Macrocycle-Enabled Counteranion Trapping for Improved Catalytic Efficiency.

The tight host-guest binding enabled by functional macrocycles can be utilized to tune catalysis. While the strong crown ether-cation complexation has been widely applied on enhancing the reactivity of the paired anion, the complementary strategy by applying a macrocyclic anion receptor to trap a counteranion to improve the cation catalytic activity remains unexplored. To realize this strategy, a macrocycle incorporating multiple cooperative H-bonding sites was synthesized and shown to tightly trap ethanedisulfonate anion in crystals and in acetonitrile solution (K>106  m-1 ). With the strong binding tendency, the presence of as low as 0.25 mol % of the macrocycle can significantly improve the ethanedisulfonic acid-catalyzed Povarov reaction efficiency, while the acyclic analogues had diminished effect. Catalysis outcomes and binding studies taken together suggested the macrocycle promotion was through favoring the substrate protonation by trapping the counteranion of the acid catalyst.