The reaction of bis(1,2,4-tri-t-butylcyclopentadienyl)ceriumbenzyl, Cp′_2CeCH_2Ph, with methylhalides: A metathesis reaction that does not proceed by a metathesis transition state

摘要

The experimental reaction between [1,2,4-(Me_3C) _3C_5H_2]_2CeCH_2Ph and CH_3X, X = F, Cl, Br, and I, yields the metathetical exchange products, [1,2,4-(Me_3C)_3C_5H_2] _2CeX and CH_3CH_2Ph. The reaction is complicated by the equilibrium between the benzyl derivative and the metallacycle [1,2,4-(Me_3C)_3C_5H_2][(Me _3C)_2C_5H_2C(CH_3) _2CH_2]Ce, plus toluene since the metallacycle reacts with CH_3X. 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 (C_5H_5) _2CeCH_2Ph and CH_3F, 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 Cp_2Ce fragment so that at the transition state the para-carbon of the benzene ring is attached to the Cp_2Ce fragment while the CH_2 fragment of the benzyl group attacks CH_3F that is activated by coordination to the metal ion. As a result the mechanism is classified as an associative interchange process.