Synthesis of C,Sn-chiral triorganotin halides via C-chiral arylcopper and arylgoldlithium intermediates. Crystal and molecular structure of one {2-[1-(S)-(dimethylamino)ethyl]phenyl}methylphenyltin bromide diastereomer

The crystal and molecular structure and absolute configuration of 2-[1-(S)-Me₂NCH(Me)]C₆H₄SnMePhBr, which is the first example of a diastereomerically pure chiral triorganotin halide, are determined by a single-crystal x-ray diffraction study. Crystals are orthorhombic, space group P2₁2₁2₁ with Z = 4 in a unit cell of dimensions a = 9.370 (4), b = 13.361 (3), and c = 14.283 (4) Å. The final R value is 0.048 for 2784 reflections. The tin atom has a distorted trigonal bipyramidal geometry with the C,N-bonded Me₂NCH(Me)C₆H₄ group spanning one equatorial (C) and one axial (N) site. The Ph and Me groups reside in the two equatorial positions resulting in an (S)_sn configuration at the tin atom (cf. Figure 1). The configuration at Sn was unambiguously determined by using anomalous diffraction effects which likewise resulted in an independent check of the configuration at the α-C atom (S configuration). The (S)_C(S)_Sn diastereomer (mp 119-123°C) was separated by preferential crystallization from a 40/60 molar equilibrium mixture of the (S)_C(S)_Sn and (S)_C(R)_Sn diastereomers, respectively. The latter mixture was obtained by the reaction of MePhSnBr₂ with the C-chiral arylcopper and arylgoldlithium intermediates {2-[1-(S)-Me₂NCH(Me)]C₆H₄}_nM (M = Cu, n = 1, or M = AuLi, = 2). Slow epimerization of the (S)_C(S)_Sn diastereomer, which involves inversion of configuration at the tin atom, takes place in solution. On the basis of the dynamic NMR resonance patterns and the geometric constraints of the five-membered chelate ring [Me₂NCH(Me)C₆H₄Sn: C-Sn-N 75.2°] it is concluded that epimerization proceeds in a hexacoordinate intermediate (transition state) formed by association of two diastereomers.