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
Jg. Knight et al., 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, ORGANOMETAL, 19(24), 2000, pp. 4957-4967
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%).