The hydroboration of alkenes is a fundamental reaction in organic chemistry used for the synthesis of organic compounds due to its high regioselectivity and stereoselectivity. The reaction involves adding a boron hydride compound, typically a borane-tetrahydrofuran (BH₃-THF) complex, to an alkene double bond to form a boron-alkyl intermediate. This intermediate then reacts with various nucleophiles to produce a wide range of organic compounds such as pharmaceuticals, agrochemicals, and polymers. Pioneering works on hydroboration stated that free BH₃ is directly implicated in the reaction, ruling out the role of THF solvent. Herein, we studied the proposed mechanism of ethylene hydroboration from the BH₃-THF adduct using density functional theory. The mechanism begins with the dissociation of the adduct, which transfers the BH₃ from the THF to the ethylene with a Gibbs free energy barrier of 23.0 kcal/mol in implicit solvent. Because these hydroboration reactions occur at room temperature and in a short time, we wonder if this is the most viable pathway. Contrary to the literature, we also noted that THF stabilizes the organoborane products formed, making the reaction exergonic. Since NaBH₄ is present in commercial BH₃-THF as a stabilizer, we then added NaBH₄ to the BH₃-THF + C₂H₄ reaction. Interestingly, we found that the barrier is reduced to 16.6 kcal/mol. In the second part of this project, we carried out molecular dynamics simulations using the AMOEBA polarizable force field and QM/MM calculations. In this work, we will show the differences between considering an implicit solvent in the gas phase or an explicit solvent in QM/MM calculations.

Energy profile of the ethylene hydroboration mechanism from BH3-THF calculated at the SMD-M06HF/def2-TZVP level.

Gabriel Merino

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