Homogeneous catalysis of olefin isomerisation. Part III. The isomerisation of pent-1-ene catalysed by solutions of tetrakis(triethyl phosphite)-nickel(o) and of hydridochlorocarbonyltris(triphenylphosphine)osmium(II)

Abstract

The isomerisation of pent-1-ene to cis- and trans-pent-2-ene is catalysed at 35 °C by solutions of Ni{P(OEt)₃}₄ in benzene containing trifluoroacetic acid, and at 80 °C by solutions of OsHCl(CO)(PPh₃)₃ in pure benzene. The initial rate of the nickel-catalysed reaction is not diminished by the addition of triethyl phosphite, and NiH{P(OEt)₃}₄⁺ is suggested as the catalyst. The initial rate of the osmium-catalysed reaction is retarded by the addition of triphenylphosphine, and catalyst generation is suggested to occur by dissociation of triphenylphosphine from the original complex.

The initial cis:trans-ratios in the pent-2-ene were 2·3(Ni) and 6·0(Os).

Isomerisation of [1,2-²H₂]pent-1-ene has shown that, for each reaction, isotopic redistribution in the reactant is rapid, and involves only the intermolecular exchange of hydrogen and deuterium bonded to the olefinic carbon atoms. Initially, pent-2-ene contained more deuterium than the starting material; no deuterium entered the ethyl group.

The suggested mechanism involves pentyl and 1-methylbutyl intermediates. Dissociation or displacement of pent-2-ene from the catalyst occurs before the formation of 1-ethylpropyl complexes. Steric hindrance about the carbon-metal bond in 1-methylbutyl complexes is suggested in order to reconcile the conclusion (i) that the formation of 1-methylbutyl complexes from pent-1-ene is a favoured process, and (ii) that the rate of conversion of 1-methylbutyl complexes into pent-2-ene is relatively slow.