Activation\nof Methyltrioxorhenium for Olefin Metathesis\nby a Frustrated Lewis Pair

Methyltrioxorhenium (MTO) supported\non Al<sub>2</sub>O<sub>3</sub> or SiO<sub>2</sub>–Al<sub>2</sub>O<sub>3</sub> is an efficient\nheterogeneous alkene metathesis catalyst that works at room temperature\nand tolerates various functional groups. Surface studies found that\nMTO interacts with highly Lewis-acidic aluminum centers and that its\nmethyl group is probably C–H activated resulting in rhenium-methylidene\nspecies. The exact structure of the catalyst resting state and the\nactive species is subject to scientific debate. Here, we report on\nthe activation of MTO by 2,6-lutidine and tris­(pentafluorophenyl)­borane\n(B­(C<sub>6</sub>F<sub>5</sub>)<sub>3</sub>), a frustrated Lewis pair\n(FLP) in solution. The MTO/FLP catalyst is active in ring-opening\nmetathesis polymerization of norbornene and in cross-metathesis of\ninternal olefins under mild conditions. ESI-MS and NMR studies found\nthat MTO is deprotonated in the presence of the FLP to yield a rhenium-methylidene\nspecies. While this initially activated methylidene eluded detection,\nspraying reaction mixtures with structurally constrained olefins in\nESI-MS allowed for the detection of on-cycle rhenium-alkylidene species.\nTime-course measurements showed that the modest catalytic activity\ncould be attributed to a rapid catalyst deactivation step. One possible\ndeactivation pathway was identified to be a second deprotonation step\nof the metathesis-active methylidene, yielding a rhenium-methylidyne.\nKinetic experiments have shown that it can be reactivated for olefin\nmetathesis by protonation in solution. Additionally, several irreversible\ncatalyst deactivation pathways leading to permanently deactivated\ncatalyst species are hypothesized. We propose that the MTO/FLP system\nconstitutes a homogeneous model system for the heterogeneous MTO catalysts.