Limonene-derived phosphines in the cobalt-catalysed hydroformylation of alkenes

Abstract

Cobalt complexes involved in the hydroformylation of alkenes using (4R,S)-4,8-dimethyl-2-octadecyl-2-phospha-bicyclo[3.3.1]nonane (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 [Co₂(CO)_8−n(LIM-18)_n] (n = 1 or 2) and [Co(CO)₃(LIM-18)₂][Co(CO)₄] containing the different diastereomers. An X-ray structure of a complex of the form [Co(CO)₃(LIM-18)]₂ shows 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, [Co(C(O)C₅H₉)(CO)₃(LIM-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-Bu^t gives a selectivity similar to that obtained using LIM-18.