Of benzoquinones

Volatility in steam. Add about 0 1 g. of benzoquinone to 3 ml. of water in a test tube and boil gently. The benzoquinone dissolves to give a yellow solution, which rapidly darkens in colour. Note the irritating and characteristic odour of benzoquinone which has volatilised in the steam. Also given by />-toluquinone and 1,4 naphthoquinone but not by the other quinones mentioned above. 

To a cold aqueous solution of benzoquinone, add 1 drop of sulphurous acid solution (SOj-water) the solution turns deep green-brown owing to the intermediate formation of quinhydrone, CeH402,CeIl4(0H)2. Now add excess of sulphurous acid the solution becomes colourless owing to the formation of hydroquinone. Add a few drops of FeClj solution the reaction is reversed and the deep yellow colour (distinct from that of FeCl ) is restored. 

Sodium hydroxide. Dissolve a few crystals of benzoquinone in 10% aqueous NaOH solution and shake gently. The solution turns rapidly brown and then almost black owing to atmospheric oxidation. 

The silyl enol ethers 209 and 212 are considered to be sources of carbanions. and their transmetallation with Pd(OAc)2 forms the Pd enolate 210. or o.w-tt-allylpalladium, which undergoes the intramolecular alkene insertion and. 1-elimination to give 3-methylcyclopentenone (211) and a bicyclic system 213[199], Five- and six-membered rings can be prepared by this reaction[200]. Use of benzoquinone makes the reaction catalytic. The reaction has been used for syntheses of skeletons of natural products, such as the phyllocladine intermediate 214[201], capnellene[202], the stemodin intermediate 215[203] and hir-sutene [204]. 

In order to make these oxidative reactions of 1,3-dienes catalytic, several reoxidants are used. In general, a stoichiometric amount of benzoquinone is used. Furthermore, Fe-phthalocyanine complex or Co-salen complex is used to reoxidize hydroquinone to benzoquinone. Also, it was found that the reaction is faster and stereoselectivity is higher when (phenylsulflnyl)benzoquinone (383) is used owing to coordination of the sulfinyl group to Pd, Thus the reaction can be carried out using catalytic amounts of PdfOAcji and (arylsulfinyl)benzoquinone in the presence of the Fe or Co complex under an oxygen atmosphere[320]. Oxidative dicyanation of butadiene takes place to give l,4-dicyano-2-butene(384) (40%) and l,2-dicyano-3-butene (385)[32l]. 

Treatment of benzoquinone with ethyl-4-methyl-A-4-thiazoline-2-thione-5-carboxylate yields ethyl-2-(2.5-dihydroxyphenylthio)-4-methyl-5-thiazolyl carboxylate (78) (Scheme 37) (165). 

Oxidation of aniline with a mixture of manganese dioxide and sulfuric acid has been used commercially for production of -benzoquinone [106-51-4] (35). 

The Nenitzescu process is presumed to involve an internal oxidation-reduction sequence. Since electron transfer processes, characterized by deep burgundy colored reaction mixtures, may be an important mechanistic aspect, the outcome should be sensitive to the reaction medium. Many solvents have been employed in the Nenitzescu reaction including acetone, methanol, ethanol, benzene, methylene chloride, chloroform, and ethylene chloride however, acetic acid and nitromethane are the most effective solvents for the process. The utility of acetic acid is likely the result of its ability to isomerize the olefinic intermediate (9) to the isomeric (10) capable of providing 5-hydroxyindole derivatives. The reaction of benzoquinone 4 with ethyl 3-aminocinnamate 35 illustrates this effect. ... 

Treatment of -benzoquinone with 1-pyrrolidinylamine provides a convenient synthesis of the immunoregulator and antibacterial agent, azarole (11)... 

Benzoquinone is an excellent dienophile in the Diels-Alder reaction. What product would you expect from reaction of benzoquinone with 1 equivalent of 1,3-butadiene Prom reaction with 2 equivalents of 1,3-butadiene ... 

The HQ reaction with oxygen proceeds through several steps, and the precise reaction path is pressure-dependent. The first step is the formation of benzoquinone is shown in the following section. Further reactions result in the formation of low MW alcohols, ketones, andother compounds. 

Larock has developed a new catalyst system for the Pd-catalyzed cyclization of olefinic tosylamides. Whereas typical conditions require either stoichiometric amounts of Pd(II) salts or catalytic amounts of Pd(II) in the presence of benzoquinone as a reoxidant, the new catalyst system utilizes catalytic Pd(OAc)2 under an atmosphere of O2 in DMSO with no additional reoxidant <96JOC3584>. Although o-vinylic tosylamides 76 can be cyclized to Af-tosylindoles 77 using this catalyst system, PdCla/benzoquinone is more effective for such cyclizations. Interestingly, in the case of o-allylic tosylanilides, the cyclization can be modulated to afford either dihydroindole or dihydroquinoline products. In a related approach involving a common 7i-aUyl Pd-intermediate, 2-iodoanilines were coupled with vinylic cyclopropanes or cyclobutanes in the presence of a Pd catalyst to afford dihydroindoles <96T2743>. 

Disappearance of pairs of inhibitor radicals by disporportionation leads to the regeneration of a molecule of benzoquinone or a substituted quinone. This molecule may terminate a subsequent chain. Hence, if the inhibitor radicals disappear exclusively by disproportionation,... 

Benzoquinone moistened by water gives rise to an important thermal load. A DTA showed that the exothermic peak is reached at 40-50 C and the compound decomposes at a temperature starting at 60-70°C. When a bottle of benzoquinone is exposed to sunlight, its temperature reaches 50-60°C rapidly. 

Lin, A. J. Shansky, C. W. Sartorelli, A. C. Potential bioreductive alkylating agents. 3. Synthesis and antineoplastic activity of acetoxymethyl and corresponding ethyl carbamate derivatives of benzoquinones. J. Med. Chem. 1974, 17, 558-561. 

The quinone-hydroquinone system represents a classic example of a fast, reversible redox system. This type of reversible redox reaction is characteristic of many inorganic systems, such as the interchange between oxidation states in transition metal ions, but it is relatively uncommon in organic chemistry. The reduction of benzoquinone to hydroquinone... 

Wassermann (6) has studied the Diels-Alder reaction of benzoquinone (B) and cyclopen-tadiene (C) at 25 °C. 

For the case where the cascade consists of only a single reactor, only a single straight line of the form of equation 8.3.31 is involved in the graphical solution. One merely links the point on curve M corresponding to the effluent concentration of benzoquinone with the point on the abscissa corresponding to the feed concentration. The slope of this line is equal to ( — 1/t) or — io/VR). In the present instance the slope is equal to... 

For the specified degree of conversion the effluent concentration of benzoquinone must be equal to (1 — 0.875)(0.08) = 0.01 kmole/m3. Case I—Single CSTR... 

The use of electrochemical transmittance spectroscopy in both the UV-visible and IR regions of the spectrum is elegantly shown by the work of Ranjith et al. (1990) who employed an OTTLE cell to study the reduction of benzoquinone, BQ. The authors were the first to report the UV-visible spectrum of BQ2- and to demonstrate the quantitative aspects of the technique by reporting extinction coefficients for the major bands of BQT and BQ2- in both the UV-visible and IR. 

Increase in cytochrome c reduction in the presence of benzoquinone could be due to Reaction (5). 

An example where electron transfer from PhO- is important comes from a related publication on the reaction of phenol with O2 where [Ru(bpy)3]2+ is used as a photosensitizer (14). In acidic media the reaction involves generation of 02 by quenching of excited [Ru(bpy)3]2+ reaction of 2 with phenol leads to the production of benzoquinone. The quantum yields for benzoquinone production are highly pH dependent, showing a sharp peak at pH 8.4. This unusual pH dependence arises from the competition of several pathways, and one of the most important being the electron-transfer quenching of [ Ru(bpy)3]2+ by PhO- ... 

Surface Oraanometallic Chemistry of Hvdroauinone. Previous studies (2) have established that the interaction of aqueous HQ with smooth polycrystalline Pt is concentration-dependent. Below 0.1 m M, HQ undergoes spontaneous oxidation to form a chemisorbed monolayer of flat-oriented (ne) benzoquinone (BQ) similar to that in the homogeneous coordination of benzoquinone to tris (triethylphosphine) Pt (0) (2) ... 

Direct production of benzoquinone (BQ) from benzene is one of the targets in industrial chemistry. Considerable efforts have been made to develop the electrochemical oxidation of benzene to p-benzoquinone to the industrial scale thus forming a basis for a new hydroquinone process [40]. Benzene in aqueous emulsions containing sulfuric acid (1 1 mixture of benzene and 10% aqueous H2S04) forms, at the anode, p-benzoquinone which can be reduced cathodically to yield hydroquinone in a paired synthesis. A divided cell with Pb02 anodes is used. 

Indeed, it has been found that the Diels-Alder reaction of benzoquinone 47 and cyclohexadiene 48 is accelerated by encapsulation in the hydroxysoftball 7-7.[251 It is known that the capsule in its resting state is occupied with two benzoquinone molecules,... 

Figure 8. Catalytic cycles for the Diels-Alder reactions of benzoquinone 47 with cyclohexadiene 48 (top) and tetramethyl thiophene dioxide 51. While the reaction of 47 with 48 suffers from product inhibition, adduct 52 is easily replaced by new reactants inside the capsule.

In Australian tenebrionid beetles, defensive compounds and their patterns seem to be of only low chemotaxonomic value. However, the aforementioned aromatic compounds are restricted to the genus Tribolium. Abdominal defensive compounds were used as chemosystematic characters in order to construct a phylogenetic tree for the genus Tribolium [330]. The defensive secretion of adults of Tenebrio molitor was shown to contain toluquinone 7 and m-cresol 89 [333]. The quantification of benzoquinones in single individuals of Tribolium castaneum at different days after adult eclosion indicates that the amount of toxic quinone only shows a maximum subsequent to cuticle sclerotization. Obviously, there is a need for an adequate cuticular barrier for self-protection from these defensive compounds [334]. 

For example, reductive elimination of benzonitrile from (Et3P)2Pd(CN)Ph is promoted by the addition of triethyl phosphite [4], addition of benzoquinone promotes reductive eliminations in palladium chemistry (Chapter 12), etc., but in all cases different roles can be envisaged, such as simple ligand exchange. 

Again we see that an alkene isomerisation reaction has taken place. Another important, useful reagent applied in this field is also pictured in Figure 15.7, viz. the use of benzoquinone as the re-oxidant for palladium. Quinone takes the role of dioxygen as oxidising agent. It is very efficient and both quinone and hydroquinone are inert towards many substrates. Furthermore, no water is formed, as is the case when dioxygen is used. 

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