Homogeneous catalysis of olefin isomerisation. Part IV. The isomerisation of pent-1-ene catalysed by solutions of IrH(CO)(PPh3)3, IrHCl2(PEt2Ph)3, IrCl(CO)(PPh3)2 IrCl(O2)(CO)(PPh3)2, and of PtH(SnCl3)(PPh3)2

Abstract

The isomerisation of pent-1-ene to pent-2-ene at 80 °C is catalysed by solutions of (i) IrH(CO)(PPh₃)₃, (ii) IrHCl₂(PEt₂Ph)₃, (iii) IrCl(CO)(PPh₃)₂ or IrCl(O₂)(CO)(PPh₃)₂ each in the presence of an oxygen source, and (iv) PtH(SnCl₃)(PPh₃)₂ in benzene. Systems (i), (ii), and (iv) catalyse the preferential formation of cis-pent-2-ene (initial cis:trans ratios typically 2·0 to 3·5) whereas (iii) provides preferential formation of trans-pent-2-ene (initial cis:trans ratio ca. 0·6).

The isomerisation of [1, 2-²H₂]pent-1-ene catalysed by (i), (iii), and (iv) is reported. Intermolecular hydrogen transfer typical of isomerisation via the formation of pentyl complexes was observed in each case. Stepwise movement of the double bond occurred in systems (i) and (iii), whereas multiple movement was observed in (iv). The relative probabilities of the three elementary reactions (viz : formation of pent-1-yl complex, formation of a pent-2-yl complex, and isomerisation) are calculated for each system.

Gross variations of the cis:trans ratio are interpreted in terms of the presence or absence of steric interaction between bulky ligands and the pent-2-yl group.