Atropisomer

Atropisomer refers to a type of stereoisomer that arises when rotation about a specific single bond is hindered sufficiently for interconversion between conformers to be slow on the timescale of observation. In many cases, atropisomerism is axial, producing enantiomeric forms that differ in the orientation of bulky groups around an axis, commonly in biaryl systems.

The effect is especially common in biaryl compounds where ortho substituents create steric hindrance to rotation about the aryl–aryl bond. If the rotational barrier is high enough, the two conformers are isolable as separate atropisomers at room temperature. If the barrier is low, rapid interconversion occurs and distinct atropisomers cannot be observed.

Examples include 2,2'-disubstituted biphenyls and substituted binaphthyl derivatives. 1,1'-Binaphthyl-derived ligands such as BINAP are well-known axially chiral atropisomers employed in asymmetric catalysis. Atropisomerism also appears in other axially chiral systems, including certain heterobiaryls and substituted biaryl motifs.

Properties and analysis involve separating atropisomers by chiral chromatography and assigning configuration around the axis. The rotational barrier can be estimated experimentally by variable-temperature spectroscopy or inferred from kinetics and computational methods. X-ray crystallography can confirm axial configuration when single crystals are available.

Applications of atropisomers include use as chiral ligands and building blocks in asymmetric synthesis, contributions to drug discovery as chiral scaffolds, and involvement in materials science where axial chirality influences properties such as optical activity and chiral recognition.