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

Triethoxysilane was found an efficient catalyst for the synthesis of E-alkenyl- and alkyldi-boronate esters by the single and sequential hydroboration of terminal alkynes respectively with pinacolborane. Mechanistic studies support that the formation of diboronate esters proceeds by a double hydroboration pathway with the steric and electronic profile at Si being key to enable the second hydroboration step. Weak non-covalent interactions involving the Si and the C≡C or C=C bonds in the alkynes or alkenylboronate esters have been identified as responsible for substrate activation toward the addition of HBPin, and the reaction of HSi(OEt)3 with HBPin as a catalyst deactivation pathway.