Photonenergy-Controlled Symmetry Breaking with Circularly Polarized Light

Symmetry Breaking by Consecutive Amplification: Efficient Paths to Homochirality

To understand chiral symmetry breaking on the molecular level, we developed a method to efficiently investigate reaction kinetics of single molecules. The model systems include autocatalysis as well as a reaction cascade to gain further insight into the prebiotic origin of homochirality. The simulated reactions start with a substrate and only a single catalyst molecule, and the occurrence of symmetry breaking was examined for its degree of dependence on randomness. The results demonstrate that interlocking processes, which e.g., form catalysts, autocatalytic systems, or reaction cascades that build on each other and lead to a kinetic acceleration, can very well amplify a statistically occurring symmetry breaking. These results suggest a promising direction for the experimental implementation and identification of such processes, which could have led to a shift out of thermodynamic equilibrium in the emergence of life.

Novel highly efficient absolute optical resolution method by serial combination of two asymmetric reactions from acetylene monomers having racemic substituents

For general optical resolution, an optical resolution agent is necessary, and the best agent should be selected for each racemic compound. In this study, we will report that a novel optical resolution method by circularly polarized light (CPL) without any optical resolution agents has been developed by serially connecting two enantioselective reactions. These reactions we developed are the enantiomer-selective helix-sense-selective polymerization (ES-HSSP) and helix-sense-selective highly selective photocyclic aromatization (SCAT) by CPL (HS-SCAT). Since this significantly unique EPHS method (EPHS = ES-HSSP + HS-SCAT) does not need any optical resolution agents, any cocatalysts, and solvents for the selective decomposition reaction (HS-SCAT), this process is quite simple and convenient. Since this process does not include any decomposition of the target racemates themselves, both enantiomers could be obtained. The optical yields for isolated compounds that were enantiomerically separated by the EPHS method were very high, for example, 78%ee, 93%ee, and 85%ee for menthol, phenethyl alcohol, and 2-butanol, respectively. In addition, their chemical yields were around 85% to 94%. Therefore, the EPHS method was found to show an excellent performance and can be applied to actual optical resolution for a wide range of racemic compounds. This is the first absolute optical resolution by CPL showing high optical and chemical yields and expected to become a practical optical resolution method.

Prebiotically Plausible Organocatalysts Enabling a Selective Photoredox α-Alkylation of Aldehydes on the Early Earth

Organocatalysis is a powerful approach to extend and (enantio-) selectively modify molecular structures. Adapting this concept to the Early Earth scenario offers a promising solution to explain their evolution into a complex homochiral world. Herein, we present a class of imidazolidine-4-thione organocatalysts, easily accessible from simple molecules available on an Early Earth under highly plausible prebiotic reaction conditions. These imidazolidine-4-thiones are readily formed from mixtures of aldehydes or ketones in presence of ammonia, cyanides and hydrogen sulfide in high selectivity and distinct preference for individual compounds of the resulting catalyst library. These organocatalysts enable the enantioselective α-alkylation of aldehydes under prebiotic conditions and show activities that correlate with the selectivity of their formation. Furthermore, the crystallization of single catalysts as conglomerates opens the pathway for symmetry breaking.

Circularly Polarized Luminescence from Simple Organic Molecules

This article aims to show the identity of "circularly polarized luminescent active simple organic molecules" as a new concept in organic chemistry due to the potential interest of these molecules, as availed by the exponentially growing number of research articles related to them. In particular, it describes and highlights the interest and difficulty in developing chiral simple (small and non-aggregated) organic molecules able to emit left- or right-circularly polarized light efficiently, the efforts realized up to now to reach this challenging objective, and the most significant milestones achieved to date. General guidelines for the preparation of these interesting molecules are also presented.

Molecular chirality in meteorites and interstellar ices, and the chirality experiment on board the ESA cometary Rosetta mission

Life, as it is known to us, uses exclusively L-amino acid and D-sugar enantiomers for the molecular architecture of proteins and nucleic acids. This Minireview explores current models of the original symmetry-breaking influence that led to the exogenic delivery to Earth of prebiotic molecules with a slight enantiomeric excess. We provide a short overview of enantiomeric enhancements detected in bodies of extraterrestrial origin, such as meteorites, and interstellar ices simulated in the laboratory. Data are interpreted from different points of view, namely, photochirogenesis, parity violation in the weak nuclear interaction, and enantioenrichment through phase transitions. Photochemically induced enantiomeric imbalances are discussed more specifically in the topical context of the "chirality module" on board the cometary Rosetta spacecraft of the ESA. This device will perform the first enantioselective in situ analyses of samples taken from a cometary nucleus.