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.