The reaction of bis(1,2,4-tri-t-butylcyclopentadienyl)ceriumbenzyl, Cp′2CeCH2Ph, with methylhalides: a metathesis reaction that does not proceed by a metathesis transition state

Abstract

The experimental reaction between [1,2,4-(Me₃C)₃C₅H₂]₂CeCH₂Ph and CH₃X, X = F, Cl, Br, and I, yields the metathetical exchange products, [1,2,4-(Me₃C)₃C₅H₂]₂CeX and CH₃CH₂Ph. The reaction is complicated by the equilibrium between the benzyl derivative and the metallacycle [1,2,4-(Me₃C)₃C₅H₂][(Me₃C)₂C₅H₂C(CH₃)₂CH₂]Ce, plus toluene since the metallacycle reacts with CH₃X. Labelling studies show that the methyl group of the methylhalide is transferred intact to the benzyl group. The mechanism, as revealed by DFT calculations on (C₅H₅)₂CeCH₂Ph and CH₃F, does not proceed by way of a four-center mechanism, a σ-bond metathesis, but by a lower barrier process involving a haptotropic shift of the Cp₂Ce fragment so that at the transition state the para-carbon of the benzene ring is attached to the Cp₂Ce fragment while the CH₂ fragment of the benzyl group attacks CH₃F that is activated by coordination to the metal ion. As a result the mechanism is classified as an associative interchange process.