Transition-metal–diene complexes. Part 3. Isomerization of methyl-substituted acyclic and cyclic 1,3- and 1,4-dienes co-ordinated to rhodium(I)

Abstract

A range of methyl-substituted acyclic and cyclic 1,4-dienes co-ordinated ton Rh^I in the complexes [Rh(cp)(diene)](cp = cyclopentadienyl) undergo virtual quantitative isomerization when heated in non-protic solvents. Reactants and products have been characterized by chemical and physical methods, particularly by ¹H n.m.r. spectroscopy. Reaction products are exclusively conjugated dienes and with one exception, cis,cis-hexa-2,4-diene, are co-ordinated to Rh^I. For the case of the acyclic 1,4-dienes the primary isomerizations, generally to mixtures of cis- and trans-1,3-isomers, are followed by a slower isomerization of the cis-primary product dienes to the trans isomers. For the isomerization of the cyclohexa-1,4-dienes, mixtures of 1,3-isomers are generally obtained in a primary process only. First-order rate constants for the decay of reactant 1,4-diene are reported. From an analysis of the kinetics of the reactions and of product distributions, a mechanism involving a (η³-allyl) hydrido-intermediate is proposed for the primary isomerizations in which the first step is the dissociation of one double bond from the metal. For the secondary isomerization of cis- to trans- 1,3-dienes a η³–η¹–η³ interconversion of anti and syn configurations is suggested as the key step. The primary reactions are shown to be under kinetic control. However, addition of an external proton source leads to rapid formation of the thermodynamically most stable products via a metal hydride addition–elimination mechanism.