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

Abstract

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−Büchi transformation. The alternative cycloaddition via the specific activation of the carbonyl component (as a commonly applied procedure in Paterno−Büchi 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 the ns/μs time scale. The spin multiplicities of the critical ion-pair intermediate [ S • +, Q • - ] in the two photoactivation methodologies determine the time scale of the reaction sequences (which are otherwise the same), and thus the efficiency of the relatively slow ion-pair collapses ( k C ≅ 10 8 s - 1 ) to the 1,4-biradical that ultimately leads to the oxetane product.