Copper-Catalyzed Metallation Reactions of Unsaturated Carbon-Carbon Bonds

A copper complex has proven to be an excellent catalyst for diborylation of alkynes and arynes. Under the copper catalysis, the reaction using propargyl ethers affords tri- or tetraborylation products, in which the C-O bonds as well as the C-C triple bonds are borylated all in one pot. A key intermediate of the reaction, a beta-borylalkenylcopper species, has been also found to be captured by a tin or a carbon electrophile, leading to three-component borylstannylation or carboboration. Regioselective installation of a boryl moiety into an internal carbon of terminal alkynes has been achieved by use of a masked diboron as a boron source in hydroboration and borylstannylation. In addition, a copper complex has been demonstrated to efficiently catalyze distannylation, hydrostannylation and silylstannylation. Total synthesis of biologically active molecules based on the metallation reactions has also been reported.