The oxidation of 2,6-xylenol catalysed by polymer supported copper complexes
TL;DR: In this article, the synthesis and characterisation of copolymers containing tertiary nitrogen to which Cu(I) or Cu(II) species are anchored is described, and the anchored species function as catalysts for the oxidation of 2,6-xylenol by O 2 under basic conditions.
About: This article is published in Polymer.The article was published on 1998-01-01. It has received 7 citations till now. The article focuses on the topics: 2,6-Xylenol & Catalyst support.
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TL;DR: The chemistry of copper is extremely rich because it can easily access Cu0, CuI, CuII, and CuIII oxidation states allowing it to act through one-electron or two-Electron processes, which feature confer a remarkably broad range of activities allowing copper to catalyze the oxidation and oxidative union of many substrates.
Abstract: The chemistry of copper is extremely rich because it can easily access Cu0, CuI, CuII, and CuIII oxidation states allowing it to act through one-electron or two-electron processes. As a result, both radical pathways and powerful two-electron bond forming pathways via organmetallic intermediates, similar to those of palladium, can occur. In addition, the different oxidation states of copper associate well with a large number of different functional groups via Lewis acid interactions or π-coordination. In total, these feature confer a remarkably broad range of activities allowing copper to catalyze the oxidation and oxidative union of many substrates.
Oxygen is a highly atom economical, environmentally benign, and abundant oxidant, which makes it ideal in many ways.1 The high activation energies in the reactions of oxygen require that catalysts be employed.2 In combination with molecular oxygen, the chemistry of copper catalysis increases exponentially since oxygen can act as either a sink for electrons (oxidase activity) and/or as a source of oxygen atoms that are incorporated into the product (oxygenase activity). The oxidation of copper with oxygen is a facile process allowing catalytic turnover in net oxidative processes and ready access to the higher CuIII oxidation state, which enables a range of powerful transformations including two-electron reductive elimination to CuI. Molecular oxygen is also not hampered by toxic byproducts, being either reduced to water, occasionally via H2O2 (oxidase activity) or incorporated into the target structure with high atom economy (oxygenase activity). Such oxidations using oxygen or air (21% oxygen) have been employed safely in numerous commodity chemical continuous and batch processes.3
However, batch reactors employing volatile hydrocarbon solvents require that oxygen concentrations be kept low in the head space (typically <5–11%) to avoid flammable mixtures, which can limit the oxygen concentration in the reaction mixture.4,5,6 A number of alternate approaches have been developed allowing oxidation chemistry to be used safely across a broader array of conditions. For example, use of carbon dioxide instead of nitrogen as a diluent leads to reduced flammability.5 Alternately, water can be added to moderate the flammability allowing even pure oxygen to be employed.6 New reactor designs also allow pure oxygen to be used instead of diluted oxygen by maintaining gas bubbles in the solvent, which greatly improves reaction rates and prevents the build up of higher concentrations of oxygen in the head space.4a,7 Supercritical carbon dioxide has been found to be advantageous as a solvent due its chemical inertness towards oxidizing agents and its complete miscibility with oxygen or air over a wide range of temperatures.8 An number of flow technologies9 including flow reactors,10 capillary flow reactors,11 microchannel/microstructure structure reactors,12 and membrane reactors13 limit the amount of or afford separation of hydrocarbon/oxygen vapor phase thereby reducing the potential for explosions.
Enzymatic oxidizing systems based upon copper that exploit the many advantages and unique aspects of copper as a catalyst and oxygen as an oxidant as described in the preceding paragraphs are well known. They represent a powerful set of catalysts able to direct beautiful redox chemistry in a highly site-selective and stereoselective manner on simple as well as highly functionalized molecules. This ability has inspired organic chemists to discover small molecule catalysts that can emulate such processes. In addition, copper has been recognized as a powerful catalyst in several industrial processes (e.g. phenol polymerization, Glaser-Hay alkyne coupling) stimulating the study of the fundamental reaction steps and the organometallic copper intermediates. These studies have inspiried the development of nonenzymatic copper catalysts. For these reasons, the study of copper catalysis using molecular oxygen has undergone explosive growth, from 30 citations per year in the 1980s to over 300 citations per year in the 2000s.
A number of elegant reviews on the subject of catalytic copper oxidation chemistry have appeared. Most recently, reviews provide selected coverage of copper catalysts14 or a discussion of their use in the aerobic functionalization of C–H bonds.15 Other recent reviews cover copper and other metal catalysts with a range of oxidants, including oxygen, but several reaction types are not covered.16 Several other works provide a valuable overview of earlier efforts in the field.17 This review comprehensively covers copper catalyzed oxidation chemistry using oxygen as the oxidant up through 2011. Stoichiometric reactions with copper are discussed, as necessary, to put the development of the catalytic processes in context. Mixed metal systems utilizing copper, such as palladium catalyzed Wacker processes, are not included here. Decomposition reactions involving copper/oxygen and model systems of copper enzymes are not discussed exhaustively. To facilitate analysis of the reactions under discussion, the current mechanistic hypothesis is provided for each reaction. As our understanding of the basic chemical steps involving copper improve, it is expected that many of these mechanisms will evolve accordingly.
1,326 citations
TL;DR: In this paper, the molar ratio of the bipyridine unit of the polymer ligand to Cu was unity, i.e., N/Cu = 2, and the best results were obtained.
Abstract: Oxidation of 2,6-disubstituted 4-methylphenols with dioxygen by using a CuCl2-poly(4-methyl-4′-vinyl-2,2′-bipyridine) catalyst gave the corresponding 4-hydroxybenzaldehydes in high yields. The activity of the catalyst and the selectivity of the products significantly depended on the reaction conditions and the composition of the catalyst. When the molar ratio of the bipyridine unit of the polymer ligand to Cu was unity, i.e., N/Cu = 2, the best results were obtained. Moreover, the reaction is likely to be promoted by coordination of the products to the catalyst. Similarly, 2,3,6-trimethylphenol and related compounds were converted to p-benzoquinones selectively with a CuCl2-poly(4-vinylpyridine) catalyst. These polymer-supported catalysts were readily recovered and are reusable without noticeable decrease of their activity.
38 citations
TL;DR: In this article, two chitosan-bound nitrobenzaldehyde metal complexes (m-CNBDM and o-NBMn) were characterized by infrared, X-ray photoelectron spectroscopy, solid-state 13C-NMR cross-polarity/magic-angle spinning, inductively coupled plasma, and elemental analysis.
Abstract: Chitosan-bound nitrobenzaldehyde metal complexes (m-CNBDM and o-CNBDM, where M is Mn or Ni) were prepared and characterized by infrared, X-ray photoelectron spectroscopy, solid-state 13C-NMR cross-polarity/magic-angle spinning, inductively coupled plasma, and elemental analysis. The complexes were found to be catalysts for the oxidation of hydrocarbons with molecular oxygen under mild conditions. o-CNBDNi has a certain catalytic activity in the oxidation of n-propylbenzene and isopropylbenzene and has no activity in the oxidation of ethylbenzene. Both o-CNBDMn and m-CNBDNi catalyze the oxidation of all the aforementioned hydrocarbons, whereas m-CNBDMn has no catalytic activity. The main oxidative products of ethylbenzene and n-propylbenzene are the same as α-ol and α-one, but they are 2-benzyl-isopropynol and isopropylbenzene peroxide for isopropylbenzene. A mechanism for the catalytic oxidative process is proposed. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 83: 2188–2194, 2002
31 citations
TL;DR: In this paper, the effect of various factors on the catalysis of the main reactions used in organic synthesis (hydrogenation, polymerization, and redox processes) are analyzed.
Abstract: The current status, trends, and a specific role for macroligands in catalysis by heterogenized metallopolymeric complexes are considered. Relations between homogeneous catalysis, enzyme catalysis, and catalysis by heterogenized metal complexes are traced. The effects of various factors on the catalysis of the main reactions used in organic synthesis—hydrogenation, polymerization (in particular, under the action of immobilized metallocene and postmetallocene catalysts), and redox processes (such as the catalysis of oxygenation, hydroperoxide oxidation, epoxidation, and hydroformylation)—are analyzed. In this review, attention is focused on the nondestructive identification of intermediates and catalytically active species in heterogenized systems. Experimental evidence is presented in support of the fact that the high activity, stability, and selectivity of immobilized catalysts are associated with a dramatic inhibition of concerted reactions in the coordination sphere of a transition metal, which result in catalyst deactivation, as well as with substrate enrichment. Prospects for the development of these highly organized hybrid systems and possibilities to consider the main requirements imposed on metal complex catalysis even at the stage of designing them are predicted.
25 citations
TL;DR: In this paper, the catalytic activity of Cu(II) complexes with Schiff base immobilized on the synthesized supports were tested in the oxidation reaction of 2,6-di-tert-butylphenol (DTBP) to diphenoquinone (PQ) with tertbutylhydroperoxide.
Abstract: Benzoquinone, diphenoquinones and its derivatives are important intermediates for industrial synthesis of a wide variety of special chemicals, such as pharmaceuticals, dyes and agricultural chemicals. The useful catalyst were obtained by aminolysis of vinylbenzyl chloride/divinylbenzene copolymer with ethylenediamine (1) or urotropine (2) and then modification by salicylaldehyde (1A, 2A) or picolinaldehyde (1B, 2B). The catalytic activity of Cu(II) complexes with Schiff base immobilized on the synthesized supports were tested in the oxidation reaction of 2,6-di-tert-butylphenol (DTBP) to diphenoquinone (PQ) with tert-butylhydroperoxide. The best oxidation degree of DTBP (60-70%) and the selectivity towards PQ (80%) is revealed by Cu(II) complexes with long Schiff base ligands derived from salicylaldimine (1A), which have CuL structure (EPR measurement).
17 citations
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TL;DR: In this article, the catalytic effects of the Cu-1 complex are discussed on the basis of the kinetic parameters and spectroscopic measurements, and it is shown that a bulky 1-ligand obstructs the coordination of the substrate to the cupric ion.
Abstract: 2,6-Xylenol (2) was oxidatively polymerized using a copper complex of partially diethylaminomethylated polystyrene (1) as catalyst in a homogeneous o-dichlorobenzene solution. The catalytic effects of the Cu-1 complex are discussed on the basis of the kinetic parameters and spectroscopic measurements. The catalytic activity of the Cu-1 complex was higher in the dimerization of 2, compared to the Cu-triethylamine catalyst, but the Cu-1 catalyzed polymerization of oligo(2) proceeded slower. The smaller value of K1 (the formation constant of the intermediate complex) and the lower molecular weight of the polymer obtained with the Cu-1 catalyst system suggests that a bulky 1-ligand obstructs the coordination of the substrate to the cupric ion. The higher catalytic activity of the Cu-1 complex is caused by the fact that the rate constant of the electron-transfer step (ke) is larger and the activation enthalpy is smaller. This is due to an acceleration of the electron transfer step by the strain in the chain of ligand 1.
2,6-Dimethylphenol (2) wurde mit Kupfer-Komplexen von partiell diethylaminomethyliertem Polystyrol (1) in homogener o-Dichlorbenzollosung oxydativ polymerisiert. Die katalytische Wirkung des Cu-1-Komplexes wird anhand der kinetischen Konstanten und der Spektren diskutiert. Die katalytische Aktivitat des Cu-1-Komplexes war bei der Dimerisierung von 2 hoher als die des Cu-Triathylamin-Katalysators, aber die Cu-1 katalysierte Polymerisation von Oligo (2) verlief langsamer. Der kleinere K1-Wert (Komplexbildungskonstante zwischen Katalysator und Substrat) und das niedrigere Molekulargewicht des erhaltenen Polymeren mit dem Cu-1-Katalysatorsystem deuten an, das der raumerfullende 1-Ligand die Koordination des Substrates an das Kupferion erschwert. Die hohere katalytische Aktivitat des Cu-1-Komplexes wird durch die grosere Geschwindigkeitskonstante der Elektronenubertragung (ke) und durch die kleinere Aktivierungsenthalpie verursacht. Dies wird dadurch erklart, das der Ubertragungsschritt durch die Spannung in der Kette des Liganden 1 beschleunigt wird.
8 citations
TL;DR: In this paper, two kinds of branching reactions occur (viz. branching at end groups and at other groups) which have a different influence on the distribution width, and the combination of both effects allows the calculation of the distribution wide as a function of conversion, using the characteristic velocity constants of the branching reaction as parameters.
Abstract: Verzweigungsreaktionen beeinflussen die Molekulargewichtsverteilung; aus dieser kann man daher die Zahl der Verzweigungspunkte berechnen, wenn der Bildungsmechanismus des Polymeren bekannt ist. Statt der Molekulargewichtsverteilung kann man die aus dieser abgeleitete Uneinheitlichkeit verwenden, die theoretisch und experimentell leichter zuganglich ist.
Bei der Radikalpolymerisation treten zwei Arten von Verzweigungsreaktionen auf (Mittelgruppen- bzw. Endgruppenverzweigung), deren Einflus auf die Uneinheitlichkeit verschieden ist. Der Einflus dieser Reaktionen kann – fusend auf Arbeiten von STOCKMAYER sowie BAMFORD und TOMPA – rechnerisch erfast werden. Die Kombination der beiden Effekte erlaubt die Berechnung der Uneinheitlichkeit als Funktion des Umsatzes, wobei die charakteristischen Geschwindigkeitskonstanten der Verzweigungsreaktionen als Parameter benutzt werden. Der Vergleich mit experimentellen Werten der Uneinheitlichkeit – erhaltlich aus Messungen von Mw und Mn – erlaubt die Ermittlung der Zahlenwerte dieser Geschwindigkeitskonstanten.
Chain branching reactions influence the molecular weight distribution; the latter can therefore be used to calculate the number of branch points if the mechanism of the polymerization reaction is known. Instead of the complete molecular weight distribution the “ununiformity” (Pw/Pn–1), which is more readily accessible both theoretically and experimentally, can be used.
In radical polymerizations, two kinds of branching reactions occur (viz. branching at end groups and at other groups) which have a different influence on the distribution width. On the basis of papers by STOCKMAYER and by BAMFORD and TOMPA, the influence of these reactions can be calculated. The combination of both effects allows the calculation of the distribution width as a function of conversion, using the characteristic velocity constants of the branching reaction as parameters. Comparison with experimentally determined values of the distribution width (obtainable from measurements of Mw and Mn) allows a determination of the numerical values of these velocity constants.
7 citations
TL;DR: In this article, the copper cystamine complex is used for homogeneous catalysis of oxygen cathodic reduction in aqueous solutions and a steady state cathodic current density of 3.2 mA/cm2 at 0.2 V and an overvoltage decrease of 400 mV was observed in oxygen-saturated solutions of equimolar concentrations of CuII ions and Cystamine (5 × 10−2 mol dm−3) at pH 4.5.
Abstract: The copper cystamine complex is a suitable system for the homogeneous catalysis of oxygen cathodic reduction in aqueous solutions. A steady state cathodic current density of 3.2 mA/cm2 at 0.2 V and an overvoltage decrease of 400 mV was observed in oxygen-saturated solutions of equimolar concentrations of CuII ions and cystamine (5 × 10–2 mol dm–3) at pH 4.5. For a given value of Cl– ion concentration and pH the catalytic current can be expressed by: Icat=k[CuII]0[O2].The optical, magnetic and kinetic properties of the catalyst were investigated. The results suggest that the CuII ion, although initially reduced to CuI, is stabilized by the disulphide bridge. It is also suggested that because the bridge is a two electron mediator for the reduction of oxygen, the potential barrier of the first electron reduction can be overcome.
6 citations
4 citations
TL;DR: In this article, the authors investigated the optimal conditions of preparation of poly2,6-dimethyl-1,4-phenylenather (DMPPO) by oxydative coupling of 2,6dimethylphenol with high molecular weights and showed that it is also possible to get DMPPO with a high molecular weight if the catalyst which is a Cu-pyridin-complex (PynCuClX (X = OH, OCH3, O1/2) is suspended e. g. in a mixture from xylene and
Abstract: Versuche zur Darstellung von Poly-2,6-dimethyl-1,4-phenylenather (DMPPO) durch oxydative Kupplung von 2,6-Dimethylphenol zeigten, das die Reaktion auch dann zu DMPPO mit hohen Molekulargewichten fuhrt, wenn der als Katalysator verwendete Cu-Pyridin-Komplex Pyn CuClX (X = OH, OCH3 oder O1/2) in suspendierter Form vorliegt, z. B. in einer Mischung von Xylol und Pyridin. Obwohl der Katalysator in solchen Mischungen (bis zu 20 Vol.-% Pyridin) so wenig loslich ist, das das Filtrat mit Dimethylphenol und O2 kein Polymeres liefert, und der Katalysatorkomplex nach der Reaktion in unveranderter Form wiedergewonnen werden kann, verlauft die Polykondensation als homogene Reaktion. Wie die Versuche zeigten, wird zu Beginn der Synthese ein Teil des Katalysators durch Reaktion mit dem Monomeren gelost (Bildung eines Cu-Phenolat-Pyridin-Komplexes) und katalysiert die weitere Reaktion. Wenn die Monomerkonzentration immer geringer wird, scheidet sich der ursprungliche Komplex wieder aus.
Die Kinetik der Reaktion wurde zur Ermittlung der optimalen Reaktionsbedingungen untersucht. Die Abhangigkeit der Anlaufzeit und der maximalen Reaktionsgeschwindigkeit von verschiedenen Reaktionsparametern wird diskutiert.
Versuche mit einer Ruhrkesselkaskade zeigten, das die kontinuierliche Darstellung des Polymeren mit suspendiertem Katalysator moglich ist.
Bei Fixierung des Katalysators an saure Ionenaustauscherharze wurden bislang keine Polymeren erhalten.
Experiments to prepare poly-2,6-dimethyl-1,4-phenylenether (DMPPO) by oxydative coupling of 2,6-dimethylphenol have shown that it is also possible to get DMPPO with high molecular weights if the catalyst which is a Cu-pyridin-complex – PynCuClX (X = OH, OCH3, O1/2) – is suspended e. g. in a mixture from xylene and pyridine. Although the catalyst is so slightly soluble in such mixtures (up to 20 vol % pyridine) that the filtrate does not give polymers with dimethyl- phenol and oxygen and the catalyst can be recovered after the polycondensation, the reaction is homogeneous. In the first step of the synthesis a part of the catalyst is dissolved by a reaction with the monomer (formation of a Cu-phenolate-pyridine-complex). This complex can catalyse the polycondensation. When the monomer concentration decreases the catalyst precipitates from the solution.
The kinetics of the reaction were investigated to find out the optimal conditions of preparation.
The dependence of the induction periode and the maximum reaction rate on various parameters are discussed.
Experiments with a series of backmix-reactors showed that the continuous preparation of the polymer with suspended catalysts is possible.
When the catalyst was fixed on acid ion exchange resins, it was not possible to prepare polymers.
2 citations