Kinetics and mechanisms of the reactions of Si=C and Ge=C double bonds

The kinetics and mechanisms of a number of characteristic reactions of Si=C and Ge=C double bonds have been studied in solution, using laser flash pho tolysis of sila- and germacyclobutanes for generation of the silene and ger mene reactive intermediates under conditions where they can be detected dir ectly. Substituent, solvent, isotope, and temperature effects on the rate c onstants for reaction of Ph2Si=CH2 with alcohols, amines, carboxylic acids and ketones suggest that these reagents react by a common mechanism, involv ing initial nucleophilic attack at silicon followed by proton transfer with in the initially formed Lewis acid-base complex between the silene and the nucleophile. The reactivity of Ph2Ge=CH2 is significantly lower than that o f the silene. Reaction with alcohols in acetonitrile solution proceeds by a mechanism leading to a second order dependence of the rate on alcohol conc entration, probably involving initial complexation followed by general base catalysed proton transfer. Reaction with primary amines in the same solven t is faster, and follows a first order dependence on amine concentration. T he photochemistry of the corresponding 1,1,3,3-tetraphenyldimetallacyclobut anes, formed by dimerization of the silene and germene, affords some insigh t into the mechanism of dimerization.