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Spontaneous transfer of chirality in an atropisomerically enriched two-axis system
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Spontaneous transfer of chirality in an atropisomerically enriched two-axis system
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Journal Article
Spontaneous transfer of chirality in an atropisomerically enriched two-axis system
2014
Overview
An enantioselective reaction involving a molecule with two axes of stereochemical consequence produces four stereoisomers, and rather than racemizing as the system approaches equilibrium, one of the diastereomeric pairs drifts spontaneously to a higher enantiomeric ratio.
Chirality enters a new dimension
Chiral molecules — those that are not identical to their mirror image — contain a stereogenic chiral centre, typically a tetrahedral atom and usually carbon, connected to four different substituents. This paper describes a novel catalytic reaction that produces a more complex type of chirality in which two axes of chirality are present within the same molecule, so that an enantioselective reaction produces four stereoisomers. Rather than racemizing as the system approaches equilibrium, one of the diastereomeric pairs drifts spontaneously to a higher enantiomeric ratio.
One of the most well-recognized stereogenic elements in a chiral molecule is an
sp
3
-hybridized carbon atom that is connected to four different substituents. Axes of chirality can also exist about bonds with hindered barriers of rotation; molecules containing such axes are known as atropisomers
1
. Understanding the dynamics of these systems can be useful, for example, in the design of single-atropisomer drugs
2
or molecular switches and motors
3
. For molecules that exhibit a single axis of chirality, rotation about that axis leads to racemization as the system reaches equilibrium. Here we report a two-axis system for which an enantioselective reaction produces four stereoisomers (two enantiomeric pairs): following a catalytic asymmetric transformation, we observe a kinetically controlled product distribution that is perturbed from the system’s equilibrium position. As the system undergoes isomerization, one of the diastereomeric pairs drifts spontaneously to a higher enantiomeric ratio. In a compensatory manner, the enantiomeric ratio of the other diastereomeric pair decreases. These observations are made for a class of unsymmetrical amides that exhibits two asymmetric axes—one axis is defined through a benzamide substructure, and the other axis is associated with differentially
N
,
N
-disubstituted amides. The stereodynamics of these substrates provides an opportunity to observe a curious interplay of kinetics and thermodynamics intrinsic to a system of stereoisomers that is constrained to a situation of partial equilibrium.
Publisher
Nature Publishing Group UK,Nature Publishing Group
