Atropisomerism, definition

As it can be seen, lUPAC definition covers more situations than the present review that is restricted to biphenyl heterocyclic analogues. Natta and Farina (72M11) offer an interesting discussion on this subject (they used the term atropisomerism, also found in the old references). In the authoritative book by Eliel et al. (94MI1), Chapter 14-5 is devoted to biphenyls atropisomerism. They report that this type of enantiomerism was discovered by Christie and Kenner in 1922 (22JCS614) in the case of 6,6 -dinitro-2,2 -diphenic acid (1) that they were able to resolve. It was later called (33MI1) atropisomerism. An important aspect of all the concepts related to a barrier is (we quote) It is immediately obvious that the term suffers from all the problems discussed previously How slow must be the interconversion of the enantiomers (i.e., how long is their half-life) before one speaks of atropisomerism At what temperature is the measurement to be made Does atropisomerism still exists when isolation of stereoisomers becomes difficult or impossible but their existence can be revealed by NMR (or other spectral) study and so on. ... 

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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