Kinetics and mechanisms of the reactions of Si1⁄4C and Ge1⁄4C double bonds

The kinetics and mechanisms of a number of characteristic reactions of Si1⁄4C and Ge1⁄4C double bonds have been studied in solution, using laser flash photolysis of silaand germacyclobutanes for generation of the silene and germene reactive intermediates under conditions where they can be detected directly. Substituent, solvent, isotope, and temperature effects on the rate constants for reaction of Ph2Si1⁄4CH2 with alcohols, amines, carboxylic acids and ketones suggest that these reagents react by a common mechanism, involving initial nucleophilic attack at silicon followed by proton transfer within the initially formed Lewis acid-base complex between the silene and the nucleophile. The reactivity of Ph2Ge1⁄4CH2 is significantly lower than that of the silene. Reaction with alcohols in acetonitrile solution proceeds by a mechanism leading to a second order dependence of the rate on alcohol concentration, probably involving initial complexation followed by general base catalysed proton transfer. Reaction with primary amines in the same solvent is faster, and follows a first order dependence on amine concentration. The photochemistry of the corresponding 1,1,3,3tetraphenyldimetallacyclobutanes, formed by dimerization of the silene and germene, affords some insight into the mechanism of dimerization.