focus on the transformation of terminal alkynes, such as phenylacetylene, as these are more sterically accessible than 1,2-disubstituted alkynes.

Homogeneous Catalyst

In 2013, the Itoh group reported the aerobic photo-oxidative cleavage of C≡C bonds;[83] this was a follow-up study to their work on C=C bond cleavage in various olefins using CBr₄ as the catalyst under photoirradiation to give aldehyde (or ketone) products (Section 3.1).[51] The use of alkynes as substrates under these conditions allowed the synthesis of carboxylic acids. The presence of two intermediates, namely 2,2-dibromoacetophenone and phenylglyoxylic acid, were confirmed as the reaction product was obtained upon using either as a substrate. Based on the presence of these intermediates, the mechanism is proposed to follow one of three pathways all of which ended in the capture of O₂ by an acyl radical, after which consecutive HAT and HBr-mediated elimination of HOBr delivers the carboxylic acid product. The substrate scope showed that primarily substituted phenylacetylenes were tolerated, however also some 1,2-disubstituted alkynes were converted into their respective carboxylic acid in moderate to high yields. Reactions of heteroaryl-substituted acetylenes or an aliphatic alkyne gave products in less than 34% yield.

The Lee group reported a synthetic procedure for the formation of azulen-1-yl ketones via oxidative cleavage of the C=C bond in N-sulfonyl-enamides via reaction with Cs₂CO₃, under air and sunlight and in the absence of a photosensitizer.[56] The N-sulfonyl-enamide substrates were also prepared in situ via a tandem Cu-catalyzed [3+2] cycloaddition, Rh-catalyzed arylation, photooxygenation, and ring-opening reaction in one pot under air and sunlight. Using a literature procedure, a wide range of N-tosyl-enamides were prepared by the reaction of azulene with triazoles.[84] The one-pot reaction of phenylacetylene with tosyl azide in the presence of the copper catalyst, followed by addition of a rhodium complex and azulene to the reaction mixture then treatment with Cs₂CO₃ gave 1-benzoylazulene in 66% yield. Surprisingly, no alkyne substrate scope was performed.

In 2016, Hwang and co-workers designed a copper(I)-catalyzed oxidative coupling of 2-aminopyridine with terminal alkynes via visible-light-promoted C≡C bond cleavage.[85] The copper(I) phenylacetylide complex that is obtained through oxidative addition of the alkyne to the copper(I) center, is excited by visible light, allowing for a SET to molecular oxygen to generate O₂ ^•– and copper(II) phenylacetylide.[86] After nucleophilic addition of 2-aminopyridine, a Cu(III) complex is formed which undergoes consecutive reductive elimination and reaction with oxygen to afford a pyridine ketoamide intermediate and regeneration of CuCl­.[87] Free 2-aminopyridine also coordinated with Cu(I), which yielded a superoxo/-peroxo complex upon excitation and subsequent electron transfer to O₂.[88] This complex then abstracts a hydrogen atom from the pyridine ketoamide, generating an N-centered radical. Radical-assisted CO-elimination and recombination of radicals leads to the formation of the pyridyl benzamide product.

Continuing their work on copper(I)-mediated C≡C bond cleavage, Hwang and co-workers designed a novel procedure for the synthesis of α-keto esters via the photooxidative C≡C bond cleavage of terminal alkynes (Scheme [16]).[89] Since it was shown that copper(I) phenylacetylide can generate copper(II) phenylacetylide and O₂ ^•– under visible-light irradiation, they believed that both species could execute the controlled oxidation of the C≡C bond in alkynes.[90]

During their optimization, they observed that among CuCl, CuBr, and CuI, only the CuI provided the product in acceptable yields. As the ligand L, 1 equivalent of 2-picolinic acid was found to be optimal. Removal of O₂ or light led to no product formation, confirming their crucial role in the reaction mechanism. After oxidative addition of phenylacetylene (41) to the copper center, excitation under visible light allows for a SET to generate copper(II) phenylacetylene III and O₂ ^•–. Next, coordination of 2-picolinic acid to the copper center and subsequent reaction with O₂ delivers the copper(III)-superoxo complex IV, which allows for a rearrangement to provide intermediate V upon elimination of Cu^II(pic)₂. Through O–O bond cleavage of intermediate V, 2-oxo-2-phenylacetaldehyde VI is formed, which is converted into the hemiacetal VII by a nucleophilic attack of an alcohol 42. The α-keto ester product 43 is then finally obtained through a copper-catalyzed aerobic oxidation.[91] The authors provided a broad and diverse alcohol and terminal alkyne scope, both of which demonstrated the robustness of their procedure as all but 4 substrates out of 43 were converted into their respective products in >70% yield. Additionally, the synthesis of 2-phenylquinoxaline, an FLT3 inhibitor, was carried out by trapping the phenylglyoxal VI intermediate in situ using a commercially available 1,2-phenylenediamine.

In 2020, the Sharada group were able to synthesize oxamates via the visible-light-mediated photocatalytic oxidative cleavage of alkynes.[92] In this report, disubstituted, electron-deficient alkynes, instead of the conventional terminal alkynes, underwent hydroamination with various aniline-derivatives as the coupling partner. After this activation mechanism, the hydroamination product oxidatively quenches the excited photocatalyst to yield the rose bengal radical anion and an N-centered radical cation. The reduced photocatalyst then initiates a SET to tert-butyl hydroperoxide (TBHP) to generate the reactive tertiary butoxide radical and hydroxide anion, thereby closing the rose bengal catalytic cycle. On the other hand, the radical cation generated earlier undergoes consecutive radical addition by a tert-butyl­ peroxide radical and nucleophilic attack by a hydroxide anion to deliver an α-tert-butylperoxy iminium intermediate. After an intramolecular cyclization releasing t-BuOH, a dioxetane intermediate is formed which provides the oxamate product after fragmentation. In terms of substrate scope, a broad range of anilines were tolerated, unlike the alkynes where deviation from the methyl or ethyl ester scaffolds resulted in no product formation.

Heterogeneous Catalyst

In 2020, Niu, Ji, and co-workers used porous g-C₃N₄ as a photocatalyst for the one-pot aerobic oxidative cleavage of C≡C bonds in alkynes to form aldehydes.[80] In 2016, Niu, Ni, Wang, and co-workers performed a similar reaction in which they reacted diazonium salts with terminal alkynes using eosin Y to generate α-chloro and α-alkoxy aryl ketones.[93] In both cases, the diazonium salt acted as a precursor to generate aryl radicals that could undergo a radical addition to the alkyne, generating an alkenyl radical. This species then reacts with O₂ ^•– that is generated by SET reduction of the photocatalyst, forming a radical dioxetane intermediate that is readily cleaved to deliver an aryl aldehyde and acyl radical. The latter is then quenched via HAT, yielding the second aldehyde product. However, the 2016 report considered the formation of the dioxetane radical intermediate as a side reaction, and O–O bond cleavage of the peroxy radical obtained after the addition of O₂ ^•– to the alkenyl radical eventually delivered the desired α-substituted aryl ketone.

Conclusion

Overall, we have provided a broad overview on the recent developments for the cleavage of carbon–carbon single, double, and triple bonds where light was crucial for the reaction. As we have demonstrated, the scope of this type of reactions is still rather limited, whereas only activated single bonds can be cleaved. Regarding double and triple bonds, the majority of the reports involve terminal alkenes and alkynes, currently restricting the application potential of these procedures in industry. However, looking at the growth that this field has seen in such a brief period, we are convinced that during the following years, major developments in the selective photocatalytic C–C bond cleavage will contribute to the facile and rapid synthesis of pharmaceuticals and other complex organic scaffolds, with photoredox catalysis playing a crucial role. We believe that this application window can be achieved due to the increasing development of new heterogeneous catalysts and their advantageous contributions in organic synthesis. For example, single atom photocatalysts (SAPCs) have only recently been investigated for their potential to generate ROS such as H₂O₂.[94] Due to the unsaturated coordination of the active metal centers, these catalysts exhibit remarkable reactivity while maintaining the recyclability of heterogeneous catalysts. We are convinced that the future of photocatalytic C–C bond cleavage will be determined by the development of novel, powerful, and selective heterogeneous catalysts which will open up the possibility for these reactions to be applied in fine chemical synthesis and the pharmaceutical industry.

Conflict of Interest

The authors declare no conflict of interest.

Acknowledgment

Special thanks to Dr. Rakesh Maiti and Mr. Robin Cauwenbergh for their kind help during the preparation of this manuscript.

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Corresponding Author

Publication History

Received: 17 October 2021

Accepted after revision: 22 November 2021

Accepted Manuscript online:
22 November 2021

Article published online:
02 February 2022

© 2021. Thieme. All rights reserved

Georg Thieme Verlag KG
Rüdigerstraße 14, 70469 Stuttgart, Germany

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