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-chiraph os)], [PtBr(S-CHBrSiMe(3))(S,S-chiraphos)], and [PtCl(R-CHClSiMe(3))(R ,R-chiraphos)] (chiraphos = 2,3-bis(diphenylphosphino)butane) have bee n determined allowing assignment of the absolute stereochemistry for a ll 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 b een observed at ambient temperature and below for X = Br or I and at e levated temperatures for X = Cl. The rates of these epimerizations hav e been monitored by P-31 NMR spectroscopy (X = Cl or Br) or polarimetr y (X = Br or I). All the epimerizations follow first order kinetics an d the rate constants k(obs) have been calculated. From these values th e following can be deduced: (i) The rates increase in the order Cl < B r < I. (ii) Addition of large amounts of halide salts has only a small effect on the rate of epimerization. (iii) The reactions are signific antly slower in MeCN than in CHCl3 or CH2Cl2. A mechanism consistent w ith these observations is proposed involving alpha-halogen migration t o the platinum(II).