Nickel-catalyzed synthesis of N-aryl-1,2-dihydropyridines by [2+2+2] cycloaddition of imines with alkynes through T-shaped 14-electron aza-nickelacycle key intermediates.

Despite there being a straightforward approach for the synthesis of 1,2-dihydropyridines, the transition-metal-catalyzed [2+2+2] cycloaddition reaction of imines with alkynes has been achieved only with imines containing an N-sulfonyl or -pyridyl group. Considering the importance of 1,2-dihydropyridines as useful intermediates in the preparation of a wide range of valuable organic molecules, it would be very worthwhile to provide novel strategies to expand the scope of imines. Herein we report a successful expansion of the scope of imines in nickel-catalyzed [2+2+2] cycloaddition reactions with alkynes. In the presence of a nickel(0)/PCy3 catalyst, a reaction with N-benzylidene-P,P-diphenylphosphinic amide was developed. Moreover, an application of N-aryl imines to the reaction was also achieved by adopting N-heterocyclic carbene ligands. The isolation of an (η(2)-N-aryl imine)nickel(0) complex containing a 14-electron nickel(0) center and a T-shaped 14-electron five-membered aza-nickelacycle is shown. These would be considered as key intermediates of the reaction. The structure of these complexes was unambiguously determined by NMR spectroscopy and X-ray analyses.