Synthesis, Structure, and Application of Self-Assembled Copper(II) Aqua Complex by H-Bonding for Acceleration of the Nitroaldol Reaction on Water

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.

Basified Graphene Oxide and PPO Composite Aerogel with Basified Graphene Oxide for Henry Reaction in Solvent-Free Conditions: A Green Approach

Novel basified graphene oxide and high-porosity monolithic composite aerogels of poly(2,6-dimethyl-1,4-phenylene oxide) (PPO) containing basified graphene oxide (b-eGO) have been prepared as recoverable and reusable catalysts for the Henry reaction in solvent-free conditions at room temperature. The results showed that, although b-eGO was able to promote the reaction, it suffered from reduced stability. On the other hand, PPO/b-eGO aerogels were able to efficiently promote the Henry reaction in solvent-free conditions. The product could be obtained pure without a purification step, and the catalyst was stable for over 15 months and could be easily recycled without losing catalytic efficiency. The stereochemical outcome was further investigated in the presence of PPO/b-eGO. Despite its negligible influence on diastereoselectivity, better efficiency and a sensible reduction of reaction time were observed.

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.

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.

Hydrosoluble Complexes Bearing Tris(pyrazolyl)methane Sulfonate Ligand: Synthesis, Characterization and Catalytic Activity for Henry Reaction

The catalytic activity of the water-soluble scorpionate coordination compounds [Cu(-NN’O-Tpms)2] (1), [Mn(Tpms)2] (2) and Li[FeCl2(-NN’N’’-Tpms)] (3) [Tpms = tris(pyrazolyl)-methane sulfonate, O3SC(pz)3], were studied towards the (Henry) reaction between benzaldehyde and nitromethane or nitroethane in aqueous medium to afford, respectively, 2-nitro-1-phenylethanol or 2-nitro-1-phenylpropanol, the latter in the syn and the anti diastereoisomeric forms. Complex 1 exhibited the highest activity under the optimum experimental conditions and was used to broaden the scope of the reaction to include several aromatic aldehydes achieving yields up to 94%.

Synthesis of chiral salalen ligands and their in-situ generated Cu-complexes for asymmetric Henry reaction

Chiral salalen ligands derived from (S)-proline and derivatives of salicyaldehydes were synthesized, and their in-situ generated Cu (II) complexes were evaluated in the asymmetric Henry reaction. Salalen ligand of different substituents on the phenyl moiety showed remarkable effect on the enantioselectivity of nitro-aldol product of 4-nitrobenzaldehyde and nitromethane. Cu (II) complex generated in situ with (S)-2-(tert-butyl)-6-((2-(((2-hydroxy-3-methylbenzylidene)amino)methyl)pyrrolidin-1-yl)methyl) phenol (10 mol%) and Cu (OAc)₂ .H₂ O (10 mol%), found to be better catalyst for nitro-aldol reaction between 4-nitrobenzaldehyde and nitromethane, gave corresponding product in 85% yield and 88% enantiomeric excess (ee) in isopropanol at 35°C after 40 hours. The catalyst also used for the Henry reaction with different substituted benzaldehydes and corresponding products were obtained in 22% to 99% yields with 66% to 92% ee. Henry reaction of 4-nitrobenzaldehyde and prochiral nitroethane gave anti-selective product (dr = 79/21; anti/syn) in a 91% yield with 80% ee.

A porous two-dimensional Zn(ii)-coordination polymer exhibiting SC–SC transmetalation with Cu(ii): efficient heterogeneous catalysis for the Henry reaction and detection of nitro explosives

The 2D Zn(ii)-CP, 1Zn and its Cu-analogue 1Cu exhibit efficient catalytic activity for the Henry reaction. 1Zn can be utilized for detecting trace amounts of nitro explosives in DMSO.

The synthesis, structure, topology and catalytic application of a novel cubane-based copper(II) metal-organic framework derived from a flexible amido tripodal acid

A novel chiral metal-organic framework, [Cu4(HL)2(H2O)4(MeO)4]n (), has been successfully synthesized from a tripodal flexible ligand (2S,2'S,2''S)-2,2',2''-(benzenetricarbonyltris(azanediyl))tripropanoic acid (H3L). Compound was characterized by IR and X-ray powder diffraction analysis. The structure was determined by X-ray single crystal diffraction analysis revealing that possesses a 3D network, featuring a tetranuclear cubane-type secondary building block [Cu4(MeO)4](4+), formed via the connection of four metal ions to four methoxide ions. These secondary building blocks are linked by four different HL(2-) ligands to construct a porous three dimensional framework of the dia topology with one-dimensional channels. Compound also acts as a heterogeneous catalyst for the diastereoselective nitroaldol (Henry) reaction, providing high yields (up to 91%) and good diastereoselectivities under ambient conditions. This catalyst can be recycled without significant loss of activity.

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.

Zinc metal-organic frameworks: efficient catalysts for the diastereoselective Henry reaction and transesterification

Three new compounds bearing different flexible side functional groups, viz. 2-acetamidoterephthalic acid (H2L1), 2-propionamidoterephthalic acid (H2L2) and 2-benzamidoterephthalic acid (H2L3), were synthesized and their coordination reactions with zinc(II) were studied. X-ray crystallography showed the formation of novel metal organic frameworks with different dimensionalities, where the side functional groups of amidoterephthalic acid and/or auxiliary ligands were found to play significant roles. These frameworks [Zn2(L1)2(4,4'-bipyridine)2(H2O)(DMF)]n (1), [Zn4(L2)3(OH)2(DMF)2(H2O)2]n (2) and [Zn(L3)(H2O)2]n·n/2(1,4-dioxane) (3) act as heterogeneous polymeric solid catalysts not only for the diastereoselective nitroaldol (Henry) reaction of different aldehydes with nitroalkanes but also for transesterification reactions. These MOF-based heterogeneous catalysts can be recycled without losing their activity.

Efficient pyrrolidine catalyzed cycloaddition of aziridines with isothiocyanates, isoselenocyanates and carbon disulfide “on water”

The cycloaddition of aziridines with isothiocyanates, isoselenocyanates and carbon disulfide is described using pyrrolidine on water. This protocol affords a potential route for the construction of the five membered heterocycles with high yields.

A nanoporous metal-organic framework with accessible Cu2+ sites for the catalytic Henry reaction

A novel 3D porous metal-organic framework containing 1D nanoscale opening channels was constructed from two kinds of pyridine carboxylates and copper(II) nodes with reachable copper sites located in the channel walls, which can be subsequently used to prompt the Henry reaction of benzaldehydes and nitroalkanes with remarkable catalytic activity compared with homogeneous copper cations.