The reaction of trimethylsilyldiazomethane with complexes of the type [PtX(CH3)(diphosphine)] (X = Cl, Br, I). Some observations on beta-hydrogen migrations in PtCHRCH3 species and organoplatinum(II)-catalysts for alkene formation from trimethylsilyldiazomethane

Treatment of [PtX(CH3)(diphos)] 1 {X = Cl, Br, I; diphos = (4R,5R)-4,5-bis(diphenylphosphinomethyl)-2,2-dimethyl-1,3-dioxolane (diop), (2S,4S)-2,4-bis(diphenylphosphino)pentane (skewphos), (2S,3S)-2,3-bis(diphenylphosphino)butane (chiraphos)} with N2CHSiMe3 gives two series of products: "alpha-products", [PtX(CH2SiMe3)(diphos)] 2 and "beta-products" [PtX(CH2CH2SiMe3)(diphos)] 3. Which product is formed and their stability depends on the ancillary ligands X and diphos. Treatment of [PtCl(CH3)(diop)] 1a with an excess of N2CHSiMe3 gives the alpha-product [PtCl(CH2SiMe3)(diop)] 2a in high yield. The structure of 2a was confirmed by X-ray crystallography. Under similar conditions [PtCl(CH3)(skewphos)] 1d reacts with an excess of N2CHSiMe3 to give the beta-product [PtCl(CH2CH2SiMe3)(skewphos)] 3d as shown unambiguously by a combination of H-1-COSY and P-31 NMR spectroscopy. It is established that the reaction sequence is 1 --> 3 --> 2 and the conversion of 3 --> 2 is via a beta-hydrogen migration and elimination of CH2=CHSiMe3. The stability of 3 with respect to beta-hydrogen elimination is in the order Cl > Br > I and chiraphos > skewphos > diop; a mechanism is proposed based on five-coordinate platinum(II) intermediates to rationalize these trends. The reactions of [PtX(CH3)(diphos)] with N2CHSiMe3 and N2CHCOOEt are contrasted and it is concluded that in PtCHRMe species, a SiMe3 group facilitates beta-hydrogen migration while a CO2Et group retards beta-hydrogen migration. The complexes 2 are catalysts for the conversion of N2CHSiMe3 to Me3SiCH=CHSiMe3.