The mechanism of the hydroalkoxycarbonylation of ethene and alkene-CO copolymerization catalyzed by Pd(II)-diphosphine cations.
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 (31)P{(1)H} and (13)C{(1)H} 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)(CH(3)CN)(2)](OTf)(2) (dibpp = 1,3-(iBu(2)P)(2)C(3)H(6)) with Et(3)N and CH(3)OH affords [Pd(dibpp)(OCH(3))(CH(3)CN)]OTf, which, on exposure to CO, gives [Pd(dibpp){C(O)OCH(3)}(CH(3)CN)]OTf immediately. Labeling studies show the reaction to be readily reversible. However, the back reaction is strongly inhibited by PPh(3), indicating an insertion/deinsertion pathway. Ethene reacts with [Pd(dibpp){C(O)OCH(3)}(CH(3)CN)]OTf at 243 K to give [Pd(dibpp){CH(2)CH(2)C(O)OCH(3)}]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)CH(3)}L]X (L = CO, CH(3)OH; X = CF(3)SO(3) (-) (OTf(-)), CH(3)C(6)H(4)SO(3) (-) (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 beta 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 beta 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.
Liu
Jianke
J
Department of Chemistry, Donnan and Robert Robinson Laboratories, University of Liverpool, P.O. Box 147, Liverpool, L69 7ZD, UK.
Heaton
Brian T
BT
Iggo
Jonathan A
JA
Whyman
Robin
R
Bickley
Jamie F
JF
Steiner
Alexander
A