| Autor: |
Polas, Anastasios, Wilton-Ely, James D. E. T., Slawin, Alexandra M. Z., Foster, Douglas F., Steynberg, Petrus J., Green, Michael J., Cole-Hamilton, David J. |
| Zdroj: |
Dalton Transactions; 2003, Vol. 2003 Issue: 24 p4669-4677, 9p |
| Abstrakt: |
Cobalt complexes involved in the hydroformylation of alkenes using 4R,S-4,8-dimethyl-2-octadecyl-2-phospha-bicyclo3.3.1nonane LIM-18, which consists of a mixture of two diastereomers, have been studied. Complexation to cobalt has been used to separate or enrich the enantiomers so that spectroscopic parameters can be determined for complexes of the form Co2CO8−nLIM-18nn 1 or 2 and CoCO3LIM-182CoCO4 containing the different diastereomers. An X-ray structure of a complex of the form CoCO3LIM-182shows it to contain the 4R isomer, but since this was isolated from a mixed solution of diastereomers, this does not definitively identify whether the 4R or 4S isomer is the more strongly coordinating isomer. Experimental studies backed up with molecular modelling suggest that steric and electronic effects determine which isomer reacts preferentially in coordination, protonation and methylation reactions. The 4R isomer is the more strongly coordinating and is metallated preferentially, although the 4S isomer is the more basic. Variable temperature and high pressure NMR studies on the acyl complex, CoCOC5H9CO3LIM-18, suggest that product elimination may be accompanied by isomerisation to branched acyl species. Modified cobalt catalysed hydroformylation of reactions suggest that the two diastereomers of LIM-18 do not give greatly different rates or selectivities and that a long chain secondary alkyl substituent on the LIM gives lower linear selectivity and a faster rate than LIM-18. LIM-Butgives a selectivity similar to that obtained using LIM-18. |
| Databáze: |
Supplemental Index |
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