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 the ns/mu s 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) similar or equal to 10(8) s(-1)) to the 1,4-biradical that ultimately leads to the oxetane product.