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 (phi(2)) and 63.3 degrees (phi(2)) and the C=C-Si bond angle alpha(4) is 133.8 degrees. The two-ring trip barrier for the correlated rotation of the two mesityl rings around the Mes-C= bonds is Delta G(c)(double dagger) = 10.2 kcal mol(-1). The structures of enols Mes(2)C=C(OH)R, R = H, Me, Et, Pr, Bu(t) (1a-1e), Me(3)Si (1f), (Me(3)Si)(3)Si (1g) and (Me(3)Si)(3)C (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 alpha(4) for 1b-1e and a shorter =C-Si distance in 1g. From the linear correlations between the observed cos phi(2) or Delta G(c)(double dagger) 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(3)Si)(3)Si. MM2* calculations gave an A value for (Me(3)Si)(3)Si 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 (Me(3)Si)(3)Si groups in these specific environments. (Me(3)Si)(3)C is significantly larger (A = 13.3 kcal mol(-1); estimated E(s) = -3.7).