Oxidation by benzoquinone

The species truw5-[0s NN(CH2)40 (tpy)(Cl)2] can be protonated to give the hydrazido(l-) species truw5-[0s N(H)N(CH2)40 (tpy)(Cl)2]. The protonated species undergoes rapid oxidation by benzoquinone, Q, to give tra i-[0s NN(CH2)40 (tpy)(Cl)2] and H2Q. This reaction occurs with a remarkable N— H/N—kinetic isotope effect of 41.4, consistent with a proton-coupled electron transfer mechanism. 

Fundamental Studies of Palladium(O) Oxidation by Benzoquinone and Dioxygen... 

Aniline Oxidation. Even though this is quite an old process, it still has limited use to produce hydroquinone on a commercial scale. In the first step, aniline is oxidized by manganese dioxide in aqueous sulfuric acid. The resulting benzoquinone, isolated by vapor stripping, is reduced in a second step by either an aqueous acidic suspension of iron metal or by catalytic hydrogenation. 

Sta.rting from Phenol. Phenol can be selectively oxidized into -benzoquinone with oxygen. The reaction is catalyzed by cuprous chloride. At low catalyst concentration, the principal drawback of this method is the high pressure of oxygen that is required, leading to difficult safety procedures. It appears that a high concentration of the catalyst (50% of Cu(I)—phenol) allows the reaction to proceed at atmospheric pressure (58). 

Another method that determines both ascorbic acid and dehydroascorbic acid first reduced the dehydroascorbic acid to ascorbic acid and then retains the ascorbic acid on an anionic Sephadex column (82). The ascorbic acid is oxidized on the column to dehyroascorbic acid by -benzoquinone, which simultaneously elutes the dehydroascorbic acid. The dehydroascorbic acid is reacted with 4-iiitro-l,2-phenylenediainine and absorbance of the resulting yeUow solution produced is measured at 375 nm. 

Neither the mechanism by which benzene damages bone marrow nor its role in the leukemia process are well understood. It is generally beheved that the toxic factor(s) is a metaboHte of benzene (107). Benzene is oxidized in the fiver to phenol [108-95-2] as the primary metabolite with hydroquinone [123-31-9] catechol [120-80-9] muconic acid [505-70-4] and 1,2,4-trihydroxybenzene [533-73-3] as significant secondary metabolites (108). Although the identity of the actual toxic metabolite or combination of metabolites responsible for the hematological abnormalities is not known, evidence suggests that benzene oxide, hydroquinone, benzoquinone, or muconic acid derivatives are possibly the ultimate carcinogenic species (96,103,107—112). 

The addition of the anion of a-bromo-a-nitrotoluerie (564) to cyclohexene gave the hexahydro derivative (565) of 3-phenyl-l,2-benzisoxazole (75TL2131). An unusual hexahydro derivative (566) was produced by the bis addition of benzonitrile N-oxide to benzoquinone (67AHC(8)277). 

Phenanthrene-l,4-diones have been prepared [52] by cycloaddition of a-substituted styrenes with an excess of 1,4-benzoquinone (Equation 2.18). Initial cycloadducts are oxidized by 1,4-benzoquinone. 

I.3.4.2. Intermolecular Cycloaddition at C=X or X=Y Bonds Cycloaddition reactions of nitrile oxides to double bonds containing heteroatoms are well documented. In particular, there are several reviews concerning problems both of general (289) and individual aspects. They cover reactions of nitrile oxides with cumulene structures (290), stereo- and regiocontrol of 1,3-dipolar cycloadditions of imines and nitrile oxides by metal ions (291), cycloaddition reactions of o-benzoquinones (292, 293) and aromatic seleno aldehydes as dipolarophiles in reactions with nitrile oxides (294). 

Excess of polymer E-E has also been found and in some cases only E-E forms, for instance during the initial stage of catalysis by Pd(dapp)2+ in the presence of an oxidant, usually benzoquinone or naphtoquinone (BQ, NQ) [37]. The oxidant favours the formation of Pd - OCH3+ at the expense of Pd - H+ [15] and in the copolymerisation process one molecule of oxidant is... 

Palladium salts will attack C-H bonds in functionalised aromatics such as acetoaniline to form palladium-carbon bonds that subsequently undergo insertion of alkenes [31], (3-Hydride elimination gave styryl derivatives and palladium hydride, which requires re-oxidation of palladium by benzoquinone. The reaction can be regarded as a combined Murai reaction (C-H activation, if electrophilic) and a Heck reaction (arylalkene formation), notably without the production of salts as the cross-coupling reactions do. An example is shown in Figure 19.15. 

The protection of a reactive intermediate complex by the DNA double helix versus a neutral oxidising agent in solution, has also been demonstrated by studying a photo-electron transfer process. In this example the intermediate complex is produced photochemically on the DNA, and is examined spectroscopically after a laser pulsed excitation [73]. Thus Ru(TAP)2(HAT) physically bound to nucleic acid is photo-reduced by hydroquinone during the laser pulse. The intermediate [Ru(TAP)2(HAT)] so-produced, detected by its absorption at 480 nm, is reoxidised by benzoquinone purposely added as oxidant to the solution. It is shown that this reoxidation of the mono-reduced complex is slower in the presence of polynucleotide than in its absence, indicating a protection of the transient mono-reduced complex in the DNA grooves. 

The stability of [Fe4S4] core was utilized for electron-transfer oxidation catalysis. With 1,4-benzoquinone as oxidant, benzoin was catalytically oxidized by... 

Tetraphenylporphin (H2TPP) was prepared from pyrrole and benzaldehyde (25). The tetraphenylchlorin contamination was oxidized by use of 2,3-dichloro-5,6-dicyano-p-benzoquinone (26). Octaethylporphin (H2-OEP)was prepared by the method of Paine et al.(27) from 3,4-diethyl-2-ethoxy-carbony1-5-methyl-pyrrole (28). Zinc(II)-tetraphenylporphin (ZnTPP) was prepared by refluxing H2TPP and zinc acetate in dimethyl-formamide (29). ZnTPP was dissolved at a concentration of 10 -10 ... 

Treatment of 2-methoxy-1,4-benzoquinone with hydrogen chloride yields a highly colored compound formulated as the internal quinhydrone 167, which on further reaction with hydrogen chloride yields the dibenzofuran 168. 2-Methoxy-l,4-dihydroxybenzene is oxidized by 1,4-benzoquinone... 

As part of an extensive study of the 1,3-dipolar cycloadditions of cyclic nitrones, Ali et al. (392-397) found that the reaction of the 1,4-oxazine 349 with various dipolarophiles afforded the expected isoxazolidinyloxazine adducts (Scheme 1.78) (398). In line with earlier results (399,400), oxidation of styrene-derived adduct 350 with m-CPBA facilitated N—O cleavage and further oxidation as above to afford a mixture of three compounds, an inseparable mixture of ketonitrone 351 and bicyclic hydroxylamine 352, along with aldonitrone 353 with a solvent-dependent ratio (401). These workers have prepared the analogous nitrones based on the 1,3-oxazine ring by oxidative cleavage of isoxazolidines to afford the hydroxylamine followed by a second oxidation with benzoquinone or Hg(ll) oxide (402-404). These dipoles, along with a more recently reported pyrazine nitrone (405), were aU used in successful cycloaddition reactions with alkenes. Elsewhere, the synthesis and cycloaddition reactions of related pyrazine-3-one nitrone 354 (406,407) or a benzoxazine-3-one dipolarophile 355 (408) have been reported. These workers have also reported the use of isoxazoles with an exocychc alkene in the preparation of spiro[isoxazolidine-5,4 -isoxazolines] (409). 

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