Density functional theory studies of the mechanistic aspects of olefinmetathesis reactions

Abstract

The potential energy surfaces of the reactions of Cl4MCH2(M=Cr, Mo, W, Ru, Re) with ethylene,models of potential chain-carrying catalysts and olefins respectively in the transition metal-catalyzedolefin metathesis reaction, have been explored using density functional theory at the B3LYP/LACVP*level of theory. In Cl4MCH2(M=Cr, Ru), the carbenoid complexes Cl3MCH2–Cl have been found tobe more stable than the corresponding carbene Cl4M=CH2complexes whereas in Cl4MCH2(M=Mo,W, Re) the carbene complexes are more stable than the carbenoid complexes. The carbenoid complexeshave been found not to favor the formation of metallacyclobutanes, a key step in the olefin metathesisreaction according to the Herrison–Chauvin mechanism, indicating that the active species for themetathesis reaction is a carbene complex and not a carbenoid complex. Therefore, even though theformation of metallacyclobutanes through formal [2+2] cycloaddition has been found to be alow-barrier process with each of the metal carbene complexes investigated, metathesis is likely to occuronly in Cl4MCH2(M=Mo, W, Re) but not in Cl4MCH2(M=Cr, Ru) since the reaction surface in thelatter complexes is likely to be populated by the carbenoid complex rather than the carbene complex. InCl4W=CH2the kinetic and thermodynamic preference of the productive [2+2] pathway leading to themetallacyclobutane over the [3+2] pathway is unambiguous whereas in Cl4MoCH2the [3+2] pathway islikely to be competitive with the [2+2] pathway. The metallacyclobutane formation in W and Re hasbeen found to have lower barriers than in Mo, suggesting that the W and Re complexes may have agreater metathesis activity than the Mo complex