Silver(I)−Carbene Complexes/Ionic Liquids:  Novel N-Heterocyclic Carbene Delivery Agents for Organocatalytic Transformations

N-Heterocyclic carbene (NHC) complexes with silver were investigated as sources of unsaturated NHC carbene catalysts via thermal decomposition. The NHC complex (1-ethyl-3-methylimidazol-2-ylidene)silver(I) chloride is an ionic liquid, and was found to catalyze the ring-opening polymerization of lactide at elevated temperatures to give narrowly dispersed polylactide of predictable molecular weight. Silver−carbene complexes can also be used for the catalysis of small molecule transesterification reactions. Thermolysis of the silver complexes in the presence of CS₂ yielded the zwitterionic CS₂ adducts of the carbene, implicating the intermediacy of the free carbene in these reactions.