N-Heterocyclic olefin catalysis for the ring opening of cyclic amidine compounds: a pathway to the synthesis of ε-caprolactam and γ-lactam-derived amines.

For the first time, 1,2-dimethyl-3-ethylimidazolium iodide (1a) catalyzes the ring opening of the bicyclic amidine system of DBU (1,8-diazabicyclo[5.4.0]undec-7-ene) or DBN (1,5-diazabicyclo[4.3.0]non-5-ene) on reaction with aldehydes. The mechanism here proposed involves a N-heterocyclic olefin (NHO) catalytic species that acts as a nucleophile to promote the cyclic amidine ring opening. The resulting ε-caprolactam and γ-lactam-derived imines were obtained in moderate to excellent yields (28-99%) and reduced to the corresponding amines by sodium borohydride. Confirmation of the imine product was achieved via single crystal X-ray diffraction studies.