Chiral Phosphoric Acids as Versatile Catalysts for Enantioselective Carbon-Carbon Bond Forming Reactions

An inventive approach to the development of chiral Bronsted acid catalysis, to enable catalysts which possess strong acid functionalities, has been accomplished. Among the various organic Bronsted acids surveyed, phosphoric acids have become the focus of our attention as potential chiral Bronsted acid catalysts because of their unique structural and chemical features. The desirable features of phosphoric acids as chiral Bronsted acid catalysts are summarized as follows. 1) Phosphoric acids are expected to have relatively strong yet appropriate acidity. 2) The phosphoryl oxygen would function as a Bronsted basic site and hence it is anticipated that it would convey acid/base dual function even to monofunctional phosphoric acid catalysts. 3) An acidic functionality is available even with the introduction of a ring system which effectively restricts the conformational flexibility of the chiral backbone. 4) Substituents can be introduced to the ring system to provide an efficient chiral environment for enantioselective transformations. It is anticipated that an efficient substrate recognition site would be constructed around the activation site due to the acid/base dual function and the steric and electronic influence of the substituents introduced at the ring system. In this context, we developed 1,1'-bi- 2-naphthol (BINOL)-derived monophosphoric acids as chiral Bronsted acid catalysts. The chiral phosphoric acids thus developed functioned as efficient enantioselective catalysts for a variety of carbon―carbon bond forming reactions via activation of a series of functionalities, affording enantioenriched products in excellent selectivities. In this article, we review our recent achievements in developing enantioselective reactions using the chiral phosphoric acid catalysts. The contents are arranged according to the type of functionality, including imines, hemiaminal ethers, aldehydes, and electron-rich double bonds, followed by specific reaction types.