Quantum Chemical Study of Diels–Alder Reactions Catalyzed by Lewis Acid Activated Oxazaborolidines

The catalytic activity of Lewis acid activated oxazaborolidines in the Diels–Alder reaction between cyclopentadiene and methacrolein is investigated by using the DFT method. Oxazaborolidine is not able to coordinate to methacrolein in the absence of AlBr₃ because the bonding stabilization is too small to cover the destabilization arising from the deformation of the two species. Accordingly, oxazaborolidine hardly catalyzes the cycloaddition by itself. The calculations show that the attachment of AlBr₃ to the nitrogen atom of oxazaborolidine enhances the Lewis acidity of its boron center and enables it to coordinate to methacrolein. When the AlBr₃-assisted oxazaborolidine is once coordinated, the catalytic activity originates mainly from the oxazaborolidine framework, and to a smaller extent from the attached AlBr₃ part. The Lewis acid AlBr₃ plays an additional role to facilitate the reaction by reducing the overlap repulsion between the diene and the dienophile. The attachment of AlBr₃ to the oxygen atom, another Lewis basic site in oxazaborolidine, also gives a stable AlBr₃–oxazaborolidine complex, but the reaction catalyzed by this complex is not preferred to that catalyzed by the complex in which AlBr₃ is attached to the nitrogen atom. The electrophilicity of boron center in oxazaborolidine and those in the AlBr₃–oxazaborolidine complexes are compared in terms of localized reactive orbitals