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Title: Computational studies of the mechanistic aspects of olefin metathesisreactions involving metal oxo-alkylidene complexes
Authors: Tia, Richard
Adei, Evans
Keywords: Metathesis
Density functional theory
Issue Date: 15-Sep-2011
Publisher: Elsevier B.V.
Citation: R. Tia, E. Adei/Computational and Theoretical Chemistry 971 (2011) 8–18. Computational studies of the mechanistic aspects of olefin metathesisreactions involving metal oxo-alkylidene complexes. doi:10.1016/j.comptc.2011.05.032
Abstract: The reactions of Cl2(O)MCH2(M = Cr, Mo, W, Ru, Re) with ethylene, models of potential chain-carryingcatalysts and olefins respectively in the metal-catalyzed olefin reaction, have been studied using hybriddensity functional theory at the B3LYP/LACVP and MO6/LACVP levels of theory. It was found that for-mation of the metallacyclobutane, a key intermediate in the olefin metathesis reaction according tothe Herrison–Chauvin mechanism, is a low-barrier process for each of the complexes studied, the highestbarrier occurring in Ru (13.78 kcal/mol by B3LYP calculations and 4.74 kcal/mol by MO6 calculations) andthe lowest barrier occurring in W (0.38 kcal/mol by B3LYP and 0.28 kcal/mol by MO6). However, forM = Cr the [3+2] addition of the olefinic bond of ethylene across the CrAO and CrAC bonds of the complexhas a lower activation barrier than the metallacyclobutane formation step. Since the potential chain-terminating side reaction is more feasible than the metallacyclobutane formation step, Cl2(O)CrCH2may not promote olefin metathesis. Also, for M = Ru the activation barrier of the [3+2] addition of ethyl-ene across the RuAO and RuACl bonds is 3.61 kcal/mol lower than the barrier of the metallacyclobutaneformation step, at the B3LYP level of theory, suggesting that Cl2(O)RuCH2may not promote olefin metath-esis. 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 RuAO and RuACl bonds, suggesting that Cl2(O)R-uCH2may catalyze metathesis. At either level of theory, the difference in activation barriers betweenthe productive [2+2] route and the [3+2] route is not high enough to preclude either pathway from man-ifesting. 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 prod-uct, i.e. a dead end in the catalytic cycle. For M = Mo, W, and Re, the metallacyclobutane formation path-way 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 theB3LYP level and 13.82, 18.81 and 4.06 kcal/mol respectively at the MO6 level. Thus, the selectivity ofthe metallacyclobutane formation, which is marginal in Re, is in the order: W > Mo > Re. However, themetallacyclobutane 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.
Description: An article published by Elsevier B.V. and also available at doi:10.1016/j.comptc.2011.05.032
URI: http://hdl.handle.net/123456789/12679
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