Neighbouring group participation of platinum(II) in the substitution of the α-halogen in complexes [PtI(CHXSiMe3)(R,R-chiraphos)] [X = Cl or Br; chiraphos = 2,3-bis(diphenylphosphino)butane] by iodide. An example of an SN1 substitution at sp3 carbon with inversion of configuration†

Abstract

The complexes [PtX(R-CHXSiMe₃)(R,R-chiraphos)] and [PtX(S-CHXSiMe₃)(R,R-chiraphos)] [X = Cl or Br; chiraphos = 2,3-bis(diphenylphosphino)butane], 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 ³¹P NMR spectroscopy and shown to occur in two stages in each case: a rapid substitution at platinum to give [PtI(R-CHXSiMe₃)(R,R-chiraphos)] and [PtI(S-CHXSiMe₃)(R,R-chiraphos)] (X = Cl or Br) as intermediates, followed by much slower substitution at carbon to give the diiodo products. Attempts to isolate pure monoiodo intermediates by treatment of [PtX(R/S-CHXSiMe₃)(R,R-chiraphos)] (X = Cl or Br) with 1 equivalent of NaI led to products contaminated by the diiodo complexes. Thus the monoiodo complexes were generated in situ and the kinetics of the substitutions at carbon was investigated by ³¹P NMR spectroscopy or polarimetry. The substitutions at carbon are first-order reactions; the rates for the [PtI(R-CHXSiMe₃)(R,R-chiraphos)] (the more stable diastereomer) are slower than for [PtI(S-CHXSiMe₃)(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 involving platinum–carbene intermediates are invoked in order to explain the kinetic and stereochemical results.