Bifunctional Activation and Racemization in the Catalytic Asymmetric Aza-Baylis−Hillman Reaction

The mechanism of bifunctional activation in the asymmetric aza-Baylis−Hillman (aza-BH) reaction was studied using NMR spectroscopic techniques. The reaction involves rate-limiting proton transfer in the absence of added protic species, but exhibits no autocatalysis. Brønsted acidic additives lead to substantial rate enhancements through acceleration of the elimination step. Furthermore, it was found that phosphine catalysts either alone or in combination with protic additives can cause racemization of the aza-BH product by proton exchange at the stereogenic center. This indicates that the spatial arrangement of a bifunctional chiral catalyst for the asymmetric aza-BH reaction is crucial not only for the stereodifferentiation within the catalytic cycle but also for the prevention of subsequent racemization.