Electron and Hydrogen Atom Transfer Mechanisms for the Photoreduction of o-Quinones. Visible Light Induced Photoreaction of β-Lapachone with Amines, Alcohols, and Amino Alcohols

Article abstract of DOI:10.1021/ja00186a029

β-Lapachone (1), a substituted o-naphthoquinone absorbing into the visible (λ_max=424 nm in benzene), is cleanly and efficiently reduced to the corresponding semiquinone radical upon photolysis in degassed solutions with alcohols, amines, and β-amino alcohols.The course and products of these photoreactions have been followed by NMR, ESR, fluorescence, and absorption spectroscopy.For all three types of reductant the overall reaction involves 2e^- oxidation of the donor, and the quantum efficiencies show a dependence upon quinone concentration indicative of therole of a second dark reduction of 1 by products of the primary photolysis.For amines and amino alcohols the reaction is initiated by single electron transfer quenching of triplet 1.For triethylamine the mechanism is indicated to be a sequence of two electron transfer-proton transfer steps culminating in two semiquinone radicals and the enamine Et2NCH=CH2.For amino alcohols a C-C cleavage concurrent with deprotonation of the alcohol (oxidative photofragmentation) occurs, in competition with reverse electron transfer, following the quenching step.For both amines and amino alcohols, limiting efficiencies of reaction approach 2 (for QH^. formation).In contrast, both 2-propanol and benzyl alcohol are oxidized by excited states of 1 with much lower efficiency.The probable mechanism for photooxidation of the alcohols involves a H atom abstraction quenching of the excited state followed by an electron transfer-proton transfer sequence in which a ground-state 1 is reduced.Lower limiting efficiencies for photoreduction of 1 by the alcohols are attributed to inefficiencies of net H-atom transfer in the quenching step.