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

Abstract

To investigate the effects of substituents attached to the silicon atom on the thermal rearrangement reactions of α-silyl alcohols, the thermal rearrangement reactions of dimethylsilyl methanol (CH3)2SiHCH2OH and vinylsilyl methanol CH2CHSiH2CH2OH 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 (CH3)2SiHCH2OH or CH2CHSiH2CH2OH 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 (CH3)2SiHOCH3 or methoxylvinylsilane CH2CHSiH2OCH3; 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 (CH3)3SiOH or methylvinylsilanol CH3SiH(OH)CHCH2. The G3 barriers of the reactions A and B were computed to be 312.8 and 241.4 kJ/mol for (CH3)2SiHCH2OH, and 317.6 and 233.7 kJ/mol for CH2CHSiH2CH2OH, 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 small-curvature 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. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2007