Catalytic Stereoselective SN1-Type Reactions Promoted by Chiral Phosphoric Acids as BrOnsted Acid Catalysts

In recent years, the field of organocatalysis has extensively explored the use of carbenium ions for practical C-C bond forming reactions. In order to control the formation of new stereocenters, the generation of carbenium ions was investigated by exploring asymmetric BrOnsted acid catalysis. Oxocarbenium ions, iminium ions, and other carbenium ions stabilized by heteroatoms could also be generated by BrOnsted acid catalysis, through the departure of suitable leaving groups. The aforementioned reactivity has recently been exploited leading to remarkable achievements. The application of chiral BrOnsted acids in the generation of heteroatom stabilized carbenium ions has recently been investigated, with the demonstration of important results. Although diverse architectural arrays of BrOnsted acid catalysts were conceived, BINOL-derived phosphoric acids have played a dominant role in this area of research. Their low pK(a) values combined with the possibility to alter their architecture, and their ability to assemble ordered transition states was exploited in different S(N)1-type reactions allowing the formation of a variety of products with high enantiomeric excesses. In addition, the generation of highly stabilized chiral counter anions allowed the exploitation of counterion asymmetric catalysis with various nucleophiles.