Oxetanes from [2+2] Cycloaddition of Stilbenes to Quinone via Photoinduced Electron Transfer†

The photochemical coupling of various stilbenes (S) and chloranil (Q) is effected by the specific charge-transfer (CT) activation of the precursor electron donor−acceptor (EDA) complex [S, Q], and the [2+2] cycloaddition is established by X-ray structure elucidation of the crystalline trans-oxetanes formed selectively in high yields. Time-resolved (fs/ps) spectroscopy reveals the (singlet) ion-radical pair 1[S•+, Q•-] to be the primary reaction intermediate and thus unambiguously establishes for the first time the electron-transfer pathway for this typical Paterno−Buchi transformation. The alternative cycloaddition via the specific activation of the carbonyl component (as a commonly applied procedure in Paterno−Buchi couplings) leads to the same oxetane regioisomers in identical molar ratios. As such, we conclude that a common electron-transfer mechanism applies via the quenching of the photoactivated quinone acceptor by the stilbene donor to afford triplet ion-radical pairs 3[S•+, Q•-] which appear on th...