Rapid Regio- and Diastereoselective Paternò−Büchi Reaction of Alkyl Phenylglyoxylates

Triplet excited states of alkyl phenylglyoxylates react rapidly (k = 9.4 x 10(9) M(-)(1) s(-)(1)) with electron rich alkenes forming oxetanes with high regio and stereoselectivity. The well-known intramolecular gamma-hydrogen abstraction (Norrish type II) cannot compete. When less electron-rich alkenes are used, the Norrish type II reaction becomes competitive. Intramolecular Paternò-Büchi reactions predominate in those alkyl phenylglyoxylates containing properly situated electron-rich alkene groups. The regioselectivity of this reaction is explained by the stability of the intermediate 1,4-biradical. The appropriate conformation at the instant of intersystem crossing determines the stereoselectivity of the products. The priority of the Paternò-Büchi over the Norrish type II reaction is understood by considering the conformation of phenylglyoxylate esters.