Triethoxysilane-Catalyzed Sequential Regioselective Hydroboration of Terminal Alkynes: Sustainable Access to E-Alkenylboronate and Alkyl Gem-Diboronate Esters

The commercial reagent, HSi(OEt)3, was an efficient catalyst for the synthesis of E-alkenylboronate esters and alkyl gem-diboronate esters by single or double hydroboration of terminal alkynes with HBPin (pinacolborane). The reaction time controlled whether a single or a double addition of HBPin to the alkynes occurred. Aromatic terminal alkynes containing strong electron-donating or -withdrawing substituents at different positions as well as aliphatic alkynes were efficiently mono- and dihydroborated. Mechanistic studies suggested that the formation of diboronate esters proceeds by a double hydroboration with the second hydroboration cycle being the rate determining. The reaction of HSi(OEt)3 with HBPin has been identified as a catalyst deactivation pathway operative in catalysis. Screening of other silicon compounds for this transformation supported that the steric and electronic profile of HSi(OEt)3 is key to promote the second hydroboration step.