Chiral oxazoline ligands containing a 1,2,4-triazine ring and their application in the Cu-catalyzed asymmetric Henry reaction

Chiral amido-oxazoline functionalized MCM-41: A sustainable heterogeneous catalyst for enantioselective Kharasch-Sosnovsky and Henry reactions

In this study, a series of chiral amido-oxazoline ligands was synthesized with a primary focus on immobilizing the most effective ligands on MCM-41 mesoporous material. Following several attempts, the para-nitro group of the chiral amido-oxazoline ligands was successfully reduced to amino group, enabling their immobilization on MCM-41. The resulting chiral heterogeneous amido-oxazoline ligands were characterized using various techniques, including FT-IR, XRD, TGA, SEM, TEM, EDX, and BET-BJH, confirming the successful immobilization of the amido-oxazoline ligands. A comparison of the efficiency of the homogeneous and heterogeneous amido-oxazoline-based ligands in the Kharasch-Sosnovsky and Henry reactions revealed better performance of the heterogeneous ligand. The immobilized amido-oxazoline-copper complexes exhibited remarkable catalytic activity, achieving excellent yields and enantioselectivities (up to 88 % ee) in the Kharasch-Sosnovsky reaction, and delivering excellent yields with moderate enantioselectivities in the Henry reaction. Notably, the Henry reaction proceeded with moderate diastereoselectivity, favoring the syn diastereomer, under solvent-free conditions, highlighting the sustainability of the process. The heterogeneous nature of the catalysts facilitated effortless recovery and efficient reusability.

Chiral pyridine oxazoline and 1,2,4-triazine oxazoline ligands incorporating electron-withdrawing substituents and their application in the Cu-catalyzed enantioselective nitroaldol reaction

Eight pyridine-containing and four 1,2,4-triazine-containing chiral oxazoline ligands incorporating electron-withdrawing substituents have been synthesized by two-step route including Buchwald–Hartwig amination. Enantio-inducing activity of the ligands has been assessed in the copper-catalyzed asymmetric nitroaldol reactions and the influence of the electron-withdrawing substituents on the ligands' activity has been investigated.

Graphical abstract

Synthesis and structures of three new pyridine-containing oxazoline ligands of complexes for asymmetric catalysis

Three new chiral pyridine-containing oxazoline derivatives with fluorine and perfluoromethyl groups, namely, 2-({2-[(4S)-4-phenyl-4,5-dihydro-1,3-oxazol-2-yl]phenyl}amino)-5-(trifluoromethyl)pyridine, C₂₁H₁₆F₃N₃O, 2-({5-fluoro-2-[(4S)-4-isopropyl-4,5-dihydro-1,3-oxazol-2-yl]phenyl}amino)-5-(trifluoromethyl)pyridine, C₁₈H₁₇F₄N₃O, and 2-({2-[(3aR,8aS)-8,8a-dihydro-3aH-indeno[1,2-d]oxazol-2-yl]phenyl}amino)-5-(trifluoromethyl)pyridine, C₂₂H₁₆F₃N₃O, as chiral ligands in metal-catalysed asymmetric reactions, were synthesized and characterized by spectral and X-ray diffraction methods. The conformation of the molecules is influenced by strong N-H...N hydrogen bonding and weak C-H...X (X = O and N) interactions. There are no intermolecular hydrogen bonds in the crystal structures of the analysed compounds. Hirshfeld surface analysis showed that the H...H contacts constitute a high percentage of the intermolecular interactions. The conformational analysis was performed by theoretical calculations using the density functional theory (DFT) method. The mechanism of complex formation in terms of the electron-withdrawing effect of the substituents on the oxazoline ring and the ligand conformation is discussed.

Hybrid nanomaterials for asymmetric purposes: green enantioselective C–C bond formation by chiralization and multi-functionalization approaches

We present a simple and novel strategy for synthesis of a MOF-based chiral multi-functional hybrid nanomaterial through chiralization and multi-functionalization approaches for asymmetric purposes.

Copper-catalyzed [4 + 2] annulation reaction of β-enaminones and aryl diazonium salts without external oxidant: synthesis of highly functionalized 3H-1,2,4-triazines via homogeneous or heterogeneous strategy

Herein, both homogeneous and heterogeneous strategies were developed to access highly functionalized 3H-1,2,4-triazines using Cu-catalyzed [4 + 2] annulation.

Chiral oxazoline ligands with two different six-membered azaheteroaromatic rings – synthesis and application in the Cu-catalyzed nitroaldol reaction

Synthesis and catalytic activity of chiral ligands 5,6-diphenyl-3-{3-[(4S/R)-4-R/Ar-4,5-dihydro-1,3-oxazol-2-yl]pyridin-2-yl}amino-1,2,4-triazines 2 and their analogs 3 possessing an N-oxide function in the pyridine ring are described. The pivotal step in the synthesis of ligands 2 is the Buchwald-Hartwig Pd-catalyzed cross-coupling reaction between 3-bromo-5,6-diphenyl-1,2,4-triazine (7a) and enantiopure 3-(4,5-dihydro-1,3-oxazol-2-yl)pyridin-2-amines 6a–d. Aromatic nucleophilic substitution of chlorine in 3-chloro-5,6-diphenyl-1,2,4-triazine (7b) with 3-(4,5-dihydro-1,3-oxazol-2-yl)pyridin-2-amine 1-oxides 12 was investigated as the key reaction in the synthesis of ligands 3. Two undesired derivatives 13 or 14, resulting from unexpected reactions of 3, were isolated depending on reaction conditions. Compounds 2 and 3 as well as the side products 13 and 14 were screened as chiral ligands in the copper catalyzed enatioselective nitroaldol reaction.

Structural characterization of copper complexes with chiral 1,2,4-triazine-oxazoline ligands

The crystal structure determination of oxazoline-1,2,4-triazine ligand

Synthesis of 1,2,4-triazine derivatives via [4 + 2] domino annulation reactions in one pot

Simple and efficient [4 + 2] domino annulation reactions have been developed for the synthesis of 1,2,4-triazine derivatives.

Amide functionalized metal–organic frameworks for diastereoselective nitroaldol (Henry) reaction in aqueous medium

The metal–organic frameworks [Zn₂(L)₂(1,4-BDC)]_n·2n(DMF) (L = 4-(pyridin-4-ylcarbamoyl)benzoate; BDC = benzenedicarboxylate) and [Cu(L)₂]_n·4n(DMF)·n(H₂O) work as catalysts for the diastereoselective nitroaldol reaction in aqueous medium.

Sulfonated Schiff base dinuclear and polymeric copper(ii) complexes: crystal structures, magnetic properties and catalytic application in Henry reaction

Aqueous medium syntheses and nitroaldol catalytic studies of three pseudohalide bridged copper(ii) complexes characterized by single crystal X-ray structure analysis and variable temperature magnetic studies are reported.