Change in Spin State and Enhancement of Redox Reactivity of Photoexcited States of Aromatic Carbonyl Compounds by Complexation with Metal Ion Salts Acting as Lewis Acids. Lewis Acid-Catalyzed Photoaddition of Benzyltrimethylsilane and Tetramethyltin via Photoinduced Electron Transfer

The lowest excited state of aromatic carbonyl compounds (naphthaldehydes, acetonaphthones, and 10-methylacridone) is changed from the n,π* triplet to the π,π* singlet which becomes lower in energy than the n,π* triplet by the complexation with metal ions such as Mg(ClO4)2 and Sc(OTf)3 (OTf = triflate), which act as Lewis acids. Remarkable positive shifts of the one-electron reduction potentials of the singlet excited states of the Lewis acid−carbonyl complexes (e.g., 1.3 V for the 1-naphthaldehyde−Sc(OTf)3 complex) as compared to those of the triplet excited states of uncomplexed carbonyl compounds result in a significant increase in the redox reactivity of the Lewis acid complexes vs uncomplexed carbonyl compounds in the photoinduced electron-transfer reactions. Such enhancement of the redox reactivity of the Lewis acid complexes leads to the efficient C−C bond formation between benzyltrimethylsilane and aromatic carbonyl compounds via the Lewis-acid-promoted photoinduced electron transfer. The quantum yield determinations, the fluorescence quenching, and direct detection of the reaction intermediates by means of laser flash photolysis experiments indicate that the Lewis acid-catalyzed photoaddition reactions proceed via photoinduced electron transfer from benzyltrimethylsilane to the singlet excited states of Lewis acid−carbonyl complexes.