The first nickel(0)-catalyzed [2 + 2 + 1] carbonylative cycloaddition reaction of imines and alkynes or norbornene has been achieved by employing phenyl formate as a CO source. With this method, a variety of N-benzenesulfonyl, -tosyl, and -phosphoryl-substituted γ-lactams can be prepared in good to high yields.
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.
Key Intermediates. -In the presence of Ni(cod) 2 and IMes or IPr, various N-aryl imines are found to undergo [2 + 2 + 2] cycloaddition with internal alkynes to afford dihydropyridines in good to high yields. Additionally, it is shown that the reaction of the phosphinic imine (IX) is successfully catalyzed by Ni (cod)2/PCy3. -(HOSHIMOTO, Y.; OHATA, T.; OHASHI, M.; OGOSHI*, S.; Chem. -Eur. J. 20 (2014) 14, 4105-4110, http://dx.
Nickel(0)-Catalyzed [2 + 2 + 1] Carbonylative Cycloaddition of Imines and Alkynes or Norbornene Leading to -Lactams. -A key feature of the process is the use of phenyl formate as a CO source that enables carbonylation and prevents quenching of the catalyst. -(HOSHIMOTO, Y.; OHATA, T.; SASAOKA, Y.; OHASHI, M.; OGOSHI*, S.; J. Am. Chem. Soc. 136 (2014) 45, 15877-15880, http://dx.
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