Photoredox Catalysis for the Generation of Carbon Centered Radicals

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Photoredox Catalysis for the Generation of Carbon Centered Radicals Published as part of the Accounts of Chemical Research special issue “Photoredox Catalysis in Organic Chemistry”. Jean-Philippe Goddard,*,† Cyril Ollivier,*,‡ and Louis Fensterbank*,‡ †

Université de Haute-Alsace, Ecole Nationale Supérieure de Chimie de Mulhouse, Laboratoire de Chimie Organique et Bioorganique EA 4566, 3 Bis rue Alfred Werner, 68093 Mulhouse Cedex, France ‡ UPMC Univ-Paris 06 − Sorbonne Universités, Institut Parisien de Chimie Moléculaire (UMR CNRS 8232), 4 Place Jussieu, C. 229, 75005 Paris, France CONSPECTUS: Radical chemistry has witnessed over the last decades important advances that have positioned it as a methodology of choice in synthetic chemistry. A number of great attributes such as specific reactivities, the knowledge of the kinetics of most elementary processes, the functional group tolerance, and the possibility to operate cascade sequences are clearly responsible for this craze. Nevertheless, at the end of the last century, radical chemistry appeared plagued by several hurdles to overcome such as the use of environmentally problematic mediators or the impossibility of scale up. While the concept of photocatalysis was firmly established in the coordination chemistry community, its diffusion in organic synthetic chemistry remained sporadic for decades until the end of the 2000s with the breakthrough merging of organocatalysis and photocatalysis by the MacMillan group and contemporary reports by the groups of Yoon and Stephenson. Since then, photoredox catalysis has enjoyed particularly active and intense developments. It is now the topic of a still increasing number of publications featuring various applications from asymmetric synthesis, total synthesis of natural products, and polymerization to process (flow) chemistry. In this Account, we survey our own efforts in this domain, focusing on the elaboration of new photocatalytic pathways that could lead to the efficient generation of C-centered functionalized alkyl and aryl radicals. Both reductive and oxidative manifolds are accessible through photoredox catalysis, which has guided us along these lines in our projects. Thus, we studied the photocatalytic reduction of onium salts such as sulfoniums and iodoniums for the production of the elusive aryl radical intermediates. Progressing to more relevant chemistry for synthesis, we examined the cleavage of C−O and the C−Br bonds for the generation of alkyl C-centered radicals. Activated epoxides could serve as valuable substrates of a photocatalyzed variant of the Nugent−RajanBabu−Gansäuer homolytic cleavage of epoxides. Using imidazole based carbamates, we could also devise the first photocatalyzed Barton−McCombie deoxygenation reaction. Finally, bromophenylacetate can be reduced using the [Au2(μ-dppm)2]Cl2 photocatalyst under UVA or visible-light. This was used for the initiation of the controlled atom transfer radical polymerization of methacrylates and acrylates in solution or laminate. Our next endeavors concerned the photocatalyzed oxidation of stabilized carbanions such as enolates of 1,3-dicarbonyl substrates, trifluoroborates, and more extensively bis-catecholato silicates. Because of their low oxidation potentials, the later have proved to be exquisite sources of radical entities, which can be engaged in diverse intermolecular reactions such as vinylation, alkynylation, and conjugate additions. The bis-catecholato silicates were also shown to behave as excellent partners of dual photoredox−nickel catalysis leading in an expeditious manner to libraries of cross coupling products. preface,2 the advances in radical chemistry have been observed in a quantic mode through a series of waves such as the Kharasch reactions in the 1930s, the physical organic chemistry advances

1. INTRODUCTION Since Gomberg discovered the triphenylmethane radical in 1900, organic radical chemistry has witnessed intense developments, which have gradually allowed its privileged position in the repertoire of synthetic methodologies.1 Interestingly, and as underlined by Stephenson, Studer, and Curran in a recent © XXXX American Chemical Society

Received: June 10, 2016

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DOI: 10.1021/acs.accounts.6b00288 Acc. Chem. Res. XXXX, XXX, XXX−XXX

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Accounts of Chemical Research

organic chemistry by Kellog,3 Pac,4 and Deronzier.5 In 2007, MacMillan opened a new era in this chemistry by merging organocatalysis with photocatalysis to devise an asymmetric alkylation of aldehydes.6 While at first glance photocatalysis could be perceived as déjà vu or from a more positive prospect as a renaissance, it is clear now that it is much more than a new method to conduct radical chemistry.7 The possibility to merge photocatalysis with organocatalysis or organometallic catalysis is for instance a very attractive feature of this new field.8 Also relevant is the generally improved environmental impact inherent to this chemistry since it involves catalysis, a key motto of green chemistry. The net result is that it is no longer needed to use stoichiometric toxic mediators. The number of catalysts available, including dyes,9 renders this chemistry quite applicable in different settings. Also of importance, scale up is possible through flow chemistry.10 Based on our interest in radical chemistry over the last two decades,11 we focused some of our activity on the development of new mediating systems for conducting homolytic chemistry. Aiming at replacing the prevalent tributyltin hydride, we found in collaboration with Dennis Curran (U. Pittsburgh) that NHCboranes possess interesting hydrogen donating ability and that the corresponding B-centered radical can propagate a radical chain.12 Going to catalytic systems, we also discovered that the FeCl2/NaBH4 system could promote radical reactivities,13 and we are pursuing some mechanistic studies with Anny Jutand (ENS Paris). The emerging photoredox catalysis strongly attracted us, and we concentrated our efforts on a series of new practical methods for the generation of C-centered radical intermediates such as aryl and alkyl radicals by either reductive or oxidative pathways.

(1960−1970s), the polymerization development (1940s), and the synthetic breakthroughs of the 1960s and 1980s. The current wave of activity in radical chemistry is a huge one and relies on the utilization of photoredox catalysis for the generation of radical species. The principles of this chemistry were known for a long time in the inorganic community with sporadic entries in Scheme 1. Copper(I)-Photocatalyzed Reduction of Diaryliodoniums

2. ARYL RADICALS VIA PHOTOREDUCTION OF ONIUM SALTS Aryl radicals are important synthetic intermediates that can also be used for the functionalization of surfaces.14 Photocatalytic reduction of aryl iodides was known15 and more recently could be extended by König to aryl chlorides by consecutive visible light-induced single electron transfer (SET) processes using perylene bisimide.16 Nevertheless, we examined onium salts as

Figure 1. NMR monitoring of the Cu(I)-catalyzed photoreductive process and proposed mechanism. B


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which render them harder to reduce through SET. Generally developed under electrochemical or photochemical conditions, the generation of aryl radicals from aryl iodoniums has been intensively used in the field of polymerization but sparsely for synthetic applications.18 We thus investigated the reduction of the simple diphenyliodonium hexafluorophosphate using a set of photocatalysts (Scheme 1, 1).19 Acetonitrile was chosen as solvent for optimized solubilization of all partners. Diisopropylethylamine (DIPEA), also known as Hünig’s base, was added in order to serve as sacrificial electron donor to reduce either the excited state of the photocatalyst or its oxidized state to propagate the catalytic cycle. The formation of the phenyl radical was probed by addition to an allyl sulfone as radical trap. In these reactions, only 0.5 mol % of the photocatalyst (ruthenium or iridium) was required for the formation of the allylation compound in 80% yield. While this reaction seemed to be very efficient, we considered the opportunity to examine the more sustainable copper-based photocatalysts, which were underexplored at that time.20 The interesting photophysical and redox properties of copper(I)-bisphenanthrolines Cu(dap)2+ (dap = 2,9-bis(para-anisyl)-1,10-phenanthroline) and Cu(dpp)2+ (dpp = 2,9-diphenyl-1,10-phenanthroline) were disclosed by

precursors since their charged character could be interesting in some special settings.17 They also display interesting redox properties. Aryl iodonium and diazonium salts show a range of reduction potentials between −0.2 V to −0.1 V (SCE), in contrast to −1.2 V (SCE) for the corresponding aryl sulfoniums, Scheme 2. Photoreduction of Triarylsulfonium Salts and C−C Bond Formation Processes

Scheme 3. Photocatalytic Reductive Opening of Epoxides, Aziridines, and a Cyclopropane

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Accounts of Chemical Research McMillin21 in the 1970s. The excited states of *[Cu(dap)2]+ (Cu(II)/Cu(I)*, −1.43 V, SCE) and *[Cu(dpp)2]+ (Cu(II)/ Cu(I)*, −1.11 V, SCE) have lower reduction potentials than *[Ru(bpy)3]2+ (Ru(III)/Ru(II)*, −0.85 V, SCE), rendering them more prone to be oxidized to the Cu(II) state.22 Following the seminal work by Sauvage on the photocatalyzed coupling of benzylic halide,23 several groups have recently reported synthetic applications of these photoactive copper complexes.24 In our hands, [Cu(dpp)2]PF6 and [Cu(dap)2]PF6 were very active for the reduction of iodonium salts since the allylation products were obtained in yields above 80%. In order to gain in selectivity and avoid uncatalyzed reactions, longer wavelength was used for irradiation, and a green LED (530 nm) was selected as favorite light source. We focused on the cheaper and more accessible [Cu(dpp)2]PF6 complex for the rest of our study. Substitution of the ester group on the olefin by other groups (phenyl, p-tolyl, methyl, or chlorine), as well as electronic variation on the aromatic ring of the iodonium salts completed the scope of the reaction with no evidence of a significant electronic effect. No selectivity was found in the formation of the aryl radicals from unsymmetrical iodonium salts bearing strong electronic and steric discrimination. To gain insight into the mechanism, we did some NMR monitoring of the irradiated reaction mixture (Figure 1). When 1 equiv of diphenyliodonium was added to [Cu(dpp)2]PF6 (40 mol %) in CD3CN, we observed an instantaneous disappearance of all the [Cu(dpp)2]PF6 signals, and the formation of broad signals was attributed to the corresponding Cu(II) complex. We also identified the formation of benzene and iodobenzene as strong evidence of the iodonium SET reduction. The reaction mixture still contained some iodonium salt. When we added 2 equiv of i-Pr2NEt, instantaneous and full consumption of the starting iodonium salt and recovery of the [Cu(dpp)2]PF6 signals took place. Presumably, the iodonium salt would oxidize the excited Cu(dpp)2+ into a Cu(II) complex. The resulting iodanyl radical decomposes into iodobenzene and a phenyl radical. Radical addition can occur to generate the allylation product. Based on this mechanism, copper(I) photocatalyzed ATRP processes have been developed by Gigmes and Lalevée.25 Due to their lower reduction potential (see above), the SET reduction of sulfonium salts needs stronger reductants.17 Mostly used for their ability to initiate photopolymerization processes, sulfonium salts have not been deeply studied as carbon-centered radical sources in a synthetic context. It is important to recall the work of Kellog who showed that the reduction of phenacylsulfonium salts by Hantzsch esters under light was accelerated by the

addition into the reaction mixture of Ru(bpy)2Cl2 or an organic dye.3 More recently, Umemoto’s reagent has been involved in photocatalyzed reductions under visible light as a source of trifluoromethyl radical.26 Our objective was therefore to develop a mild reduction of nonactivated triarylsulfonium salts to generate reactive aryl radicals. Here also, the catalytic system was optimized through the allylation of the intermediate aryl radical with allyl sulfone radical acceptors (Scheme 2).27 Ru(bpy)2Cl2 (5 mol %) proved to be an adequate photocatalyst, and i-Pr2NEt (5 equiv) was required as a sacrificial electron donor. Yields from 42% to 68% of addition products were obtained. Interestingly, 2-chloroallyl chloride proved to be as reactive as allyl sulfones. 1,1-Diphenylethylene also appeared to be a very good aryl radical acceptor. We evaluated the influence of the sulfonium counterion. Due to a poorer solubility, triphenylsulfonium bromide led to lower yields (30%) than the triflate, tetrafluoroborate, and hexafluorophosphate salts. p-Me, p-tBu, and p-F substituents on the arylsulfoniums did not affect the efficiency of the reaction. The supposed mechanism involves a SET reduction of the excited photocatalyst with i-Pr2NEt to access a Ru(I) complex, which would give one electron to the Scheme 5. Photocatalytic Reductive Deoxygenation of Secondary Alcohols

Scheme 4. Photoallylation of Epoxides and Aziridines

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titanium(III), initially developed by Nugent and RajanBabu28 and more recently rendered catalytic and asymmetric by Gansaüer.29 However, if we consider the low redox potential of stilbene oxide (−2.35 V, Ag/AgI), compared with those of photocalysts (up to −2.19 V vs SCE for Ir(III)(ppy)3/Ir(II)(ppy)3−), the direct SET to the oxirane seems challenging. For these reasons, we decided to use an aryl ketone moiety (−1.3 V, SCE) as a relay, reduction of which would generate a ketyl radical that can undergo ring-opening to afford a valuable radical anion intermediate (Scheme 3). A precedent by Hasegawa supported the viability of this approach showing that an α-ketoepoxide can be reduced by a benzimidazoline under irradiation in the presence of Ru(bpy)3Cl2.30 But the reaction apparently required UV irradiation, and only one substrate was reduced in low conversion with no functionalization of the radical intermediate. For this reason, we decided to revisit this transformation in light of the contemporary developments of visible-light photoreduction.31 Based on the reaction conditions developed by Stephenson for the photoreduction of halides,32 epoxy chalcone (R1 = Ph, R2 = H) was treated with 5 mol % of Ru(bpy)3Cl2 and 1 equiv of Hantzsch ester in DMSO under 14 W fluorescent light bulb irradiation and gave the expected β-hydroxyketone in 91% yield. Various epoxy chalcones bearing electron-donating or -withdrawing substituents gave the corresponding products in fair to good yields. Interestingly, a flow version of this reaction was devised by Seeberger.10b With an oxirane substituted by a butyl chain, the reduction proceeded similarly. Limitations were encountered with trisubstituted and sterically hindered epoxyketones. The use of the more reducing photocatalyst Ir(dtbbpy)(ppy)2PF6 solved this lack of reactivity. This method was successfully extended to the photoreduction of ketoaziridines and a ketocyclopropane (using fac-Ir(ppy)3 and to give 60% of the corresponding ketone33). More relevant from a synthetic point of view, the intermediate photogenerated radical was trapped successfully with allylsulfones. For this transformation, the iridium catalyst Ir(dtbbpy)(ppy)2PF6 appeared the most efficient, and we designed a methylated (in position 4) Hantzsch ester derivative to slow the

sulfonium salt. The resulting sulfuranyl radical decomposes to an aryl radical and the corresponding thioether.

3. ALKYL RADICALS VIA PHOTOREDUCTION Alkyl radicals are highly reactive chemical species that have widely contributed to the development of synthetic radical chemistry over the last decades.1 The most typical and straightforward approaches for generating them are the abstraction of halides by a tin or tin-free radical chain carrier and the reduction of alkyl halides by a stoichiometric amount of a SET agent (metallic complexes or electron rich organic substrates).1 More recently, catalytic versions have been proposed with the emergence of photoredox catalysis.7 In our case, we focused on C−O bond homolytic cleavage and investigated the photocatalytic version of the SET reduction of epoxides, a well-known process using Scheme 6. Mechanism of the Photocatalytic Deoxygenation

Scheme 7. Photo-ATRP of Acrylates Catalyzed by [Au2(μ-dppm)2]Cl2.

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Accounts of Chemical Research Scheme 8. Photocatalytic Oxidation of 1,3-Dicarbonyl Derivatives

The mechanism is still in debate. The activated photocatalyst could be reduced by the Hantzsch ester to give a stronger reductant, which can transfer an electron to the epoxy chalcone and regenerate the photocatalyst.31 Alternatively, as proposed by MacMillan,7a after uncatalyzed initiation, hydrogen donation from Hantzsch ester generates a cyclohexadienyl radical that would reduce the excited photocatalyst.

hydrogen donation and favor the allylation process. A variety of α-epoxyketones and tosylaziridines could be allylated in the presence of 3 equiv of allyl sulfone in good yields and with excellent diastereoselectivities. The structure of the major diastereomer was determined by X-ray analysis and proved to be syn. This selectivity was consistent with Guindon’s model for the radical allylation of β-alkoxy esters under nonchelation control34 (Scheme 4). F


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Accounts of Chemical Research Scheme 9. Silicates as Partners of Photocatalytic Oxidation

Another relevant example of C−O bond cleavage for the generation of alkyl radical is the radical deoxygenation of alcohols. Initially reported by Barton and McCombie35 with O-thiocarbamates in tin hydride conditions, this precious transformation36 would highly benefit from a photocatalytic version.37 After optimization, we found that the imidazolyl thiocarbamate of 1-(phenylsulfonyl)piperidin-4-ol treated with 1 mol % of fac-Ir(ppy)3, i-Pr2NEt in acetonitrile under blue LED irradiation at room temperature gave the product of deoxygenation in 53% yield (same yield with the Boc derivative) (Scheme 5). By comparison, the same transformation performed under typical tin hydride conditions gave the deoxygenation product in lower yield (46% yield). The reaction was extented to other secondary thiocarbamates derived from the corresponding alcohol of mono- and bicyclic acetals, furanoses bearing an acetal or a benzoyl group, lithocholic methyl ester, and the acyclic 4-phenyl2-butanol. An example of reduction of a tertiary thiocarbamate

was highlighted with a derivative of D-(−)-quinic acid giving 60% yield of deoxygenated product. To gain some insight into the mechanism, we first tested the reactivity of the prototypical O-thiocarbamate of piperidin-4-ol without i-Pr2NEt (Scheme 6). Using 1 mol % of photocatalyst, the product of thiocarbamate radical transfer was obtained in 15% yield. This experiment suggests an electron donation from the activated iridium complex to the thiocarbamate. To confirm this, fluorescence quenching experiments were accomplished and showed a decrease of the intensity of fluorescence upon gradual addition of thiocarbamate, which is consistent with an energy or an electron transfer between fac-Ir(ppy)3* and the substrate. To conclude, we determined by cyclic voltammetry the reduction potentials of a set of thiocarbamates, which were measured between −1.11 and −1.73 V (SCE). Thus, facIr(ppy)3* would transfer an electron to the thiocarbamate moiety and the photocatalyst is regenerated thanks to i-Pr2NEt. The thiocarbamate radical anion then collapses by fragmentation G


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experiments, and we showed that EBPA is an oxidative quencher of the photocatalyst. We were able to synthesize block copolymers with low dispersity, confirming the controlled, living character of this process and also to reinitiate the polymerization of 2,2,3,3,4,4,4-heptafluorobutyl acrylate (HFBA) on a PMMA film. This was determined by the surface modification from hydrophilic to hydrophobic.

to liberate the intermediate carbon-centered radical, which can abstract a hydrogen from the amine radical cation (a byproduct of the catalyst regeneration).38 In the same vein, the photoreduction of 3,5-bis(trifluoromethyl)benzoate esters of activated alcohols and N-(acyloxy)phthalimide oxalates of tertiary alcohols, as reported by Reiser39 and Overman,40 respectively, constitute other methods of photocatalyzed C−O bond cleavage. Photoredox catalysis has proven to be a powerful methodology for molecular synthesis, but it has also recently found applications in polymerization. Among various polymerization modes, ATRP has appeared to be quite well adapted for photoredox catalysis.41 In this context, we investigated new catalytic systems and focused our attention on the intriguing digold bis(diphenylphosphino)methanedichloride [Au2(μ-dppm)2]Cl2. This complex was studied by Schmidbaur42 and Che,43 and its synthetic potential was unveiled by Barriault.44 We extended the scope of this promising catalyst to the photo-ATRP of acrylates (Scheme 7).45 Upon UVA activation and using ethylα-bromophenylacetate (EBPA) as initiator, a highly responsive controlled living radical polymerization is observed. In collaboration with Jacques Lalevée, we studied this catalytic system by different spectroscopic analyses and “on−off” switchable

4. ALKYL RADICALS VIA PHOTOOXIDATION 1,3-Dicarbonyl compounds are key intermediates in organic synthesis. While their oxidation with stoichiometric amounts of metal complexes, such as manganese(III), cerium(IV) or iron(III), is well established,1 the development of a photocatalytic variant is of great interest. The strategy is based on the oxidation of the enol form or a corresponding enolate by a photocatalytic system.46 First, we showed that ethyl benzoyl acetate treated with Ru(bpy)3Cl2 and K2CO3 in DMF under visible light and air gave the dimerization product in 56% yield. To improve this result, other oxidative quenchers such as diazoniums or methyl viologen were tested but without success. Tritylium was also considered. In presence of air, the dimer was obtained quantitatively

Scheme 10. Radical Chemistry Using Silicates

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silicon chemistry55 drove us to the use of hypervalent silicon derivatives as potential substrates for photooxidation. We chose bis-catecholato silicates as precursors because of their higher solubility compared with organopentafluorosilicates.50 Some prior works by Nishigaishi describing the photoallylation of benzyl derivatives56 and dicyanoarenes57 using allylbiscatecholato silicates under photoinduced electron transfer conditions suggested the feasibility of our approach. We prepared a series of alkyl potassium and ammonium bis-catecholato derivatives based on literature procedures.58 Potassium silicates proved less stable than ammonium ones; however, when isolated in the presence of 1 equiv of 18-C-6, they became rock stable. Their lower oxidation potential (from +0.34 V to +0.89 V vs SCE in DMF, see Scheme 9) than trifluoroborates (>1.1 V vs SCE)53 augured well for their oxidation. A series of stoichiometric reagents (Cu(II), DMP, tritylium, and Ledwith−Weitz salt) showed some reactivity,52 but we concentrated on photocatalysis. While Fukuzumi acridinium52,54 has so far not given satisfactory results, Ru(bpy)3Cl2·6H2O and [Ir(dF(CF3)ppy)2(bpy)]PF6 complexes proved particularly appropriate (Scheme 9). Gratifyingly, the radical process proved to be highly efficient with various alkyl precursors generating relatively stabilized radicals (benzyl, allyl, α-amino) to highly reactive primary ones. All these species could be engaged in various transformations: allylation, vinylation, alkynylation, and conjugate additions. Remarkably, an α-chloro radical could be allylated in good yield (Scheme 10). Fluorescence quenching studies of Ir(dF(CF3)ppy)2(bpy)*, noted *IrIII-dF(CF3), by benzyl silicate and cyclohexyl silicate showed a significant decrease of fluorescence intensity upon

(Scheme 8). Control experiments confirmed that no reaction took place in the absence of light, photocatalyst, or base. Similarly no dimerization took place only with oxygen in a basic medium or with tritylium. This dimerization reaction was then extended to substituted ethyl benzoyl acetates giving products in 43−85% yields. With 2 equiv of TEMPO, the oxyamination products were obtained in 57−89% yields. Of note, oxyamination of β-keto ester was previously reported by Tan47 and Akita48 using Rose Bengal and Ir(ppy)2(dtbbpy)PF6 as photocatalysts, respectively. Allylation of the malonyl radical by allylstannane and allylsulfone reagents was also possible. Inspired by the works of Snider with manganese(III) acetate,49 two examples of cascade reactions were accomplished in conditions requiring the presence of 2 equiv of tritylium. The corresponding tricyclic products were obtained with moderate yields, suggesting that further optimization would be required. While the oxidation of organometallic compounds such as Grignard or lithium reagents to provide C-centered radicals is well-known, this approach suffers from stringent reaction conditions and poor compatibility with a variety of functional groups. Inspired by the seminal work of Kumada on organopentafluorosilicate compounds,50 we surmised that the oxidation of softer organometallic derivatives was possible, and we initially oxidized organotrifluoroborates with copper(II) and Dess− Martin periodinane,51 and more recently with tritylium and Ledwith Weitz aminium salts.52 The logical next step was to develop the photooxidation of these derivatives but many groups were faster than us,53 and it is only recently that we could bring some preliminary results on their photooxidation52 with Fukuzumi catalyst.54 Moreover, our long-standing interest in Scheme 11. Proposed Mechanism

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Accounts of Chemical Research Scheme 12. Dual Photoredox/Nickel Catalysis with Silicates

complementary findings with ammonium silicates. The latter could be used in arylation61 and also vinylation reactions.62

gradual addition of the silicates. Stern−Volmer relationships were obtained, yielding quenching rate constants around 1010 M−1·s−1, comparable with the quenching rate constants of the Ru(bpy)32+/methylviologene system. This led us to the following mechanism proposal, also consistent with the oxidation potentials. The silicates appear as very efficient reductive quenchers of the photoexcited state of *[IrIII]. Electron donation from silicates would trigger a Si−C bond rupture, liberating radical R. The resulting IrII-dF(CF3) would be oxidized to regenerate the IrIII-dF(CF3) catalytic species by reacting whether with TEMPO or the sulfonyl or halide radicals generated in the allylation and related reactions (Scheme 11). DFT calculations were performed by Etienne Derat. While the excited photocatalyst alone in its triplet state is characterized by one unpaired electron centered on the metal and the other one located on the bpy ligand, the situation becomes different after interaction with the silicate. Indeed, one unpaired electron is still found on the bpy ligand, but the second one is now located on the catechol moiety of the silicate. Therefore, a SET from a catechol moiety of the silicate to the metallic center of the photocatalyst took place, a process corresponding to an intermolecular ligand−metal charge transfer (LMCT). This oxidation strongly weakens the strength of the Si−C bond by a factor of 5.8 for benzyl silicate, the bond dissociation energy (BDE) being calculated in that case to be as low as 13.8 kcal/mol so that fragmentation takes place easily. Due to the very efficient radical generation from silicates, we examined the possibility of dual catalysis, focusing on photoredox/nickel Csp2−Csp3 cross coupling reactions.8,59 The reaction proved very productive. A series of primary potassium alkyl silicates were engaged with various bromides and iodides of aryl and heteroaryl platforms and provided generally quite satisfactory yields of coupling products (Scheme 12).60 We checked at that occasion that there was no difference of reactivity between potassium silicates and the ones stabilized by 18-C-6. Following our preliminary report, the group of Molander reported

5. CONCLUDING REMARKS Photocatalysis has very rapidly been established as a method of choice for the generation of a variety of C-centered radicals, gradually supplanting the previous methods of radical generation. In this Account, we have described the use of catalytic photoredox processes for the efficient production of radical intermediates from readily available substrates: aryl radicals from sulfoniums and iodoniums by reduction; alkyl radicals from ketoepoxides, aziridines, and thiocarbamates by reduction or from 1,3-dicarbonyl and alkyl silicates by photooxidation. All the protocols we have developed are compatible with postfunctionalization notably via C−C bond formation. Among all the substrates studied, silicates stand out since they are excellent partners for photoredox/nickel dual catalysis.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. *E-mail: [email protected]. Fax: (+33) 1-4427-7360. Notes

The authors declare no competing financial interest. Biographies Jean-Philippe Goddard obtained his Ph.D. in 2002 from Université Paris Sud (Orsay) with Charles Mioskowski. Then, he moved to Bern to work with Jean-Louis Reymond as a postdoctoral fellow. In 2004, he obtained a temporary lecturer position at UPMC and then became assistant professor in the group of Max Malacria and Louis Fensterbank. In 2013, he obtained a full professor position at the Université de HauteAlsace in Mulhouse, and in 2015 he was nominated junior member of the Institut Universitaire de France. J


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(9) Nicewicz, D. A.; Nguyen, T. M. Recent Applications of Organic Dyes as Photoredox Catalysts in Organic Synthesis. ACS Catal. 2014, 4, 355−360. (10) (a) Tucker, J. W.; Zhang, Y.; Jamison, T. F.; Stephenson, C. R. J. Visible-Light Photoredox Catalysis in Flow. Angew. Chem., Int. Ed. 2012, 51, 4144−4147. (b) Bou-Hamdan, F. R.; Seeberger, P. H. Visible-LightMediated Photochemistry: Accelerating Ru(bpy)32+-Catalyzed Reactions in Continuous Flow. Chem. Sci. 2012, 3, 1612−1616. (11) For recent references, see: (a) Larraufie, M.-H.; Ollivier, C.; Fensterbank, L.; Malacria, M.; Lacôte, E. Radical Synthesis of Guanidines from N-Acyl Cyanamides. Angew. Chem., Int. Ed. 2010, 49, 2178−2181. (b) Larraufie, M.-H.; Courillon, C.; Ollivier, C.; Lacôte, E.; Malacria, M.; Fensterbank, L. Radical Migration of Substituents of Aryl Groups on Quinazolinones Derived from N-Acyl Cyanamides. J. Am. Chem. Soc. 2010, 132, 4381−4387. (12) Solovyev, A.; Ueng, S.-H.; Monot, J.; Fensterbank, L.; Malacria, M.; Lacôte, E.; Curran, D. P. Estimated Rate Constants for Hydrogen Abstraction from N-Heterocyclic Carbene-Borane Complexes by an Alkyl Radical. Org. Lett. 2010, 12, 2998−3001. (13) Ekomié, A.; Lefèvre, G.; Fensterbank, L.; Lacôte, E.; Malacria, L.; Ollivier, C.; Jutand, A. Iron-Catalyzed Reductive Radical Cyclization of Organic Halides in the Presence of NaBH4: Evidence of an Active Hydrido Iron(I) Catalyst. Angew. Chem., Int. Ed. 2012, 51, 6942−6946. (14) (a) Bouriga, M.; Chehimi, M. M.; Combellas, C.; Decorse, P.; Kanoufi, F.; Deronzier, A.; Pinson, J. Sensitized Photografting of Diazonium Salts by Visible Light. Chem. Mater. 2013, 25, 90−97. (b) Vase, K. H.; Holm, A. H.; Norrman, K.; Pedersen, S. U.; Daasbjerg, K. Electrochemical Surface Derivatization of Glassy Carbon by the Reduction of Triaryl- and Alkyldiphenylsulfonium Salts. Langmir 2008, 24, 182−188. (15) (a) Nguyen, J. D.; D’Amato, E. M.; Narayanam, J. M. R.; Stephenson, C. J. Engaging Unactivated Alkyl, Alkenyl and Aryl Iodides in Visible-Light-Mediated Free Radical Reactions. Nat. Chem. 2012, 4, 854−859. (b) Kim, H.; Lee, C. Visible-Light-Induced Photocatalytic Reductive Transformations of Organohalides. Angew. Chem., Int. Ed. 2012, 51, 12303−12306. (16) Ghosh, I.; Ghosh, T.; Bardagi, J. I.; König, B. Reduction of Aryl Halides by Consecutive Visible Light-Induced Electron Transfer Processes. Science 2014, 346, 725−728. (17) Fensterbank, L.; Goddard, J.-P.; Malacria, M.; Ollivier, C. Homolytic Reduction of Onium Salts. Chimia 2012, 66, 425−432. (18) (a) Neufeldt, S. R.; Sanford, M. S. Combining Transition Metal Catalysis with Radical Chemistry: Dramatic Acceleration of PalladiumCatalyzed C-H Arylation with Diaryliodonium Salts. Adv. Synth. Catal. 2012, 354, 3517−3522. (b) Liu, Y.-X.; Xue, D.; Wang, J.-D.; Zhao, C.-J.; Zou, Q.-Z.; Wang, C.; Xiao, J. Room Temperature Arylation of Arenes and Heteroarenes with Diaryliodonium Salts by Photoredox Catalysis. Synlett 2013, 24, 507−513. (19) Baralle, A.; Fensterbank, L.; Goddard, J.-P.; Ollivier, C. Aryl Radical Formation by Copper(I) Photocatalyzed Reduction of Diaryliodonium Salts. Chem. - Eur. J. 2013, 19, 10809−10813. (20) Paria, S.; Reiser, O. Copper in Photocatalysis. ChemCatChem 2014, 6, 2477−2483. (21) McMillin, D. R.; Buckner, M. T.; Ahn, B. T. A Light-induced Redox Reaction of Bis(2,9-dimethyl-1,10-phenanthroline)Copper(I). Inorg. Chem. 1977, 16, 943−945. (22) Armaroli, N.; Accorsi, G.; Cardinali, F.; Listorti, A. Photochemistry and Photophysics of Coordination Compounds: Copper. Top. Curr. Chem. 2007, 280, 69−115. (23) Kern, J.-M.; Sauvage, J.-P. Photoassisted C−C Coupling via Electron Transfer to Benzylic Halides by a Bis(di-imine) Copper(I) Complex. J. Chem. Soc., Chem. Commun. 1987, 546−548. (24) (a) Knorn, M.; Rawner, T.; Czerwieniec, R.; Reiser, O. [Copper(phenanthroline) (bisisonitrile)]+-Complexes for the VisibleLight-Mediated Atom Transfer Radical Addition and Allylation. ACS Catal. 2015, 5, 5186−5193. (b) Fumagalli, G.; Rabet, P. T. G.; Boyd, S.; Greaney, M. F. Three-Component Azidation of Styrene-Type Double Bonds: Light-Switchable Behavior of a Copper Photoredox Catalyst. Angew. Chem., Int. Ed. 2015, 54, 11481−11484.

Cyril Ollivier obtained his Ph.D. in 2000 from the University of Fribourg (Switzerland) and UPMC under the guidance of Philippe Renaud and Max Malacria. He was awarded a Swiss National Foundation Fellowship to work with Philippe Magnus at the University of Texas at Austin. In 2002, he was appointed Chargé de Recherche CNRS at Aix-Marseille University and in 2007, he moved to UPMC where he is now Directeur de Recherche. Louis Fensterbank obtained his Ph.D. in 1993 from SUNY Stony Brook with Scott Sieburth. After a temporary lecturer position at UPMC in 1994, he was appointed Chargé de Recherche CNRS in 1995 in Max Malacria’s team. In 2004, he obtained a professorship position at UPMC, and in 2008, he was nominated junior member of the Institut Universitaire de France.

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ACKNOWLEDGMENTS We are grateful to the following funding agencies and institutions: UPMC, UHA, CNRS, ANR, IUF, Région Ile de France, Région Martinique, Labex MiChem, and COST Action CM1201. We warmly thank all our collaborators who have participated in the photocatalysis efforts and whose names have appeared in the quoted publications and notably research associates from our team: Alexandre Baralle, Abdulkader Baroudi, Lise-Marie Chamoreau, Ludwig Chenneberg, Vincent Corcé, Marion Daniel, Etienne Derat, Simon Donck, Emmanuel Lacôte, Christophe Lévêque, Marie-Hélène Laraufie, Max Malacria, Rémi Pellet.

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Photoredox Catalysis for the Generation of Carbon Centered Radicals

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