Chiral Phosphoric Acid-Catalyzed Enantioselective Synthesis of Axially Chiral Compounds Involving Indole Derivatives

Indoles are one of the most ubiquitous subclass of N-heterocycles and are increasingly incorporated to design new axially chiral scaffolds. The rich profile of reactivity and N-H functionality allow chemical derivatization for enhanced medicinal, material and catalytic properties. Although asymmetric C-C coupling of two arenes gives the most direct access of axially chiral biaryl scaffolds, this chemistry has been the remit of metal catalysis and works efficiently on limited substrates. Our group has devoted special interest in devising novel organocatalytic arylation reactions to fabricate biaryl atropisomers. In this realm, indoles and derivatives have been reliably used as the arylation partners in combination with azoarenes, nitrosonapthalenes and quinone derivatives. Their efficient interaction with chiral phosphoric acid catalyst as well as the tunability of electronics and sterics have enabled excellent control of stereo-, chemo- and regioselectivity to furnish diverse scaffolds. In addition, indoles could act as nucleophiles in desymmetrization of 1,2,4-triazole-3,5-diones. This account provides a succinct illustration of these developments.

Catalytic Atroposelective Electrophilic Amination of Indoles

Reported here is the first catalytic atroposelective electrophilic amination of indoles, which delivers functionalized atropochiral N-sulfonyl-3-arylaminoindoles with excellent optical purity. This reaction was furnished by 1,6-nucleophilic addition to p-quinone diimines. Control experiments suggest an ionic mechanism that differs from the radical addition pathway commonly proposed for 1,6-addition to quinones. The origin of 1,6-addition selectivity was investigated through computational studies. Preliminary studies show that the obtained 3-aminoindoles atropisomers exhibit anticancer activities. This method is valuable with respect to enlarging the toolbox for atropochiral amine derivatives.

Enantioselective Aromatic Amination?

The atroposelective formation of C-N bonds has recently emerged within the field of amination reactions. On first sight, it may seem quite surprising that such an ancient class of organic coupling reactions (Gabriel, Ullmann, Goldberg, Buchwald, Hartwig and many others) has so few enantioselective solutions, and this in spite of asymmetric synthesis being now a mature concept and field. Why should enantioselective C-N bond formation be so difficult? This question and some of the first examples that promise an imminent change of paradigm are herein discussed.