Quinonoid

Aminoazobenzene is a very weak base, and consequently it will not form salts with weak organic acids, such as acetic acid, although it will do so with the strong mineral acids, such as hydrochloric acid. Aminoazobenzene is a yellowish-brown compound, whilst the hydrochloride is steel blue. The colour of the latter is presumably due to the addition of the proton to the phenyl-N-atom, the cation thus having benzenoid and quinonoid forms ... 

Dissolve ca. 0 2 g. of product (I) in cold ethanol, and add with shaking 1-2 drops of dilute sulphuric acid. A deep purple coloration appears at once. This shows that salt formation has occurred on the quinoline nitrogen atom to form the cation (Ha), which will form a resonance hybrid with the quinonoid form tils). [Note that the forms (IIa) and (11b) differ only in electron position, and they are not therefore tautomeric.] If, hoAvever, salt formation had occurred on the dimethylaniino group to give the cation (III), thrs charge separiition could not occur, and the deep colour would be absent. 

Quinones are exceptions. When one or more atoms of quinonoid oxygen have been replaced by >NH or >NR, they are named by using the name of the quinone followed by the word imine (and preceded by proper affixes). Substituents on the nitrogen atom are named as prefixes. Examples are... 

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. 

Depending on the reaction conditions, the product can be isolated in either the lactoid form A [2321-07-5] (2) or the quinonoid form B [56503-30-1] (3). These 9-phenylxanthenes are closely related stmcturaHy to the triphenyl methane dyes (4) and, like them, are cationic resonance hybrids. 

Quinonoid compounds have been thoroughly reviewed (4,6). More recent trends in quinone addition and substitution chemistry were reviewed in 1993 (7). The quinone system in natural products has also been covered (8) there has been a fascinating discussion of the quinone problem (9). 

The close electrochemical relationship of the simple quinones, (2) and (3), with hydroquinone (1,4-benzenediol) (4) and catechol (1,2-benzenediol) (5), respectively, has proven useful in ways extending beyond their offering an attractive synthetic route. Photographic developers and dye syntheses often involve (4) or its derivatives (10). Biochemists have found much interest in the interaction of mercaptans and amino acids with various compounds related to (3). The reversible redox couple formed in many such examples and the frequendy observed quinonoid chemistry make it difficult to avoid a discussion of the aromatic reduction products of quinones (see Hydroquinone, resorcinol, and catechol). 

An especially interesting case of oxygen addition to quinonoid systems involves acidic treatment with acetic anhydride, which produces both addition and esterification (eq. 3). This Thiele-Winter acetoxylation has been used extensively for synthesis, stmcture proof, isolation, and purification (54). The kinetics and mechanism of acetoxylation have been described (55). Although the acetyhum ion is an electrophile, extensive studies of electronic effects show a definite relationship to nucleophilic addition chemistry (56). 

The synthesis of natural products containing the quinonoid stmcture has led to intensive and extensive study of the classic diene synthesis (77). The Diels-Alder cycloaddition of quinonoid dienophiles has been reported for a wide range of dienes (78—80). Reaction of (2) with cyclopentadiene yields (79) [1200-89-1] and (80) [5439-22-5]. The analogous 1,3-cyclohexadiene adducts have been the subject of C-nmr and x-ray studies, which indicate the endo—anti—endo stereostmcture (81). 

The oxidation of 4-bromophenols to quinones can also be accompHshed using periodic acid (113). A detailed study of this reagent with stericaHy hindered phenols provided insight about the quinonoid product (114). The highest yield of 2,6-di-/-butyl-l,4-ben2oquinone [719-22-2] is for the case of R = OCH. The stilbene stmcture [2411-18-9] is obtained in highest yield for R = H. 

On more severe thionation, a third thiamine ring is formed to give a sulfur black. However, if hydroxyl groups instead of amino groups are attached at positions 2 and 2, no ring closure would take place and the blue dye would be stable to heat. These formulas are general expressions for the nuclear stmctures of the blue-to-black sulfur dyes they do not take into consideration the quinonoid formation of each dye and other aspects. 

Polycyclic Aromatic Carbonyl Dyes. StmcturaHy, these dyes contain one or more carbonyl groups linked by a quinonoid system. They tend to be relatively large molecules built up from smaller units, typically anthraquinones. Since they are appHed to the substrate (usually cellulose) by a vatting process, the polycycHc aromatic carbonyl dyes are often called the anthraquinonoid vat dyes. 

Preliminary IR spectral studies were said to suggest that pyrimidinones existed as pyrimidinols <50JCS3062) but this conclusion was promptly reversed <52JCS168) on better experimental evidence subsequent comparison with their N- and O-methyl derivatives showed that the pyrimidinones (39a R = H) and (40a R = H) along with their A-methyl derivatives (39a R = Me), (40a R = Me) and (40b R = Me) all exhibited vqo in the range 1600-1700 cm, whereas the methoxypyrimidines (39b R = Me) and (40c R = Me) showed no such absorptions <53JCS33l, 55JCS211). Closer analysis of the spectra for pyrimidin-4-one (40a R= H) showed that the ort/jo-quinonoid form (40a R = H) is the predominant tautomer (see Section 2.13.1.4). 

Quinazolin-2(and 4)-one were first shown to exist as 0x0 tautomers by IR and UV spectroscopic comparisons akin to those above (52JA4834, 51JCS3318) like its pyrimidinone analogue, the quinazolinone (41) prefers that configuration to the pczra-quinonoid tautomer (57JCS4874), a conclusion upheld by the NMR study of simple analogues (69T783). 

Finally, a quinonoid 6,7,8-trihydropterin structure (49) absorbing at 303 nm plays an important role as a labile intermediate in the tetrahydrobiopterin-dependent enzymatic hydroxylation of phenylalanine <67JBC(242)3934). 

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. 

Pterin, 7-chloro-reduction, 3, 293 Pterin, 6-chloromethyl-synthesis, 3, 312 Pterin, 5,8-diacetyl-5,8-dihydro-synthesis, 3, 306 Pterin, N, N -dibenzyl-debenzylation, 3, 295 Pterin, 6-(dibromomethyl)-synthesis, 3, 302 Pterin, 6-(diethoxymethyl)-synthesis, 3, 312 Pterin, 6,7-dihydro-quinonoid, 3, 306 Pterin, 7,8-dihydro-electrochemistry, 3, 285 quaternization, 3, 305 reactions... 

The diffusion, location and interactions of guests in zeolite frameworks has been studied by in-situ Raman spectroscopy and Raman microscopy. For example, the location and orientation of crown ethers used as templates in the synthesis of faujasite polymorphs has been studied in the framework they helped to form [4.297]. Polarized Raman spectra of p-nitroaniline molecules adsorbed in the channels of AIPO4-5 molecular sieves revealed their physical state and orientation - molecules within the channels formed either a phase of head-to-tail chains similar to that in the solid crystalline substance, with a characteristic 0J3 band at 1282 cm , or a second phase, which is characterized by a similarly strong band around 1295 cm . This second phase consisted of weakly interacting molecules in a pseudo-quinonoid state similar to that of molten p-nitroaniline [4.298]. 

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