Chemistry of metal hydrides. Part XIX. Stereochemical changes at platinum during the insertion of acetylenes with platinum(II) hydrides

Abstract

The reactions of trans-[PtH(PEt3)2(acetone)]+ and trans-[PtH(PEt3)2(CO)]+ with a variety of acetylenes are described. The geometries of the products with respect to the platinum centre and to the stereochemistry of the alkenyl group have been determined on the basis of their 1H nmr parameters. For reactions with the cation containing acetone, and particularly from a variable temperature 1H nmr study of its reaction with 2-butyne in the presence of water, the insertion pathway is shown to be entirely analogous to that for olefin insertion, involving an initial displacement of the trans ligand by acetylene, isomerization to a cis-like intermediate, migratory insertion rearrangement, and re-combination with an appropriate ligand to give either cis or trans platinum products. The insertion pathway with the carbonyl cation is dependent on the nature of the acetylene; for non-activated or weakly activated acetylenes insertion occurs via five-coordinate intermediates, but with strongly activated acetylenes, the same mechanism as with the acetone cation appears to occur, involving reversible loss of carbon monoxide.