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Reductive Openings of Acetals: Explanation of Regioselectivity in Borane Reductions by Mechanistic Studies

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journal contribution
posted on 18.07.2008, 00:00 by Richard Johnsson, Dan Olsson, Ulf Ellervik
The mechanisms of regioselective reductive openings of acetals were investigated in several model systems by a combination of Hammett plots, kinetic experiments, density functional calculations, and 11B NMR. The regioselectivity of borane reductions of cyclic acetals can be controlled by the choice of borane. Lewis acid activation of BH3·NMe3 increases the reaction rate and renders the borane the most electrophilic species, which associates to the more electron-rich oxygen of the acetal. In contrary, without activation, the regioselectivity is instead directed by the Lewis acid, as exemplified by the reaction with BH3·THF.