On the Origin of -Selectivity in the Ring-Opening Metathesis Polymerization with Molybdenum Imido Alkylidene 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 -selective ROMP of norborn-2-ene (NBE) with Mo(-2,6-Me-CH)(CHCMe)(IMes)(OTf) (, IMes = 1,3-dimesitylimidazol-2-ylidene) as a first step to stereoselective polymerization. Four different reaction pathways based on an ene or ene approach of NBE to either the - or -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 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 -selectivity and point toward a substrate-specific reaction mechanism in olefin metathesis with neutral Mo imido alkylidene -heterocyclic carbene bistriflate complexes.