Intramolecular C−H Activation of a Bisphenolate(benzene)-Ligated Titanium Dibenzyl Complex. Competing Pathways Involving α-Hydrogen Abstraction and σ-Bond Metathesis

A titanium dibenzyl complex featuring a ligand with two phenolates
linked by a benzene-1,3-diyl group was found to undergo thermal decomposition
to give toluene and a cyclometalated dimeric complex. The thermal
decomposition followed first-order kinetics and was studied at a number
of temperatures to determine the activation parameters (ΔH⧧= 27.2(5) kcal/mol and ΔS⧧ = −6.2(14)
cal/(mol K)). Deuterated isotopologues were synthesized to measure
the kinetic isotope effects. The complexes with deuterium in the benzyl
methylene positions decomposed more slowly than the protio analogues.
Isotopologues of toluene with multiple deuteration positions were
observed in the product mixtures. These data are consistent with competing
α-abstraction and σ-bond metathesis.