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

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 (CH 3 ) 2 SiHCH 2 OH and vinylsilyl methanol CH 2 CHSiH 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 CHSiH 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 CHSiH 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)CHCH 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 CHSiH 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 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