Pyridyl Directed Catalyst-Free <i>trans</i>-Hydroboration of Internal Alkynes

We report the first examples of straightforward <i>trans</i>-hydroboration of internal alkynes at room temperature with 9-BBN, producing five-membered BN-heterocycles. Contrary to conventional <i>cis</i>-hydroboration, we demonstrate that the introduction of a pyridyl group switches the stereoselectivity of the reaction. A hydride migration mechanism has been proposed and supported by DFT calculations for the <i>trans</i>-hydroboration. This new hydroboration approach allows facile construction of new blue fluorescent BN-heterocyclic compounds