A Density Functional Study on the Coordination of Aldehydes to N-Sulfonyl 1,3,2-Oxazaborolidin-5-one

The coordination of four aldehydes (formaldehyde, acetaldehyde, benzaldehyde, and s-trans-acrolein) to N-sulfonyl 1,3,2-oxazaborolidin-5-one has been studied by means of theoretical calculations. The effect of alkyl substituents on the ring has also been examined. Coordination can take place on each ring face, the energy minima presenting different types of interaction:  hydrogen-bonds (SO···H−C or B−O···H−C) or syn-periplanar H−B···OC arrangements. In contrast with previous models, a preference for coordination on the top face has been found (by 2.0−2.3 kcal mol^-1). The largest interaction energies hold for complexation with benzaldehyde ( ∼7 kcal mol^-1). The configuration of the major products experimentally obtained in Diels−Alder and Mukaiyama-aldol reactions can be explained by means of two reaction models. Our results are consistent with available experimental data for enantiomeric excess. Thus, an enantiomeric excess of 99% is predicted for benzaldehyde reactions, in good agreement with the highest experimental values (98%). The only known case where the proposed models are not valid corresponds to reactions catalyzed by oxazaborolidinones bearing aromatic substituents, because of the stabilization of a structure presenting a B−O···H−C hydrogen-bond induced by the formation of a charge-transfer complex between the aromatic ring and the aldehyde.