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.
The experimental reaction between [1,2,4-(Me(3)C)(3)C(5)H(2)](2)CeCH(2)Ph and CH(3)X, X = F, Cl, Br, and I, yields the metathetical exchange products, [1,2,4-(Me(3)C)(3)C(5)H(2)](2)CeX and CH(3)CH(2)Ph. The reaction is complicated by the equilibrium between the benzyl derivative and the metallacycle [1,2,4-(Me(3)C)(3)C(5)H(2)][(Me(3)C)(2)C(5)H(2)C(CH(3))(2)CH(2)]Ce, plus toluene since the metallacycle reacts with CH(3)X. 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(5)H(5))(2)CeCH(2)Ph and CH(3)F, does not proceed by way of a four-center mechanism, a sigma-bond metathesis, but by a lower barrier process involving a haptotropic shift of the Cp(2)Ce fragment so that at the transition state the para-carbon of the benzene ring is attached to the Cp(2)Ce fragment while the CH(2) fragment of the benzyl group attacks CH(3)F that is activated by coordination to the metal ion. As a result the mechanism is classified as an associative interchange process.
Werkema
Evan L
EL
Department of Chemistry and Chemical Sciences Division of Lawrence Berkeley National Laboratory, University of California, Berkeley, California 94720-1460, USA.
Andersen
Richard A
RA
Maron
Laurent
L
Eisenstein
Odile
O