Computational studies of the mechanistic aspects of olefin metathesis reactions involving metal oxo-alkylidene complexes

Abstract

The reactions of Cl 2(O)MCH 2(M = Cr, Mo, W, Ru, Re) with ethylene, models of potential chain-carrying catalysts and olefins respectively in the metal-catalyzed olefin reaction, have been studied using hybrid density functional theory at the B3LYP/LACVP * and MO6/LACVP * levels of theory. It was found that formation of the metallacyclobutane, a key intermediate in the olefin metathesis reaction according to the Herrison–Chauvin mechanism, is a low-barrier process for each of the complexes studied, the highest barrier occurring in Ru (13.78 kcal/mol by B3LYP calculations and 4.74 kcal/mol by MO6 calculations) and the lowest barrier occurring in W (0.38 kcal/mol by B3LYP and 0.28 kcal/mol by MO6). However, for M = Cr the [3+2] addition of the olefinic bond of ethylene across the Cr O and Cr C bonds of the complex has a lower activation barrier than the metallacyclobutane formation step. Since the potential chain-terminating side reaction is more feasible than the metallacyclobutane formation step, Cl 2(O)CrCH 2 may not promote olefin metathesis. Also, for M = Ru the activation barrier of the [3+2] addition of ethylene across the Ru O and Ru Cl bonds is 3.61 kcal/mol lower than the barrier of the metallacyclobutane formation step, at the B3LYP level of theory, suggesting that Cl 2(O)RuCH 2 may not promote olefin metathesis. However, the MO6 results indicate that the barrier for metallacyclobutane formation is 3.55 kcal/mol lower than that for the [3+2] addition across the Ru O and Ru Cl bonds, suggesting that Cl 2(O)RuCH 2 may catalyze metathesis. At either level of theory, the difference in activation barriers between the productive [2+2] route and the [3+2] route is not high enough to preclude either pathway from manifesting. Thus, metallacyclobutane formation may occur in the Ru complex, albeit very unselectively. However, the metallacyclobutane intermediate formed is very stable (31.45 kcal/mol and 42.57 kcal/mol by B3LYP and MO6 respectively) and may not easily undergo cycloreversion to form the final product, i.e. a dead end in the catalytic cycle. For M = Mo, W, and Re, the metallacyclobutane formation pathway is more favorable than the potential side reactions, the differences between the productive [2+2] pathway and the lowest-barrier side reactions being 19.30, 25.87, 3.54 kcal/mol respectively at the B3LYP level and 13.82, 18.81 and 4.06 kcal/mol respectively at the MO6 level. Thus, the selectivity of the metallacyclobutane formation, which is marginal in Re, is in the order: W > Mo > Re. However, the metallacyclobutane formed in Re may be too stable to allow cycloreversion to form the final product. Thus metathesis is predicted to occur in Mo and W but not in Cr, Ru or Re.