Diboration of alkenes and alkynes with a carborane-fused four-membered boracycle bearing an electron-precise B-B bond.

Small ring compounds are fascinating molecules and have been used as valuable compounds in organic synthesis. In this study, a carborane-fused four-membered boracycle bearing an electron precise B-B bond, 1,2-[BBrSMe]--CBH, was synthesized the reaction of 1,2-Li--carborane with BBr(SMe). This novel boracycle can be used as a "strain-release" compound to achieve diboration of alkenes and alkynes, leading to the generation of ring-expansion products. Interestingly, when bis(trimethylsilyl) acetylene was employed, an allene-functionalized six-membered boracycle was obtained. Moreover, DFT calculations were conducted to shed light on the reaction mechanism.