Photoreduction of phenacyl halides by NADH analogues. Origins of different mechanistic pathways

Photoreduction of phenacyl halides (PhCOCH₂X) by an NADH analogue, 10-methyl-9-acridone (AcrH₂) in acetonitrile (MeCN) under irradiation of light (λ > 320 nm) proceeds via the rate-determining photoinduced electron transfer from the singlet excited state of AcrH ₂ to PhCOCH₂X to give the radical ion pair (AcrH ₂^+. PhCOCH₂X^-.). The subsequent fast reaction is completed within the complex by proton transfer from AcrH ₂^+. to PhCOCH₂X^-., followed by electron transfer from AcrH^. to PhĊ(OH)CH₂X, yielding 10-methylacridinium ion (AcrH⁺) and acetophenone (PhCOCH ₃). However, photoreduction of PhCOCH₂X by another NADH analogue, 1-benzyl-1,4-dihydronicotinamide (BNAH), in MeCN under irradiation with visible light of λ > 360 nm proceeds by a different reaction pathway; electron-transfer radical chain reactions, in which the chain carrier radicals are produced by photoinduced electron transfer from the singlet excited state of BNAH to PhCOCH₂X. The origins of these different mechanistic pathways dependent on the NADH analogues are discussed.