Enantioselective reductive cyclization of 1,6-enynes via rhodium-catalyzed asymmetric hydrogenation: C-C bond formation precedes hydrogen activation

Asymmetric hydrogenation of 1,6-enynes using chirally modified cationic rhodium precatalysts enables enantioselective reductive cyclization to afford alkylidene-substituted carbocycles and heterocycles in a completely atom economical fashion. Good to excellent yields and exceptional levels of asymmetric induction are observed across a structurally diverse set of substrates. Mechanistic studies involving hydrogen-deuterium crossover experiments, along with the observance of nonconjugated cycloisomerization products 14c and 15c, suggest rhodium(III) metallocyclopentene formation occurs in advance of hydrogen activation. This oxidative coupling-hydrogenolytic cleavage motif should play a key role in the design of related hydrogen-mediated couplings.