Imine Amidation Catalyzed by a Chiral VAPOL Calcium Phosphate

Over the last 16 years, chiral phosphoric acids (CPAs) have been shown to be excellent asymmetric catalysts, very effectively used in constructing chiral molecules with high enantiocontrol. In 2010, Ishihara et al. discovered that chiral metal phosphate complexes (or salts) could be found in substantial quantities, as contaminates, in some reported CPA-catalyzed reactions (Hatano, M.; Moriyama, K.; Maki, T.; Ishihara, K. Angew. Chem., Int. Ed. 2010, 49, 3823-3826). These metal phosphates were shown to actually catalyze the reactions in addition to CPAs. In this work, we have investigated in depth a reaction first reported to be catalyzed by CPAs based on a vaulted bis-phenanthrol (VAPOL) backbone. We have found that VAPOL metal phosphates were, in fact, superior catalysts for this reaction. Upon optimization, a wide substrate scope, low catalyst loading, and mild conditions could provide intermolecular imine amidation reactions producing chiral N,N'-aminal products in high yields and with excellent ee values (up to >99% yield, >99% ee).