Spontaneous Deracemizations

Abstract

Spontaneous deracemizations is a challenging, multidisciplinary subject in current chirality research. In the absence of any chiral inductors, an achiral substance or a racemic composition is driven into an enantioenriched or even homochiral state through a selective energy input, e.g., chemical potential, photoirradiation, mechanical grinding, ultrasound waves, thermal gradients, etc. The most prominent examples of such transformations are the Soai reaction and the Viedma deracemization. In this review, we track the most recent developments in this topic and recall that many other deracemizations have been reported for solutions from mesophases to conglomerate crystallizations. A compiled set of simply available achiral organic, inorganic, organometallic, and MOF compounds, yielding conglomerate crystals, should give the impetus to realize new experiments on spontaneous deracemizations. Taking into account thermodynamic constraints, modeling efforts have shown that structural features alone are not sufficient to describe spontaneous deracemizations. As a guideline of this review, particular attention is paid to the physicochemical origin and symmetry requirements of such processes.

Absence of Non‐Linear Effects Despite Evidence for Catalyst Aggregation

“…[7,8] Nonlinear scaling in enantioselectivity is also widely discussed in relation to the origin of biological homochirality, in which (+)-NLEs as steps of chiral amplification might have played a role. [9][10][11] Although the concepts of non-linear effects are also valid in stoichiometric reactions and kinetic resolution processes, they have been mostly studied for catalytic reactions. This is mainly due to their usefulness in the analysis of catalytic reaction mechanisms: the presence of a NLE is generally considered to be a footprint of catalyst aggregation or catalysis by multiligand-complexes.…”

“…Langenbeck and Triem reported a NLE in the double addition of menthol to oxalyl chloride, along with the conceptual basics for asymmetric amplification and an outlook to catalytic reactions [7,8] . Nonlinear scaling in enantioselectivity is also widely discussed in relation to the origin of biological homochirality, in which (+)‐NLEs as steps of chiral amplification might have played a role [9–11] …”

Biased Symmetry Breaking and Chiral Control by Self‐Replicating in Achiral Tetradentate Platinum (II) Complexes

“…So far, most researches on supramolecular chirality have been carried out, and attempts have been made to explain the origin of homochirality. The supramolecular chirality is mainly obtained by self‐assembly of inherent chiral molecules, or by methods of chiral induction, such as chiral templates, chiral environments, and chiral additives, but sometimes by self‐assembly of exclusive achiral components and Viedma deracemization [8–12] . Chiral self‐assembly derived from achiral components via spontaneous symmetry breaking has been a fascinating field of research [8, 11–13] .…”

“…The supramolecular chirality is mainly obtained by self‐assembly of inherent chiral molecules, or by methods of chiral induction, such as chiral templates, chiral environments, and chiral additives, but sometimes by self‐assembly of exclusive achiral components and Viedma deracemization [8–12] . Chiral self‐assembly derived from achiral components via spontaneous symmetry breaking has been a fascinating field of research [8, 11–13] . Spontaneous symmetry breaking, which refers to the macro‐optical activity and particular helical stacking, is generated from racemic mixtures or achiral molecules via non‐covalent interactions in supramolecular phases, such as crystal, gel, specific liquid phase [14–20] .…”

“…However, two inseparable domains with opposite handedness (chiral conglomerates) in the whole regional area are present simultaneously, which limits their application to some extent. Therefore, the homochiral self‐assemblies with controlled handedness are essential for material applications [12, 21–25] …”