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Atropisomer
Atropisomers are stereoisomers arising because of hindered rotation about a single bond, where energy differences due to steric strain or other contributors create a barrier to rotation that is high enough to allow for isolation of individual conformers.〔Anslyn and Dougherty, ''Modern Physical Organic Chemistry'', University Science Books, 2006, ISBN 978-1-891389-31-3〕 The word ''atropisomer'' (Gr., άτροπος, ''atropos'', meaning "without turn") was coined in application to a theoretical concept by German biochemist Richard Kuhn for Karl Freudenberg's seminal ''Stereochemie'' volume in 1933. Atropisomerism was first experimentally detected in a tetra substituted biphenyl, a diacid, by George Christie and James Kenner in 1922. Michinori Ōki further refined the definition of atropisomers taking into account the temperature-dependence associated with the interconversion of conformers, specifying that atropisomers interconvert with a half-life of at least 1000 seconds at a given temperature, corresponding to an energy barrier of 93 kJ mol−1 (22 kcal mol −1) at 300 K (27 °C).〔Ōki, Michinori (1983) Recent Advances in Atropisomerism, in ''Topics in Stereochemistry'', Vol. 14 (N. L. Allinger, E. L. Eliel and S. H. Wilen, Eds.), Hoboken, NJ:John Wiley & Sons, pp. 1-82; ''published online in 2007'', DOI: 10.1002/9780470147238.ch1, see () and (), accessed 12 June 2014.〕
Three basic factors contribute to the stability of individual atropisomers: the repulsive interactions (e.g., steric bulk) of substituents near the axis of rotation, the length and rigidity of the single bond, a largely sp2-sp2 type of bond joining the aryl rings, and whether there are photochemical or other mechanisms to induce rotation in addition to thermal pathways.〔〔 A variety of methods are employed to study atropisomers, including (from more general to more specific/structural), dipolemetry, titrimetry, electronic and infrared spectroscopy, and X-ray crystallography and nuclear magnetic resonance spectroscopy, the last two being primary means of structure characterization of organic systems, and the last being an ideal means of studying dynamics when the system is amenable to it;〔 inferences from theory and results of reaction outcomes and yields also contribute.
The importance of atropisomers arises to significant degree because with sufficient stability of a conformer, they can display axial chirality (planar chirality). Atropisomers that display axial chirality often have substituents ortho to the bond joining the aryl rings, substituents that cause significant steric repulsion that hinders rotation about the bond. The degree of hindrance correlates with the van der Waals radii of the particular substituents, and other properties that contribute to their repulsive potentials.〔 As the Ōki refinement of the atropisomer definition suggests, atropisomers are involved in a chemical equilibrium that, for a given structure, is thermally controlled; they differ in this way from most other types of chiral structures, where interconversion involves a chemical isomerization (i.e., with breaking and reforming of covalent bonds).
== Stereochemical assignment ==
Determining the axial stereochemistry of biaryl atropisomers can be accomplished through the use of a Newman projection along the axis of hindered rotation. The ortho, and in some cases meta substituents are first assigned priority based on Cahn–Ingold–Prelog priority rules. Starting with the substituent of highest priority in the closest ring and moving along the shortest path to the substituent of highest priority in the other ring, the absolute configuration is assigned P for clockwise and M for counterclockwise. In the example shown, A has priority over B.〔
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