We and others have previously reported the intermolecularumpolungreactions of Michaelacceptorscatalyzed by an N-heterocycliccarbene (NHC). The representative tail-to-tail dimerization of methyl methacrylate (MMA) has now been intensively investigated, leading to the following conclusions: (1) The catalysis involves the deoxy-Breslow intermediate, which is quite stable and remains active after