Showing papers on "Pyridine published in 2004"

Asymmetric catalysis with "planar-chiral" derivatives of 4-(dimethylamino)pyridine.

Abstract: Whereas chiral Lewis acid catalysis has been intensively investigated, chiral Lewis base (nucleophilic) catalysis has been comparatively neglected. We have developed “planar-chiral” derivatives of 4-(dimethylamino)pyridine (DMAP), a highly versatile nucleophilic catalyst, that are effective in a diverse array of processes, including the Staudinger synthesis of β-lactams, the acylation of silyl ketene acetals, and the kinetic resolution of amines.

Syntheses, structures, and photoluminescence of a novel class of d10 metal complexes constructed from pyridine-3,4-dicarboxylic acid with different coordination architectures.

Abstract: Two novel d10 metal coordination polymers [Zn(PDB)]n (1) and [Cd3(PDB)2(OH)2(H2O)2]n (2) (H2PDB = pyridine-3,4-dicarboxylic acid) have been synthesized under hydrothermal conditions and characterized by elemental analysis, IR, TG analysis, and single-crystal X-ray diffraction. Crystal data for 1: C7H3NO4Zn, orthorhombic Pna21, a = 8.423(17) A, b = 6.574(13) A, c = 12.899(3) A, Z = 4. Crystal data for 2: C14H12N2O12Cd3, monoclinic C2/c, a = 20.130(4) A, b = 6.692(13) A, c = 13.081(3) A, β = 102.78(3)°, Z = 4. Both compounds exhibit novel three-dimensional frameworks. Compound 1 not only possesses a one-dimensional rectangular channel but also contains infinite double-stranded helical chains. Compound 2 has two different types of channels, one being built up from pyridine rings and {CdO5N} and {CdO6} building units and the other being constructed from pyridine rings and {CdO5N} building units. Furthermore, both compounds show strong photoluminescence properties at room temperature.

An effective use of benzoic anhydride and its derivatives for the synthesis of carboxylic esters and lactones: A powerful and convenient mixed anhydride method promoted by basic catalysts

Abstract: Various carboxylic esters are obtained at room temperature in excellent yields with high chemoselectivities from nearly equimolar amounts of carboxylic acids and alcohols using 2-methyl-6-nitrobenzoic anhydride with triethylamine by the promotion of a basic catalyst such as 4-(dimethylamino)pyridine. A variety of lactones are also prepared in high yields at room temperature from the corresponding ω-hydroxycarboxylic acids with use of 2-methyl-6-nitrobenzoic anhydride in the presence of 4-(dimethylamino)pyridine. A similar reaction occurs with triethylamine when using a catalytic amount of 4-(dimethylamino)pyridine 1-oxide as an effective promoter for the intramolecular condensation reaction. These methods are successfully applied to the synthesis of erythro-aleuritic acid lactone and an eight-membered-ring lactone moiety of octalactins A and B. The efficiency of the cyclizations is compared to those of other reported lactonizations.

Formation of water-soluble pincer silver(I)-carbene complexes: a novel antimicrobial agent.

Abstract: Silver(I)-2,6-bis(ethanolimidazolemethyl)pyridine hydroxide (4a) and silver(I)-2,6-bis(propanolimidazolemethyl)pyridine hydroxide (4b) are water-soluble silver(I)-carbene complexes that were synthesized in high yield by reacting silver(I) oxide with N-substituted pincer ligands 3 (a = 2,6-bis(ethanolimidazoliummethyl)pyridine diiodide, b = 2,6-bis(propanolimidazoliummethylpyridine)pyridine dibromide). The X-ray crystal structure of 4a is a one-dimensional linear polymer, whereas the mass spectroscopy confirms a monomer in the gas phase. A change in the anion of 4a from a hydroxide to a hexafluorophosphate formed a silver(I)-carbene complex 4c that is dimeric in structure and insoluble in water. The bactericidal activities of the water-soluble silver(I)-carbene complexes were found to be improved over that of silver nitrate.

Homogeneous Palladium Catalyst Suppressing Pd Black Formation in Air Oxidation of Alcohols

Abstract: In homogeneous catalyst systems, there is the persistent problem that metal aggregation and precipitation cause catalyst decomposition and considerable loss of catalytic activity. Pd black formation is a typical example. Pd catalysts are known to easily aggregate and form Pd black, although they realize a wide variety of useful reactions in organic synthesis. In order to overcome this intrinsic problem of homogeneous Pd catalysis, we explored a new class of Pd catalyst by adopting aerobic oxidation of alcohols as a probe reaction. Herein we report a new catalyst system that suppresses the Pd black formation even under air and with a high substrate to catalyst molar ratio (S/C: more than 1000) in oxidation of alcohols. The novel pyridine derivatives having a 2,3,4,5-tetraphenylphenyl substituent and its higher dendritic unit at the 3-position of the pyridine ring were found to be excellent ligands with Pd(OAc)2 in the palladium-catalyzed air (balloon) oxidation of alcohols in toluene at 80 °C. Comparison ...

Chemistry of covalent inhibition of the gastric (H+, K+)-ATPase by proton pump inhibitors.

Abstract: Proton pump inhibitors (PPIs), drugs that are widely used for treatment of acid related diseases, are either substituted pyridylmethylsulfinyl benzimidazole or imidazopyridine derivatives. They are all prodrugs that inhibit the acid-secreting gastric (H(+), K(+))-ATPase by acid activation to reactive thiophiles that form disulfide bonds with one or more cysteines accessible from the exoplasmic surface of the enzyme. This unique acid-catalysis mechanism had been ascribed to the nucleophilicity of the pyridine ring. However, the data obtained here show that their conversion to the reactive cationic thiophilic sulfenic acid or sulfenamide depends mainly not on pyridine protonation but on a second protonation of the imidazole component that increases the electrophilicity of the C-2 position on the imidazole. This protonation results in reaction of the C-2 with the unprotonated fraction of the pyridine ring to form the reactive derivatives. The relevant PPI pK(a)'s were determined by UV spectroscopy of the benzimidazole or imidazopyridine sulfinylmethyl moieties at different medium pH. Synthesis of a relatively acid stable analogue, N(1)-methyl lansoprazole, (6b), allowed direct determination of both pK(a) values of this intact PPI allowing calculation of the two pK(a) values for all the PPIs. These values predict their relative acid stability and thus the rate of reaction with cysteines of the active proton pump at the pH of the secreting parietal cell. The PPI accumulates in the secretory canaliculus of the parietal cell due to pyridine protonation then binds to the pump and is activated by the second protonation on the surface of the protein to allow disulfide formation.

Metal chelation-controlled twisted intramolecular charge transfer and its application to fluorescent sensing of metal ions and anions.

Abstract: Two fluorescent ligands, N-(2-(5-cyanopyridyl))cyclen (L5) and N-(2-pyridyl)cyclen (L6) (cyclen = 1,4,7,10-tetraazacyclododecane), were designed and synthesized to control twisted intramolecular charge transfer (TICT) by metal chelation in aqueous solution. By complexation with Zn2+, L6 exhibited TICT emissions at 430 nm (excitation at 270 nm) in 10 mM HEPES (pH 7.0) with I = 0.1 (NaNO3) at 25 °C due to the perpendicular conformation of a pyridine ring with respect to a dialkylamino group, which was fixed by Zn2+−N(pyridine) coordination, as proven by potentiometric pH, UV, and fluorescence titrations and X-ray crystal structure analysis. We further describe that the 1:1 complexation of ZnL6 with guests such as succinimide, phosphates, thiolates, and dicarboxylates, which compete with a nitrogen in the pyridine ring for Zn2+ in ZnL6, induces considerable emission shift from TICT emissions (at 430 nm) to locally excited emissions (at ca. 350 nm) in neutral aqueous solution at 25 °C.

Pyridine substituted N-heterocyclic carbene ligands as supports for Au(I)-Ag(I) interactions: formation of a chiral coordination polymer.

Abstract: Reaction of 1,3-bis(2-pyridinylmethyl)-1H-imidazolium tetrafluoroborate, [H(pyCH2)2im]BF4, with silver oxide in dichloromethane readily yields [Ag((pyCH2)2im)2]BF4, 1·BF4 1·BF4 is converted to the analogous Au(I)-containing species, [Au((pyCH2)2im)2]BF4, 3, by a simple carbene transfer reaction in dichloromethane Further treatment with two equivalents of AgBF4 produces the trimetallic species [AuAg2((pyCH2)2im)2(NCCH3)2](BF4)3, 4, which contains two silver ions each coordinated to the pyridine moieties on one carbene ligand and to an acetonitrile molecule in a T-shaped fashion Monometallic [Ag((py)2im)2]BF4, 5, and [Au((py)2im)2]BF4, 6, are made analogously to 1·BF4 and 3 starting from 1,3-bis(2-pyridyl)-imidazol-2-ylidene tetrafluoroborate, [H(py)2im]BF4 Addition of excess AgBF4 to 6 yields the helical mixed-metal polymer, {[AuAg((py)2im)2(NCCH3)](BF4)2}n, 7 which contains an extended Au(I)−Ag(I) chain with short metal−metal separations of 28359(4) and 29042(4) A Colorless, monometallic [Hg((pyCH2

A New Tridentate Pincer Phosphine/N-Heterocyclic Carbene Ligand: Palladium Complexes, Their Structures, and Catalytic Activities

Abstract: A new imidazolium salt, 1,3-bis(2-diphenylphosphanylethyl)-3H-imidazol-1-ium chloride (2), for the phosphine/N-heterocyclic carbene-based pincer ligand, PC(NHC)P, and its palladium complexes were reported. The complex, [Pd(PC(NHC)P)Cl]Cl (4), was prepared by the common route of silver carbene transfer reaction and a novel direct reaction between the ligand precursor, PC(NHC)P.HCl and PdCl(2) without the need of a base. Metathesis reactions of 4 with AgBF(4) in acetonitrile produced [Pd(PC(NHC)P)(CH(3)CN)](BF(4))(2) (5). The same reaction in the presence of excess pyridine gave [Pd(PC(NHC)P)(py)](BF(4))(2) (6). The X-ray structure determination on 4-6 revealed the chiral twisting of the central imidazole rings from the metal coordination plane. In solution, fast interconversion between left- and right-twisted forms occurs. The twisting reflects the weak pi-accepting property of the central NHC in PC(NHC)P. The uneven extent of twisting among the three complexes further implies the low rotational barrier about the Pd-NHC bond. Related theoretical computations confirm the small rotational energy barrier about the Pd-NHC bond (ca. 4 kcal/mol). Catalytic applications of 4 and 5 have shown that the complexes are modest catalysts in Suzuki coupling. The complexes were active catalysts in Heck coupling reactions with the dicationic complex 5 being more effective than the monocationic complex 4.

Efficient asymmetric hydrogenation of pyridines.

Abstract: available and the reaction has the potential for the simultaneous creation of multiple stereocenters. Here we describe an efficient and unprecedented auxiliary-based method for the asymmetric hydrogenation of substituted pyridines (YR= N), which enables the stereoselective formation of piperidines with up to four new chiral centers in a single operation (Scheme 2a). The heterogeneous catalytic hydrogenation of pyridines is usually performed in acidic media. Protonation not only activates the pyridines for hydrogenation, it also suppresses catalyst poisoning by the resulting piperidines. We reasoned that single-point attachment of chiral oxazolidinones 4 for ease of introduction of the auxiliary in the 2-position of the pyridine would be ideal (Scheme 2a). Moreover, it occurred to us that whereas conformation 2 should be strongly preferred for unprotonated pyridines due to dipole-moment minimization, upon protonation hydrogen bonding between the pyridinium and the oxazolidinone moiety would favor conformation 5, in which the auxiliary is oriented coplanar with the pyridine ring but rotated by 1808. Indeed, on hydrogenation the iPr substituent shields one of the diastereotopic p-faces and selective hydrogen transfer to the opposite side leads to aminal 6 (Scheme 2b). Substrates 2 can be readily synthesized from oxazolidinones and the corresponding 2-bromoor chloropyridines 1 by copper catalysis (Scheme 2a). Gratifyingly, hydrogenation of pyridine 2d in acetic acid under a hydrogen atmosphere of 100 bar with PtO2 as the catalyst led to the formation of (S)-3-methyl piperidine (3d) in 85% ee. Different catalysts were screened, and Pd(OH)2/C was identified as the optimum catalyst, providing 3d in 98% ee (Table 1, entry 4). Importantly, the reaction does not stop at aminal 6d, but leads directly to piperidine 3d and oxazolidinone 4. Evidently traceless cleavage of the auxiliary occurs under the reaction conditions, thereby combining chirality transfer from and release of the auxiliary into a single operation. We were pleased to find that after treatment of the crude reaction mixture with hydrochloric acid, separation and purification of the less soluble piperidine hydrochloride 3d and the more soluble auxiliary could be achieved efficiently by simple extraction with ether/hexanes mixtures. The piperidinium hydrochloride 3d was obtained in 90% yield (98% ee) and 4 was recovered unchanged (93% yield, > 99% ee), allowing the recycling of the auxiliary. This method for the stereoselective synthesis of piperidines has been applied successfully to a large number of substrates, whereby the oxazolidinone with a tBu group often resulted in slightly improved ee values relative to those obtained with the iPr group (Table 1). Substituents in the 4, 5or 6-position of the 2-oxazolidinone-substituted pyridine can be used to create stereocenters at the corresponding positions (entries 1–7). Even multiple stereocenters can be generated, as exemplified by the stereoselective formation of diand trisubstituted piperidines in near-quantitative yield and excellent enantioselectivities (entries 9, 10). As far as we know this is the first highly asymmetric hydrogenation of an aromatic compound that selectively generates three stereocenters. Under milder conditions we even succeeded in the stereoselective synthesis (> 95:5) of aminal 6j, which bears four new chiral centers (entry 11). Furthermore, functional groups on the pyridine ring are well tolerated (entries 3, 5–6). The versatility of the process can be increased still further, since hydrogenation of 2d in the presence of acetaldehyde or acetic anhydride results in the formation of the corresponding (S)-N-ethylpiperidine 7 (entries 12, 13). The only present limitation concerns 3-substitution of the pyridine ring. A methyl substituent in the 3-position leads to a less reactive substrate, presumably because the oxazolidinone is rotated out of the plane of the pyridine ring thereby shielding both pfaces. Hydrogenation of 2h results in a nearly racemic product (entry 8). Finally, we were pleased to find that our method gives easy access to coniine (3b), the poisonous hemlock alkaloid, in excellent yield and enantiomeric excess (entry 2). Scheme 1. Stereoselective hydrogenation of (hetero)aromatic compounds (YR=CR, N).

Mechanistic Characterization of Aerobic Alcohol Oxidation Catalyzed by Pd(OAc)2/Pyridine Including Identification of the Catalyst Resting State and the Origin of Nonlinear [Catalyst] Dependence

Abstract: The Pd(OAc)(2)/pyridine catalyst system is one of the most convenient and versatile catalyst systems for selective aerobic oxidation of organic substrates. This report describes the catalytic mechanism of Pd(OAc)(2)/pyridine-mediated oxidation of benzyl alcohol, which has been studied by gas-uptake kinetic methods and (1)H NMR spectroscopy. The data reveal that turnover-limiting substrate oxidation by palladium(II) proceeds by a four-step pathway involving (1) formation of an adduct between the alcohol substrate and the square-planar palladium(II) complex, (2) proton-coupled ligand substitution to generate a palladium-alkoxide species, (3) reversible dissociation of pyridine from palladium(II) to create a three-coordinate intermediate, and (4) irreversible beta-hydride elimination to produce benzaldehyde. The catalyst resting state, characterized by (1)H NMR spectroscopy, consists of an equilibrium mixture of (py)(2)Pd(OAc)(2), 1, and the alcohol adduct of this complex, 1xRCH(2)OH. These in situ spectroscopic data provide direct support for the mechanism proposed from kinetic studies. The catalyst displays higher turnover frequency at lower catalyst loading, as revealed by a nonlinear dependence of the rate on [catalyst]. This phenomenon arises from a competition between forward and reverse reaction steps that exhibit unimolecular and bimolecular dependences on [catalyst]. Finally, overoxidation of benzyl alcohol to benzoic acid, even at low levels, contributes to catalyst deactivation by formation of a less active palladium benzoate complex.

A Family of Manganese Rods: Syntheses, Structures, and Magnetic Properties

Abstract: The reaction of the mixed-valent metal triangles [Mn3O(O2CR)6(py)3] (R = CH3, Ph, C(CH3)3) with the tripodal ligands H3thme (1,1,1-tris(hydroxymethyl)ethane) and H3tmp (1,1,1-tris(hydroxymethyl)propane) in MeCN, produces a family of manganese rodlike complexes whose structures are all derived from a series of edge-sharing triangles Variable temperature direct current (dc) magnetic susceptibility data were collected for all complexes in the 18−300 K temperature range in fields up to 70 T Complex 1, [Mn12O4(OH)2(PhCOO)12(thme)4(py)2], has an S = 7 ground state with the parameters g = 198 and D = −013 K Complex 2, [Mn8O4((CH3)3CCO2)10(thme)2(py)2] has a ground state of S = 6, with g = 181 and D = −036 K Complex 3, [Mn7O2(PhCO2)9(thme)2(py)3], has a spin ground states of S = 7 with the parameters g = 178 and D = −020 K The best fit for complex 4, [Mn6((CH3)3CCO2)8(tmp)2(py)2], gave a spin ground state of S = 3 with the parameters g = 173 and D = −075 K, but was of poorer quality than that norma

New entries in Lewis acid-Lewis base bifunctional asymmetric catalyst: catalytic enantioselective Reissert reaction of pyridine derivatives.

Abstract: The first catalytic enantioselective Reissert reaction of pyridine derivatives that affords products with excellent regio- and enantioselectivity is described. The key for success is the development of new Lewis acid−Lewis base bifunctional asymmetric catalysts containing an aluminum as a Lewis acid and sulfoxides or phosphine sulfides as a Lewis base. These reactions are useful for the synthesis of a variety of chiral piperidine subunits, and catalytic enantioselective formal synthesis of CP-293,019, a selective D4 receptor antagonist, was achieved. Preliminary mechanistic studies indicated that both sulfoxides and phosphine sulfides can activate TMSCN as a Lewis base. In addition, the sulfoxides with appropriate stereochemistry might stabilize a highly enantioselective bimetallic complex (a presumed active catalyst) through internal coordination to aluminum.

Chiral pyridine N-oxides: Useful ligands for asymmetric catalysis

Abstract: The synthesis and applications in asymmetric catalysis of chiral pyridine N-oxide derivatives is reviewed.

Series of Trinuclear NiIILnIIINiII Complexes Derived from 2,6-Di(acetoacetyl)pyridine: Synthesis, Structure, and Magnetism

Abstract: A series of trinuclear CuIILnIIICuII complexes with the bridging ligand 2,6-di(acetoacetyl)pyridine have been prepared by one-pot reaction with Cu(NO3)2·3H2O and Ln(NO3)3·nH2O in methanol. X-ray crystallographic studies for all the complexes indicate that two L2- ligands selectively sandwich two Cu(II) ions with the 1,3-diketonate entities and one Ln(III) ion with the 2,6-acetylpyridine entity to form a trinuclear CuLnCu core bridged by the enolate oxygen atoms. Cryomagnetic properties of the complexes are studied with respect to the electronic structure of the Ln ion.

New Chromium Complexes for Ethylene Oligomerization: Extended Use of Tridentate Ligands in Metal-Catalyzed Olefin Polymerization

Abstract: A family of chromium complexes bearing tridentate pyridine-based ligands are disclosed as highly active precatalysts for the oligomerization of ethylene. The ligands are comprised of two distinct types: Type 1, in which both ketone groups of 2,6-diacetylpyridine are converted to imines to produce pyridine bisimine NNN ligands; and Type 2, in which only one ketone group of 2,6-diacetylpyridine is condensed with an aniline derivative to give monoimine NNO coordination sets. Ligands of either type are coordinated to chromium(II) or chromium(III) chlorides, and activation of the resultant complexes with methylaluminoxane (MAO) produces highly active ethylene oligomerization and polymerization catalysts. Catalysts of Type 1 (NNN set) generally produce 1-butene when only two ortho alkyl substituents are present but switch to making waxes or polyethylene when the size and/or number of ortho substituents are increased. Catalysts of Type 2 (NNO set) produce waxes and polyethylene under all of the substitution pat...

N,N'-Ethylenebis(pyridoxylideneiminato) and N,N'-Ethylenebis(pyridoxylaminato): Synthesis, Characterization, Potentiometric, Spectroscopic, and DFT Studies of Their Vanadium(IV) and Vanadium(V) Complexes

Abstract: The Schiff base N,N'-ethylenebis(pyridoxylideneiminato) (H(2)pyr(2)en, 1) was synthesized by reaction of pyridoxal with ethylenediamine; reduction of H(2)pyr(2)en with NaBH(4) yielded the reduced Schiff base N,N'-ethylenebis(pyridoxylaminato) (H(2)Rpyr(2)en, 2); their crystal structures were determined by X-ray diffraction The totally protonated forms of 1 and 2 correspond to H(6)L(4+), and all protonation constants were determined by pH-potentiometric and (1)H NMR titrations Several vanadium(IV) and vanadium(V) complexes of these and other related ligands were prepared and characterized in solution and in the solid state The X-ray crystal structure of [V(V)O(2)(HRpyr(2)en)] shows the metal in a distorted octahedral geometry, with the ligand coordinated through the N-amine and O-phenolato moieties, with one of the pyridine-N atoms protonated Crystals of [(V(V)O(2))(2)(pyren)(2)]2 H(2)O were obtained from solutions containing H(2)pyr(2)en and oxovanadium(IV), where Hpyren is the "half" Schiff base of pyridoxal and ethylenediamine The complexation of V(IV)O(2+) and V(V)O(2) (+) with H(2)pyr(2)en, H(2)Rpyr(2)en and pyridoxamine in aqueous solution were studied by pH-potentiometry, UV/Vis absorption spectrophotometry, as well as by EPR spectroscopy for the V(IV)O systems and (1)H and (51)V NMR spectroscopy for the V(V)O(2) systems Very significant differences in the metal-binding abilities of the ligands were found Both 1 and 2 act as tetradentate ligands H(2)Rpyr(2)en is stable to hydrolysis and several isomers form in solution, namely cis-trans type complexes with V(IV)O, and alpha-cis- and beta-cis-type complexes with V(V)O(2) The pyridinium-N atoms of the pyridoxal rings do not take part in the coordination but are involved in acid-base reactions that affect the number, type, and relative amount of the isomers of the V(IV)O-H(2)Rpyr(2)en and V(V)O(2)-H(2)Rpyr(2)en complexes present in solution DFT calculations were carried out and support the formation and identification of the isomers detected by EPR or NMR spectroscopy, and the strong equatorial and axial binding of the O-phenolato in V(IV)O and V(V)O(2) complexes Moreover, the DFT calculations done for the [V(IV)O(H(2)Rpyr(2)en)] system indicate that for almost all complexes the presence of a sixth equatorial or axial H(2)O ligand leads to much more stable compounds

Influence of Stacking on Hydrogen Bonding: Quantum Chemical Study on Pyridine-Benzene Model Complexes

Abstract: The present work focuses on the influence of aromatic stacking on the ability of an aromatic nitrogen base to accept a hydrogen bond. Substituent effects were studied at the MP2 level for 10 complexes of a substituted benzene stacked with pyridine in a parallel offset conformation. The interaction energies between each substituted benzene and pyridine were analyzed in terms of Hartree−Fock, correlation, and electrostatic contributions. It appears that the basicity of pyridine is directly related to the electrostatic interaction between the cycles. It increases with increasing electron donating character of the benzene substituents. Also, density functional theory based descriptors such as global and local hardnesses and the benzene ring polarizability are found to adequately predict the interaction energy. These findings may be important in the study of DNA/RNA chains.

Generation and Characterization of Well-Defined Zn2+ Lewis Acid Sites in Ion Exchanged Zeolite BEA

Abstract: Bifunctional, Lewis, and Bronsted acidic molecular sieves were prepared by stepwise zinc ion exchange of zeolite BEA. The relation between the location of the Zn2+ cations in the zeolite structure and the Lewis acidity of the metal cations was explored. Several techniques were used for the detailed characterization of the material, including IR spectroscopy, with pyridine and acetonitrile as probe molecules, temperature programmed desorption, with ammonia and 2-propylamine as probe molecules, X-ray absorption spectroscopy, and 27Al MAS and MQMAS NMR spectroscopy. At low zinc concentrations (<0.15 Zn/Al) the cations are preferentially incorporated in the vicinity of two framework aluminum atoms. With increasing zinc loading (0.15 ≤ Zn/Al ≤ 0.26), additional cation sites are created at nearby framework aluminum pairs with two zinc cations being bridged by an oxygen atom. At higher zinc loading (0.26 < Zn/Al < 0.77) zinc oxide is formed in addition to the other two Zn2+ species.

Aqueous Biphasic Oxidation: A Water‐Soluble Polyoxometalate Catalyst for Selective Oxidation of Various Functional Groups with Hydrogen Peroxide

Abstract: A “sandwich” type polyoxometalate, Na12[(WZn3(H2O)2][(ZnW9O34)2], was used as an oxidation catalyst in aqueous biphasic reaction media to effect oxidation of alcohols, diols, pyridine derivatives, amines and aniline derivatives with hydrogen peroxide. The catalyst was shown by 183W NMR to be stable in aqueous solutions in the presence of H2O2 and showed only minimal non-productive decomposition of the oxidant. Secondary alcohols were selectively oxidized to ketones, while primary alcohols tended to be oxidized to the corresponding carboxylic acids, although secondary alcohols were selectively oxidized in the presence of primary alcohols. Vicinal diols yielded carbon-carbon bond cleavage products in very high yields. Pyridine derivatives were oxidized to the respective N-oxides, but strongly electron-withdrawing moieties inhibited the oxidation reaction. Primary amines were oxidized to the oximes, but significantly hydrolyzed in situ. Aniline derivatives were oxidized to the corresponding azoxy or nitro products depending on the substitution pattern in the aromatic ring. Catalyst recovery and recycle was demonstrated.

New pyridine carboxamide ligands and their complexation to copper(II). X-Ray crystal structures of mono-, di, tri- and tetranuclear copper complexes

Abstract: Seven new pyridine dicarboxamide ligands H2L1–7 have been synthesised from condensation reactions involving pyridine-2,6-dicarboxylic acid (H2dipic), pyridine-2,6-dicarbonyl dichloride or 2,6-diaminopyridine with heterocyclic amine or carboxylic acid precursors. Crystallographic analyses of N,N′-bis(2-pyridyl)pyridine-2,6-dicarboxamide monohydrate (H2L8·H2O), N,N′-bis[2-(2-pyridyl)methyl]pyridine-2,6-dicarboxamide and N,N′-bis[2-(2-pyridyl)ethyl]pyridine-2,6-dicarboxamide monohydrate revealed extensive intramolecular hydrogen bonding interactions. 2,6-Bis(pyrazine-2-carboxamido)pyridine (H2L6) and 2,6-bis(pyridine-2-carboxamido)pyridine (H2L7) reacted with copper(II) acetate monohydrate to give tricopper(II) complexes [Cu3(L)2(μ2-OAc)2]. X-Ray crystallography confirmed deprotonation of the amidic nitrogen atoms and that the (L6,7)2− ligands and acetate anions hold three copper(II) ions in approximately linear fashion. H2L8. Reacted with copper(II) tetrakis(pyridine) perchlorate to give [Cu(L8)(OH2)]2·2H2O, in which (L8)2− was tridentate through the nitrogen atoms of the central pyridine ring and the deprotonated carboxamide groups at one copper centre, with one of the terminal pyridyl rings coordinating to the other copper atom in the dimer. The corresponding reaction using H2L7 gave [Cu3(L7)2(py)2][ClO4]2, which transformed during an attempted recrystallisation from ethanol under aerobic conditions to a tetracopper(II) complex [Cu4(L7)2(L7–O)2].

Synthesis of photo-luminescent Zn(II) Schiff base complexes and its derivative containing Pd(II) moiety

Abstract: An efficient method was developed for the preparation of a series of zinc Schiff base complexes. Introduction of a pyridyl group as a bridging unit as well as incorporation of ethynyl and electron-donating groups into the salicylidene moiety of these complexes moderately enhances the photoluminescence intensity and quantum yield. Electron-rich palladium groups possibly influence the photophysical character through the bridging CC bond. The crystal structure of the pyridine adduct of a salen Zn complex is determined by X-ray diffraction analysis.