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

Abstract

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: “α-products“, [PtX(CH₂SiMe₃)(diphos)] 2 and “β-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 α-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 β-product [PtCl(CH₂CH₂SiMe₃)(skewphos)] 3d as shown unambiguously by a combination of ¹H-COSY and ³¹P NMR spectroscopy. It is established that the reaction sequence is 1 → 3 → 2 and the conversion of 3 → 2 is via a β-hydrogen migration and elimination of CH₂CHSiMe₃. The stability of 3 with respect to β-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 β-hydrogen migration while a CO₂Et group retards β-hydrogen migration. The complexes 2 are catalysts for the conversion of N₂CHSiMe₃ to Me₃SiCH=CHSiMe₃.