The fields of C-H functionalization and photoredox catalysis have garnered enormous interest and utility in the past several decades. Many different scientific disciplines have relied on C-H functionalization and photoredox strategies including natural product synthesis, drug discovery, radiolabeling, bioconjugation, materials, and fine chemical synthesis. In this Review, we highlight the use of photoredox catalysis in C-H functionalization reactions. We separate the review into inorganic/organometallic photoredox catalysts and organic-based photoredox catalytic systems. Further subdivision by reaction class-either sp2 or sp3 C-H functionalization-lends perspective and tactical strategies for use of these methods in synthetic applications.

Photoredox-Catalyzed C-H Functionalization Reactions.

Photoinduced Electron Transfer Bond Fragmentations

Features of the oxidative cleavage reactions of diastereomers of dimeric lignin model compounds, which are models of the major types of structural units found in the lignin backbone, were examined. Cation radicals of these substances were generated by using SET-sensitized photochemical and Ce(IV) and lignin peroxidase promoted oxidative processes, and the nature and kinetics of their C−C bond cleavage reactions were determined. The results show that significant differences exist between the rates of cation radical C1−C2 bond cleavage reactions of 1,2-diaryl-(β-1) and 1-aryl-2-aryloxy-(β-O-4) propan-1,3-diol structural units found in lignins. Specifically, under all conditions C1−C2 bond cleavage reactions of cation radicals of the β-1 models take place more rapidly than those of the β-O-4 counterparts. The results of DFT calculations on cation radicals of the model compounds show that the C1−C2 bond dissociation energies of the β-1 lignin model compounds are significantly lower than those of the β-O-4 mod...

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Nature and Kinetic Analysis of Carbon−Carbon Bond Fragmentation Reactions of Cation Radicals Derived from SET-Oxidation of Lignin Model Compounds

Photosensitized (electron transfer) carbon-carbon bond cleavage of radical cations : The diphenylmethyl system

Removal of an electron from a bonding orbital or addition of an electron to an antibonding orbital of a diamagnetic molecule activates the resulting radical ion for fragmentation. Such reactive radical ions may be generated by photoinduced electron transfer (PET). There are two alternative ways to accomplish the transfer of an electron: (1) the local excitation of a donor or an acceptor which is well described by the empirical Weller equation and (2) the excitation of the charge-transfer complexes according to the Mulliken theory. The fragmentation reaction competes with back-electron transfer (BET) within the photogenerated radical ion pairs. The back electron transfer is well described by the Marcus theory. In most PET systems the rate of BET decreases with the increasing exergonicity and the rate is faster within contact ion pairs than with solvent separated ion pairs. The exergonicity of BET as well as ion pair solvation and spin multiplicity are predetermined by the method of ion-pair generation. These factors, in addition to the rate of cleavage, are critical in determining the overall efficiency of the PET fragmentation.

Fragmentations by photoinduced electron transfer. Fundamentals and practical aspects

Several organic substrates (donors) known to undergo oxidative C-C bond cleavage upon steady-state irradiation under electron-transfer photosensitization have been examined by 337.1 nm laser flash photolysis for photoproduction of substrate radicals/radical cations and sensitizer triplet as a result of charger-transfer interaction with 1,4-dicyanonaphthalene (DCN) single (acceptor). The transient-absorption phenomena and quantitative estimates of electron-transfer-mediated yields show that for substrates such as 1-(diphenylmethyl)cyclohepta-2,4,6-triene and aryl-substituted pinacols and pinacol-ethers characterized by relatively unstable, short-lived, radical cations, a substantial fraction of arylmethyl radicals are generated fast via back electron transfer in the photogenerated ion pair. In other cases, longer lived radical cations (solvated) that either undergo intramolecular electron transfer leading to C-C bond fragmentation (e.g., with 1,1,2,2-tetraphenylethane) or diffusional back donation of electron from the DCN radical anion (e.g., with p-methoxy- and p-methyl-substituted bibenzyls). Within small groups of closely related quenchers, radical and radical ion yields are found to depend systematically on the structures, chemical nature, and oxidation potentials of the substrates. 18 references, 5 figures, 1 table.

Electron-transfer-sensitized C-C bond cleavage. Facile homolytic fission via geminate back electron transfer in photogenerated ion pairs

Steady-state irradiation of phenyl glycopyranosides under conditions of photosensitization (electron transfer) by 1,4-dicyanonaphthalene (DCN) leads to the formation of the corresponding monosaccharides (or their methyl derivatives, if methanol is present in the solvent). Mechanistic aspects of this photoreaction have been examined by steady-state fluorescence quenching measurements and 337.1-nm laser flash photolysis. Formation of the DCN radical anion (with yields of 0.18 to 0.35 delta/sub ion/) as well as the DCN triplet (with yields of 0.14 to 0.27 delta/sub T/) is observed as a result of the electron-transfer quenching of the DCN singlet by phenyl glycopyranosides in acetonitrile and a methanol-acetonitrile (1:10) mixture. Phenoxy radical is also shown to be a transient photoproduct in the latter solvent. Various features of the reaction scheme involving the fragmentation of electron-transfer-derived radical cations of phenyl glycopyranosides into the phenoxy radical and carbonium ions and possible roles of oxygen and methanol in enhancing or inhibiting the yields are discussed.

H. F. Davis

Papers

Electron-transfer-sensitized C-C bond cleavage. Facile homolytic fission via geminate back electron transfer in photogenerated ion pairs

Mechanistic aspects of 1,4-dicyanonaphthalene-sensitized phototransformation of aryl glycopyranosides