Transition metal-diene complexes. Part II. Isomerization of rhodium-complexed penta-1,4- and cis-penta-1,3-dienes

Several 1,4-dienes and cis-methyl-substituted 1,3-dienes co-ordinated to RhI in the complexes [(diene)RhX][X =η-cyclopentadienyl (cp), pentane-2,4-dionate (pd), or bridging Cl] undergo virtually quantitative isomerization when heated in inert solvents. Co-ordinated cis-penta-1,3-diene and cis-3-methylpenta-1,3-diene yield complexes of the trans-isomers as sole products. Co-ordinated cyclohexa-1,4-diene gives co-ordinated cyclo-hexa-1.3-diene; a 1,3-hydrogen shift has been demonstrated for this reaction by means of deuterium labelling. Co-ordinated penta-1.4-diene and 3-methylpenta-1.4-diene isomerizeto the corresponding co-ordinated cis- and trans-1,3-isomers at equal rates, the cis product subsequently converting into the trans in a secondary process. First-order rate constants and activation parameters for some of the reactions in o-dichlorobenzene are derived. Reaction rates increase with increase in the electron-withdrawing nature of the counter ligand X. The results are examined in relation to current theories concerning the mechanism of metal-assisted alkene isomerization. A new mechanism is proposed in which the rate-determinjng step is formation of a symmetrical σ-allyl complex, this being a common transition state for the isomerization of the acyclic 1,4-dienes and the η-σ-η interconversipn of anti- and syn-η allys the key step in the isomerization of cis to trans-1,3-dienes. An important conclusion which probably has general significance is that the conversion of an η-alkene to a η-allyl complex involves prior formation of an σ-allyl complex.