Correlated rotation of the Me3Si groups in tris(trimethylsilyl)methanes

Low-temperature NMR spectra of (Me3Si)(3)CH (1) allowed the restricted rota tion of the Me,Si groups to be detected; the barrier was 5.1 kcal mol(-1). On the basis of molecular mechanics calculations, this motion was assigned to a correlated disrotatory process with an eclipsed, staggered, staggered (ESS) transition state. In (Me3Si)(3)CMe (2), both a correlated ESS disrota tory process and a correlated conrotatory SSS process were distinguished, w ith rotational barriers of 6.7 and 5.15 kcal mol(-1), respectively. Thus, 2 appears to adopt a chiral conformation (point group C-3) at low temperatur e. For more hindered derivatives such as (Me2Si)(3)CCH2Me (3) and (Me2Si)(2 )C-CH2Et (4), the ESS and SSS processes involving rotation about the Si-C b onds were also detected, but in addition a third process became evident. Th is process corresponds to restricted rotation about the C-C bond and seems to be correlated with the SSS conrotatory process at the C-Si bond, since b oth display essentially the same barrier. Although in (Me3Si)(3)CCH2Ph (5) the effects of restricted rotation about the C-CH2Ph bond could not be obse rved, the same chiral conformation adopted by 3 and 4 (point group C-1) was nonetheless assigned on the basis of molecular mechanics calculations and X-ray diffraction studies. The C-13 spectra (CPMAS) of crystalline 5 also s howed that the pairs of ortho and meta phenyl carbon atoms are diastereotop ic due to restricted Ph-CH2 bond rotation in the solid state.