Axially Chiral Sulfonic Acids for Atroposelective Synthesis of C–N Axially Chiral 1,2‐Diarylbenzimidazoles

Axially chiral sulfonic acids are introduced as a new class of organocatalysts for atroposelective synthesis. Cyclization of structurally diverse o ‐phenylenediamines with aromatic and heteroaromatic aldehydes efficiently afforded 29 C–N axially chiral 1,2‐diarylbenzimidazoles in enantiomeric ratios of up to 16:84 using only 2.5 mol % catalyst. Investigation of substrate effects revealed that substituents located in the immediate vicinity of the developing C–N chiral axis play a key role in stereocontrol. The enantiomeric ratios were readily enhanced by simple recrystallization, exceeding 99:1 in several cases.