Photochemical Cycloaddition of Singlet and Triplet Diphenylvinylene Carbonate with Conjugated Dienes

The singlet and triplet excited states of diphenylvinylene carbonate (V) react with conjugated dienes to produce 2 + 2 cycloadducts. The singlet reaction occurs with retention of diene stereochemistry via a singlet exciplex intermediate. Singlet cycloaddition is only moderately regioselective and stereoselective, indicative of a loosely bound exciplex intermediate. The triplet cycloaddition occurs with substantial loss of diene stereochemistry and high regioselectivity. The high triplet regioselectivity is a consequence of selective collapse of the more substituted biradical rather than selective bonding to the less substituted diene terminus. The regioselectivity of initial bonding is the same for the singlet and triplet reactions.