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 Dip
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- 作者：Julian G. Knight ; Simon Doherty ; Anthony Harriman ; Edward G. Robins ; Michael Betham ; Graham R. Eastham ; Robert P. Tooze ; Mark R. J. Elsegood ; Paul Champkin ; and William Clegg
- 刊名：Organometallics
- 出版年：2000
- 出版时间：November 27, 2000
- 年：2000
- 卷：19
- 期：24
- 页码：4957 - 4967
- 全文大小：151K
- 年卷期：v.19,no.24(November 27, 2000)
- ISSN：1520-6041
文摘
The C4-bridged diphosphines cis- and trans-1,2-bis(diphenylphosphinomethyl)cyclohexane (cis, 1a; trans, 1b) and endo,endo-2,3-bis(diphenylphosphinomethyl)norbornane andexo,endo-2,3-bis(diphenylphosphinomethyl)norbornane (endo,endo, 1c; exo,endo, 1d) and theircorresponding 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 thecis-disubstituted cyclohexyl ring in the expected chair conformation. Variable-temperature31P{1H} NMR studies of 2a have revealed a dynamic process that interchanges stereoisomersvia conformational changes in the six-membered ring, often characteristic of cis-disubstitutedcyclohexane derivatives. The free energy of activation associated with this exchange hasbeen determined (
G
= 48 kJ mol-1, 2a). Variable-temperature 31P{1H} and 31P NOESYNMR studies have shown that 4a exist as an interconverting mixture of isomers which differin 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 (G = 44.2kJ mol-1), as determined from a line shape analysis of the 31P{1H} NMR spectra in thetemperature range 213-308 K, is similar to that for 2a and consistent with a processinvolving inversion of the cyclohexane ring. At 213 K, the equilibrium population of theseisomers 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, whichcorresponds to an equilibrium constant of 5 and is fully consistent with that obtained usingNMR 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 methyland 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. Methanolsolutions of 3a-c and methane sulfonic acid are selective for the copolymerization of ethylenewith carbon monoxide, generating low molecular weight polymers. Surprisingly, the activityof catalyst systems based on cis-1,2-bis(diphenylphosphinomethyl)cyclohexane (1a) ismarkedly higher than those based on its trans-isomer 1b. In contrast, catalyst systems formedfrom exo,endo-2,3-bis(diphenylphosphinomethyl)norbornane (1d) are highly selective for theproduction of methyl propanoate (>90%).
G = 48 kJ mol-1, 2a). Variable-temperature 31P{1H} and 31P NOESYNMR studies have shown that 4a exist as an interconverting mixture of isomers which differin 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 (G = 44.2kJ mol-1), as determined from a line shape analysis of the 31P{1H} NMR spectra in thetemperature range 213-308 K, is similar to that for 2a and consistent with a processinvolving inversion of the cyclohexane ring. At 213 K, the equilibrium population of theseisomers 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, whichcorresponds to an equilibrium constant of 5 and is fully consistent with that obtained usingNMR 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 methyland 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. Methanolsolutions of 3a-c and methane sulfonic acid are selective for the copolymerization of ethylenewith carbon monoxide, generating low molecular weight polymers. Surprisingly, the activityof catalyst systems based on cis-1,2-bis(diphenylphosphinomethyl)cyclohexane (1a) ismarkedly higher than those based on its trans-isomer 1b. In contrast, catalyst systems formedfrom exo,endo-2,3-bis(diphenylphosphinomethyl)norbornane (1d) are highly selective for theproduction of methyl propanoate (>90%).