On the Origin ofE-Selectivity in the Ring-Opening Metathesis Polymerization with Molybdenum Imido AlkylideneN-Heterocyclic Carbene Complexes

The understanding and control of stereoselectivity is a central aspect in ring-opening metathesis polymerization (ROMP). Herein, we report detailed quantum chemical studies on the reaction mechanism of E -selective ROMP of norborn-2-ene (NBE) with Mo( N -2,6-Me 2 -C 6 H 3 )(CHCMe 3 )(IMes)(OTf) 2 ( 1 , IMes = 1,3-dimesitylimidazol-2-ylidene) as a first step to stereoselective polymerization. Four different reaction pathways based on an ene syn or ene anti approach of NBE to either the syn - or anti -isomer of the neutral precatalyst have been studied. In contrast to the recently established associative mechanism with a terminal alkene, where a neutral olefin adduct is formed, NBE reacts directly with the catalyst via [2 + 2] cycloaddition to form molybdacyclobutane with a reaction barrier about 30 kJ mol –1 lower in free energy than via the formation of a catalyst–monomer adduct. However, the direct cycloaddition of NBE was only found for one out of four stereoisomers. Our findings strongly suggest that this stereoselective approach is responsible for E -selectivity and point toward a substrate-specific reaction mechanism in olefin metathesis with neutral Mo imido alkylidene N -heterocyclic carbene bistriflate complexes.