Atropisomer, definition

The traditional limitation of the stereoisomer concept to isomeric compounds with different configurations stems from the idea that configurations are not interconvertible under the observations conditions. ) A definition of stereoisomers on this basis is questionable as its application is then dependent upon observation conditions. It also fails to account for the fact that many mobile interconversions of configurations (e.g. configurations involving tricoordinate Nitrogen or pentacoordinate Phosphorus) are known, as well as thermally stable conformations (e.g. the atropisomers). 

Atropisomerism is significant because it introduces an element of chirality in the absence of stereogenic atoms. Axial chirality is observed with stereoisomers (or atropisomers) that result from hindered rotation about a single C—C or C—N bond. The barrier of rotation between atropisomers must be high enough to allow for their isolation. A minimum of three or/ho-substituents are generally required for an axially chiral biphenyl to have substantial stability toward racemization at room temperature. For general definitions and descriptions, see references [3-5]. 

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