Photochemical reactions of sulfide-containing alkyl phenylglyoxylates

Article abstract of DOI:10.1016/S0040-4020(97)00420-1

Ethylthioalkyl phenylglyoxylates (2) underwent regioselective photocyclization to produce up to 13-membered ring thiacyclols (3) in good yields. Cyclization occurred between the excited carbonyl group and the carbon a to the sulfur on the remote side. The more methylenes placed between the carbonyl group and the sulfide, the lower the yield of 3 while intermolecular hydrogen abstraction induced reductive dimerization and intramolecular Norrish Type II photolysis become competitive, The remote cyclization can be rationalized by a photoinduced electron transfer from the sulfur atom to the excited carbonyl group followed by proton transfer and subsequent cyclization of the resulting biradical. Rate constants for electron transfer increase as the chain connecting the electron donor and acceptor becomes longer and more flexible.