Boric acid as a precatalyst for BH 3 -catalyzed hydroboration.

We report that boric acid, BO 3 H 3 , is a good precatalyst for the BH 3 -catalyzed hydroboration of esters using pinacolborane as a borylation agent. Using microwave irradiation as an energy source, we demonstrated that a dozen esters were converted into the corresponding boronate ethers in good yields. It was also possible to use boric acid as a precatalyst to reduce carbonates and alkynes. Considering the hazardous and pyrophoric nature of BH 3 solutions, boric acid proves to be a safe and green precatalyst for the metal-free reduction of unsaturated species.