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 C-4-bridged diphosphines

The C-4-bridged diphosphines cis- and trans-1,2-bis(diphenylphosphinomethyl )cyclo hexane (cis, 1a; trans, 1b) and endo,endo-2,3-bis(diphenylphosphinom ethyl)norbornane and exo,endo-2,3-bis(diphenylphosphinomethyl)norbornane (e ndo,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 a nalysis of 2a reveals that 1a forms a seven-membered chelate ring with the cis-disubstituted cyclohexyl ring in the expected chair conformation. Varia ble-temperature P-31{H-1} NMR studies of 2a have revealed a dynamic process that interchanges stereoisomers via conformational changes in the six-memb ered ring, often characteristic of cis-disubstituted cyclohexane derivative s. The free energy of activation associated with this exchange has been det ermined (DeltaG(double dagger) = 48 kJ mol(-1), 2a). Variable-temperature P -31{H-1} and P-31 NOESY NMR studies have shown that 4a exist as an intercon verting 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 (DeltaG(double d agger) = 44.2 kJ mol(-1)), as determined from a line shape analysis of the P-31{H-1} NMR spectra in the temperature range 213-308 K, is similar to tha t for 2a and consistent with a process involving inversion of the cyclohexa ne ring. At 213 K, the equilibrium population of these isomers is surprisin gly 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 consist ent with that obtained using NMR spectroscopy. Single-crystal X-ray analysi s of 4a has revealed a square-planar geometry, such that the axial diphenyl phosphinomethyl group occupies the site traits to the methyl and the equato rial diphenylphosphinomethyl group occupies the position trans to the chlor ide. The single-crystal X-ray structures of compounds 2d and 4c are also re ported. Methanol solutions of 3a-c and methane sulfonic acid are selective for the copolymerization of ethylene with carbon monoxide, generating low m olecular weight polymers. Surprisingly, the activity of catalyst systems ba sed on cis-1,2-bis(diphenylphosphinomethyl)cyclohexane (1a) is markedly hig her 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%).