A study of the mechanism of platinum(II)/tin(II) dichloride mediated hydrogenation of alkynes and alkenes employing parahydrogen-induced polarization

The mechanism of hydrogenation of alkynes catalyzed by the [(PR3)(2)PtHX]/SnX2 system (PR3 = PPh3, PMePh2; X = Cl, Br) has been studied by means of parahydrogen-induced polarization of IH spectra (PHIP). Dihydride intermediates confirming the stepwise hydrogenation at room temperature were observed when the reaction was run in acetone. The obtained H-1-PHIP spectra, together with NMR data for related species, are consistent with the formulation of these intermediates as cis-[H2Pt(PR3) (SnX3) (sigma-alkenyl) (acetone)], where the (sigma-alkenyl ligand originates from an insertion reaction of the alkyne (1-phenyl-1-propyne, 1-phenyl-1-butyne, diphenylacetylene, 3,3-dimethylbutyne). At elevated temperatures, the hydrogenation in acetone proceeds as a cis-synchronous transfer of the two hydrogen atoms of parahydrogen to the substrate molecule. A mechanism for this synchronous hydrogenation is suggested.