Triethoxysilane-Catalyzed Hydroboration of Alkenes, Nitriles, Ketones and Esters: From Non-Covalent Interactions to a Hydrosilylation/Si-B Exchange Pathway

Triethoxysilane was found an efficient precatalyst for the hydroboration of alkenes, nitriles, ketones and esters with pinacolborane (HBPin). The catalyst showed an excellent substrate scope including aromatic and aliphatic substrates affording the products in good to excellent yields. The system was found less efficient for the hydroboration of alkenes and Ealkenylboronate esters than that of alkynes, which could be traced back to the strength of the interactions between the multiple CC bonds of the substrates with HSi(OEt)3. The nature of the multiple CC bond (C=C vs C≡C) played the largest role in catalyst efficiency, the presence of a BPin substituent in the E-alkenylboronate esters not impacting the substrate conversion compared to that of terminal alkenes. The nature of aryl substituents in aromatic substrates exerted distinct effects on catalytic efficiency depending on the substrate class. Whereas the catalyst was sensitive to both the electronic and steric properties of terminal alkynes, catalytic efficiency in the hydroboration of aromatic nitriles was governed primarily by steric effects, with minimal sensitivity to electronic variation. Competition experiments allowed to unravel the catalyst preferences for the hydroboration of different types of functional groups, HSi(OEt)3 showing a significant preference for alkynes and alkenes over nitriles and ketones. Unlike in the HSi(OEt)3-catalyzed sequential hydroboration of alkynes, the fluorinated analog HSi(OCH2CF3)3 showed a comparable efficiency to HSi(OEt)3 for the hydroboration of nitriles, suggesting that the Lewis acidity at Si did not play a role in catalyst efficiency. The significant differences between alkynes and heteroatom-containing substrates could be traced back to the presence of a catalytically active species different from HSi(OEt)3 in the hydroboration of nitriles, ketones and esters. Additionally, intermediates from hydrosilylation of the multiple bond of nitriles were identified during catalytic turnover, supporting a mechanism involving H-Si bond addition across the multiple bond followed by Si-B exchange steps.