Enantioselective 1,3-dipolar cycloaddition of C,N-cyclic azomethine imines to unsaturated nitriles catalyzed by NiII-Pigiphos.

The asymmetric 1,3-dipolar cycloaddition reaction of C,N-cyclic azomethine imines with small unsaturated nitriles using a dicationic Ni(II) complex containing the chiral triphosphine ligand bis{(R)-1-[(Sp)-2-(diphenylphosphino)ferrocenyl]ethyl}cyclohexylphosphine [(R,Sp)-Pigiphos] as a catalyst has been developed. A variety of new chiral cyanopyrazolidines were obtained regio- and diastereoselectively in good to excellent yields with moderate to excellent enantioselectivities. Thus, N-benzoylimino-3,4-dihydro-6-methylisoquinolinium betaine (1a) reacts at RT with acrylonitrile in the presence of 1-5 mol % catalyst to afford 3,4-trans-1-benzoyl-4-cyano-2,3-(tetrahydroisoquinoline)tetrahydropyrazole (2a) in up to 84% yield and 98% ee. The regio- and stereoselectivity were confirmed in the case of compound 2a and 3,4-trans-1-benzoyl-4-cyano-2,3-(6-bromotetrahydroisoquinoline)tetrahydropyrazole (2e) by X-ray crystallography.