Redox-Photosensitized Reactions. 11. Ru(bpy)32+-Photosensitized Reactions of 1-Benzyl-1,4-dihydronicotinamide with Aryl-Substituted Enones, Derivatives of Methyl Cinnamate, and Substituted Cinnamonitriles: Electron-Transfer Mechanism and Structure-Reactivity Relationship

Article abstract of DOI:10.1021/jo00175a006

Reactions of 1-benzyl-1,4-dihydronicotinamide (BNAH) with aryl-substituted enones and derivatives of methyl cinnamate and cinnamonitrile (1a-u) are photosensitized by Ru(bpy)3²⁺ (bpy = 2,2'-bipyridine).The reduction of carbon-carbon double bonds commonly requires the substitution of either an electron-withdrawing aryl group or two phenyl groups at the β-carbon atom of 1.With enones which possess one aryl substituent with no extra electron-withdrawing group at the β position, the photosensitized reactions result in no two-electron reductions but give 1:1 adducts (4d-h) along with half-reduced dimers of olefins (3d and 3g) and a half-oxidized dimer of BNAH (5).The observed results can be easily interpreted by assuming the intervention of 1-benzyl-3-carbamoyl-1,4-dihydropyridin-4-yl radical (BNA.) and half-reduced species (.1-H) as key intermediates that are formed by mediated electron-proton transfer from BNAH to 1 in which Ru(bpy)3²⁺ acts as a one-electron shuttle upon photoexcitation in the initial electron transfer.Whether BNA. undergoes electron transfer to or a radical-coupling reaction with .1-H depends on steric and electronic properties of .1-H which should be affected by the substituents at the radical center.Mechanistic implications for thermal reactions of NADH models with olefins in the dark are briefly discussed on the basis of these observations.