Control over the oxidative reactivity of metalloporphyrins. Efficient electrosynthesis of meso,meso-linked zinc porphyrin dimer

The electrochemical oxidation of zinc(II) 5,15-p-ditolyl-10-phenylporphyrin at its first oxidation potential leads to the formation of the corresponding meso-meso porphyrin dimer as the main product. The number of electrons abstracted, the addition of the hindered base 2,6-lutidine as well as operating in DMF, instead of a CH(2)Cl(2)/CH(3)CN mixture are the key parameters to obtain high yields of the desired coupling product. Indeed, when the electrolyses are carried out in the CH(2)Cl(2)/CH(3)CN mixture, the unexpected zinc(II) 5-chloro-10,20-p-ditolyl-15-phenyl porphyrin is produced as a by-product, the chlorine atom originating from the CH(2)Cl(2) solvent. The monomer and the dimer are characterised by electrochemical analysis. The signature of the dimer is clearly distinguished on the cyclic voltammogram of the monomer on condition of the prior addition of 2,6-lutidine as a hindered base, indicating that the dimerisation process is thus strongly accelerated. Besides, unprecedented X-ray crystallographic structures of the monomer and the meso-meso dimer are presented and their respective structural parameters are compared.     

Anodic oxidation of 1-naphthylamine in acetonitrile

The anodic oxidation of 1-naphthylamine (ArNH₂) has been studied at the platinum electrode in acetonitrile by controlled potential electrolysis, chronopotentiometry, cyclic voltammetry, cyclic voltammetry and spectrometric methods. From the electrochemical data a complex ECE type of mechanism is inferred with an overall efficiency of one electron per molecule of ArNH₂. The electrochemical steps are reversible and the chemical process seems to follow second order kinetics and is extremely fast. The cation radical produced in the first charge transfer step suffers fast decomposition to give dimers which are easier to oxidize than ArNH₂. The dimeric products were identified by comparison with authentic samples. Further complications arise due to slow reactions of the oxidized dimers. Electrochemical studies in basic media (pyridine and 2,6-lutidine) and acid media (anhydrous trifluoroacetic acid) were also performed.     

Mechanistic details of metal-free cyclization reaction of organophosphorus oxide with alkynes mediated by 2,6-lutidine and Tf2O

Theoretical investigations have elucidated the mechanism of metal-free electrophilic phosphinative cyclization of alkynes reaction reported by Miura and coworkers. Two competitive mechanisms I and II were explored without or with 2,6-lutidine. Both of I and II involve transformation of P(V) to P(III), electrophilic addition, ring opening and cyclization/cyclization, hydrogen-transfer, and oxidation. The rate-determining step of mechanism I and competitive less-step II is electrophilic [2 + 1] cycloaddition and electrophilic addition via single C P bond formation with activation barrier of 13.5 and 10.6 kcal/mol, respectively. Our calculation results suggested that the cumulative effect of the isomer of 2,6-lutidine and Tf₂O as well as TfO⁻ affects the title reaction to some extent, and simultaneously activates key reaction sites and reverses the polarities of them via the formation of abundant noncovalent interactions to decrease activation barriers of TSs. In addition, the effects of two series substituents on reactivity of phosphine oxide were investigated. Therefore, our study will serve as useful guidance for more efficient metal-free synthesis of organophosphorus compounds mediated by pyridine reagents.     

Transformation,par 1′anhydride acètique,d'aldèhydes à caractére èlectrophile prononcè; catalyse par des pyridines. 1. Acyloïnes et hydrates di-o-acètylès d'aldèhydes

2-Benzothiazole-carbaldehyde is transformed into di-O-acetyl-enol-(benzothiazolecarboxyl-2)-oin in the presence of acetic anhydride and of pyridine as catalyst. Without pyridine or with 2,6-lutidine no reaction occurs. A mechanism of this reaction is proposed. No reaction was observed in the case of benzaldehyde. Choral reacts with acetic anhydride in the presence of pyridine as well as of 2,6-lutidine as catalyst to give 1,1-diacetoxy-2,2,2-trichloro-ethane. A mechanism is proposed, in which in an intermediate state the acetate ion (and not pyridine, for steric reasons) attacks the carbon of the carbonyl function of the conjugate acid with the acetylium cation to yield 1,1-diacetoxy-2,2,2-trichloro-ethane. These two reactions occur only with aldehydes whose carbonyl is very electrophilic, and seem to be a possible way to point out the presence of an acylium cation in pyridine medium.     

多金属ZSM-5催化剂的制备及其催化氨化合成2,6-二甲基吡啶

通过掺杂制备了一系列多金属改性的ZSM-5催化剂,并用于丙酮和甲醇氨化合成2,6-二甲基吡啶的反应中.在固定床反应器上筛选出催化性能良好的催化剂6%Pb-0.5%Fe-0.5%Co/ZSM-5(200),探讨了过渡金属掺杂的促进作用,并考察了反应温度、氨醇比、酮醇比、水含量和停留时间对反应性能的影响.结果表明,该催化剂...     

Synthesis, structure, and characterization of new mononuclear Mn(II) complexes. Electrochemical conversion into new oxo-bridged Mn(2)(III,IV) complexes. Role of chloride ions

Two Mn(II) complexes are isolated and X-ray characterized, namely, cis-[(L(2))Mn(II)(Cl)(2)] (1) and [(L(3))Mn(II)Cl(OH(2))](ClO(4)) (2(ClO(4))), where L(2) and L(3) are the well-known tetradentate N,N'-dimethyl-N,N'-bis(2-pyridylmethyl)ethane-1,2-diamine and N,N'-dimethyl-N,N'-bis(2-pyridylmethyl)propane-1,3-diamine ligands, respectively. The crystal structure reveals that whereas the ligand L(2) is in the cis-alpha conformation in complex 1, the ligand L(3) is in the more unusual cis-beta conformation in 2. EPR spectra are recorded on frozen solutions for both complexes and are characteristic of Mn(II) species. Electrochemical behaviors are investigated on acetonitrile solution for both complexes and show that cation 2 exists as closely related Mn(II) species in equilibrium. For both complexes exhaustive bulk electrolyses of acetonitrile solution are performed at oxidative potential in various experimental conditions. In the presence of 2,6-lutidine and after elimination of chloride ligands, the formation of the di-mu-oxo mixed-valent complexes [(L(2))Mn(III)(mu-O)(2)Mn(IV)(L(2))](3+) (3a) and [(L(3))Mn(III)(mu-O)(2)Mn(IV)(L(3))](3+) (4) is confirmed by UV-vis and EPR spectroscopies and cyclic voltammetry. In addition crystals of 4(ClO(4))(3) were isolated, and the X-ray structure reveals the cis-alphaconformation of L(3). In the absence of 2,6-lutidine and without elimination of the exogenous chloride ions, the electrochemical oxidation of 1 leads to the formation of the mononuclear Mn(III) complex, namely, [(L(2))Mn(III)(Cl)(2)](+) (5), as confirmed by UV-vis as well as parallel mode EPR spectroscopy and cyclic voltammetry. In the same conditions, the electrochemical oxidation of complex 2 is more intricate, and a thorough analysis of EPR spectra establishes the formation of the binuclear mono-mu-oxo mixed-valent [(L(3))ClMn(III)(mu-O)Mn(IV)Cl(L(3))](3+) (6) complexes. Electrochemical conversion of Mn(II) complexes into mixed-valent Mn(2)(III,IV) oxo-bridged complexes in the presence of 2,6-lutidine is discussed. The role of the chloride ligands as well as that of L(3) in the building of oxo bridges is discussed. Differences in behavior between L(2) and L(3) are commented on.     

Vapor-phase catalytic oxidation of aβ-picoline fraction

The vapor-phase catalytic oxidation with atmospheric oxidation of a -picoline fraction consisting of a mixture of 3- and 4-methylpyridines and 2,6-lutidine containing small amounts of 2-methylpyridine and pyridine has been studied. On a vanadium oxide catalyst at 440° C with a contact time of 1.2 sec and a molar ratio of oxygen to steam to methylpyridine of 7 54 1, a high degree of conversion of the 4-methylpyridine and the 2,6-lutidine into pyridine-4-aldehyde and 6-methylpyridine-2-aldehyde, respectively, has been achieved. The main component of the aqueous solution of the catalyzate (after the isolation of thepyridine aldehydes in the form of oximes) is 3-methylpyridine.     

Synthesis of N-acyl-N,O-acetals from N-aryl amides and acetals in the presence of TMSOTf

Secondary amides undergo in situ silyl imidate formation mediated by TMSOTf and an amine base, followed by addition to acetal acceptors to provide N-acyl-N,O-acetals in good yields. An analogous, high-yielding reaction is observed with 2-mercaptothiazoline as the silyl imidate precursor. Competing reduction of the acetal to the corresponding methyl ether via transfer hydrogenation can be circumvented by the replacement of CY₂NMe with 2,6-lutidine under otherwise identical reaction conditions.     

Theoretical study of the mechanism of oxoiron(IV) formation from H2O2 and a nonheme iron(II) complex: O-O cleavage involving proton-coupled electron transfer

It has recently been shown that the nonheme oxoiron(IV) species supported by the 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane ligand (TMC) can be generated in near-quantitative yield by reacting [Fe(II)(TMC)(OTf)(2)] with a stoichiometric amount of H(2)O(2) in CH(3)CN in the presence of 2,6-lutidine (Li, F.; England, J.; Que, L., Jr. J. Am. Chem. Soc. 2010, 132, 2134-2135). This finding has major implications for O-O bond cleavage events in both Fenton chemistry and nonheme iron enzymes. To understand the mechanism of this process, especially the intimate details of the O-O bond cleavage step, a series of density functional theory (DFT) calculations and analyses have been carried out. Two distinct reaction paths (A and B) were identified. Path A consists of two principal steps: (1) coordination of H(2)O(2) to Fe(II) and (2) a combination of partial homolytic O-O bond cleavage and proton-coupled electron transfer (PCET). The latter combination renders the rate-limiting O-O cleavage effectively a heterolytic process. Path B proceeds via a simultaneous homolytic O-O bond cleavage of H(2)O(2) and Fe-O bond formation. This is followed by H abstraction from the resultant Fe(III)-OH species by an ?OH radical. Calculations suggest that path B is plausible in the absence of base. However, once 2,6-lutidine is added to the reacting system, the reaction barrier is lowered and more importantly the mechanistic path switches to path A, where 2,6-lutidine plays an essential role as an acid-base catalyst in a manner similar to how the distal histidine or glutamate residue assists in compound I formation in heme peroxidases. The reaction was found to proceed predominantly on the quintet spin state surface, and a transition to the triplet state, the experimentally known ground state for the TMC-oxoiron(IV) species, occurs in the last stage of the oxoiron(IV) formation process.     

Unexpected highly chemoselective deprotection of the acetals from aldehydes and not ketones: TESOTf-2,6-lutidine combination

Acetal functions are recognized as good protecting groups of carbonyl groups. Although many deprotecting methods of acetals to carbonyl functions have already been developed, there is no methodology which can deprotect acetals in the presence of ketals because the usual acidic or radical reactions occur more easily via the more stable cationic or radical intermediates from the ketals. On the other hand, this new method can proceed in a reverse manner to that described in previous reports. That is, the method can deprotect aliphatic acetals in the presence of ketals. The reaction condition is common for silylation, i.e., the TESOTf-2,6-lutidine combinations. Although the TMSOTf-2,6-lutidine combination can also deprotect acetals, it lacks chemoselectivity in deprotection of the acetals from aldehydes and ketones. The treatment of acetals with TESOTf and 2,6-lutidine in CH2Cl2 followed by a H2O workup gave the corresponding aldehydes. Of course, the compounds, which have both acetal and hydroxyl functions afforded the compounds obtained by the usual silylation of an alcohol and deprotection of an acetal without any problem. However, deprotection of the ketals from ketones was not observed during the conversion reaction of acetals from aldehydes. This chemoselectivity was confirmed in the reactions of the compounds that have the acetal and ketal in the same molecule. In both cases, the acetal functions were deprotected to give aldehydes with intact ketals. Furthermore, under the conditions described here, many functional groups such as methoxy, acetoxy, allyl alcohol, and silyloxy ether are intact. This method is very mild and available for many compounds.     

N-donor effects on carboxylate binding in mononuclear iron(II) complexes of a sterically hindered benzoate ligand

Using the sterically hindered 2,6-dimesitylbenzoate ligand Mes2ArCO2-, a series of mononuclear Fe(II) carboxylate complexes has been obtained with the general formula (Mes2ArCO2)2Fe(base)2 (base = 1-methylimidazole (MeIm), pyridine (Py), 2-picoline (2-Pic), 2,5-lutidine (2,5-Lut), 2,6-lutidine (2,6-Lut), (base)2 = N,N,N',N'-tetramethylethylenediamine (TMEDA)). For the monodentate base adducts, single-crystal X-ray diffraction studies revealed several different structural types ranging from distorted tetrahedral to distorted octahedral that correlate with the degree of alpha-substitution of the N-donors. Increasing alpha-substitution leads to the lengthening of the Fe-N bond, which in turn results in a change in carboxylate binding mode from eta 1 to eta 2. We surmise that this change is due to an electrostatic effect and is driven by increasing the Lewis acidity of the Fe center. Such a simple process for inducing carboxylate shifts could play a critical role in biological systems.     

Proton transfer reactions of methylanthracene radical cations with pyridine bases under non-steady-state conditions. Real kinetic isotope effect evidence for extensive tunneling.

The kinetics of the proton transfer reactions between the 9-methyl-10-phenylanthracene radical cation (MPA(+)(.)) with 2,6-lutidine were studied in acetonitrile-Bu(4)NBF(4) (0.1 M) using derivative cyclic voltammetry. Comparisons of extent of reaction-time profiles with theoretical data for both the simple single-step proton transfer and a mechanism involving the formation of a donor-acceptor complex prior to unimolecular proton transfer were made. The experimental extent of reaction-time profiles deviated significantly from those simulated for the single-step mechanism, while excellent fits of experimental to theoretical data, in the pre-steady-state period, for the complex mechanism were observed. In this time period, the apparent deuterium kinetic isotope effects (KIE(app)) were observed to vary significantly with the extent of reaction as predicted by the complex mechanism. Resolution of the apparent rate constants into the microscopic rate constants for the complex mechanism resulted in a real kinetic isotope effect (KIE(real)) equal to 82 at 291 K. Arrhenius activation parameters (252-312 K) for the reactions of MPA(+)(*) with 2,6-lutidine in acetonitrile-Bu(4)NBF(4) (0.1 M) revealed E(a)(D) - E(a)(H) equal to 2.89 kcal/mol and A(D)/A(H) equal to 2.09. In this temperature range, KIE(real) varied from 46 at the highest temperature to 134 at the lowest. The large KIE(real), along with the Arrhenius parameters, are indicative of extensive tunneling for the proton transfer steps.     

Oxidant-free direct coupling of internal alkynes and 2-alkylpyridine via double C-H activations by alkylhafnium complexes

We have developed a novel oxidant-free direct cross-coupling reaction of 2,6-lutidine and internal alkynes leading to five-membered carbocyclic compounds mediated by nonmetallocene cationic hafnium alkyl complexes. Mechanistic studies of the coupling reaction showed that the reaction begins with C(sp(3))-H bond activation via σ-bond metathesis, after which the coordinatively unsaturated hafnium center mediates further insertion, migration, and β-H elimination reactions to give five-membered carbocycles from readily available substrates.     

Physicochemical Properties of Complexes of Zinc Iodide with Pyridine N-Oxides

Polycrystalline 1 : 2 complexes of ZnI₂ with N-oxides of pyridine, picoline, and 2,6-lutidine were studied by IR spectroscopy, dielectrometry, and conductometry. An equilibrium between three solid phases was observed. These phases are characterized by different enthalpies of formation of intermolecular bonds and different mechanism of electronic effect transmission via these bonds. Gas-like thermal molecular motion in one of the phases of the 2,6-lutidine N-oxide complex was observed. Reversible chemical reactions on the surface of the crystalline 3-picoline N-oxide complex are initiated on exposure to an alternating electric field. Two complexes, similarly to the ZnCl₂ complexes, are ionized in the cumulative mode on fast heating from 18-19 to 26-45°C. Partial activation energies of electrical conductivity of different ions were determined for a number of complexes and inorganic salts.     

Complexes of Copper (II) Aryl Carboxylates with 2, 6-Lutidine

Copper (II) aryl carboxylates are known to form co-ordination complexes with various oxygen and nitrogen donors such as pyridine-N-oxide¹⁾, quinoline and isoquinoline²⁾ diethylamine and dipropylamine¹⁾. There is no reference in literature regarding the preparation of complexes of copper (II) aryl carboxylates with 2,6-lutidine. The present communication describes the preparation of complexes of various copper (II) aryl carboxylates with 2,6-lutidine in acetone or ethylacetate medium.     

Study of the electrical oxidation of 2,6-dimethyl-3,5-diethoxy-carbonyl-1,4-dihydropyridines in acetonitrile by means of a rotating disk electrode with a ring

It is shown that the electrochemical oxidation of esters of 2,6-dimethyl-1,4-dihydropyridine-3,5-dicarboxylic acids in anhydrous acetonitrile takes place as a one-electron process, while the final products of the transformations (pyridines or pyridinium salts) are formed as a result of disproportionation of the intermediate radicals. When 1–2% (by volume) water is added, the oxidation mechanism changes substantially, and the electrochemical process becomes a two-electron process.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 9, pp. 1263–1267, September, 1980.     

Electrochemical oxidations: Part IV. Electrochemical investigations into the behaviour of 2,6-di-tert-butyl-4-(4-dimethylaminophenyl)-phenol part 1. Phenol and the species derived from it: Phenoxy radical,phenolate anion and phenoxenium cation

The anodic oxidation of 2,6-di-tert-butyl-4-(4-dimethylaminophenyl)phenol (IIa) was investigated by cyclic voltammetry. The stable products resulting from the oxidation are phenoxenium cation (IId) (which itself is reduced in two one-electron steps to the phenolate anion (IIc) via the phenoxy radical, IIb) and the “protonated phenol (III) (which is oxidized to (IId) in a second peak at higher potential). A mechanism for the electrochemical oxidation of phenol (IIa) is suggested.     

Disposable Glucose Sensors for Flow Injection Analysis Using Substituted 1,4‐Benzoquinone Mediators

The redox chemistry of several substituted benzoquinones was investigated by cyclic voltammetry at a glassy carbon electrode and candidates for inclusion in a mediated biosensor for use in flow analysis were selected on the basis of oxidation potential, electrochemical reversibility and solubility. Glucose sensors constructed by sequential deposition onto a carbon pellet of 2,6‐dimethyl‐1,4‐benzoquinone, 2,3,5,6‐tetramethyl‐1,4‐benzoquinone or phenyl‐1,4‐benzoquinone mediator solution, followed by glucose oxidase in polyvinylalcohol‐Nafion solution, were tested for response to glucose using flow injection analysis. Sensors prepared from 2,6‐dimethyl‐1,4‐benzoquinone gave highest sensitivity, with a linear range of response to glucose of 2.5–40 mM. The use of an enzyme‐free comparative electrode to eliminate the response from interferents was investigated.     

Kinetic Study of the Electro‐Catalytic Oxidation of Acetaldehyde on Copper Electrode

Electrocatalytic oxidation of acetaldehyde was investigated on a copper electrode in alkaline solution. The process of oxidation involved and its kinetics were established by using cyclic voltammetry and chronoamperometry techniques as well as steady state polarization measurements. It has been found that in the course of an anodic potential sweep the electro‐oxidation of acetaldehyde follows the formation of Cu(III) and is catalysed by this species through a mediated electron transfer mechanism. A mechanism based on the electrochemical generation of Cu(III) active sites and their subsequent consumption by the acetaldehyde in question was also investigated.     

Mechanism of the hydroxycarbonylation of 1-hexene catalyzed by immobilized rhodium complexes of pyridine ligands

Summary In this work, a mechanistic study of the hydroxycarbonylation of 1-hexene to heptanoic acid and the water gas shift reaction (WGSR) catalyzed by the rhodium(I) complexes, [Rh(COD)(amine)₂](PF₆) (COD = 1,5-cyclooctadiene, amine = 4-picoline, 3-picoline, 2-picoline, pyridine, 3,5-lutidine or 2,6-lutidine) immobilized on poly(4-vinylpyridine) in contact with water under CO is discussed. Catalytic cycles for these reactions bearing common Rh-H catalytic species are proposed.