NEIGHBORING GROUP PARTICIPATION OF PLATINUM(II) IN THE SUBSTITUTION OF THE ALPHA-HALOGEN IN COMPLEXES [PTI(CHXSIME3)(R,R-CHIRAPHOS)] [X = CL OR BR, CHIRAPHOS = 2,3-BIS(DIPHENYLPHOSPHINO)BUTANE] BY IODIDE - AN EXAMPLE OF AN S(N)1 SUBSTITUTION AT SP(3) CARBON WITH INVERSION OF CONFIGURATION

The complexes [PtX(R-CHXSiMe3)(R,R-chiraphos)] and [PtX(S-CHXSiMe3)(R, R-chiraphos)] [X = Cl or Br; chiraphos = 2,3-bis(diphenylphosphino)but ane], as single diastereomers, reacted with NaI in CH3CN to give [PtI( CHISiMe3)(R,R-chiraphos)] as an equilibrium mixture of diastereomers. The reactions were monitored by P-31 NMR spectroscopy and shown to occ ur in two stages in each case: a rapid substitution at platinum to giv e [PtI(R-CHXSiMe3)(R,R-chiraphos)] and [PtI(S-CHXSiMe3)(R,R-chiraphos) ] (X = Cl or Br) as intermediates, followed by much slower substitutio n at carbon to give the diiodo products. Attempts to isolate pure mono iodo intermediates by treatment of [PtX(R/S-CHXSiMe3)(R,R-chiraphos)] (X = Cl or Br) with 1 equivalent of NaI led to products contaminated b y the diiodo complexes. Thus the monoiodo complexes were generated in situ and the kinetics of the substitutions at carbon was investigated by P-31 NMR spectroscopy or polarimetry. The substitutions at carbon a re first-order reactions; the rates for the [PtI(R-CHXSiMe3)(R,R-chira phos)] (the more stable diastereomer) are slower than for [PtI(S-CHXSi Me3)(R,R-chiraphos)] by factors of ca. 100 for X = Cl and ca. 40 for X = Br. For the most reactive complex [PtI(S-CHBrSiMe3)(R,R-chiraphos)] inversion of configuration upon substitution was detected, the extent of which was extrapolated to be initially ca. 95%. Mechanisms involvi ng platinum-carbene intermediates are invoked in order to explain the kinetic and stereochemical results.