Hydroboration of alkynes initiated by sodium triethylborohydride

Hydroboration of terminal and internal alkynes with pinacolborane promoted by sodium triethylborohydride is reported. The optimised process requires no additional solvent and yields vinylboronate esters in yields of up to 99%, starting from both aromatic and aliphatic alkynes. [Display omitted] Sodium triethylborohydride, a popular hydride transferring reagent, is commonly used as an activating agent for hydroboration catalysts based on the first-row transition metal complexes. It has been found that NaHBEt3 can be a selective catalyst for hydroboration of terminal alkynes with pinacolborane. Hydroboration of aromatic and aliphatic alkynes in the presence of 10 mol% of NaHBEt3 proceeded in a highly selective manner to give (E)-vinylboronate esters with high yields, whereas ethynylsilanes seem to be less reactive in this process. Internal alkynes can also be transformed into corresponding vinylboronate esters, however, this process require prolonged reaction times compared to their terminal counterparts.