Chiral Schiff Base Complexes Research Articles

In Situ Generated DBU⋅HF Acts as a Fluorinating Agent in a Hexafluoroisobutylation Tandem Reaction: An Effective Route to 5,5,5,5’,5’,5’‐Hexafluoroleucine

AbstractWe report the direct incorporation of the hexafluoroisobutyl group on a chiral glycine Schiff base complex mediated by 1,8‐diazabicyclo[5.4.0]undec‐7‐ene (DBU). The fluoroalkylation involves 2‐(bromomethyl)‐1,1,1,3,3,3‐hexafluoropropane reagent, which generates in situ hexafluoroisobutylene (HFIB), and reacts then with the enolate through a tandem allylic shift/hydrofluorination process. We showed that the use of neutral organic base DBU generates in situ an original DBU⋅HF salt, which preserves the fluoride nucleophilicity and acts as a fluorinating agent. This fluoride salt promotes the hydrofluorination of the pentafluorinated alkene overcoming the usual fluoride β‐elimination observed with α‐CF3‐vinyl reagents. With alkali metal bases, by contrast, the hydrofluorination is disfavored and the pentafluorinated alkene intermediate is obtained predominantly. This study highlights the critical role of the fluoride counter ion to preserve its nucleophilicity. The protocol is amenable to multidecagram scale synthesis of enantiopure (S)‐ and (R)‐5,5,5,5’,5’,5’‐hexafluoroleucine and their N‐Fmoc or N‐Boc derivatives in good overall yield.

An indirect approach for encapsulation of chiral cobalt catalyst in microporous Zeolite-Y

Hierarchical structure in microporous zeolite-Y was introduced to overcome the diffusional restriction for direct encapsulation of chiral cobalt (II) Schiff base complexes. Mesopores with cylindrical channels of ∼5 nm dimensions were formed on post modification of zeolite-Y and was evident from the transmission electron microscopy and surface area analysis. Two different chiral cobalt (II) Schiff base complexes with minor variation in the ligand structure were constructed inside the mesoporous zeolite-Y cavity. The formation of the metal complexes was ascertained from the decrease in surface area and mesoporous pore volume. The two synthesized heterogeneous chiral cobalt Schiff base complexes were used as catalysts for asymmetric Henry reaction. At −35 °C, more than 80% ee with S-major nitro-aldol product was achieved within 3 h. Initial activation of the reaction mixture with microwave irradiation was found to have dramatic influence on the catalytic performance of the two catalysts.

Fe3O4@dopa (dopa = dopamine hydrochloride) functionalized Mn(III) Schiff base complex: A promising magnetically separable heterogeneous catalyst for oxidative transformations

A chiral Schiff base complex has been prepared by treating (R)-1,2-diaminopropane with 3,5-dichlorosalicylaldehyde in ethanol, followed by addition of manganese chloride hexahydrate to generate a homogeneous catalyst, [MnL(Cl)(H2O)] (HMN). Crystal structure of the complex reveals its mononuclear nature. Circular dichroism (CD) studies indicate that the ligand and its corresponding complex contain an asymmetric center. The catalytic activity of HMN toward epoxidation of alkenes, oxidation of alcohols and oxidation of alkanes has been investigated in the presence of iodosylbenzenediacetate (PhI(OAc)2), in acetonitrile. In the present work we found yields to be much higher compared to our previous approaches. For further adaptation, we attached our efficient homogeneous catalyst with surface modified magnetic nanoparticles (Fe3O4@dopa) and thereby obtained a new magnetically separable nanocatalyst Fe3O4@dopa@MnLCl (FDM). This catalyst has been characterized and its oxidation ability assessed in similar conditions as those used for the homogeneous catalyst. Enantiomeric excess in epoxide yield reveals retention of chirality of the active site of Fe3O4@dopa@MnLCl. The catalyst can be recovered by magnetic separation and recycled several times without significant loss of catalytic activity.

New Cu(II) coordination polymer by chiral tridentate Schiff base ligand

The present research reports the synthesis, X-ray, magnetic and electronic properties for novel coordination polymer based upon copper (II) with chiral tridentate Schiff base ligand synthesized at condensation of acetylacetone and l-leucine amino acid. The investigation was also conducted by quantum mechanical calculations. The large energy gap indicates a high kinetic stability. Magnetic measurement gives predominant antiferromagnetic interactions within the chain. Results reveals further insight into copper(II) chiral tridentate Schiff base complexes.

Electronic, Conjugation, and Confinement Effects on Structure, Redox, and Catalytic Behavior of Oxido-Vanadium(IV) and -(V) Chiral Schiff Base Complexes

Change in oxophilicity, redox, and catalytic behavior with the modification of ligand system was observed for four oxido-vanadium Schiff base complexes. Mononuclear V(IV) complexes were obtained with methyl-substituted Schiff base ligand and one having higher conjugation. Electronic and conjugation effects reduce the oxophilicity of vanadium and led to mononuclear (LVIV═O, L = Schiff base ligand) species. However, μ-oxido bridged dinuclear [O═VVOL]2 complexes were formed with Schiff base ligands without such electronic effect under identical condition. Such structural variation also changed the electronic and redox properties of the metal complexes and brought about substantial differences in the catalytic activities of the vanadium Schiff base complexes. Confinement effect imparted by the zeolite-Y framework on the metal complexes synthesized inside the cavity by the “ship in a bottle synthesis” method was also found to alter the catalytic behavior of the metal complexes. All of the mononuclear and μ-oxi...

ChemInform Abstract: Oxidative Coupling of 2‐Naphthol to (R)/(S)‐BINOL by MCM‐41 Supported Mn‐Chiral Schiff Base Complexes.

AbstractEnantiopure BINOL is obtained by enantioselective oxidation of 2‐naphthol (I) in the presence of oxygen as oxidant using three Mn(III)‐chiral Schiff base complexes supported on MCM‐41 as effective reusable catalysts.

Enantioselective Epoxidation of Styrene by Manganese Chiral Schiff Base Complexes Immobilized on MCM-41.

Two chiral MnIII complexes synthesized and characterized by single-crystal X-ray diffraction were grafted onto mesoporous silica MCM-41. The MCM-41-immobilized solid chiral catalysts were characterized by various spectrochemical and physicochemical methods (XRD, FTIR, 29 Si NMR, TGA, UV/Vis/DRS, EPR, cyclic voltammetry, SEM, BET). Both the homogeneous and heterogeneous MnIII chiral complexes are found to be effective catalysts for epoxidation of styrene. MCM-41-supported chiral catalysts are found to retain their catalytic activity up to five cycles. When an ionic liquid was implemented to enhance the catalytic recyclability of the homogeneous catalysts, they can be recycled up to three times. However, after the third cycle the catalytic activities of the complexes were found to decrease due to rupturing of the chiral catalyst.

Synthesis, characterization, reactivity and catalytic activity of a novel chiral manganese Schiff base complex

A new type of manganese-Schiff base complex was synthesized and used as a catalyst for the oxidation of alcohols to the carbonyl compounds.

X-ray structure analysis, electronic and vibrational circular dichroism of chiral-at-metal dioxidovanadium(V) complexes with amino acids derived Schiff base ligands

Five new chiral dioxidovanadium Schiff base complexes derived from l-valine, l-leucine, l-histidine, l-threonine and d-threonine were synthesized and characterized by elemental analysis, 1H and 51V NMR and IR spectra. The absolute configurations of the complexes were determined by X-ray single crystal analysis in solid state and by electronic and vibrational circular dichroism in CH3CN solution. The l-threoninato complex, which contains two stereogenic centers in the amino acid site, does not show epimerization of the amino acid, in contrast with the analogical isoleucine-derived complexes reported recently. Conglomerate crystallization was observed in racemic dl-valine- and dl-threonine-derived complexes.

Effect of Nanospace Confinement on the Catalytic Activity and Stability of a Chiral Schiff Base Complex (CuL; L=C22 H24 N2 O4 ): A Combined Experimental and Theoretical Study.

Emerging trends in recyclable homogeneous chiral catalysts and their application in asymmetric synthesis are prompting a renewed interest in the field of catalysis as well as in industry. However, owing to fatal disadvantages of the homogeneous catalyst, they are not easily recoverable and recyclable. Herein, a comparison is made of the recyclability and stability of a homogeneous chiral CuII Schiff base complex of general formula CuL, in which L=C22 H24 N2 O4 , with that of a zeolite-Y-confined heterogeneous catalyst. On the basis of our experimental evidence, we demonstrate how self-decomposition and bimetallic formation disrupt the catalytic activity of a homogeneous catalyst. By exploiting the special structure of faujasite zeolite, self-decomposition of the chiral catalyst is fully controlled and efficiently used for the asymmetric nitroaldol reaction. The use of ionic liquid results in catalytic enhancement and provides a convenient way to recycle the homogeneous catalyst up to three cycles. When encapsulated in a cavity of faujasite by means of the "ship-in-a-bottle" synthesis method, it shows much better catalytic activity with enhanced enantioselectivity, recyclability, and stability. The heterogeneous catalyst is recycled up to nine cycles and retains the catalytic activity for a longer period of time relative to its homogeneous counterpart. The synthesized heterogeneous CuII complex provides the nitroaldol product with a maximum of 84 % yield and 87 % enantiomeric excess (ee; R isomer). Structural and reactivity differences between the homogeneous and heterogeneous chiral complexes are substantiated by density functional theory (DFT) calculations.

Synthesis and characterization of uranyl(VI) chiral Schiff-base complexes derived from salicylaldehyde and L-aminoacids

Complexes of uranyl(VI) (U(VI)) with tridentate ligands derived from salicylaldehyde and aminoacids are reported, bis 2-((2-hydroxybenzylidene)amino)acetic acid U(VI), UO2(SalGly)2 (1); tri aqua 2-((2-hydroxybenzylidene)amino)propanoic acid U(VI) nitrate, [UO2(SalAla)(H2O)3]NO3 (2); tri aqua 3-hydroxy-2-((2-hydroxybenzylidene)amino)propanoic acid U(VI) nitrate, [UO2(SalSer)(H2O)3]NO3 (3); tri aqua 2-((2-hydroxybenzylidene)amino)-3-methylbutanoic acid U(VI) nitrate, [UO2(SalVal)(H2O)3]NO3 (4), and tri aqua 2-((2-hydroxybenzylidene)amino)-4-(methylthio)butanoic acid U(VI) nitrate, [UO2(SalMet)(H2O)3]NO3 (5). The characterization of the complexes was performed by electrospray mass spectroscopy, ultraviolet-visible spectroscopy, Fourier transform infrared spectroscopy, nuclear magnetic resonance, fluorescence spectroscopy, and thermal analysis. The complexes are electrolytes (1 : 1) having a molar ratio M : L = 1 : 1, except 1, which is a nonelectrolyte with a molar ratio M : L = 1 : 2. The fluorescence intensity of the complexes is more intense than uncomplexed uranyl, except 1 and 5. High fluorescence of the uranyl SalAla complex (2) was used to assess complexes–DNA interaction based on fluorescence quenching assay. The interaction of 2 with DNA was assessed by measuring the relative fluorescence quenching of the complex when nucleic acids were added.

The role of chiral dopants in organic/inorganic hybrid materials containing chiral Schiff base Ni(II), Cu(II) and Zn(II) complexes

Chiral Schiff base complexes containing azo-groups, bis( N- R-1-cyclohexylethyl-4-phenyldiazenylsalicydenaminato) nickel(II), copper(II) and zinc(II) complexes, and without azo-groups, bis( N- R-1-cyclohexylethyl-3,5-dichlorosalicydenaminato) nickel(II), copper(II) and zinc(II) complexes, affording a distorted square planar trans-[MN 2O 2] coordination geometry were prepared. Organic/inorganic hybrid materials in polymethylmethacrylate ( PMMA) spincoat films of the complexes (both the azobenzene ( AZ) containing type and the latter complexes of the AZ separated type) were assembled for a comparison of polarized UV light induced molecular arrangement caused by the Weigert effect. Investigation of the parameters for the optical anisotropy of the metal complexes as well as AZ suggested that the degree of increasing optical anisotropy of the containing type was higher than that of the separated type based on π–π ∗ (of which a characteristic band appeared around 380 nm) and n–π ∗ bands of polarized absorption electronic spectra. In the AZ containing type, the rigid nickel(II) or zinc(II) complexes easily increase the optical anisotropy compared to the flexible copper(II) complexes. In the AZ separated type, interestingly, enhancement of some CD bands suggests the role of chiral dopants of some complexes without azo-groups for AZ.

Chiral discrimination asserted by enantiomers of Ni (II), Cu (II) and Zn (II) Schiff base complexes in DNA binding, antioxidant and antibacterial activities

Chiral Schiff base ligands ( S)-H 2L and ( R)-H 2L and their complexes ( S-Ni-L, R-Ni-L, S-Cu-L, R-Cu-L, S-Zn-L and R-Zn-L) were synthesized, characterized and examined for their DNA binding, antioxidant and antibacterial activities. The complexes showed higher binding affinity to calf thymus DNA with binding constant ranging from 2.0 × 10 5 to 4.5 × 10 6 M −1. All the complexes also exhibited remarkable superoxide (56–99%) and hydroxyl scavenging (45–89%) activities as well as antibacterial activities against gram (+) and gram (−) bacteria. However, none of the complexes showed antifungal activity. Conclusively, S enantiomers of the complexes were found to be relatively more efficient for DNA interaction, antioxidant and antibacterial activities than their R enantiomers. This study reveals the possible utilization of chiral Schiff base complexes for pharmaceutical applications.

Polarized spectroscopy and hybrid materials of chiral Schiff base Ni(II), Cu(II), Zn(II) complexes with included or separated azo-groups

Chiral Schiff base complexes containing azo-groups, bis(N-R-1-naphtylethyl-4-phenyldiazenylsalicydenaminato) nickel(II), copper(II), and zinc(II) complexes affording a distorted square planar trans-[MN2O2] coordination geometry were prepared newly. Organic/inorganic hybrid materials in polymethylmethacrylate (PMMA) cast films of them (a containing type) or the analogous chiral Schiff base complexes, bis(N-R-1-phenylethyl-3,5-dichlorosalicydenaminato) nickel(II), copper(II), and zinc(II), and azobenzene (AZ) (a separated type) were assembled for comparison of polarized UV light induced molecular arrangement caused by Weigert effect. Investigation of parameters for optical anisotropy of metal complexes as well as AZ suggested that the degree of increasing optical anisotropy of the containing type is higher than that of the separated type based on π–π∗ (of which characteristic band appeared around 380nm), n–π∗, and d–d bands of polarized absorption electronic spectra. Rigid nickel(II) or zinc(II) complexes are easy to increase optical anisotropy than flexible copper(II) complexes for both types.

ChemInform Abstract: Dimeric and Polymeric Chiral Schiff Base Complexes and Supported BINOL Complexes as Potential Recyclable Catalysts in Asymmetric Kinetic Resolution and C—C Bond Formation Reactions

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Rational synthesis of chiral layered magnets by functionalization of metal simple hydroxides with chiral and non-chiral Ni(ii) Schiff base complexes

Synthesis of new heterometallic layered magnets with controlled chirality have been achieved by insertion of chiral and non-chiral salen-type Ni(II) complexes into copper and cobalt layered simple hydroxides.

From homogeneously to heterogeneously catalyzed cyclopropanation reactions: New polymeric membranes embedding cobalt chiral schiff base complexes

In this paper we report the preparation of catalytic polymeric membranes having the complex [( N,N′- bis(3- tert-butyl-5-(heptadecafluorooctyl)salicylidene)- trans-1,2-cyclohexanediamine)Co] embedded into Hyflon AD60X, PES and PSf and their use in the cyclopropanation reaction of olefins with ethyl diazoacetate (EDA). A study of the reaction in the homogeneous phase was also performed. The kinetics of the cyclopropanation reaction is first-order in the diazoacetate and the catalyst and zero order in the olefin, but we observed a more complex dependence on the additive, NMI ( N-methylimidazole). The positive effect of the additive is completely reversed at NMI concentration higher than 6·10 −2 M.

Synthesis, Structure, Magnetic Properties and Aqueous Optical Citrate Detection of Chiral Dinuclear CuII Complexes

AbstractTwo chiral dimeric CuII Schiff base complexes, Na2[Cu(S‐lys)2·(H2O)2]·(ClO4)2 (1) and [Cu(S‐ornH)2·(H2O)2]·(ClO4)2 (2) have been prepared by an easy one‐pot method in which the L‐lysine and L‐ornithine‐based Schiff base ligands are generated in situ. These complexes have been characterized by various spectroscopic techniques and single‐crystal X‐ray diffraction. The molecular structures show that both complexes are indeed dimeric with CuII centers that are phenolate‐bridged. Variable‐temperature magnetic analysis revealed that a strong antiferromagnetic coupling interaction was mediated through the phenolate bridge between the two CuII centers. Since these compounds were highly soluble in water, we explored carboxylate sensing capability through colorimetric indicator displacement assays (IDAs). By using pyrocatechol violet (PV) as a displaceable colorimetric indicator, citrate ion selectivity was demonstrated in 100 % water solution in the physiological pH range.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)

Polarized spectroscopy of hybrid materials of chiral Schiff base cobalt(II), nickel(II), copper(II), and zinc(II) complexes and photochromic azobenzenes in PMMA films

A chiral Schiff base complex, bis( N- R-1-phenylethyl-3,5-dichlorosalicydenaminato) cobalt(II) was prepared newly and characterized to be a distorted tetrahedral trans-[CoN 2O 2] coordination geometry. Organic/inorganic hybrid materials containing the related cobalt(II), nickel(II), copper(II), and zinc(II) complexes and photochromic azobenzene in polymethylmethacrylate (PMMA) cast films were assembled for comparison of their flexibility and molecular arrangement in the photofunctional medium. Characterization of each component and hybrid materials was carried out by means of absorption and CD spectra and thermal analysis (TG–DTA and DSC). Moreover, we have attempted to observe changes of conformation and/or molecular arrangement of the complexes or azobenzene induced by cis– trans photoisomerization of azobenzene after alternate irradiation of polarized UV and visible light. Gradual increase of optical anisotropy was observed for all the hybrid materials regardless of flexibility of Schiff base complexes, and the degree of dichroism and weak intermolecular interactions were discussed based on polarized absorption electronic spectra.

Dimeric & Polymeric Chiral Schiff Base Complexes and Supported BINOL Complexes as Potential Recyclable Catalysts in Asymmetric Kinetic Resolution and C–C Bond Formation Reactions

This review is based on dimeric and polymeric chiral Schiff base metal complexes and chiral BINOL supported metal complexes as potential recyclable catalysts for kinetic resolution of racemic and meso epoxide and asymmetric C–C bond formation reactions e.g., asymmetric addition of diethylzinc to aldehydes, enantioselective addition of phenylacetylene to aldehydes, asymmetric nitro-Aldol reactions and asymmetric cyanation reaction.