Rhodium-Catalyzed Atroposelective [2+2+2] Cycloaddition of Ortho-Substituted Phenyl Diynes with Nitriles: Effect of Ortho Substituents on Regio- and Enantioselectivity

Axially chiral 3-(2-halophenyl)pyridines were successfully synthesized in high yields with excellent enantioselectivity by the cationic rhodium(I)/(S)-H-8-BINAP complex-catalyzed atroposelective [2 + 2 + 2] cycloaddition of (o-halophenyl)diynes with nitriles. Interestingly, regio- and enantioselectivity highly depend on ortho substituents on the phenyl group of diynes. When the ortho substituents were methoxy and methoxycarbonyl groups, axially chiral 3-arylpyridines were obtained as a major product, while enantioselectivity was lowered significantly. On the other hand, when the ortho substituents were alkyl groups, regioselectivity was switched to give achiral 6-arylpyridines in high yields.