Remarkable Differences in Catalyst Activity and Selectivity for the Production of Methyl Propanoate versus CO−Ethylene Copolymer by a Series of Palladium Complexes of Related C4-Bridged Diphosphines

Abstract

The C4-bridged diphosphines cis- and trans-1,2-bis(diphenylphosphinomethyl)cyclohexane (cis, 1a; trans, 1b) and endo,endo-2,3-bis(diphenylphosphinomethyl)norbornane and exo,endo-2,3-bis(diphenylphosphinomethyl)norbornane (endo,endo, 1c; exo,endo, 1d) and their corresponding palladium complexes [(P−P)PdCl2] (2a−d), [(P−P)Pd(OAc)2] (3a−d), and [(P−P)PdMeCl] (P−P = 1a, 4a; P−P = 1c, 4c) have been prepared and characterized. Single-crystal X-ray analysis of 2a reveals that 1a forms a seven-membered chelate ring with the cis-disubstituted cyclohexyl ring in the expected chair conformation. Variable-temperature 31P{1H} NMR studies of 2a have revealed a dynamic process that interchanges stereoisomers via conformational changes in the six-membered ring, often characteristic of cis-disubstituted cyclohexane derivatives. The free energy of activation associated with this exchange has been determined (ΔG⧧ = 48 kJ mol-1, 2a). Variable-temperature 31P{1H} and 31P NOESY NMR studies have shown that 4a exist as an interconverting mixture of isomers which differ in the position of the Pd−Me, trans either to an axial or to an equatorial diphenylphosphinomethyl group. The free energy of activation associated with this interchange (ΔG⧧ = 44.2 kJ mol-1), as determined from a line shape analysis of the 31P{1H} NMR spectra in the temperature range 213−308 K, is similar to that for 2a and consistent with a process involving inversion of the cyclohexane ring. At 213 K, the equilibrium population of these isomers is surprisingly disparate. Full energy minimization calculations at the ZINDO(1) level gave an energy difference between the two minimized structures of 4 kJ mol-1, which corresponds to an equilibrium constant of 5 and is fully consistent with that obtained using NMR spectroscopy. Single-crystal X-ray analysis of 4a has revealed a square-planar geometry, such that the axial diphenylphosphinomethyl group occupies the site trans to the methyl and the equatorial diphenylphosphinomethyl group occupies the position trans to the chloride. The single-crystal X-ray structures of compounds 2d and 4c are also reported. Methanol solutions of 3a−c and methane sulfonic acid are selective for the copolymerization of ethylene with carbon monoxide, generating low molecular weight polymers. Surprisingly, the activity of catalyst systems based on cis-1,2-bis(diphenylphosphinomethyl)cyclohexane (1a) is markedly higher than those based on its trans-isomer 1b. In contrast, catalyst systems formed from exo,endo-2,3-bis(diphenylphosphinomethyl)norbornane (1d) are highly selective for the production of methyl propanoate (>90%).