Nickel-Catalyzed Cyclizative trans-Carboboration of Alkynes through Activation of Boron-Chlorine Bonds by Using Organometallic Reagents as Donors of Organic Groups

Cyclizative alkynylboration and alkenylboration of carbon-carbon triple bonds proceeds in the presence of a nickel catalyst. Chloroboranes bearing homopropargyloxy or bishomopropargyloxy groups were used as substrates with alkynylstannane, vinylstannane, and alkenylzirconium reagents as donors of organic groups. The reaction mechanism may involve the following elementary steps: oxidative addition of the B-Cl bond to a nickel(0) catalyst, insertion of the triple bond into the Ni-B bond, Z-to-E isomerization of the alkenylnickel intermediate, transmetalation from organostannane or organozirconium reagents, and reductive elimination of the B-C bond. An intermediary nickel complex was isolated and identified by a single-crystal X-ray analysis.