Dimerization mechanisms of heterocyclic carbenes

The dimerization reactions of a series of heterocyclic carbenes based on the 1,3-dimethylimidazol-2-ylidene template were studied extensively using molecular orbital calculations. The carbenes studied were the 1,3-XY five-membered ring heterocyclic carbenes with all 10 possible combinations of X,Y = NCH3, PCH3, O and S. Two different mechanisms for dimerization were studied: a direct carbene plus carbene dimerization reaction and a proton-catalysed dimerization. The parent carbene with XY = NN is found to be kinetically and thermodynamically stable under both mechanisms, whereas XY = NS is predicted to be kinetically stable under aprotic conditions. All remaining carbenes were predicted to be not particularly stable towards dimerization. Indirect schemes for calculating the stability of carbenes towards dimerization were found to give good estimates of the enthalpy of dimerization, but do not take into account the activation barriers. Copyright (c) 2004 John Wiley & Sons, Ltd.