Near‐Enantiopure Trimerization of 9‐Ethynylphenanthrene on a Chiral Metal Surface

Enantioselectivity in heterogeneous catalysis strongly depends on the chirality transfer between catalyst surface and all reactants, intermediates, and the product along the reaction pathway. Herein we report the first enantioselective on‐surface synthesis of molecular structures from an initial racemic mixture and without the need of enantiopure modifier molecules. The reaction consists of a trimerization via an unidentified bonding motif of prochiral 9‐ethynylphenanthrene (9‐EP) upon annealing to 500 K on the chiral Pd₃‐terminated PdGa{111} surfaces into essentially enantiopure, homochiral 9‐EP propellers. The observed behavior strongly contrasts the reaction of 9‐EP on the chiral Pd₁‐terminated PdGa{111} surfaces, where 9‐EP monomers that are in nearly enantiopure configuration, dimerize without enantiomeric excess. Our findings demonstrate strong chiral recognition and a significant ensemble effect in the PdGa system, hence highlighting the huge potential of chiral intermetallic compounds for enantioselective synthesis and underlining the importance to control the catalytically active sites at the atomic level.     

Highly Enantioselective Adsorption of Small Prochiral Molecules on a Chiral Intermetallic Compound

Intrinsically chiral surfaces of intermetallic compounds are shown to be novel materials for enantioselective processes. Their advantage is the significantly higher thermal and chemical stability, and therefore their extended application range for catalyzed chiral reactions compared to surfaces templated with chiral molecular modifiers or auxiliaries. On the Pd₁‐terminated PdGa(111) surface, room‐temperature adsorption of a small prochiral molecule (9‐ethynylphenanthrene) leads to exceptionally high enantiomeric excess ratios of up to 98 %. Our findings highlight the great potential of intrinsically chiral intermetallic compounds for the development of novel, enantioselective catalysts that can be operated at high temperatures and potentially also in harsh chemical environments.     

Self‐Replication and Amplification of Enantiomeric Excess of Chiral Multifunctionalized Large Molecules by Asymmetric Autocatalysis

Self‐replication of large chiral molecular architectures is one of the great challenges and interests in synthetic, systems, and prebiotic chemistry. Described herein is a new chemical system in which large chiral multifunctionalized molecules possess asymmetric autocatalytic self‐replicating and self‐improving abilities, that is, improvement of their enantioenrichment in addition to the diastereomeric ratio. The large chiral multifunctionalized molecules catalyze the production of themselves with the same structure, including the chirality of newly formed asymmetric carbon atoms, in the reaction of the corresponding achiral aldehydes and reagent. The chirality of the large multifunctionalized molecules controlled the enantioselectivity of the reaction in a highly selective manner to construct multiple asymmetric stereogenic centers in a single reaction.     

Catalytic Asymmetric Construction of Axially Chiral Indole-Based Frameworks: An Emerging Area

Axially chiral indole-based frameworks have been recognized as a class of important five-membered heterobiaryls and developing catalytic asymmetric approaches for constructing these frameworks in an enantioselective manner is highly desirable. In recent years, synthetic chemists have paid much attention to this research field, and rapid developments have occurred. At this point, a range of axially chiral indole-based scaffolds have been constructed via various catalytic asymmetric reactions based on different strategies. Thus, the catalytic asymmetric construction of axially chiral indole-based frameworks has become an emerging area. This minireview summarizes the rapid advances in this field and gives some insights into future developments, which will help this research field to thrive.     

Construction of Chiral Cyclic Compounds Enabled by Enantioselective Photocatalysis

Chiral cyclic molecules are some of the most important compounds in nature, and are widely used in the fields of drugs, materials, synthesis, etc. Enantioselective photocatalysis has become a powerful tool for organic synthesis of chiral cyclic molecules. Herein, this review summarized the research progress in the synthesis of chiral cyclic compounds by photocatalytic cycloaddition reaction in the past 5 years, and expounded the reaction conditions, characters, and corresponding proposed mechanism, hoping to guide and promote the development of this field.     

Cyclotrimerization‐Induced Chiral Supramolecular Structures of 4‐Ethynyltriphenylamine on Au(111) Surface

Cyclotrimerization‐induced chiral supramolecular structures of 4‐ethynyltriphenylamine (ETPA) have been synthesized on the Au(111) surface through alkyne‐based reactions. Whereas the ETPA molecules adsorbed on the Au(111) surface remain inert and form a close‐packed self‐assembled structure at room temperature, the combination of scanning tunneling microscopy observations and theoretical calculations unambiguously reveal that the ETPA molecules cyclotrimerize to form new trimer‐like species—1,3,5‐tris[4‐(diphenylamino)phenyl]benzene (TPAPB)—after annealing at 323 K. Further annealing drives these cyclotrimerized TPAPB molecules to form chiral hexagonal supramolecular structures with an extraordinary self‐healing ability.     

Stereogenic‐Only‐at‐Metal Asymmetric Catalysts

Chirality is an essential feature of asymmetric catalysts. This review summarizes asymmetric catalysts that derive their chirality exclusively from stereogenic metal centers. Reported chiral‐at‐metal catalysts can be divided into two classes, namely, inert metal complexes, in which the metal fulfills a purely structural role, so catalysis is mediated entirely through the ligand sphere, and reactive metal complexes. The latter are particularly appealing because structural simplicity (only achiral ligands) is combined with the prospect of particularly effective asymmetric induction (direct contact of the substrate with the chiral metal center). Challenges and solutions for the design of such reactive stereogenic‐only‐at‐metal asymmetric catalysts are discussed.     

Organocatalytic Formation of Chiral Trisubstituted Allenes and Chiral Furan Derivatives

A novel reaction that provides chiral allenes by amino catalytic activation of either aldehydes or α,β‐unsaturated aldehydes for reaction with alkynyl‐substituted enones is presented. The reaction forms a variety of trisubstituted allenes in high yields and with excellent stereoselectivities. The utility of the reaction concept is demonstrated by the synthesis of chiral furan derivatives in high yields and stereoselectivities.     

Stereochemical Communication within a Chiral Ion Pair Catalyst

Ionic interactions are increasingly appreciated as a key, asymmetry‐inducing factor in enantioselective catalytic transformations, including those involving Brønsted acid or base catalysis, phase‐transfer catalysis, and related processes. However, a detailed understanding of these interactions is often lacking. Herein, we show how an enantiopure anion enforces a chiral conformation onto a catalytically relevant achiral cation. Specifically, we use vibrational circular dichroism (VCD) spectroscopy to monitor the transmission of stereochemical information from a chiral phosphate anion to a flexible manganese(III)–salen cation. We show that VCD can be used to study solvent effects and that the obtained chiroptical data directly and quantitatively correlate with the experimentally observed enantioselectivity in an asymmetric olefin epoxidation reaction.     

Chiral Thin Films of Metal Oxide

In this paper, we describe for the first time the synthesis of new chiral nanosized metal oxide surfaces based on chiral self‐assembled monolayers (SAMs) coated with metal oxide (TiO₂) nanolayers. In this new type of nanosize chiral surface, the metal oxide nanolayers enable the protection of the chiral self‐assembled monolayers while preserving their enantioselective nature. The chiral nature of the SAM/TiO₂ films was characterized by variety of unique techniques, such as second‐harmonic generation circular dichroism (SHG‐CD), quartz crystal microbalance, and chiral adsorption measurements with circular dichroism spectroscopy. The chiral resolution abilities of the SAMs coated with metal oxide (TiO₂) nanolayers were investigated in the crystallization of a racemic mixture of threonine and glutamic acid. Our proposed methodology for the preparation of nanoscale chiral surfaces described in this article could open up opportunities in other fields of chemistry, such as chiral catalysis.     

Catalytic and highly enantioselective reactions of alpha-sulfonyl carbanions with chiral bis(oxazoline)s

The enantioselective reactions of lithiated benzyl trifluoromethyl sulfones with a substoichiometric amount of a bis(oxazoline) and various aldehydes is disclosed. The products were formed with excellent diastereo- and enantioselectivities. Fluorination of the sulfone with N-fluorobenzenesulfonimide and a stoichiometric amount of a bis(oxazoline) gave products with extremely high enantioselectivities (up to 99% ee; ee=enantiomeric excess). The enantioselective reaction was confirmed to proceed through a dynamic thermodynamic resolution pathway.