Theoretical study of thermal rearrangements of α‐silylalcohols: Effects of substituents attached to the silicon atom on the reactions

Abstract

ABSTRACT:To investigate the effects of substituents attached to the silicon atom on the thermal rearrangement reactions of ␣-silyl alcohols, the thermal rearrangement reactions of dimethylsilyl methanol (CH 3 ) 2 SiHCH 2 OH and vinylsilyl methanol CH 2 ACHSiH 2 CH 2 OH were studied by ab initio calculations at the G3 level. Geometries of various stationary points were fully optimized at the MP2(full)/6-31G(d) and MP2(full)/6-311G(d,p) levels, and harmonic vibrational frequencies were calculated at the same levels. The reaction paths were investigated and confirmed by intrinsic reaction coordinate (IRC) calculations at the MP2(full)/6-31G(d) level. The results show that two dyotropic reactions could occur when (CH 3 ) 2 SiHCH 2 OH or CH 2 ACHSiH 2 CH 2 OH is heated. One is Brook rearrangement reaction (reaction A), and the dimethylsilyl or vinylsilyl groups migrates from carbon atom to oxygen atom coupled with a simultaneous migration of a hydrogen atom from oxygen atom to carbon atom passing through a double three-membered ring transition state, forming dimethylmethoxylsilane (CH 3 ) 2 SiHOCH 3 or methoxylvinylsilane CH 2 ACHSiH 2 OCH 3 ; the other is a hydroxyl group migration (reaction B) from carbon atom to silicon atom, coupled with a simultaneous migration of a hydrogen atom from silicon atom to carbon atom, via a double three-membered ring transition state, forming trimethylsilanol (CH 3 ) 3 SiOH or methylvinylsilanol CH 3 SiH(OH)CHACH 2 . The G3 barriers of the reactions A and B were computed to be 312.8 and 241.4 kJ/mol for (CH 3 ) 2 SiHCH 2 OH, and 317.6 and 233.7 kJ/mol for CH 2 ACHSiH 2 CH 2 OH, respectively. On the basis of the MP2(full)/6-31G(d) optimized parameters, vibrational frequencies, and G3 energies, the reaction rate constants k(T) and equilibrium constants K(T) were calculated using canonical variational transition state theory (CVT) with centrifugal-dominant smallcurvature tunneling (SCT) approximation over a temperature range of 400 -1800 K. The influences of methyl and vinyl groups attached to the silicon atom on reactions are discussed.

Dyotropic Reactions: Mechanisms and Synthetic Applications

“…Silanes 67 lead to the formation of compounds 69 and/or 71 (Scheme ). − Two main reaction pathways have been explored for these transformations: the Brook rearrangement (pathway A) and the 1,2-dyotropic process (pathway B). Experimental and computational ( vide infra ) studies indicate that the dyotropic process is both thermodynamically and kinetically favored, with the Brook rearrangement being competitive only at high temperatures.…”

“…Yu and Feng have reported theoretical studies on the thermal rearrangements of silylmethanamine 228 to form methylsilanamine 230 via a concerted transition state 229 or to form (methylamino)silane 234 through a stepwise mechanism. This stepwise mechanism involves transition structures 231 and 233 and the intermediate aminosilane−carbene complex 232 (Scheme ). − These reactions are the model reactions for the dyotropic transformations shown in Scheme . The computed activation barrier for the concerted dyotropic rearrangement via transition structure 229 (corresponding to a mechanism of type (a), Scheme ) has an activation barrier of 66.8 kcal/mol.…”

“…In contrast, the 228 → 232 and 232 → 234 steps (corresponding to a mechanism of type (b), Scheme ) have activation barriers of 59.2 and 12.4 kcal/mol, respectively. Interestingly, when the same reaction takes place with α-silylalcohols, the mechanism of both dyotropic rearrangements is concerted …”