Polymer-Supported Highly Enantioselective Catalyst for Nitro-Michael Addition: Tuning through Variation of the Number of H-Bond Donors and Spacer Length

Two series of polymer-bound bifunctional organocatalysts. based on chiral diamine scaffolds, were prepared by solid-phase synthesis. The series, incorporating a single hydrogen bond donor carbamate functionality. was notably more enantioselective in the Michael reaction of acetone and nitrostyrene than the series that includes a double hydrogen bond donor urea moiety, or the polymer-bound diamine that lacks sufficiently acidic protons. The best aminocarbamate catalyst promoted the addition of acetone to nitroolefins with enantioselectivity unmatched by known heterogeneous catalysts. Introduction of a short linear spacer between the support and the scaffold improves the activity but reduces the selectivity of the catalyst. Alternatively, an increase in the reaction yield could be induced by the benzoic acid additive.