Silylation products of cyclic tri-aminal carbanions and their lithiation

Abstract

The structure of 1,3,5-trimethyl-1,3,5-triaza-cyclohexane (TMTAC) was determined by single crystal X-ray diffraction and compared with earlier gas-phase data. It shows a preference for an aee-conformation in all phases. Lithiated TMTAC, [(RLi)₂·(RH)] (1) (R = 2,4,6-trimethyl-2,4,6-triaza-cyclohex-1-yl), was reacted with Et₃SiCl, Ph₃SiCl and PhMe₂SiCl to afford the substituted silanes Et₃SiR (1), Ph₃SiR (2) and PhMe₂SiR (3) in moderate yields. They were characterised by NMR spectroscopy (¹H, ¹³C, ²⁹Si). 1 reacts with Me₂SiCl₂ and Ph₂SiCl₂ to give Me₂SiR₂ (5) and Ph₂SiR₂ (6) which were characterised by NMR spectroscopy. 5 was also identified by crystal structure determination. Analogous triple substitution could not be observed by employing trichlorosilanes. Quantumchemical calculations explain this by sterical overcrowding of the silicon atom. The reaction of 1 with SiCl₄ did not yield fourfold substitution but a formal insertion product of SiCl₂ into a C–N bond of the TMTAC ring (2,4,6-trimethyl-2,4,6-triaza-1,1-dichloro-1-sila-cycloheptane, 7) in very small quantities. It was identified by X-ray crystallography and shows an intramolecular Si⋯N dative bond. The reactions of (3) and (5) with n-butyl lithium afforded lithiation of the silicon bound methyl groups in both cases. The products, 8 and 9, were characterised by NMR spectroscopy (¹H, ¹³C, ²⁹Si), 8 was also characterised by X-ray crystallography.