The Mechanism of the Hydroalkoxycarbonylation of Ethene and Alkene–CO Copolymerization Catalyzed by PdII–Diphosphine Cations

Abstract

All the intermediates in the “carboalkoxy” pathway, and their interconversions giving complete catalytic cycles, for palladium–diphosphine‐catalyzed hydroalkoxycarbonylation of alkenes, and for alkene–CO copolymerization, have been demonstrated using ³¹P{¹H} and ¹³C{¹H} NMR spectroscopy. The propagation and termination steps of the “hydride” cycles and the crossover between the hydride and carboalkoxy cycles have also been demonstrated, providing the first examples of both cycles, and of chain crossover, being delineated for the same catalyst. Comparison of the propagation and termination steps in the pathways affords new insight into the selectivity‐determining steps. Thus, reaction of [Pd(dibpp)(CH₃CN)₂](OTf)₂ (dibpp = 1,3‐(iBu₂P)₂C₃H₆) with Et₃N and CH₃OH affords [Pd(dibpp)(OCH₃)(CH₃CN)]OTf, which, on exposure to CO, gives [Pd(dibpp){C(O)OCH₃}(CH₃CN)]OTf immediately. Labeling studies show the reaction to be readily reversible. However, the back reaction is strongly inhibited by PPh₃, indicating an insertion/deinsertion pathway. Ethene reacts with [Pd(dibpp){C(O)OCH₃}(CH₃CN)]OTf at 243 K to give [Pd(dibpp){CH₂CH₂C(O)OCH₃}]OTf, that is, there is no intrinsic barrier to alkene insertion into the PdC(O)OMe bond, as had been proposed. Instead, termination is proposed to be selectivity determining. Methanolysis of the acyl intermediate [Pd(dibpp){C(O)CH₃}L]X (L = CO, CH₃OH; X = CF₃SO₃⁻ (OTf⁻), CH₃C₆H₄SO₃⁻ (OTs⁻)) is required in the hydride cycle to give an ester and occurs at 243 K on the timescale of minutes, whereas methanolysis of the β chelate, required to give an ester from the carbomethoxy cycle, is slow on a timescale of days, at 298 K. These results suggest that slow methanolysis of the β chelate, rather than slow insertion of an alkene into the Pdcarboalkoxy bond, as had previously been proposed, is responsible for the dominance of the hydride mechanism in hydroalkoxycarbonylation.