Abstract
Chiral bifunctional dual H-bond donor catalysts have become one of the pillars of organocatalysis. They include squaramide, thiosquaramide, thiourea, urea, and even selenourea-based catalysts combined with chiral amines, cinchona alkaloids, sulfides, phosphines and more. They can promote several types of reactions affording products in very high yields and excellent stereoselectivities in many cases: conjugate additions, cycloadditions, the aldol and Henry reactions, the Morita–Baylis–Hilman reaction, even cascade reactions, among others. The desire to understand mechanisms and the quest for the origins of stereoselectivity, in attempts to find guidelines for developing more efficient catalysts for new transformations, has promoted many mechanistic and theoretical studies. In this review, we survey the literature published in this area since 2015.
Highlights
The activation of reactants by hydrogen bonding and other noncovalent interactions has enabled the development of an enormous variety of reactions in recent years, and the production of chiral substances for several applications
The largest number of papers encountered covering studies falling within the scope of this review were developments on the Michael addition reaction and its variants [14,15]
The first report on utilizing a bifunctional thiourea-based organocatalyst in synthesis was by Takemoto and coworkers in 2003 [16]
Summary
The activation of reactants by hydrogen bonding and other noncovalent interactions has enabled the development of an enormous variety of reactions in recent years, and the production of chiral substances for several applications. This was possible due to the discovery of an array of very efficient chiral bifunctional catalysts, mostly thiourea- or squaramide-based, usually combined with chiral amines or cinchona alkaloids, and more recently amides, phosphines or sulfides [1,2,3,4]. The ability of thioureas to recognize cations is more limited [6].2Aofs5a4 result of their ambivalent hydrogen bonding characteristics, squaramides form dimeric species.
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