Nickel-catalyzed [2+2+2] cycloaddition of two alkynes and an imine

Abstract

The reaction of N-benzenesulfonylbenzaldimine with Ni(cod)2 and PCy3 gave the corresponding η2-iminenickel complex quantitatively. Diphenylacetylene reacted with the η2-iminenickel complex to generate five-membered aza-nickelacycle. Insertion of second alkynes into the five-membered aza-nickelacycle led to the formation of the corresponding seven-membered aza-nickelacycles. Heating the solution of the seven-membered aza-nickelacycles induced the reductive elimination to give 1,2-dihydropyridine. In the presence of 10 mol % of Ni(cod)2 and PMetBu2 at 100 °C, the intermolecular [2+2+2] cycloaddition of N-benzenesulfonylbenzaldimine and 2-butyne occurred to give the expected 1,2-dihydropyridine in 87 % yield. In the presence of PCy3, the reaction also proceeded catalytically, however, PMetBu2 gave better results. Less bulky or less basic phosphine, PnBu3 or P(o-tol)3, was not efficient for the reaction. Although Ni(0)-NHC complex was a good catalyst for [2+2+2] cycloaddition of two alkynes and a ketone or an aldehyde, this reaction did not proceed in the presence of an NHC ligand, 1,3-bis-(2,4,6-trimethylphenyl)imidazol-2-ylidene.