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DOI: 10.1039/A705058E (Paper) Reference Section for: J. Chem. Soc., Dalton Trans., 1997, 4719-4724

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[hair space]†

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Roberto Argazzi, Paola Bergamini, Emiliana Costa and Paul G. Pringle

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.

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