Mechanistic investigations in α‐hydroxycarbonyls reduction by BH₄^‐

Abstract

BH4‐, a well‐known and widely used reducing agent for carbonyl compounds, has been reported to have the ability to participate in dihydrogen bonding, an interaction with applications in catalysis, stereoselectivity and crystal engineering. Specifically, α‐hydroxycarbonyls are activated for reduction by dihydrogen bonding that occurs between BH4‐ and hydroxyl group. We explored the effect of the interaction on the mechanism of these reactions by examining their activation parameters. We found that dihydrogen bonding activates α‐hydroxycyclopentanone for reduction with NBu4BH4 by lowering the activation enthalpy by 6.6 kcal/mol. While the activation entropy is a significant component of the barrier, the changes resulting from the occurrence of dihydrogen bonding are manifested predominantly in the enthalpy term. Computational studies suggest that, while internal hydrogen bonding is allowed by the flexibility of the carbon backbone, that interaction is outweighed by dihydrogen bonding once BH4‐ is present in the system. Experimentally, a red shift of the hydroxyl frequency is observed upon addition of BH4‐ to the reaction mixture, suggesting a dihydrogen bonding interaction. The flexibility of the substrate's skeleton or the selectivity of the hydride sites in BH4‐ does not account for the lack of directing effect of the dihydrogen bonding. When a substrate with a rigid naphthalene backbone moiety, 2‐hydroxyacenaphthylen‐1(2H)‐one, is reduced, the stereochemical outcome is very similar to the one corresponding to the α‐hydroxycyclopentanone. Copyright © 2012 John Wiley & Sons, Ltd.

Dynamics of Dihydrogen Bonding in Aqueous Solutions of Sodium Borohydride

“…It has been shown that reductions with borohydride can be controlled selectively and the rate can be tuned based on manipulating the dihydrogen interactions. 41,42 Basic aqueous solutions of borohydride have also been proposed as a hydrogen storage mechanism 43 and for use in fuel cells. 44 Thus, the water and borohydride system presents a fairly simple system to study hydrogen bonding, but it also has direct relevance to a variety of systems and processes.…”

“…Given the ubiquity of water as a solvent and the widespread use of borohydride as a reducing agent, the borohydride/water system is important in its own right, but it is also useful in understanding dihydrogen bond dynamics. It has been shown that reductions with borohydride can be controlled selectively and the rate can be tuned based on manipulating the dihydrogen interactions. , Basic aqueous solutions of borohydride have also been proposed as a hydrogen storage mechanism and for use in fuel cells . Thus, the water and borohydride system presents a fairly simple system to study hydrogen bonding, but it also has direct relevance to a variety of systems and processes.…”