Activation of Methyltrioxorhenium for Olefin Metathesis by a Frustrated Lewis Pair

Methyltrioxorhenium (MTO) supported
on Al2O3 or SiO2–Al2O3 is an efficient
heterogeneous alkene metathesis catalyst that works at room temperature
and tolerates various functional groups. Surface studies found that
MTO interacts with highly Lewis-acidic aluminum centers and that its
methyl group is probably C–H activated resulting in rhenium-methylidene
species. The exact structure of the catalyst resting state and the
active species is subject to scientific debate. Here, we report on
the activation of MTO by 2,6-lutidine and tris­(pentafluorophenyl)­borane
(B­(C6F5)3), a frustrated Lewis pair
(FLP) in solution. The MTO/FLP catalyst is active in ring-opening
metathesis polymerization of norbornene and in cross-metathesis of
internal olefins under mild conditions. ESI-MS and NMR studies found
that MTO is deprotonated in the presence of the FLP to yield a rhenium-methylidene
species. While this initially activated methylidene eluded detection,
spraying reaction mixtures with structurally constrained olefins in
ESI-MS allowed for the detection of on-cycle rhenium-alkylidene species.
Time-course measurements showed that the modest catalytic activity
could be attributed to a rapid catalyst deactivation step. One possible
deactivation pathway was identified to be a second deprotonation step
of the metathesis-active methylidene, yielding a rhenium-methylidyne.
Kinetic experiments have shown that it can be reactivated for olefin
metathesis by protonation in solution. Additionally, several irreversible
catalyst deactivation pathways leading to permanently deactivated
catalyst species are hypothesized. We propose that the MTO/FLP system
constitutes a homogeneous model system for the heterogeneous MTO catalysts.