Quinonoidal structures

Diketones and tetraketones derived from aromatic compounds by conversion of two or four SCH groups into keto groups, with any necessary rearrangement of double bonds to a quinonoid structure, are named by adding the suffix -quinone and any necessary affixes. 

Substituted indazolones exist in the OH form (143b) and 2-substituted indazolones exist in the NH form (144a), whereas the structure of AC-unsubstituted indazolones varies with the physical state. This difference of behaviour, depending on the position of the A -R substituent, corresponds to the aromatic structure of the indazole derivatives compared with the quinonoid structure of isoindazoles. 

It was subsequently shown that the polymers contain semi-quinonoid structures 47 proposed to arise from a-p coupling of radicals 46 as shown in Scheme 9.15., (M It was also suggested that 47 could be subject to radical-... 

In conclusion, with regard to the structure of benzenediazonium compounds with electron donor substituents in the 2- or 4-position, the most recent experimental data, mainly X-ray analyses and 13C and 15N NMR data, are consistent with 4.4 as the dominant mesomeric structure of quinone diazides, as proposed by Lowe-Ma et al. (1988). For benzenediazonium salts with a tertiary amino group in the 4-position the data are consistent with the quinonoid structure 4.20 as the dominant mesomeric form. 

A large primary isotope effect kH/kD = 3.6 had also been found earlier by Ibne-Rasa122 in the nitrosation of 2,6-dibromophenol in the 4 position which was also shown to be base-catalysed. These values are not unexpected in view of the isotope effect found with diazonium coupling which involves a similarly unreactive electrophile, so that the rate-determining transition state will be displaced well towards products. Furthermore, the intermediate will have a quinonoid structure and will, therefore, be of low energy consequently, the energy barrier for the second step of the reaction will be high. 

Proton-coupled intramolecular electron transfer has been investigated for the quinonoid compounds linked to the ferrocene moiety by a 7r-conjugated spacer, 72 (171) and 75 (172). The complex 72 undergoes 2e oxidation in methanol to afford 74, which consists of an unusual allene and a quinonoid structure, with the loss of two hydrogen atoms from 72 (Scheme 2). The addition of CF3SO3H to an acetonitrile solution of 74 results in two intense bands around 450 nm, characteristic of a semi-quinone radical, and a weak broad band at lOOOnm in the electronic... 

Type 61b of the intensely colored quinocyclopropenes is represented by the di-cyanomethylene species 115 and 118 of p- and o-quinonoid structure. In addition to the systems 115,119, and 122 reported by Gompper1001 a series of o- and p-quino-cyclopropenes in the benzene, naphthalene, anthracene, phenanthrene, and fluorene series (718-125) were prepared75) carrying the bis-(p-anisyl)-cyclopropenyl residue, which brings about a better stabilization of the cyclopropenium moiety101 ... 

Phenyl and triphenylmethyl radicals generated from 6 contribute to the initiation and the termination, respectively, resulting in polymer 18 because of the remarkably different reactivities of these radicals (Eq. 21). The co-chain end terminated with 1 thermally redissociates to induce further polymerization. Therefore, the polymerization proceeded via a mechanism close to the model in Eq. (18). The recombination product of methyl isobutyryl radical and 1 was reported to have a quinonoide structure [82], suggesting a similar structure of the chain end, 18b. 

Quinonoid structure, synthesis of natural products containing, 21 254 Quinonoid systems, oxygen addition to, 21 249... 

Improved understanding of the interaction of visible and ultraviolet radiation with organic structures aroused interest in the tautomeric capabilities of dye molecules such as bcnzcnoid-quinonoid tautomerism in azo (1.40), anthraquinone (1.41) and triarylmethane (1.42) systems. According to Watson (1913), if a dye could show a quinonoid structure in all... 

As long as the solution which has been made alkaline retains its colour the ammonium colow base, which preserves the quinonoid structure of the dye, is still present (Hantzsch). This base is stable for a short time only and can in no case be isolated. The disappearance of the quinonoid system, which rapidly follows, occurs either through a rearrangement of the genuine base to the so-called pseudo- or carbinol-base, accompanied by the wandering of the OH-group ... 

When the colourless carbinol base is treated with acid the quinonoid structure is re-formed and the dye produced. But on dissolving cautiously, with cooling, it can he seen that the colour attains its maximum depth only gradually. Hence the exceedingly unstable colourless carbinol salt is first formed it changes into the dye with spontaneous elimination of water ... 

The classical idea of through-conjugation is revised the importance of structures 52 and 53 was criticized in particular by Hiberty and Ohanessian159. There is a tendency to explain physical and chemical properties of derivatives of p-nitroaniline without considering quinonoid structures like 52 to be the most important ones. 

In contrast to phenylcarbene, which ring-expands photochemically, 8 was found to be remarkably inert under similar conditions [73]. This supports indirectly the quinonoid structure of 8, as opposed to a biscarbene structure (such as represented by S4 of Fig. 3) with weakly or noninteracting divalent carbon centers. 

Glowiak studied the stability of the four diazophenols (59-62), which he prepared from the diazotization of the corresponding aminophenols. It was noted that (61) and (62) show higher chemical stability than (59) and (60) the latter compounds were postulated to have a quinonoid structure rather than a zwitterionic diazophenol structure. 

Amine oxidases catalyze the oxidative deamination of both xenobiotic and biogenic amines, and thus have many critical biological functions. Two distinct classes differ in the nature of their prosthetic groups [1]. The flavin-(FAD flavin adenine dinucleotide)-dependent amine oxidases include monoamine oxidases (MAO A and B) and polyamine oxidases. Amine oxidases not containing FAD, the so-called semicarbazide-sensitive amine oxidases (SSAO), include both plasma amine oxidases and tissue amine oxidases. These contain quinonoid structures as redox cofactors that are derived from posttranslationally modified tyrosine or tryptophan side chains, topaoquinone frequently playing this role [2]. 

It is assumed that the water-insoluble lignin is a polymerization and oxidation product of a Cio monomer, C9Hn03(0CH3). The brown color may be the result of loss of 0.17 methyl groups and hydrogen atoms during the formation of a quinonoid structure and loss of 0.7 moles of water. 

Figure 6. Suggested mechanism for the reaction of ortho-quinonoid structures in lignin derivatives with hydrogen peroxide under alkaline conditions (according to Gierer, ref. 3).

Recently from a comparison of the infra-red absorption spectrum of this compound with that of o-benzoquinone Glowiak [10] came to the conclusion that dinitro-benzenediazo-oxide has a quinonoid structure. Both substances show the presence of the strong absorption band of the carbonyl group 1666 cm-1 for dinitrobenzene-diazo-oxide and 1680 cm-1 for o-benzoquinone. In addition dinitrobenzenediazo-oxide gives a band with a frequency of 2190 cm-1, characteristic of a double bond between nitrogen atoms. (Some derivatives of this compound may also have the diazo structure (Ha), which is discussed later on.)... 

From their infra-red absorption spectra he ascribes a quinonoid structure to the first two compounds and a diazo structure to the last two and to their plumbous salts ... 

The compounds with a quinonoid structure differ from those with a diazo structure by their darker colour and lower chemical stability. They are for example easily decomposed by light and concentrated acids they are less resistant to heat and show a higher sensitiveness to impact, friction and flame than compounds with a diazo structure. 

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