Reaction Mechanism of Ring-Closing Metathesis with a Cationic Molybdenum Imido Alkylidene N-Heterocyclic Carbene Catalyst

DFT calculations were carried out to explore all relevant pathways for the ring-closing metathesis (RCM) reaction of a cationic molybdenum imido alkylidene N-heterocyclic carbene (NHC) catalyst with α,ω-dienes. Besides the catalytic cycle, which produces the desired cycloalkene in two consecutive metathesis steps, the initiation, a nonproductive cycle, and a degenerative catalytic path were also investigated. On the basis of the approaching face of the diene, two further possibilities were considered for all pathways: syn-and anti-addition. Steric repulsion on the metallacyclobutane ring of the trigonal-bipyramidal (TBP) intermediate appears to be one of the important influencing factors. We found the nonproductive cycle to compete with the catalytic conversion of the substrate. The TBP intermediate formed during the catalytic cycle is energetically lower than the corresponding TBP intermediate of the nonproductive cycle. However, the barriers for the nonproductive cycle are slightly lower than the barrier for the catalytic cycle. Thus, the nonproductive cycle can be expected to be used during catalysis, but it will only have a weak detrimental effect on the turnover rate because it is faster than the catalytic cycle. Of the four different initiation paths, the α-anti-addition path was found to be the energetically most favorable one. Notably, the catalyst was found to be fairly stable against degradation via β-hydride (β-H) transfer to the metal. High activation barriers for the conversion of the TBP intermediate to a square-pyramidal (SP)-based intermediate and a higher energy of a later intermediate suggest a stability of catalyst against degradation. A very high barrier for β-H transfer also disfavors the degradation reaction. The 1H NMR spectrum of the reaction between [Mo(N-2,6-Me2-C6H3)(CHCMe2Ph)(IMes)(OCH(CF3)2)+B(ArF)4–] and 1,7-octadiene confirms the absence of any β-H transfer.