The effective ‘size’ of the tris(trimethylsilyl)silyl group in several molecular environments

Abstract

The effective size of the tris(trimethylsilyl)silyl group in several molecular environments has been estimated. 2,2-Dimesityl-1-tris(trimethylsilyl)silylethanol 1g has been prepared and its structure determined by X-ray crystallography. The Mes–CC torsional angles are 59.6 (φ₂) and 63.3°(φ₂) and the CC–Si bond angle α₄ is 133.8°. The two-ring flip barrier for the correlated rotation of the two mesityl rings around the Mes–C bonds is ΔG_c ^‡= 10.2 kcal mol^–1. The structures of enols Mes₂CC(OH)R, R = H, Me, Et, Prⁱ, Bu^t(1a–1e), Me₃Si (1f), (Me₃Si)₃Si (1g) and (Me₃Si)₃C (1h) and the two-ring flip barriers have been calculated by the MM2* force-field. The calculated and the experimental values are in good agreement, except for somewhat lower calculated α₄ for 1b–1e and a shorter C–Si distance in 1g. From the linear correlations between the observed cos φ₂ or ΔG_c ^‡ values and E_s values for the enols 1a–1e, and the values observed for 1g an average E_s value of –1.46 has been calculated for (Me₃Si)₃Si. MM2* calculations gave an A value for (Me₃Si)₃Si of 4.89 kcal mol^–1. These steric parameters resemble those for the Bu^t group (E_s=–1.54; A= 4.9 kcal mol^–1) indicating a similar effective size for the Bu^t and (Me₃Si)₃Si groups in these specific environments. (Me₃Si)₃C is significantly larger (A= 13.3 kcal mol^–1; estimated E_s=–3.7).