Quinone catalysts, oxidation with

Benzoquinone ( quinone ) is obtained as the end product of the oxidation of aniline by acid dichromate solution. Industrially, the crude product is reduced with sulphur dioxide to hydroquinone, and the latter is oxidised either with dichromate mixture or in very dilute sulphuric acid solution with sodium chlorate in the presence of a little vanadium pentoxide as catalyst. For the preparation in the laboratory, it is best to oxidise the inexpensive hydroquinone with chromic acid or with sodium chlorate in the presence of vanadium pent-oxide. Naphthalene may be converted into 1 4-naphthoquinone by oxidation with chromic acid. 

Aromatic compounds are oxidized to quinones by bis(triorganosilyl) peroxides in the presence of a metal acid catalyst. Thus, 2-methylnaphthalene was oxidized with BTSP in the presence of Re207 and BU3PO in the presence of CHCI3, to a mixture of 59% 2-methyl-1,4-naphthoquinone and 8% 6-methy 1-1,4-naphthoquinone. ... 

A similar mechanism could operate in the reduction of oxygen on chelate catalysts, as in the organic cathodes with air regeneration described by Alt, Binder, Kohling and Sandstede 13-40>. These cathodes contain a reversible insoluble quinone/hydroquinone system. The quinone, which is electrochemically reducible, can be obtained either by electrochemical oxidation or by purely chemical oxidation with H2O2 or oxygen (air). A cathodic current is observed in these systems only at potentials below the redox potential, and unusually hard current/ voltage characteristic curves are obtained. 

In the studies of the synthesis of the ansamycin antibiotic rifamycin S (13S), Corey and Clark [76] found numerous attempts to effect the lactam closure of the linear precursor 132 to 134 uniformly unsuccessful under a variety of experimental conditions, e.g. via activated ester with imidazole and mixed benzoic anhydride. The crux of the problem was associated with the quinone system which so deactivates the amino group to prevent its attachment to mildly activated carboxylic derivatives. Cyclization was achieved after conversion of the quinone system to the hydroquinone system. Thus, as shown in Scheme 45, treatment of 132 with 10 equiv of isobutyl chloroformate and 1 eqtuv of triethylamine at 23 °C produced the corresponding mixed carbonic anhydride in 95% yield. The quinone C=C bond was reduced by hydrogenation with Lindlar catalyst at low temperature. A cold solution of the hydroquinone was added over 2 h to THF at 50 °C and stirred for an additional 12 h at the same temperature. Oxidation with aqueous potassium ferricyanide afforded the cyclic product 134 in 80% yield. Kishi and coworkers [73] gained a similar result by using mixed ethyl carbonic anhydride. 

Meunier has developed model chemistry for peroxidases such as ligninase with KHSO5 as oxo-transfer oxidant and Fe or Mn-tetrasulfoporphyrin in aq. MeCN. These catalysts oxidize lignin models such as veratryl alcohol (3,4-methoxybenzyl alcohol) to give biologically relevant products such as quinones. 

Secondary alcohols have been oxidized to ketones with excess ferf-butylhydroperoxide in up to 93-99% yields using a zirconium catalyst.250 Zirconium catalysts have also been used with ferf-butylhydroperoxide in the oxidation of aromatic amines to nitro compounds and of phenols to quinones. Allylic oxidation of steroids in 75-84% yields has been performed with ferf-butylhydroperoxide and cop-... 

Starting with the same material (a Norit ROX 0.8 activated carbon), different treatments were used to modify the properties of the carbon catalyst. First, the activated carbon was oxidized with different reactants in either the gas or the liquid phase. The gas-phase treatments proved to be more effective than the liquid-phase oxidations, and the higher activities of the former were explained in terms of the amount of carbonyl or quinone groups on the surface of the catalysts, together with higher surface areas. 

Oxidation of a phenol to the corresponding jC -quinone using a copper catalyst takes place at room temperature under similar conditions as those used for alcohol oxidation, with O2 as oxidant. Likewise, hydroquinones (22) can be transformed to 3-alkoxy-/7-quinones (23) when reacted in the presence of an alcohol. In the case of 4-substituted phenols (24), polymer-based catalysts composed of ligands (e.g. PVBPy) that chelate copper have been used at elevated temperatures to selectively oxidize a benzylic carbon to yield 4-hydroxybenzaldehydes (25) in good yields. ... 

Phenols —>quinones. Useful catalysts include polymer-supported vanadium salts and (PhjPljRuClj. The Rh-catalyzed reaction can oxidize p-substituted phenols, giving 4-t-butylperoxy-2,5-cyclohexadienones, which undergo rearrangement (migration of the substituent) to afford the quinones on treatment with TiCl. ... 

It was suggested that, in the oxidation of ethylbenzene and ascorbate ion, quinone catalysts with two substituents are more effective than those with many substituents which may consequently hinder contact between the active site (C=0) and the reactant as well as limiting addition of the free radicals to the C=C bonds. ... 

The intermolecular diamination of 1,3-dienes with acyclic ureas to provide monocyclic or bicydic urea derivatives has been achieved by Uoyd-Jones and Booker-Milburn through use of a palladium catalyst combined with either benzo-quinone or O2 as an oxidant [65]. For example, diene 81 was converted to urea 82 in 82% yield (Eq. (1.37)). These transformations are mechanistically distinct from the reactions described above, and appear to involve intermediate jr-allylpalladium complexes. 

Phenols (e.g., phenol itself [CeHs-OH or Ar-OH], Table 6.10, item 2) and their esters (e.g., the trifluoroacetate ester of phenol [C6H5-O2CCF3 or Ar02CCH3], Table 6.10, item 3) have been oxidized with air and oxygen (O2), in neutral and alkaUne solutions, with and without ionic and/or radical catalysts and/or irradiation and in a variety of solvents. Enzymes (this chapter and Chapter 12) from a wide variety of sources have also been used. Frequently, oxidation of aromatic systems to phenols cannot be stopped before quinones and products of ring fragmentation occur and numerous, sometimes ill-defined, products result. Thus, as shown in Equation 6.80, oxidation of the polynuclear hydrocarbon chrysene with anunonium cerium(IV) sulfate [ceric ammonium sulfate, Ce(NH,)4(S04)4] is reported to produce 6H-benzo[d]naphtho[l,2-/>]pyran-6-one (8% yield) and a quinone (23% yield). The remainder of the product(s) (69%) was unidentified. 

The experimental observations were interpreted by assuming that the redox cycle starts with the formation of a complex between the catalyst and the substrate. This species undergoes intramolecular two-electron transfer and produces vanadium(II) and the quinone form of adrenaline. The organic intermediate rearranges into leucoadrenochrome which is oxidized to the final product also in a two-electron redox step. The +2 oxidation state of vanadium is stabilized by complex formation with the substrate. Subsequent reactions include the autoxidation of the V(II) complex to the product as well as the formation of aVOV4+ intermediate which is reoxidized to V02+ by dioxygen. These reactions also produce H2O2. The model also takes into account the rapidly established equilibria between different vanadium-substrate complexes which react with 02 at different rates. The concentration and pH dependencies of the reaction rate provided evidence for the formation of a V(C-RH)3 complex in which the formal oxidation state of vanadium is +4. 

An increased selectivity for phenol in the oxidation of benzene by H202 with TS-1 catalyst in sulfolane solvent was attributed to the formation of a bulky sulfolane-phenol adduct which cannot enter the pores of TS-1. Further oxidation of phenol to give quinones, tar, etc. is thus avoided. Removal of Ti ions from the surface regions of TS-1 crystals by treatment with NH4HF2 and H202 was also found to improve the activity and selectivity (227). The beneficial effects of removal of surface Al ions on the catalytic performance of zeolite catalysts for acid-catalyzed reactions have been known for a long time. 

The titanosilicate version of UTD-1 has been shown to be an effective catalyst for the oxidation of alkanes, alkenes, and alcohols (77-79) by using peroxides as the oxidant. The large pores of Ti-UTD-1 readily accommodate large molecules such as 2,6-di-ferf-butylphenol (2,6-DTBP). The bulky 2,6-DTBP substrate can be converted to the corresponding quinone with activity and selectivity comparable to the mesoporous catalysts Ti-MCM-41 and Ti-HMS (80), where HMS = hexagonal mesoporous silica. Both Ti-UTD-1 and UTD-1 have also been prepared as oriented thin films via a laser ablation technique (81-85). Continuous UTD-1 membranes with the channels oriented normal to the substrate surface have been employed in a catalytic oxidation-separation process (82). At room temperature, a cyclohexene-ferf-butylhydroperoxide was passed through the membrane and epoxidation products were trapped on the down stream side. The UTD-1 membranes supported on metal frits have also been evaluated for the separation of linear paraffins and aromatics (83). In a model separation of n-hexane and toluene, enhanced permeation of the linear alkane was observed. Oriented UTD-1 films have also been evenly coated on small 3D objects such as glass and metal beads (84, 85). 

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