Carbonylation of [PtX(Ph)(PR3)2]. New intermediates in the CO insertion sequence

Abstract

Phosphorus-31 n.m.r. studies of the reaction of trans-[PtX(Ph)(PR₃)₂] with carbon monoxide have led to the identification of several new intermediates and reaction pathways involved in the CO insertion process to produce trans-[PtX(COPh)(PR₃)₂]. Carbonyl addition forms a metastable five-co-ordinate compound in non-polar solvents, which eliminates a halide to form ionic species trans-[Pt(Ph)(CO)(PR₃)₂]X in polar solvents. Loss of PR₃ from five-co-ordinate intermediates produces two isomers of [PtX(Ph)(CO)(PR₃)] which convert to the remaining isomer with Ph trans to PR₃ before migration of Ph proceeds to form a benzoyl complex. A direct carbonyl insertion route from a five-co-ordinate intermediate also operates, independently of the phosphine elimination pathways, and this becomes the predominant pathway with more nucleophilic PR₃ ligands. The use of elemental sulphur to remove PR₃ from the reaction mixtures was instrumental in identifying some intermediates, and this method has potential synthetic value in replacing PR₃ by weaker ligands at platinum.