Quinoidal systems

Diels-Alder Reaction. In 1928, Diels and Alder discovered that 1,3-unsaturated organic compounds reacted with quinoid systems to give partially hydrogenated, cycHc compounds. In the course of their work, they found that 1 mol of 1,4-naphthoquinone [130-15-4] reacted readily with 1 mol of 1,3-butadiene [106-99-0] to give a partially hydrogenated anthraquinone (11) l,4,4a,9a-tetrahydro-9,10-anthracenedione [56136-14-2] which, on oxidation with chromic oxide, produced anthraquinone (43) ... 

Reaction of the amine with p-dimethylaminobenzaldehyde (Ehrlich reagent) or p-diethylaminobenzaldehyde to produce a colored Schiff base [23—25]. The color formation is ascribed to a resonance hybrid between a protonated Schiff base and a quinoid system (Eq. (2)) ... 

A simple, fast and specific color test for urea nitrate was reported recently by Almog et al. It is based on the reaction between urea nitrate and ethanolic solution ofp-dimethylaminocinnamaldehyde (p-DMAC) (9) under neutral conditions [91]. A red pigment is formed within 1 min from contact. Its structure has also been determined by the same group, by X-ray crystallography [92]. It appears to be a resonance hybrid between a protonated Schiffbase (10) and a quinoid system (10a) (Eq. (14)). The limit of detection on filter paper is 0.1 mg/cm. Urea itself, which is the starting material for urea nitrate, does not react with p-DMAC under the same conditions. Other potential sources of false-positive response such as common fertilizers, medications containing the urea moiety and various amines, do not produce the red pigment with p-DMAC. 

Naturally, the product of two-electron reduction of the para derivative can be depicted (Scheme 3.60) as an anionic diborataquinoid system. In contrast to the p-diborataquinoid dianion, an anionic OT-quinoid system is impossible. Indeed, the meta-substituted isomer depicted in Scheme 3.60 has been... 

The same inertness holds for many inoperative cyclization paths that would have given highly quinoid systems such as 104 (cf 44 and 45), 105 (cf 86). 106(cf 87) and 107 c 88). Under certain circumstances such topological factors can not only modify the AO coefficients at the reacting atoms but can also result in an interchange of the usual topmost occupied MO and lowest unoccupied MO with other orbitals which prevent the photocyclization altogether. This situation has been deduced for the pentahelicenes series (Table In this series, (Table 20) benzo... 

Oxidative transformations of hindered synthetic phenols and a-tocopherol are analogous to the oxidation of various mono- and dihydric phenols of plant origin. Natural phenols present in fruits or green tea leaves are oxidized on contact with air and/or during fermentation (a process characteristic of tea leaves) and are transformed into dark colored quinoid systems, not harmful for human beings. It may be extrapolated that trace amounts of discoloring quinones or quinone methides arising from phenolic antioxidants in plastics are harmless as well. 

The reduction of azo compounds using sodium hydrosulfite (Na2S204) and NaOH is an important reaction, as it provides an indirect method for the amination of phenols and naphthols (Fig. 13.49). The reduction of nitro groups in anthraquinone compounds works best when a mild reducing agent (e.g., sodium hydrosulfide, NaSH) is used. In this way one avoids reducing the quinoid system. 

The anodic oxidation of phenylenediamines parallels that of aminophenols (see Sec. III.A.l) and has been reviewed by Adams [108]. If unsubstituted at the nitrogens, the two-electron oxidation leads to the quinone dimine. This compound either undergoes hydrolysis to the quinone inline and benzoquinone, or a 1,4-addition of a nucleophile, for example, the parent phenylenediamine itself, to the quinoidal systems occurs. Further oxidation of the products may take place. In acetonitrile, the one-electron oxidation to the cation radical predominates [109]. Under these conditions,/7-phenylenediamine also leads to 1,4-coupling products [110,111]. A-Substituted phenylenediamines are forming more stable cation radicals. For example, tetrakis(/7-bromophenyl)/7-phenylenediamine ( °= 0.91V vs. NHE) and tetrakis(2,4-dibromophenyl)-/7-phenylenediamine E° = 0.94 V vs. NHE) in acetonitrile even show reversible behavior for the second oxidation step to the dication [78]. 

To reach high reaction rates, the hydride ion abstraction must be fast and very effective. The chemical regeneration agent frequency used for NAD" " is FMN, a quinoid system. It is most applied as a stoichiometric oxidant. It can also be introduced in the presence of oxygen, thus producing hydrogen peroxide, which has to be destroyed by catalase [10,13,14,114]. In this case, in principle, less than stoichiometric amounts of FMN should be sufficient. However, due to the low reactivity, even in this case more than stoichiometric amounts of the catalyst are applied. Thus, with FMN, the problem is a... 

The most attractive detailed hypotheses (Fig. 11) suggest the formation of the erythrinane skeleton by oxidation of LXXXVIII, a symmetrical intermediate derived from two molecules of tyrosine or dihydroxyphenylalanine. The two additional bonds necessary might be formed in either order. In one hypothesis (1, 57) oxidation of one aromatic ring to the o-quinone (LXXXIX) is followed by nucleophilic addition of the amino group and further oxidation to XC (or the related o-quinone). This sequence is exactly analogous to the in vitro oxidation of dihydroxyphenylalanine itself to the quinone dopachrome (3S). Nucleophilic or radical addition of the second phenolic ring to the quinoid system would complete the spiro skeleton of XCI. 

Calixquinones can be easily reduced to the corresponding calixhydroquinones (Zn/HCl or Na2S204) . The calix[8]hydroquinone 57b, however, was prepared from the octabenzyl ether 57a obtained by one-pot condensation in a mixture with the analogous calix[6]- and -[7]arene. Oxidation of 57b to the respective octaquinone was not reported, but the ewrfo-ether 57c was obtained by exhaustive 0-propylation prior to the cleavage of the benzyl ether groups . Inherently chiral derivatives of a calix[4]arene monoquinone have been obtained by 1,4-addition of various nucleophiles to the quinoid system. 

Fig. 5.12 The influence of a quinoidal system on the phosphorus resonance of selected phosphaalkenes...

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