Activation of Si–H bonds by stable singlet carbenes? A density functional theory study on the reaction pathways

The mechanism of activation of Si–H bonds (silanes, primary, secondary, and tertiary silanes) by stable singlet carbenes (including the cyclic (alkyl)–(amino) carbenes (CAACs) and N-heterocyclic carbenes (NHCs)) have been investigated using density functional theory calculations done at the B3LYP/6-311G(d, p) level of theory. Solvent effects on these reactions have been explored by calculations that included a polarizable continuum model (PCM) for the solvent (THF). Our calculation results suggest: (1) these insertion reactions will easily yield the eventual insertion products through a three-center transition structure (TS) whether for (CAACs) or NHCs; (2) for a given CAACs will yield (R)–(S)-isomer mixture products; (3) the reactivity of silanes decreases in the order: H4Si > H3SiOCH3 > H2Si(OCH3)2 > HSi(OCH3)3 and the higher activation barriers for NHCs indicate that the activation of Si–H bonds is strongly favored kinetically in the case of the CAACs. These computational results are in consistent with the experimental observations of Bertrand et al. (G. D. Frey, J. D. Masuda, B. Donnadieu, G. Bertrand Angew. Chem. Int. Ed. 49 (2010) 9444–9447) for the activation of Si–H bonds by stable singlet carbenes.