Synthesis of trans‐Disubstituted Alkenes by Cobalt‐Catalyzed Reductive Coupling of Terminal Alkynes with Activated Alkenes

Abstract A cobalt‐catalyzed reductive coupling of terminal alkynes, RCCH, with activated alkenes, R′CHCH 2 , in the presence of zinc and water to give functionalized trans ‐disubstituted alkenes, RCHCHCH 2 CH 2 R′, is described. A variety of aromatic terminal alkynes underwent reductive coupling with activated alkenes including enones, acrylates, acrylonitrile, and vinyl sulfones in the presence of a CoCl 2 /P(OMe) 3 /Zn catalyst system to afford 1,2‐ trans ‐disubstituted alkenes with high regio‐ and stereoselectivity. Similarly, aliphatic terminal alkynes also efficiently participated in the coupling reaction with acrylates, enones, and vinyl sulfone, in the presence of the CoCl 2 /P(OPh) 3 /Zn system providing a mixture of 1,2‐ trans‐ and 1,1‐disubstituted functionalized terminal alkene products in high yields. The scope of the reaction was also extended by the coupling of 1,3‐enynes and acetylene gas with alkenes. Furthermore, a phosphine‐free cobalt‐catalyzed reductive coupling of terminal alkynes with enones, affording 1,2‐ trans ‐disubstituted alkenes as the major products in a high regioisomeric ratio, is demonstrated. In the reactions, less expensive and air‐stable cobalt complexes, a mild reducing agent (Zn) and a simple hydrogen source (water) were used. A possible reaction mechanism involving a cobaltacyclopentene as the key intermediate is proposed.