Organocatalytic acid mediated Mannich reactions and multicomponent boronate reactions to make chiral benzhydrils

Abstract

Since its discovery in 1912, the Mannich reaction has been widely utilized in organic chemistry to form C-C bonds. Reactivity of an enol with an imine allows for easy formation of a [beta]-aminoketone. Enamines have also been widely utilized as convenient nucleophiles. In our work, unexpected reactivity of the [gamma] position of [beta]-enamidoesters in a Brønsted acid environment and high enantioselectivity of a Mannich reaction were achieved through chiral phosphoramidic acid catalysis. A novel class of chiral phosphoramidic acids was designed, synthesized from the corresponding diamines, with several sulfonyl N-protecting groups, and characterized. Their unique properties arise from their Brønsted acid nature, atropisomerism and ability to form complexes via H-bond. Once prepared, such catalysts were successfully used as organocatalysts for the regio- and enantioselective Mannich reaction of [beta]-enamidoesters and imines. Their activity is described as a method to reverse the regioselectivity of the nucleophile while achieving high enantioselectivities in the formation of chiral benzhydrils. A diverse range of imines has been tested, obtaining yields of up to 93% and enantioselectivities of up to 99:1. A few substituted enamines were also tested to study the influence of substituents on the regioselectivity. A mechanism for this reaction is proposed and kinetic studies confirmed that the reaction is first order in catalyst. The ozonolysis of the product of this Mannich reaction was performed to prove the absolute stereochemistry of the product; and a new efficient methodology for the asymmetric preparation of aminoacid [beta]-phenyl-[beta]-alanine benzyl ester is described. The reduction of the enamide moiety of the Mannich product was attempted via asymmetric hydrogenation and via hydride reduction to diastereomerically obtain 1,3-diamines, which are compounds of major synthetic interest. Unfortunately our attempts in this direction were not successful. Finally, a multicomponent reaction between an aldehyde, a substituted phenol, and a styrylboronate was developed as an alternative method for the preparation of chiral benzhydrils. This process is also organocatalytic and the methodology was optimized in the presence of 3-3'-disubstituted BINOLs. Yields up to 71% and enantioselectivities up to 96:4 were achieved. A mechanism for this organocatalytic reaction is also proposed.