Quinoid derivatives

Aromatic nitro compounds are often strongly colored. They frequently produce characteristic, colored, quinoid derivatives on reaction with alkali or compounds with reactive methylene groups. Reduction to primary aryl amines followed by diazotization and coupling with phenols yields azo dyestuffs. Aryl amines can also react with aldehydes with formation of Schiff s bases to yield azomethines. 

Primary amines and substances with reactive methylene groups react with 1,2-naphthoquinone-4-sulfonate to yield intensely colored p-quinoid derivatives, which, in the case of aryl amines, are indophenol dyes [12, 13]. 

The hexoses that are the initial products of acid hydrolysis of sucrose (1) react at el vated temperature under the influence of acids to yield furfural derivatives (2). Thed condense, for example, with the phenols to yield triarylmethanes (3), these react furthei by oxidizing to yield colored quinoid derivatives (2, 4). Polyhydric phenols, e. g. resorj cinol, on the other hand, yield condensation products of Types 5 and 6 [2],... 

It is effective against both sucking and chewing insects its lDjq for rats is 150, for guinea pigs 500 mg/kg. Its action persists for 6-11 weeks on various surfaces but, on plant surfaces, it is enzymatically decomposed in a shorter time. During its photochemical decomposition 2,3,5,6-tetrahydroxypyridine is formed. This compound is oxidatively converted to quinoidal derivatives, which are finally oxidised, with loss of carbon dioxide, to noncyclic decomposition products ... 

A variety of biochemical pathways are known which may lead to reactive quinoid derivatives. They include dihydroxylation of aromatic or heterocyclic compounds and epoxide formation and hydrolysis to -diphenolic compounds (Booth and Boyland 1957) o- and p-hydroxylations of phenols or arylamines (In-SCOE et al. 1965 Miller et al. 1960 Booth and Boyland 1957) and rearrangement of -hydroxyarylamines to o-aminophenols (Miller and Miller 1960). It now appears that aromatic hydroxylations proceed via highly reactive arene oxides, i.e., compounds in which a formal aromatic double bond has undergone epoxidation. Depending on the compound, arene oxides may give rise to other electrophilic reactive species, including quinoid structures, but react as such readily with nucleophiles and thus provide a basis for understanding covalent attachment of aromatic hydrocarbon derivatives to protein and nucleic acids (Jerina and Daly 1974). 

Naphtols LIII, vinylethers LIV, LV, phenols LVII, LVIII and quinoid derivatives LIX [99] have been prepared by this way (Figures 3 and 4). 

Curing systems using quinoid derivatives (GMF or dibenzo GMF with oxidizing agents) give very fast cures, and they are used on a large scale in the CV wire insulation process aging behavior of these vulcanizates is superior to those obtained by sulfur or sulfur-donor systems. 

However, benzylidene derivatives show a strong bathochromic shift in comparison with alkylidene derivatives. Thus absorption is a result of the whole conjugated system that is comparable to that of the quinoid dyes. The color of this type of compound is sensitive to acids and bases. 

Despite the inconveniences, a certain number of studies have been carried out, particularly concerning dyes containing azomethine groups. Such as hydrazones, pyrazolones, formazans, and selenazoles quinoids. Saturated heterocycles, that is, selenazolines and selenazolidines. have also been tackled. Selenium derivatives for pharmacological or physiological applications are little developed by comparison with their thiazole homologs. 

Conversion of Aromatic Rings to Nonaromatic Cyclic Structures. On treatment with oxidants such as chlorine, hypochlorite anion, chlorine dioxide, oxygen, hydrogen peroxide, and peroxy acids, the aromatic nuclei in lignin typically ate converted to o- and -quinoid stmctures and oxinane derivatives of quinols. Because of thein relatively high reactivity, these stmctures often appear as transient intermediates rather than as end products. Further reactions of the intermediates lead to the formation of catechol, hydroquinone, and mono- and dicarboxyhc acids. 

Conversion of Cyclic to Acyclic Structures. Upon oxidation, the aromatic rings of lignin may be converted direcdy to acycHc stmctures, eg, muconic acid derivatives, or indirectly by oxidative splitting of o-quinoid rings. Further oxidation creates carboxyUc acid fragments attached to the lignin network. 

Similarly, indole itself could be converted by 2-methylsulfanyl-l,3-dithiolium iodide to its 3-dithiolium derivative, which gave 27 quantitatively with DBU. However, treatment of indoles, which bear the benzo-dithiolium moiety in the 2-position with tertiary amines, resulted in a black reaction mixture. All attempts to isolate the o-quinoid compound 28 failed (Scheme 7). 

In general, the A -methyl derivative of a given compound absorbs at longer wavelengths than the O-methyl derivative. The intensity of a band which appears in aqueous solutions beyond the maximum absorption in alcohol and which is due to the absorption of the betainic species alone, is a measure of the tautomeric equilibrium. The pA"a value of the 2-methyl-hydroxyisoquinolinium chlorides increase in the order 4-hydroxy (4.93), 8-hydroxy (5.81), 6-hydroxy (6.02), 5-hydroxy (6.90), and 7-hydroxy (7.09 in water at 25 °C, respectively) (57JCS5010). Thus, 2-methyl-4-hydroxyisoqui-nolinium chloride is the strongest acid. The UV spectra of 2-methyl-isoquinolinium-5-olate (34) and 2-methyl-isoquinolinium-8-olate (39) were also presented (61BCJ533) and the formation of a quinoid structure of 2-methyl-isoquinolinium-6-olate (38) can also be detected by means of UV-spectroscopy. 

While tocopherylacetic aicd (51), the lower Crhomologue of 3-(5-tocopheryl)-propionic acid (50) showed a changed redox behavior (see Section 6.5.1), compound 50 displayed the usual redox behavior of tocopherol derivatives, that is, formation of both ortho- and para-quinoid oxidation intermediates and products depending on the respective reaction conditions. Evidently, the electronic substituent effects that... 

Perhaps due to oxidizing quinoid type electronic structure of benzotriazol-2-yl derivatives, some of their properties are completely different from those of isomeric benzotriazol-l-yl derivatives. Thus, anions derived from 2-alkylben-zotriazoles 388 are rapidly converted to appropriate radicals that undergo coupling to form dimers as mixtures of racemic 289 and meso 390 forms <1996LA745>. When the reaction mixture is kept for an extended period of time at —78 °C, (Z)- 391 and (E)- 392 alkenes are formed. When benzophenone is added to the reaction mixture, alcohols 387 are obtained in good yields however, benzaldehyde does not react under these conditions (Scheme 63). 

However, because of the mostly very slow electron transfer rate between the redox active protein and the anode, mediators have to be introduced to shuttle the electrons between the enzyme and the electrode effectively (indirect electrochemical procedure). As published in many papers, the direct electron transfer between the protein and an electrode can be accelerated by the application of promoters which are adsorbed at the electrode surface [27], However, this type of electrode modification, which is quite useful for analytical studies of the enzymes or for sensor applications is in most cases not stable and effective enough for long-term synthetic application. Therefore, soluble redox mediators such as ferrocene derivatives, quinoid compounds or other transition metal complexes are more appropriate for this purpose. 

Quinoid-type chromophoric structures, reactions of, 21 36-37 Quinoline (QI), 12 723, 725-726 soluble dyes, 7 373t Quinoline-4-carboxylic acids, 21 190 Quinoline derivatives, 21 196-214 Quinoline-derived drugs, 21 197-198t Quinoline dyes, 21 196 Quinoline, formation of, 21 109. See also Quinolines... 

These concepts were recently applied to an understanding of the electrochemical reduction of the mono and dinitro derivatives of the nonbenzenoid hydrocarbon 24a45. Compound 24b exhibits a single one-electron wave at —1.08 V, while dinitro compound 25 exhibits two one-electron waves at —0.88 and —1.05 V46. This behavior is quite similar to that exhibited by ortfto-dinitrobenzene (21) it appears therefore that 25 is reduced to a dianion in which the quinoidal structure 26 is an important contributor to the resonance hybrid. The quinoidal structure 11 could be produced from 25 even though the... 

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