Multiple reaction paths electronic structure

Kochi and co-workers reported the photochemical addition of various stilbenes to chloroanil 53, which is controlled by the charge-transfer (CT) activation of the precursor electron-donor/acceptor (EDA) complex. The [2-1-2]-cycloaddition products 54 were established by an x-ray structure of the trans-oxetane formed selectively in high yields. Time-resolved (fs/ps) spectroscopy revealed that the (singlet) ion-radical pair is the primary reaction intermediate and established the electron-transfer pathway for this Patern6-BUchi transformation. The alternative pathway via direct electronic activation of the carbonyl component led to the same oxetane regioisomers in identical ratios. Thus, a common electron-transfer mechanism applies that involves quenching of the excited quinone acceptor by the stilbene donor to afford a triplet ion-radical intermediate, which appears on a nanosecond/microsecond time scale. The spin multiplicities of the critical ion-pair intermediates in the two photoactivation paths determine the time scale of the reaction sequences and also the efficiency of the relatively slow ion-pair collapse k = 10 s ) to the 1,4-biradical that ultimately leads to the oxetane product 54. 

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