Anchimeric assistance by platinum(II) in the epimerizations of [PtX(CHXSiMe(3))(R,R-chiraphos)]

The crystal structures of solvates of [PtCl(S-CHClSiMe(3))(R,R-chiraphos)], [PtBr(S-CHBrSiMe(3))(S,S-chiraphos)], and [PtCl(R-CHClSiMe(3))(R,R-chiraphos)] (chiraphos = 2,3-bis(diphenylphosphino)butane) have been determined allowing assignment of the absolute stereochemistry for all the species [PtX(CHXSiMe(3))(chiraphos)], 1a-c and 2a-c (X = Cl, Br, I). Inversion at the or-carbon in [PtX(CHXSiMe(3))(chiraphos)] has been observed at ambient temperature and below for X = Br or I and at elevated temperatures for X = Cl. The rates of these epimerizations have been monitored by P-31 NMR spectroscopy (X = Cl or Br) or polarimetry (X = Br or I). All the epimerizations follow first order kinetics and the rate constants k(obs) have been calculated. From these values the following can be deduced: (i) The rates increase in the order Cl < Br < I. (ii) Addition of large amounts of halide salts has only a small effect on the rate of epimerization. (iii) The reactions are significantly slower in MeCN than in CHCl3 or CH2Cl2. A mechanism consistent with these observations is proposed involving alpha-halogen migration to the platinum(II).