On the mechanism of homogeneous alkene metathesis : a computational study

Abstract

A mechanism for alkene metathesis has been proposed by Chauvin, wherein metal carbenes act as catalysts for the reaction. The use and discovery of Fischer-, Tebbe-, Grubbs- and Schrock-type metal carbenes have to a certain extent proven the general mechanism. These metal carbenes showed different activity for alkene metathesis. Only Grubbs- and Schrock-type carbenes proved to be highly active for metathesis. A lot of studies have been done on the reasons for the activity, but still the main factors are unknown. In this study a molecular modelling investigation into the mechanism of the alkene metathesis reaction is done in an attempt to identify a factor(s) that can predict activity. By defining and knowing factors that contribute to activity, new catalysts can be designed that are truly active and selective. Fischer-, Tebbe-, Grubbs- and Schrocktype metal carbenes are investigated in this regard. The results of the investigation indicate that the frontier molecular orbital theory shows a possibility for prediction of alkene metathesis activity. By observing the size and location of the atomic orbital coefficients of the molecular orbital, the site of primary overlap for formation of metathesis products could be identified. The largest atomic orbital coefficient of the LUMO should be located on the metal atom. An atomic orbital coefficient should also be present on the carbene carbon for secondary overlap for formation of the metallacyclobutane intermediate. By exchanging the ruthenium in the second generation Grubbs catalyst framework the effect of the metal could be elucidated. The results clearly showed the important influence the metal atom has on the electronic properties of the catalyst complex. The results of frontier molecular orbital calculations supported the general activity trend of the four main types of metal carbenes for the metathesis of linear alkenes. By changing the metal in known catalyst frameworks a deeper understanding can be gained for the design of new alkene metathesis catalysts.