Cobalt(I)-Catalyzed Reductive Cyclization of Enynes and Diynes Using Hydrogen Gas as a Reductant

Reductive cyclization of enynes and diynes by using H2 gas as a reductant was realized, and the corresponding cyclized products were obtained in good yields without olefin isomerization and over-reduction of the products. By an experiment using D2 instead of H2, it was confirmed that H2 unambiguously operates as a reductant in this reaction. The protocol of the reaction is very economical and user-friendly, using air- and moisture-stable CoBr2·6H2O and 1 atm of pressure of H2.