HOMOGENEOUS HYDROGENATION WITH IRIDIUM COMPLEXES - EVIDENCE FOR POLYHYDRIDE INTERMEDIATES IN THE REDUCTION OF ALPHA-PINENE

The hydrogenation of alpha-pinene by Crabtree's catalyst [(cycloocta-1,5-diene)(pyridine)(tricyclohexylphosphine)iridium hexafluorophosphate] in dichloromethane, occurs from the endo-face with high stereospecificity. Reaction is accompanied by reversible equilibration with beta-pinene, which forms 2% of the equilibrium mixture. The hydrogenation of beta-pinene is slower than its isomerization under these conditions. Reductions were carried out with deuterium in order to probe the reaction mechanism. Using the novel technique of H-2-decoupled C-13 n.m.r., it was found that a complete analysis of the isotope distribution in the product could be obtained by analysis of the C1 signal, since the possible beta- and gamma-perturbations gave rise to distinct spectral shifts. All possible isotopomers corresponding to deuterium substitution at C2, C3 and C10 were observed, and no other site of deuteration was detected. Analysis of the product demonstrated that iridium polyhydrides are likely to be involved, since the results are inconsistent with simpler mechanistic pathways.