Mechanistic Studies of Alkene Isomerization Catalyzed by CCC-Pincer Complexes of Iridium

Iridium complexes containing CCC-pincer m-phenylene-bridged
N-heterocyclic carbene ligands were examined as catalysts for alkene
isomerization. Complexes containing either mesityl or adamantyl side
groups were found to catalyze the isomerization of a number of alkenes
to the internal isomers, including 1-octene, vinylcyclohexane, and
allylbenzene. Mechanistic studies indicate a surprising dichotomy,
apparently caused by ligand steric effects. For the mesityl-substituted
catalyst, several lines of evidence provide strong support for isomerization
via an iridium allyl hydride intermediate: (1) H–D crossover
experiments indicate that 1,3-hydrogen migration is exclusively intramolecular,
(2) the catalyst resting state, a π-allyl hydride species, was
isolated and serves as a kinetically competent catalyst, (3) NMR experiments
indicate that the π-allyl hydride resting state undergoes reversible
C–H reductive elimination that is rapid relative to catalytic
turnover, and (4) kinetic studies indicate that the isomerization
reaction is first order in substrate and catalyst, consistent with
turnover-limiting ligand substitution. H–D crossover experiments
for alkene isomerization catalyzed by the adamantyl-substituted complex
show selectivity for a 1,3-deuterium shift, as well as the intermolecular
transfer of hydrogen. These results are consistent with an insertion/elimination
mechanism proceeding selectively through a secondary metal–alkyl
or with a π-allyl-type mechanism with an unknown pathway for
intermolecular hydrogen crossover.