Asymmetric cooperative catalysis in the conjugate addition of pyrazolones to nitroolefins and subsequent dearomative chlorination

Over the past decade many bifunctional amine-thioureas have been developed as active metal-free organocatalysts. Cooperative catalysis of these amino-thioureas allows high reaction rates and excellent transfer of stereochemical information. Despite these impressive advances, the design of new high-performance catalysts for applications in asymmetric catalytic reactions is of ongoing interest in organic chemistry. Herein we describe a cooperative catalyst system consisting of a chiral amine-thiourea and an achiral organic acid that promotes the conjugate addition of 4-nonsubstituted pyrazolones to nitroolefins and subsequent dearomative chlorination. The corresponding adducts and the subsequent products were obtained in high to excellent yields (up to 99%) and high stereoselectivities (up to 99/1 dr, 98% ee) under mild reaction conditions. These transformations provide an easy access to enantio-enriched pyrazole derivatives, which could possess potential pharmaceutical activity.