Atropisomer

An atropisomer is a stereoisomer that remains distinct because rotation around a single bond is hindered enough to prevent rapid interconversion. Its stereochemistry arises from restricted bond rotation rather than a conventional stereogenic center.

Axial chirality: Chirality arising from a non-tetrahedral axis, commonly a bond whose restricted rotation fixes the arrangement of attached groups. Many atropisomers are chiral because their hindered bond defines a stereogenic axis.
Stereoisomerism: Isomerism in which molecules have the same connectivity but differ in the three-dimensional arrangement of their atoms. Atropisomers are stereoisomers distinguished by persistent spatial arrangements.
BINAP: A chiral diphosphine ligand with a binaphthyl framework used in asymmetric catalysis. Its stable axial chirality makes BINAP a prominent atropisomeric catalyst ligand.
Cis–trans isomerism: Geometric isomerism in which groups occupy different relative positions across a restricted-rotation bond or ring. Both involve restricted motion, but atropisomerism typically concerns stereochemistry around a single bond.
BINOL: A chiral biaryl diol whose two naphthyl rings have restricted rotation around their connecting bond. BINOL is a classic example of a configurationally stable atropisomer.
Rotational barrier: The energy difference between a molecule's stable conformation and the transition state for rotation around a bond. A sufficiently high barrier keeps atropisomeric forms from rapidly interconverting.
Enantiomer: One of a pair of stereoisomers that are nonsuperimposable mirror images. Chiral atropisomers can occur as enantiomeric pairs.
Atroposelective synthesis: A synthesis that preferentially forms one atropisomer over another. This approach controls which hindered-rotation stereoisomer is produced.
E/Z notation: A system for specifying the relative arrangement of substituents around a stereochemically restricted double bond. It describes alkene geometry, unlike the hindered single-bond rotation defining atropisomers.
2,2′-Biphenol: A biphenyl compound bearing hydroxyl groups at the two positions adjacent to its inter-ring bond. Its ortho substituents can restrict rotation and create atropisomeric forms.

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StereoisomerBroader topic: Hindered bond rotation can make otherwise interconvertible forms isolable stereoisomers.