A theoretical study of mechanisms of 1,3-silyl migration in formylmethylsilane and related migrations. Comparison with allylsilane

Two concerted pathways leading to retention and inversion of stereochemistr y at the migrating silicon center were found for the isomerization of formy lmethylsilane to siloxyethene by ab initio molecular orbital calculations. The activation energy for the retention pathway has been calculated at the MP2/6-311++G(3df, 2p)//MP2/6-311++G** level to be 32 kcal/mol, which is ca. 30 kcal/mol smaller than that for the inversion pathway. The predicted exc lusive retention stereochemistry and the calculated activation energy are i n good agreement with the experimental results for a silylmethyl ketone wit h an optical active silyl group. Remarkable differences in the retention tr ansition structures between the 1,3-silyl migrations in formylmethylsilane and the 1,3-silyl migrations in allylsilane are revealed by detailed analys is of geometries, natural bond orbitals, and the Laplacian (del(2)rho) of t he wave function. The retention 1,3-silyl migration in formylmethylsilane i s best described as an intramolecular nucleophilic substitution at silicon, while the corresponding 1,3-silyl migration in allylsilane is as an electr ocyclic sigmatropic rearrangement controlled by subjacent orbital interacti ons. For related 1,3-migrations in HC(=O)CH2MH3 (M = C, Si, Ge, Sn), the E- a values for the retention pathway are much lower than the inversion pathwa y and they decrease in the following order: M = C much greater than Si > Ge > Sn (as expected from the relative stability of the pentacoordinate struc ture among the metals). A facile 1,3-migration from carbon to nitrogen in i minomethylsilane is predicted to occur with retention stereochemistry via a n intramolecular nucleophilic substitution, which is similar to that in for mylmethylsilane.