Origins of Asymmetric Phosphazene Organocatalysis: Computations Reveal a Common Mechanism for Nitro- and Phospho-Aldol Additions

We report a hybrid density functional theory-molecular mechanics study of the mechanism of the addition of nitroalkanes and phosphonates to benzaldehyde catalyzed by a chiral phosphacene catalyst developed by Ooi and co-workers. Our results are consistent with a reaction mechanism in which a catalyst molecule simultaneously interacts by hydrogen bonds with the nucleophile and the electrophile, transferring a proton to the aldehyde in concert with carbon-carbon bond formation. Despite the C-2 symmetry of this class of organocatalyst, substrate recognition, and asymmetric induction in both reaction classes studied relies on interactions with nonequivalent N-H bonds that break symmetry. The origin of the stereo and diastereoselectivity is discussed in terms of steric effects and of the conformations adopted by the reactants, and the most favorable transition structure results from minimal geometric distortion energies. A rational model for predicting the major stereoisomer of reactions catalyzed by this chiral phosphacene, based on the qualitative assessment of steric interactions, is given.