Quinonoid compound

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). 

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... 

As observed with the pincer-type quinone methide complexes (Section 3.2), the one-site coordinated QMs can also undergo chemical transformation to other quinonoid compounds. For instance, reaction of complex 20 with MeOTf resulted in the methylene arenium complex 21 (Scheme 3.13).22... 

Vigalok, A. Milstein, D. Advances in metal chemistry of quinonoid compounds new types of interactions between metals and aromatics. Acc. Chem. Res. 2001, 34, 798-807. 

The Chemistry ofthe Quinonoid Compounds (S. Patai, ed.), Vols. 1 and 2, Academic Press, New York(1974). 

See Other METAL NITROPHENOXIDES, NITROAROMATIC-ALKALI HAZARDS, (7C/-NITRO-QUINONOID COMPOUNDS... 

S. Patai and Z. Rapport, eds., The Chemistry of Quinonoid Compounds. Wiley, New York, 1988. 

The color of the quinonoid compounds that may be obtained by disproportionation can be sufficiently like that of the radicals to cause confusion if visual observation or broad-band spectrophotometry is used.11 For example, Preckel and Selwood, using paramagnetism as a measure of the amount of radical, reported that solutions of triphenyl-methyl derivatives more or less rapidly lost their paramagnetism. The decomposed solutions were still highly colored, but the color was no longer dependent on the temperature as it is in the case of a radical-dimer equilibrium mixture. What is more striking, and an even more subtle and dirtier trick on the part of nature, is the fact that Preckel and Selwood s non-paramagnetic solutions were still rapidly bleached by exposure to the air. It is clear that radical-like reactivity is not a safe criterion for the presence of radicals. It is also clear that the ebullioscopic method is particularly unsatisfactory in view of the excellent chance for decomposition. 

Quinone sesquiterpenes, 21 252-253 Quinone stabilizers, 20 105-106 Quinonoid compounds, 21 237 Quinonoid dienophiles, Diels-Alder cycloaddition of, 21 254... 

A one-electron oxidation study of quercetin (see structure below) and quercetin derivatives (rutin) by DPBH, CAN, or dioxygen in protic and aprotic solvents has shown that quercetin radicals quickly disproportionate to generate quercetin and produce a quinone. This quinone adds water molecules and is then degraded. Oligomerization might be a minor route in media of low water content. Oxidation of quercetin-serum albumin complex retarded water to the quercetin quinone. The role of the quercetin 3-OH was established as follows (1) allows the formation of jo-quinonoid compounds, quickly converted into solvent adducts which still react with one-electron oxidants, and (2) in its deprotonated form stabilizes radicals, allowing autoxidation to proceed under mild conditions. 

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