Enantioselective "organocatalysis in disguise" by the ligand sphere of chiral metal-templated complexes

Asymmetric catalysis holds a prominent position among the important developments in chemistry during the 20th century. This was acknowledged by the 2001 Nobel Prize in chemistry awarded to Knowles, Noyori, and Sharpless for their development of chiral metal catalysts for organic transformations. The key feature of the catalysts was the crucial role of the chiral ligand and the nature of the metal ions, which promoted the catalytic conversions of the substrates via direct coordination. Subsequently the development of asymmetric organic catalysis opened new avenues to the synthesis of enantiopure compounds, avoiding any use of metal ions. Recently, an alternative approach to asymmetric catalysis emerged that relied on the catalytic functions of the ligands themselves boosted by coordination to metal ions. In other words, in these hybrid chiral catalysts the substrates are activated not by the metal ions but by the ligands. The activation and enantioselective control occurred via well-orchestrated and custom-tailored non-covalent interactions of the substrates with the ligand sphere of chiral metal complexes. In these metal-templated catalysts, the metal served either as a template (a purely structural role), or it constituted the exclusive source of chirality (metal-centred chirality due to the spatial arrangement of achiral or chiral bi-/tridentate ligands around an octahedral metal centre), and/or it increased the Brønsted acidity of the ligands. Although the field is still in its infancy, it represents an inspiring combination of both metal and organic catalysis and holds major unexplored potential to push the frontiers of asymmetric catalysis. Here we present an overview of this emerging field discussing the principles, applications and perspectives on the catalytic use of chiral metal complexes that operate as "organocatalysts in disguise". It has been demonstrated that these chiral metal complexes are efficient and provide high stereoselective control in asymmetric hydrogen bonding catalysis, phase-transfer catalysis, Brønsted acid/base catalysis, enamine catalysis, nucleophilic catalysis, and photocatalysis as well as bifunctional catalysis. Also, many of the catalysts have been identified as highly effective catalysts at remarkably low catalyst loadings. These hybrid systems offer many opportunities in the synthesis of chiral compounds and represent promising alternatives to metal-based and organocatalytic asymmetric transformations. This journal is © The Royal Society of Chemistry

Expanding the family of octahedral chiral‐at‐metal cobalt(Iii) catalysts by introducing tertiary amine moiety into the ligand

Chiral metal‐templated complexes are attractive catalysts for organic synthetic transformations. Herein, we introduce a novel chiral cobalt(III)‐templated complex based on chiral trans‐3,4‐diamino‐1‐benzylpyrrolidine and 3,5‐di‐tert‐butyl‐salicylaldehyde which features both hydrogen bond donor and Brønsted base functionalities. The obtained complexes were fully characterized by1H,13C NMR, IR‐, UV‐vis, CD‐spectroscopy and by a single X‐ray diffraction analysis. It was shown that chlorine anion is connected with amino groups of the complex via a hydrogen bonding. DFT calculations of charges and molecular electrostatic potential of the cobalt(III) complex showed that the basicity of the complex is certainly diminished as compared with the routine tertiary amines but the acidity of the conjugated acid of the complex should be increased. Thus, the catalytic potential of the complex may be much greater as a chiral acid than a chiral base. We believe that this work opens a new way in chiral bifunctional catalyst design. © 2021 by the authors. Licensee MDPI, Basel, Switzerland

Synthesis and Investigations of Chiral NNO Type Copper(II) Coordination Polymers

Herein, we report the synthesis and characterizations of chiral copper(II) coordination polymers (CPs) based on Schiff base of (S)-2-aminomethylpyrrolidine and salicylaldehyde derivatives. Single crystal X-ray analysis showed that copper(II) complex is 1D CP, with the repeating units LCu-OAc (L=ligand) organized in a perpendicular manner, where the copper(II) atoms are six-coordinated and have a distorted octahedral geometry. Also complex with chlorine counter ion having unusual dimeric structure was synthesized and characterized. It was demonstrated that CPs catalyze the enantioselective Henry reaction with up to 78% ee. © 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinhei

The Elaboration of a General Approach to the Asymmetric Synthesis of 1,4-Substituted 1,2,3-Triazole Containing Amino Acids via Ni(II) Complexes

Herein we report a general method of the asymmetric synthesis of 1,4-substituted 1,2,3-triazole containing amino acids via copper(I) iodide catalyzed “click reaction” between azides and chiral Ni(II) complexes of Schiff bases of (S)- or (R)-BPB with alkyne side chain derivatised amino acids. The final Ni(II) complexes were decomposed with aqueous HCl and the 1,4-substituted 1,2,3-triazole containing amino acids isolated with excellent enantioselectivities (>99% ee). The chiral auxiliary ligand (BPB=N-benzyl proline benzophenone) can be recycled and reused for the synthesis of starting Ni(II) complexes. © 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinhei

Economical Synthesis of α-Amino Acids from a Novel Family of Easily Available Schiff Bases of Glycine Esters and 2-Hydroxybenzophenone

We report a novel, efficient, and easily prepared substrate/precursor family of Schiff bases of various glycine esters with 2-hydroxybenzophenone, and their use for the synthesis of amino acids in quantitative yields. The Michael addition of the substrates to methyl acrylate gave two different types of product (cyclic or chain), depending on the nature of the base. Also, we demonstrated that one of the new substrates could be involved in an asymmetric version of the alkylation reaction (70% ee). © 2018 Georg Thieme Verlag KG Stuttgart · New York

Combinatorial approach towards synthesis of 2´,3´- dideoxynucleosides and enzyme-catalysed selective hydrolysis of diethyl acetamidomalonate and amides of polyacetocy aromatic carboxylic acid

507-512Seventeen noveI 3’-alkylthio-2’,3’-dideoxynucleosides have been synthesised by Michael-type addition of alkylthiols to an ,α,β-unsaturated hexose aldehyde, followed by acetylation, nucleoside coupling and deprotection. Based on these results, a general scheme for combinatorial synthesis of libraries of 3’-substituted 2’,3’-dideoxynucleosides has been proposed. Porcine pancreatic lipase (PPL) has been found to hydrolyse the amides of polyacetoxyaromatic carboxylic acids in a highly chemoselective fashion. The enzyme exclusively hydrolyses the ester group over the amide group. Hydrolysis of diethyl acetamidomalonate in phosphate buffer in the presence of α-chymotrypsin proceeds enantioselectively affording the (+)-monoacid

The Elaboration of a General Approach to the Asymmetric Synthesis of 1,4-Substituted 1,2,3-Triazole Containing Amino Acids via Ni(II) Complexes

Herein we report a general method of the asymmetric synthesis of 1,4-substituted 1,2,3-triazole containing amino acids via copper(I) iodide catalyzed “click reaction” between azides and chiral Ni(II) complexes of Schiff bases of (S)- or (R)-BPB with alkyne side chain derivatised amino acids. The final Ni(II) complexes were decomposed with aqueous HCl and the 1,4-substituted 1,2,3-triazole containing amino acids isolated with excellent enantioselectivities (>99% ee). The chiral auxiliary ligand (BPB=N-benzyl proline benzophenone) can be recycled and reused for the synthesis of starting Ni(II) complexes. © 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinhei

Self-Assembled Ionic Composites of Negatively Charged Zn(salen) Complexes and Triphenylmethane Derived Polycations as Recyclable Catalysts for the Addition of Carbon Dioxide to Epoxides

The design and synthesis of a novel type of self-assembled ionic composite composed of negatively charged Zn(salen) complexes and triphenylmethane derived polycations is reported. These composites were applied as easily recyclable catalysts for carbon dioxide addition to epoxides. The composites functioned as bifunctional catalysts which could be easily separated and recycled by precipitation from the reaction mixture upon addition of tetrachloromethane. The same batch of the catalyst could be employed for, at least, five runs with its catalytic properties improving as it was reused. A fully heterogeneous system was also prepared by cross-linking leuco dye with para-dibromoxylene and adding to it calculated amounts of Zn(salen) complex. The heterogeneous system was catalytically competent in the reaction between styrene oxide and carbon dioxide and its activity also increased on its reuse. © 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinhei