Cationic molybdenum imido alkylidene N‐heterocyclic carbene complexes confined in mesoporous silica : tuning transition states towards Z‐selective ring‐opening cross‐metathesis

Abstract

We recently reported a method for selective macro(mono)cyclization of dienes utilizing catalysts confined inside the pores of mesoporous silica, which we believe occurs due to suppression of oligomerization due to pore size. We hypothesized, however, that the system of cationic molybdenum imido alkylidene N‐heterocyclic carbene (NHC) catalysts immobilized selectively inside the mesopores of silica materials could address much more subtle selectivity differences, such as E/Z selectivity in ring‐opening/cross‐metathesis (ROCM). Upon investigation, we observed that surface‐bound cationic molybdenum imido alkylidene NHC catalysts indeed display an increased Z‐selectivity, especially during the early stages of the reaction. This effect was present when the catalyst was confined inside a pore, as well as when the catalyst was bound to non‐porous silica, which led us to conclude it is an effect caused by the catalyst being bound directly to the surface of a silica material where the proximity of the catalyst to the surface governs the transition state. Kinetic investigations revealed that significant post‐metathesis olefin isomerization occurs, the amount of which seems to be governed by the rate of diffusion of the product away from the active catalyst, with smaller pore sizes resulting in higher Z‐selectivity at higher conversion, attributable to faster diffusion of the product out of the pore than diffusion back into the pore.