Cycloalkenones, photocycloadditions

Photocycloadditions of 2-Trimethylsilyloxy-1,3-butadiene to 2-Cycloalkenones General Procedure46 ... 

These two mechanisms, the reversible biradical intermediate and the intermediate exciplex [25] have both been useful for analysis of the regioselectivity and stereoselectivity observed in [2 -I- 2] photocycloaddition between enones and alkenes. Hoffman et al. [26], for example, describes his stereoselective photocycloadditions as arising from a biradical from the triplet excited enone which may or may not involve exciplex formation.. Ground state trans-cycloalkenones have also been proposed as the reactive intermediates which lead to [2 + 2] cycloadducts [27]. The distance between the reacting partners is clearly an issue. If the n systems are not sufficiently close, then photocycloaddition will not occur [28]. 

Participation of alkenyl boronates in intermolecular [2+2] enone-olefin photocycloadditions has been demonstrated with cycloalkenones. The major photoadducts with cyclopentenone resulted from head-to-head addition (Scheme 9.48) [101]. 

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