Copper(II)-Catalyzed Nitroaldol (Henry) Reactions: Recent Developments

Development and synthesis of a novel salen-type ligand based on phenylalanine for Mizoroki-Heck, and S-arylation cross-coupling reactions

In this article, we introduced a novel salen-type ligand precursor and applied it in the Pd-catalyzed Mizoroki-Heck and Cu-catalyzed S-arylation cross-coupling reactions. For the preparation of this structure, (DL)-phenyl alanine was employed as a starting material. These ligand precursor and related catalytic system can be readily synthesised. Various aryl halides (-I, -Br) and alkenes were applied successfully in this protocol to give the corresponding Mizoroki-Heck cross-coupling and S-arylated products in high to excellent yields.

Transition-Metal-Catalyzed Directed C-H Functionalization in/on Water

Directing group assisted C-H bond functionalization using transition-metal-catalysis has emerged as a reliable synthetic tool for the construction of regioselective carbon-carbon/heteroatom bonds. Off late, "in/on water directed transition-metal-catalysis", though still underdeveloped, has appeared as one of the prominent themes in sustainable organic chemistry. This article covers the advancements, mechanistic insights and application of the sustainable directed C-H bond functionalization of (hetero)arenes in/on water in the presence of transition-metal-catalysis.

Synthesis and Characterization of New Triazole-Bispidinone Scaffolds and Their Metal Complexes for Catalytic Applications

Bispidines are a family of ligands that plays a pivotal role in various areas of coordination chemistry, with applications in medicinal chemistry, molecular catalysis, coordination polymers synthesis, and molecular magnetism. In the present work, triazole moieties were introduced using the CuAAC click-reaction, with the aim of expanding the number of coordination sites on the bispidine core. The 1,2,3-triazole rings were thus synthesized on propargyl-derived bispidines after reaction with different alkyl azides. The new class of triazole-bispidines was characterized, and their chelation capabilities were evaluated with different metals through NMR titration, ESI-MS spectrometry, and single-crystal X-ray diffraction (SC-XRD). Finally, the suitability of these molecules as metal ligands for the catalytic Henry reaction was demonstrated with copper and zinc.

Catalytic Syn-Selective Nitroaldol Approach to Amphenicol Antibiotics: Evolution of a Unified Asymmetric Synthesis of (-)-Chloramphenicol, (-)-Azidamphenicol, (+)-Thiamphenicol, and (+)-Florfenicol

A unified strategy for an efficient and high diastereo- and enantioselective synthesis of (-)-chloramphenicol, (-)-azidamphenicol, (+)-thiamphenicol, and (+)-florfenicol based on a key catalytic syn-selective Henry reaction is reported. The stereochemistry of the ligand-enabled copper(II)-catalyzed aryl aldehyde Henry reaction of nitroethanol was first explored to forge a challenging syn-2-amino-1,3-diol structure unit with vicinal stereocenters with excellent stereocontrol. Multistep continuous flow manipulations were carried out to achieve the efficient asymmetric synthesis of this family of amphenicol antibiotics.

Copper(II) Complexes of Sulfonated Salan Ligands: Thermodynamic and Spectroscopic Features and Applications for Catalysis of the Henry Reaction

Copper(II) complexes formed with sulfonated salan ligands (HSS) have been synthesized, and their coordination chemistry has been characterized using pH-potentiometry and spectroscopic methods [UV-vis, electron paramagnetic resonance (EPR), and electron-electron double resonance (ELDOR)-detected NMR (EDNMR)] in aqueous solution. Several bridging moieties between the two salicylamine functions were introduced, e.g., ethyl (HSS), propyl (PrHSS), butyl (BuHSS), cyclohexyl (cis-CyHSS, trans-CyHSS), and diphenyl (dPhHSS). All of the investigated ligands feature excellent copper(II) binding ability via the formation of a (O^-,N,N,O^-) chelate system. The results indicated that the cyclohexyl moiety significantly enhances the stability of the copper(II) complexes. EPR studies revealed that the arrangement of the coordinated donor atoms is more symmetrical around the copper(II) center and similar for HSS, BuHSS, CyHSS, and dPhHSS, respectively, and a higher rhombicity of the g tensor was detected for PrHSS. The copper(II) complexes of the sulfosalan ligands were isolated in solid form also and showed moderate catalytic activity in the Henry (nitroaldol) reaction of aldehydes and nitromethane. The best yield for nitroaldol production was obtained for copper(II) complexes of PrHSS and BuHSS, although their metal binding ability is moderate compared to that of the cyclohexyl counterparts. However, these complexes possess larger spin density on the nitrogen nuclei than that for the other cases, which alters their catalytic activity.

Synthesis and practical applications of 2-(2-nitroalkyl)pyrroles

Functionalization of pyrroles introducing a 2-nitroalkyl moiety allows the formation of nitro-containing compounds to be used as pivotal intermediates for the synthesis of bioactive compounds. The reaction of pyrroles with nitroalkenes under the Friedel-Crafts conditions allows a direct entry to 2-(2-nitroalkyl)pyrroles. This approach can also be used for the preparation of enantioenriched derivatives exploiting asymmetric catalysis. In a complementary fashion, the Henry reaction between 2-formylpyrroles and nitroalkanes generates the corresponding nitroaldol products which upon dehydration and reduction of the intermediate 2-pyrrolylnitroethene efficiently afford 2-(2-nitroalkyl)pyrroles. This review article summarizes the most relevant procedures for the preparation of 2-(2-nitroalkyl)pyrroles during the last two decades as well as their significant practical applications.

1-Aminobicyclo[2.2.2]octane-2-carboxylic Acid and Derivatives As Chiral Constrained Bridged Scaffolds for Foldamers and Chiral Catalysts

The improvement of molecular diversity is one of the major concerns of chemists since the continuous development of original synthetic molecules provides unique scaffolds usable in organic and bioorganic chemistry. The challenge is to develop versatile platforms with highly controlled chemical three-dimensional space thanks to controlled chirality and conformational restraints. In this respect, cyclic β-amino acids are of great interest with applications in various fields of chemistry. In addition to their intrinsic biological properties, they are important precursors for the synthesis of new generations of bioactive compounds such as antibiotics, enzyme inhibitors, and antitumor agents. They have also been involved in asymmetric synthesis as efficient organo-catalysts in their free form and as derivatives. Finally, constrained cyclic β-amino acids have been incorporated into oligomers to successfully stabilize original structures in foldamer science with recent successes in health, material science, and catalysis. Over the last ∼10 years, we focused on bicyclic β-amino acids possessing a bicyclo[2.2.2]octane structure. This latter is a structural key element in numerous families of biologically active natural and synthetic products and is an interesting template for asymmetric synthesis. Nonetheless, reported studies on bicyclic carbo-bridged compounds are rather limited compared to those on bicyclic-fused and heterobridged derivatives. In this Account, we particularly focused on the synthesis and applications of the 1-aminobicyclo[2.2.2]octane-2-carboxylic acid, named, ABOC, and its derivatives. This highly constrained bicyclic β-amino acid, with a sterically hindered bridgehead primary amine and an endocyclic chiral center, displays drastically reduced conformational freedom. In addition, its high bulkiness strongly impacts the spatial orientation of the appended functionalities and the conformation of adjacent building blocks. Thus, we have first expanded a fundamental synthetic work by a wide ranging study in the field of foldamers, in the design of various stable peptide/peptidomimetic helical structures incorporating the ABOC residue (11/9-, 18/16-, 12/14/14-, and 12/10-helices). In addition, such bicyclic residue was fully compatible with and stabilized the canonical oligourea helix, whereas very few cyclic β-amino acids have been incorporated into oligoureas. In addition, we have pursued with the synthesis of some ABOC derivatives, in particular the 1,2-diaminobicyclo[2.2.2]octane chiral diamine, named DABO, and its investigation in chiral catalytic systems. Covalent organo-catalysis of the aldol reaction using ABOC-containing tripeptide catalysts provided a range of aldol products with high enantioselectivity. Moreover, the double reductive condensation of DABO with various aldehydes allowed the building of new chiral ligands that proved their efficiency in the copper-catalyzed asymmetric Henry reaction.

Recent advances in the catalytic synthesis of 3-aminooxindoles: an update

3-Substituted-3-aminooxindoles are versatile scaffolds and these motifs constitute the core structure of number of natural products and biologically active compounds.

Asymmetric catalysis in direct nitromethane-free Henry reactions

This review summarizes the current state and applications of catalytic Henry reactions involving complex nitroalkanes coupling with various carbonyl compounds to generate chiral β-nitro alcohol scaffolds with four adjacent stereogenic centers.

Four rare structurally characterized hetero-pentanuclear [Zn4Ln] bis(salamo)-type complexes: syntheses, crystal structures and spectroscopic properties

Four new hetero-pentanuclear 3d–4f complexes [Zn₄(L)₂La(NO₃)₂(OEt)(H₂O)] (1), [Zn₄(L)₂Ce(NO₃)₂(OMe)(MeOH)] (2), [Zn₄(L)₂Pr(NO₃)₂(OEt)(EtOH)] (3) and [Zn₄(L)₂Nd(NO₃)₂(OMe)(MeOH)] (4) were synthesized by the reactions of a newly synthesized octadentate bis(salamo)-based tetraoxime ligand (H₄L) with Zn(OAc)₂·2H₂O and Ln(NO₃)₃·6H₂O (Ln = La, Ce, Pr and Nd), respectively, and characterized via elemental analyses, FT-IR, UV-Vis spectroscopy and single crystal X-ray crystallography. The X-ray crystallographic investigation revealed that all Zn^II ions were located in N₂O₃ coordination spheres, and possessed a trigonal bipyramid coordination environment. The Ln^III ion lay in an O₈ coordination sphere, and adopted a distorted square antiprismatic coordination environment. Furthermore, supramolecular interactions and fluorescence properties were investigated.

Four new hetero-pentanuclear 3d–4f complexes 1–4 were synthesized and characterized. Supramolecular interactions and fluorescence properties of complexes 1–4 were investigated.

Henry Reaction Revisited. Crucial Role of Water in an Asymmetric Henry Reaction Catalyzed by Chiral NNO-Type Copper(II) Complexes

Chiral copper(II) and cobalt(III) complexes (1-5 and 6, respectively) derived from Schiff bases of (S)-2-(aminomethyl)pyrrolidine and salicylaldehyde derivatives were employed in a mechanistic study of the Henry reaction-type condensation of nitromethane and o-nitrobenzaldehyde in CH₂Cl₂ (CD₂Cl₂), containing different amounts of water. The reaction kinetics was monitored by ¹H and ¹³C NMR. The addition of water had a different influence on the activity of the two types of complexes, ranging from a crucial positive effect in the case of the copper(II) complex 2 to insignificant in the case of the stereochemically inert cobalt(III) complex 6. No experimental support was found by ¹H NMR studies for the classical Lewis acid complexation of the carbonyl group of the aldehyde by the central copper(II) ion, and, moreover, density functional theory (DFT) calculations support the absence of such coordination. On the other hand, a very significant complexation was found for water, and it was supported by DFT calculations. In fact, we suggest that it is the Brønsted acidity of the water molecule coordinated to the metal ion that triggers the aldehyde activation. The rate-limiting step of the reaction was the removal of an α-proton from the nitromethane molecule, as supported by the observed kinetic isotope effect equaling 6.3 in the case of the copper complex 2. It was found by high-resolution mass spectrometry with electrospray ionization that the copper(II) complex 2 existed in CH₂Cl₂ in a dimeric form. The reaction had a second-order dependence on the catalyst concentration, which implicated two dimeric forms of the copper(II) complex 2 in the rate-limiting step. Furthermore, DFT calculations help to generate a plausible structure of the stereodetermining transition step of the condensation.

Synthesis and structural characterization of a novel dinuclear Cu(ii) complex: an efficient and recyclable bifunctional heterogeneous catalyst for the diastereoselective Henry reaction

We describe the synthesis and structural characterization of a novel dinuclear Cu(ii) complex containing a new flexible mixed pyridine-carboxylate ligand. Single crystal X-ray diffraction reveals its uniqueness with two coordinated water molecules and four uncoordinated pyridyl groups. Utilizing the bifunctional catalytic sites, it has been found to be an efficient, robust and recyclable heterogeneous catalyst for the diastereoselective Henry (nitroaldol) reaction of aldehydes with nitroethane.

Stereodivergent synthesis of all the four stereoisomers of antidepressant reboxetine

Chiral amino alcohol-copper(ii) catalysts Cu-L1c and Cu-ent-L1c were utilized to promote the diastereoselective nitroaldol reactions of chiral aldehydes (S)-3 or (R)-3 with nitromethane, which respectively led to the preferential formation of certain stereoisomer for nitro diol derivatives 4. Using this catalytic protocol, all the four stereoisomers of the antidepressant reboxetine were divergently prepared. The highest overall yield of this synthetic route reached up to 30.5% from aldehyde (S)-3.

Application of chiral bi- and tetra-dentate bispidine-derived ligands in the copper(ii)-catalyzed asymmetric Henry reaction

Rationalized bispidinone-based ligands were reacted with copper(ii) salts, which afforded effective catalysts for asymmetric Henry reactions.

Chiral copper-salen complex grafted over functionalized mesoporous silica as an efficient catalyst for asymmetric Henry reactions and synthesis of the potent drug (R)-isoproterenol

A new chiral Cu(ii)@AFS-1 catalyzed asymmetric Henry reaction (ee = 94%, yield 96%) and diastereoselective Henry reaction have been reported. Additionally, drug (R)-(−)-isoproterenol was synthesized using this protocol.

Tri- and hexanuclear heterometallic Ni(ii)–M(ii) (M = Ca, Sr and Ba) bis(salamo)-type complexes: synthesis, structure and fluorescence properties

Tri- and hexanuclear Ni(ii)–M(ii) (M = Ca, Sr and Ba) complexes have been synthesized and characterized structurally. Fluorescence titration experiments show that the coordinating capabilities in the central O₆ site are in the order of Ca(ii) > Sr(ii) > Ba(ii).

Efficient synthesis of chiral benzofuryl β-amino alcohols via a catalytic asymmetric Henry reaction

A reliable and generally applicable method to access bioactive chiral benzofuryl β-amino alcohols via a catalytic asymmetric Henry reaction.