Benzoquinones reaction with phenols

As in the case of phenoxyl and nitroxyl radical reactions, the value of Ee0 for the quinone reaction with phenol (AriOH) is much lower than that for the reaction of Q with R1 H (AEdJ 23 kj mol ). Such a difference is the result of the high triplet repulsion in TS of the type C H and low in the TS of the type H O, as in the reactions of the nitroxyl radical. The very low value of Ee0 for the reaction Q with aromatic amine is due to a high difference in electron affinity of N and atoms in TS of the type H N. The values of rate constants of p-benzoquinone with several inhibitors were calculated by the IPM method. The parameters of the IPM model are collected in Table 18.9. 

Ab initio calculations have indicated that a new reaction mechanism, viz. a suprafacial [5,5]-sigmatropic rearrangement, is possibly involved in the formation of leuco-indoaniline from the reaction of p-benzoquinone diimine with phenolate. 

Olariu et al. have reported 1,2-dihydroxybenzene (80.4 12.1)%, 1,4-benzoquinone (3.7 1.2)%, and 2-nitrophenol (5.8 1.0)% as the main OH-initiated oxidation of phenol. 1,2-Dihydroxybenzene is formed through the addition channel as shown in the figure n-E-2. Since they have not observed the formation of 1,4-benzoquinone when phenoxy radicals were produced by reaction of Cl-atoms, they have concluded that 1,4-benzoquinone formation in the OH reaction with phenol must be occurring via the OH addition channel not the abstraction channel. The complete reaction mechanism leading to 1,4-benzoquinone is still not clear. 

Scheme 2.7). The phenols were formed during isolation (chromatography on silica gel) from the corresponding cycloadducts. In the reaction with p-benzoquinone, a product was unexpectedly obtained from a hetero-T>ie s-Alder reaction with the quinone acting as a carbonyl dienophile. 

Enthalpies, Activation Energies, and Rate Constants of Reactions of p-Benzoquinone with Phenols and Amines InH + Q > In + HQ in Hydrocarbon Solutions Calculated by IPM Method for Equations See Chapter 6 and for the Values of a, bre, and A, See Table 18.9... 

An example where electron transfer from PhO- is important comes from a related publication on the reaction of phenol with O2 where [Ru(bpy)3]2+ is used as a photosensitizer (14). In acidic media the reaction involves generation of 02 by quenching of excited [Ru(bpy)3]2+ reaction of 2 with phenol leads to the production of benzoquinone. The quantum yields for benzoquinone production are highly pH dependent, showing a sharp peak at pH 8.4. This unusual pH dependence arises from the competition of several pathways, and one of the most important being the electron-transfer quenching of [ Ru(bpy)3]2+ by PhO- ... 

At the Sn02 anode only a very small amount of highly toxic intermediates (hydroquinone, catechol, benzoquinone) is formed. These intermediates are formed in large amounts on the Pt anode probably by chemical reaction of adsorbed hydroxyl radicals with phenol. 

The UV absoibance is used for the preliminary control of the degree of decomposition. The GC/MS and HPLC analysis are used to identily intermediate and final products formed during ozonation. It was found that reaction of ozone with phenol at pH 9, in addition to catechol (C) and hydroquinone (HQ) are likely primary oxidation products, p-benzoquinone (PBQ) and o-benzoquinone (OBQ), the others are more oxidized species, and CO and water the final oxidation products. The detected degradation products are shown in Scheme 24.1. 

The same intermediate occurs in the Lehmann method463 for converting the methiodides of phenolic Mannich bases into 2,3-dihydrobenzo-furans, through reaction with dimethyloxosulfonium methylide. Thus, o-benzoquinone methide (207) leads to 2,3-dihydrobenzofuran according to Scheme 4. Similarly, 2-naphtholgives l,2-dihydronaphtho[2,l-6]furan through 208 and 209, formed and rearranged in situ.i6i The reaction has been applied to the synthesis of polycyclic benzofurans.483,485... 

Under different reaction conditions, phenols can be oxidized to p-quinones (equations 272600-602 and 273603), but in the case of phenol itself, insufficient selectivity has prevented, as yet, the commercial application of this potentially important synthesis of p-benzoquinone and hydroquin-one. The selectivity of p-benzoquinone, or p-quinol formation can be increased at the expense of oxidative coupling products by using a large excess of the copper reagent [Cu4Cl402(MeCN)3 or CuCl + 02 in MeCN] with respect to the phenolic substrate.604 The suggested mechanism involves the oxidation of the phenoxide radical (189) by a copper(II)-hydroxo species to p-quinol (190) which can rearrange (for R2 = H) to hydroquinone (191 Scheme 14), which is readily oxidizable to p-quinone.6... 

Diels-Alder reactions. Homophthalic anhydride (1) undergoes Diels-Alder reactions with some alkynes and benzoquinones at 150-200° to give linear phenols, probably via the tautomer a.2 Examples ... 

It is worthwhile to cite the pioneering work of Brackman and Havinga, carried out in the 1950s and considered as early tyrosinase models [1,2,179], Here, conditions were found to effect the straightforward catalytic o-hydroxyla-tion of phenols to give substituted o-quinones. A most interesting case occurs when copper salts are reacted with phenol, 02, and morpholine (mp) in methanol, giving insoluble morpholino-substituted o-benzoquinone. The reaction is complicated, but a Cu(II)-peroxo-phenol-mp species is seen to be an important intermediate (Scheme 17). 

Recently, the reaction of masked ortho-benzoquinone [92] with C60 was tested [93]. The [4+2] cycloaddition reaction of such electron-deficient dienes with fullerenes resulted in the formation of highly functionalized bicyclo [2.2.2] octenone-fused fullerenes. The reactants were generated in situ by the oxidation of the readily available 2-methoxy phenols with hypervalent iodine agents. For the several different masked ortho-benzoquinones that were tested, it was found that the yield of the cycloadducts depends on the nature of the starting materials and the reaction conditions. Other Diels-Alder reactions of such electron-deficient dienes with electron-poor fullerenes involved tropones [94], 1,3-butadienes substituted with electron-withdrawing groups [95], and 2-pyrone [96]. 

Identification of SeMet after being separated from other amino acids by means of paper chromatography and electrophoresis is facilitated by spraying spots with H202 or preferably by exposure to cyanogen bromide (Shepherd and Huber, 1969). Pre-derivatization of SeMet through reaction with o-benzoquinone facilitates separation and identification in the presence of Met. Both SeMet and Met react with o-benzoquinone at pH 1 to form phenolic sulfonium- or selenonium derivatives whose UV absorption spectra differ. 

A series of azophenol acerands 4 was prepared by condensation of crowned benzoquinones 10 with 2,4-dinitrophenylhydrazine in ethanol [7b], The quinone was derived from p-methoxyphenol (6) as shown in Scheme 1 [8]. By bis(hy-droxymethylation) (67% yield of 6, followed by methylation (92%) of the phenol group and Williamson-type reaction with ditosylates of oligoethyleneglycol in the presence of sodium hydride, crowned 1,4-dimethoxybenzene 9 was obtained in reasonable yields. Oxidative demethylation of 9 with ceric ammonium nitrate (CAN) in aqueous acetonitrile at 50 °C gave the desired crowned benzoquinones 10 in good yields. 

Reaction between phenol and hydroxyl yields the dihydroxybenzenes, which can then undergo further oxidation (hydroquinone to benzoquinone, further hydroxylated to hydroxybenzoquinone, catechol and resorcinol to trihydroxybenzenes [79,100]). The condensation products, phenoxyphenols and dihydroxybiphenyls, most likely originate from the reaction between phenol and the phenoxyl radical [101]. Their presence indicates that some phenoxyl forms in the system, due to the reaction of phenol with OH or NO2. The possibility for NO2 to oxidise phenol to phenoxyl has been the object of a literature debate [102,103] in the context of nitration processes. The problem can be tackled upon consideration of the reduction potentials of the various species. The reduction potential of phenoxyl to undissociated phenol is E = 1.34 V - 0.059 pH [104], while for the reduction of nitrogen dioxide to nitrite it is E = 0.90 V [105]. Accordingly oxidation of phenol to phenoxyl would be possible above pH 7.5, and of course in the presence of phenolate (pH > 10 [106]). 

In addition, the consecutive oxidation of phenol to benzoquinone was regarded as a two step reaction with hydroquinone as intermediate, thus avoiding the the need to postulate formulation of a trimolecular collision. It was further assumed that hydroquinone was... 

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