Pseudo-pentacoordination in silylcarbamates: structure-based prediction of silylating power

Abstract

N-substituted trimethylsilyl carbamates (NTSCs) are powerful silylating agents. A survey of eight available X-ray molecular structures reveals an unusually short Si⋯O C distance that seems to convey a pseudo-pentacoordinated character of the silicon atom. The molecular structure of some representatives of NTSCs and their carbon analogues have been optimized at the MP2/DZ(2)P level. The theoretical structures confirm the existence of a Si⋯O through-space interaction and the pseudo-pentacoordination at the silicon atom. A structure-based predictive model is developed in which the high reactivity of NTSCs in displacing mobile hydrogens (i.e. the silylating power) is explained by both the pseudo-pentacoordination at the silicon and the unusually long Si–O ester bond, that is, by the use of ground-state structural parameters. On this basis, O-silylcarbamimidates are predicted to show higher reactivity in nucleophilic reactions, as compared to NTSCs, due to the stronger Si⋯N C interaction. Beyond NTSCs, the current predictive model can be applied to a wider range of compounds. O-alkylcarbamimidates are predicted to be good alkyl donors in nucleophilic substitutions. This conclusion is in accordance with the fact that O-alkyl-isoureas are general alkylating agents. Preliminary experimental results on the rate constants of hydrolysis of N-silylated cyclic ureas show strong correlation with the shift toward pseudo-pentacoordination around silicon, proving the validity of the current predictive model.