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

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-C=C torsional angles are 59.6 (φ₂) and 63.3° (φ₂) and the C=C-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 Mes₂C=C(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 (Me ₃Si)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).