Structural Chemistry of C–N Axially Chiral Compounds

In the last several years, atropisomers owing to the rotational restriction around a C–N single bond (C–N axially chiral compounds) have attracted significant attention in the field of synthetic organic chemistry. In particular, the highly enantioselective synthesis of various C–N axially chiral compounds and their application to asymmetric reactions have been reported by many groups. On the other hand, studies on the structural chemistry of C–N axially chiral compounds have attracted scant attention in comparison with synthetic studies. For over 25 years, our group has explored asymmetric synthesis of C–N axially chiral compounds and their synthetic application. In the course of these synthetic studies, we found several notable structural properties in relation to the C–N bond rotation and an association of enantiomers (the relationship between the rotational stability and the structure or electronic effect, the chirality-dependent halogen bond, and the self-disproportionation of enantiomers). Furthermore, on the basis of these structural properties, the development of acid-mediated molecular rotors and the synthesis of isotopic atropisomers possessing high stereochemical purity and rotational stability were achieved. Through this Perspective, I wish to make the chemistry community aware that C–N axially chiral compounds are attractive molecules from the viewpoints of both synthetic organic chemistry and structural chemistry.