Abstract

DFT (M06-L) calculations have been used to determine the relative stabilities of the metallacyclobutane intermediates arising from the cross-metathesis reactions of terminal olefins as well as to get insights into the origin of the nondetection of the a,beta-substituted species. For that, we discuss the structures, NMR signatures, stabilities with respect to separated reactants, and experimentally proposed interconversion pathways of all potential metallacyclobutane intermediates arising from propene and styrene homocoupling. For the case of propene, the unsubstituted and mono- and disubstituted metallacycles are lower in Gibbs energy than the separated reactants under the NMR experimental conditions. Moreover, for the same number of substituents, regardless of their nature, the metallacycles presenting substituents at the Ca carbons are always lower in energy than those presenting substituents at C beta, the energy difference being between 1.7 and 8.8 kcal mol1. The computed energy barriers associated with the olefin and carbene rotation processes, two of the experimentally proposed pathways for the metallacycle interconversion, are low and are in excellent agreement with the values previously determined through NMR studies. Cycloaddition and cycloreversion energy barriers are also low, and in fact, there is not a significant difference between the barrier heights of the processes leading to observed or nonobserved intermediates. Therefore, the nondetection of metallacyclobutane intermediates with substituents in C beta seems to arise from their lower stability in comparison with the isomers with substituents in Ca, which makes their detection not feasible under thermodynamic equilibrium conditions. That is, for cross-metathesis processes involving small terminal alkenes and activated carbenes, the nature of the observed metallacycles is based on thermodynamic control. The preference of having the substituents in Ca is attributed to the formation of stronger MC and CC bonds during the cycloaddition when the substituents are in an a position due to higher charge transfer from the original alkene fragment to the metal carbene.