Quantum chemical study of Lewis acid catalyzed allylboration of aldehydes

The reaction rate enhancement in the reaction of allylboronate with benzaldehyde in the presence of AlCl3 has been studied theoretically. B3LYP calculations find a relatively high activation barrier for the reaction of pinacol allylboronate with benzaldehyde in the absence of the Lewis acid. The reaction paths that go through the transition states coordinated by an AlCl3 molecule at one of the two oxygen atoms in the boronate give significantly lower values of activation energy. An analysis of electron populations and orbitals taking part in bond formation indicates that the AlCl3 molecule attached to the boronate oxygen atom strengthens the electrophilicity of the boron center, while it weakens the nucleophilicity of the C(gamma)-C(beta) bond. The result supports the electrophilic boronate activation mechanism proposed by Rauniyar and Hall on the basis of experiments and kinetic studies. In contrast, the reaction path in which AlCl3 is coordinated to the carbonyl oxygen of benzaldehyde shows a higher activation barrier, though the initial reactant complex is more stable than those in other reaction paths. The AlCl3 molecule reduces the reactivity of aldehyde by depressing the nucleophilicity of the sigma-type lone pair of electrons on the carbonyl oxygen, though the electrophilicity of the carbonyl pi orbital is strengthened to some extent. The significance of charge polarization within allylboronate in enhancing the reactivity of boron by the Lewis acid is discussed.