Deuterium labelling evidence for a hydride mechanism in the formation of methyl propanoate from carbon monoxide, ethene and methanol catalysed by a palladium complex

Abstract

Reaction of ethene with CO in CH₃OD in the presence of a catalyst prepared in situ from [Pd(DBPMB)(DBA)] (DBPMB = 1,2-bis[(di-tert-butyl)phosphinomethyl]benzene, DBA = dibenzylideneacetone) and methanesulfonic acid under conditions of good gas mixing gives a 1 ∶ 1 mixture of CH₂DCH₂CO₂Me and CH₃CHDCO₂Me with no H incorporated into the CH₃OD. If the gas mixing is less efficient, the methyl propanoate has 0–5 D atoms incorporated in the ethyl group, CH₃OD exchanges to give increasing amounts of CH₃OH throughout the reaction and there is a slight increase in the less deuteriated products with reaction time. Significant D incorporation into unreacted ethene is also observed. These results are interpreted in terms of a hydride mechanism with the rates of the individual steps under conditions of good mixing being: reversible H migration to coordinated ethene > CO coordination ≫ C₂H₄ exchange > H/D exchange.