CYCLOADDITION AND OTHER REACTIONS OF SILENES WITH ALPHA,BETA-UNSATURATED ESTERS

Several silenes of the family (Me3Si)2Si = C(OSiMe3)R were found to react in a [2 + 4] manner with a variety of acrylate esters to give 1-sila-3-oxacyclohex-4-enes as products, as confirmed by an X-ray structure. With crotonate esters the same silenes yielded 1-sila-2-oxacyclohex-3-enes when the reaction was carried out in the dark, with minor amounts of gamma-silylcrotonate products, the result of nominal addition of the allylic C-H bond across the ends of the Si = C bond, also being formed. On the other hand, cophotolysis of the relevant acylsilane with the crotonate esters led to formation of the gamma-silylcrotonates as the major product. The structures of these species were confirmed by a crystal structure. Reactions with methacrylate esters gave acyclic alpha-(silylmethyl)acrylate products as a result of nominal insertion of the silene into the allylic C-H bond. Maleate and fumarate esters each gave a mixture of the same two diastereomers from [2 + 4] cycloaddition to the C = C-C = O(OR) system, forming 1-sila-3-oxacyclohex-4-enes. Reactions with ethyl cinnamate involved [2 + 4] cycloadditions of the opposite regiochemistry, leading to 1-sila-2-oxycyclohex-3-enes. These latter products readily hydrolyzed to silalactones, as shown by a crystal structure.