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Anchimeric Assistance by Platinum(II) in the Epimerizations of [PtX(CHXSiMe3)(R,R-chiraphos)]

The crystal structures of solvates of [PtCl(S-CHClSiMe3)(R,R-chiraphos)], [PtBr(S-CHBrSiMe3)(S,S-chiraphos)], and [PtCl(R-CHClSiMe3)(R,R-chiraphos)] (chiraphos = 2,3-bis(diphenylphosphino)butane) have been determined allowing assignment of the absolute stereochemistry for all the species [PtX(CHXSiM...

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Bibliographic Details
Published in:Organometallics 1996-12, Vol.15 (26), p.5591-5597
Main Authors: Argazzi, Roberto, Bergamini, Paola, Costa, Emiliana, Gee, Victoria, Hogg, John K, Martín, Antonio, Orpen, A. Guy, Pringle, Paul G
Format: Article
Language:English
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Summary:The crystal structures of solvates of [PtCl(S-CHClSiMe3)(R,R-chiraphos)], [PtBr(S-CHBrSiMe3)(S,S-chiraphos)], and [PtCl(R-CHClSiMe3)(R,R-chiraphos)] (chiraphos = 2,3-bis(diphenylphosphino)butane) have been determined allowing assignment of the absolute stereochemistry for all the species [PtX(CHXSiMe3)(chiraphos)], 1a − c and 2a − c (X = Cl, Br, I). Inversion at the α-carbon in [PtX(CHXSiMe3)(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 31P 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 α-halogen migration to the platinum(II).
ISSN:0276-7333
1520-6041
DOI:10.1021/om9605950