Desymmetrizing atroposelective bromination of N-arylcarbazoles enabled by cross-assembled bifunctional catalysts

Abstract

Asymmetric catalytic halofunctionalization has gained prominence for introducing carbon point chirality and halogen functionality simultaneously. Axially chiral biaryls possess a range of promising applications; however, their synthesis through asymmetric halogenation is limited. Here, we report the synthesis of C–N axially chiral N-arylcarbazoles through desymmetrizing atroposelective bromination. This process is facilitated by a catalyst blend consisting of a chiral phosphoric acid and an achiral Lewis base. The resulting bromo-carbazoles contain readily modifiable halogen handles, allowing for the introduction of additional chiral axes through diastereoselective cross-coupling. These axially chiral compounds exhibit high fluorescence quantum yields and satisfactory circularly polarized luminescence performance. Mechanistic studies indicate that the chiral phosphate and achiral pyridine catalysts cross-assemble through C–H nonclassical hydrogen bonds and C–X halogen bonds, creating a confined microenvironment for efficient enantiofacial discrimination of substrates. This research paves the way for an approach to synthesizing mimics of planar chiral arenes for applications in materials science.