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. 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).

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.

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