Oxidation of aromatic compounds

Quenching of the excited state of U02 by naphthalene can occur along two parallel pathways, exciplex formation and electron transfer to give the naphthalene radical cation In the presence of molecular oxygen, CeHg is 

P-Methylstyrene has been photooxidised to cinnamaldehyde by irradiating in the presence of 2-iodo-5-nitrothiophene, and a time-resolved study of the irradiation of a series of substituted styrenes embedded in acidic and non-acidic zeolites has appeared. ° In this latter case, the corresponding radical cation is produced for which the zeolite framework provides a stabilising effect whose strength is dependent upon the nature of the substituents. 

Rate constants have been determined for these processes, and it is concluded that in the case of the benziloxy radicals real-time monitoring provides a direct observation of the transition state for C-C bond scission. A highly selective photodecarboxylation of aralkyl carboxylic acids such as 3-indolepro-pionic acid and 1-naphthylacetic acid has been achieved by irradiating the two-component crystals formed with acridine or phenanthridine as electron acceptor at -70 Electron transfer followed by proton transfer produces a carboxylate radical and either a hydroacridine or hydrophenanthridine radical which decays by decarboxylation. 

DL-a-acetamido-P-hydroxy-p-nitropropiophenone has been prepared from L-(+)-threo-(p-nitrophenyl)-2-aminopropan-l,3-diol and acetic anhydride followed by photooxidation in the presence of acidic KBrOs and HBr. Photosensitized electron transfer reactions of tri-1-naphthyl phosphate and di-1-naphthyl methyl phosphate using 9,10-dicyanoanthracene are reported to lead to the formation of 1,1 -binaphthyl, but no reaction occurs for mono-1-naphthyl and di- or tri-phenyl esters. Irradiation of bis(3,4-methylenedioxyphenyl) methylphosphonate in the presence of DCA induces single electron oxidation followed by intramolecular rearrangement and gives 2-(3,4-methylenedioxyphenyl)-3,4-methylenedioxy-phenyl methylphosponate.  

2-Methylbenzene-l,4-dicarbonitriIe (2-methyl-BDC) and 2,5-dimethylbenzene-1,4-dicarbonitrile (2,5-dimethyl-BDC) are reported to increase the quantum yield of the photosensitized electron transfer reaction of methanolic 6,6-diphenyl-1,4-dioxaspiro[4.5]decane more than benzene-1,4-dicarbonitrile (BDC). This has been attributed to the occurrence of electron transfer between MeOH and excited BDC, a process which is suppressed in the methylated BDCs, 

The mechanism of the photohydroxylation of 1,4-benzoquinone in water, which results in the formation of equal amounts of 2-hydroxy-l,4-benzoqui-none and hydroquinone, has been re-examined. The quantum yield and laser 

Electron tramsfer photooxygenation of Ph C-C-CPh in MeCN sensitized by 9,10-dicyamoamthracene gives benzophenone amd 

1- diphenyl-2-vinylcyclopropame in oxygen-saturated MeCN with 

2- Cycloaddition with singlet oxygen is not observed.An investigation of the role of cosensitizers such as biphenyl in 

10- dicyamoamthracene-sensitized reactions has appeared. cis-trams Photoisomerisation amd photooxygenation of cis- amd trams-1.2-bisl4-methoxyphenyl cyclopropanea are sensitized by 

3- diphenyldihydro-l,4-dioxine undergo oxidation using 2,4,4,6-tetrad3romocyclohexa-2,4-dienone as electron acceptor in a process in v ich the intermediate radical cation reacts with through a radical cation chain mechamisro. DCA-sensitized photooxidation of 1,4-diphenylbuta-l,3-diene in MeCN gives am 

Oxidation of Aromatic Compounds The yields of ring cleavage products from the NO-air 

1- Methylfluorene has a stability comparable to that of fluorene itself, and 

2- nitrofluorene, although largely unaffected by direct oxidation, does undergo a replacement reaction in the presence of a chlorine source. 

Kinetic isotope effects have been measured for hydrogen atom transfer pathways in the photooxidation of various toluenes by photoactivated qui-nones The effect of added salts on the yields of the cation radicals 

Tetraphenylporphine-sensitized photooxygenation of E,E)- or ( ,Z)-l-ar-ylpenta-1,3-dienes gives most of the cw-endoperoxides, m-3-aryl-6-methy 1-1,2-dioxacyclohex-4-enes, in a process which occurs by exclusive addition of 02( Ag) to the ( , )-dienes formed by photoisomerisation of the ( , Z)-dienes, 

Following irradiation of a mixture of triarylstibines and styrenes, air oxidation gives the 2-aryl-1-phenylethanols (81 R = H, Me) in a transformation which has been rationalised in terms of valence expansion of the oxygen-antimony-styrene complex and subsequent reductive coupling. 

The dye-sensitized photo-oxidation of phenols has been examined using CIDNP techniques.148 It is concluded from the observed polarizations of nuclear spin that sensitization by xanthene dyes (such as Rose Bengal) is the result of reversible hydrogen abstraction by the triplet dye molecule from the phenolic hydroxyl group. Any resulting photochemical reactions (e.g. with oxygen) arise from irreversible reactions of the phenoxy-radicals produced, rather than from direct reactions of the phenols with 102. Dye-sensitized photo-oxidation of p-hydroxyphenylpyruvic acid (94) in solution at pH 7 (conditions under which 

A similar oxidation is employed industrially for the preparation of the terephthalic acid used in the production of polyester fibers. Approximately 5 million tons per year of p-xylene are oxidized, using air as the oxidant and Co(lll) salts as catalyst. 

The mechanism of side-chain oxidation is complex and involves reaction of C-J-l bonds at the position next to the aromatic ring to form intermediate ben-zylic radicals, tert- Butyl benzene has no benzylic hydrogens, however, and is therefore inert. 

Analogous side-chain oxidations occur in various biosynthetic pathways. The neurotransmitter norepinephrine, for instance, is biosynthesized from dopamine by a benzylic hydroxylation reaction. The process is catalyzed by the copper-containing enzyme dopamine /3-monooxygenase and occurs by a radical mechanism. A copper-oxygen species in the enzyme first abstracts the pro-R benzylic hydrogen to give a radical, and a hydroxyl is then transferred from copper to carbon. 

Problem 16.19 1 What aromatic products would you obtain from the KMn04 oxidation of the following substances  

578 CHAPTER 16 Chemistry of Benzene Electrophilic Aromatic Substitution 

Side-chain bromination at the benzylic position occurs when an alkyJbcnzene is treated with N-bromosuccinimide (NBS). For example, propylbenzene gives ri-bromopropyl)benzene in 97% deld on reaction with NBS in the presence of benzoyl peroxide, (PhC02)2, radical initiator. Bromination occurs exclu- 

The mechanism of benzylic bromination is similar to that discussed in Sec-tion 10.4 for allylic bromination of alkencs. Abstraction of a benzylic hydrogen atom generates an intermediate benzylic radical, which reacts with Br2 to yield product and a Br- radical that cycles back into the reaction to carry on the chain. The Br2 necessary for reaction with the benzylic radical is produced by a con-airrenl reaction of HBr with NBS. 

Already very early it was proposed to apply electrochemically generated and regenerated Mn(III) and Ce(IV) as oxygen carriers for the synthesis of benzal- 

Substrate Redox catalyst Electrolyte Anode Product Ref.  

Side-chain oxidations of alkyl aromatic compounds to aromatic carboxylic acids by electrogenerated and regenerated chromic acid have been studied extensively in the case of saccharin formation from o-toluene sulfonamide This 

In this case a platinum anode wich is not sensitive to organic contamination can be used giving a current yield for Cr(VI) of 94%. Electrolyses can be performed in a two-phase medium making an in-cell process possible This method has been applied to the synthesis of benzaldehyde from benzylic alcohol giving 100% current yields at 10 % conversion while the current yield drops to 60 % at high conversions. 

In addition to the synthesis of saccharin, also a number of other side-chain oxidations have been studied leading to aromatic carboxylic acids by indirect electrochemical oxidation using chromic acid as oxidizing agent. They include the oxidation of p-nitrotoluene 2,4-dinitrotoluene toluene, p-xylene, and p-tolualdehyde 

Beyond doubt, relatively little is known about the oxidation chemistry of aromatics, despite the increasing use of BTX (benzene, toluene and xylene) aromatics for improving the anti-knock behaviour of internal combustion engines. Further, the problem of polyaromatic hydrocarbons (PAH) associated with soot particles which originate particularly in diesel engines focuses attention on combustion-generated pollution. 

At low temperatures, radical attack involves addition to the ring, but above about 600 K the process is reversed, with the exception of O atom attack [114] so that phenol is an important product for benzene. 

At about 750 K, benzaldehyde is formed in high yield from toluene oxidation, almost certainly through the sequence [115]. 

QH5CHCH3 radicals are able to react with O2 to give styrene quite rapidly, with a rate constant a factor of 10 lower than for alkyl + O2. Radical-radical reactions are hence considerably reduced in importance. 

Where relatively high concentrations of H atoms exist, addition of H followed by loss of an alkyl group is an efficient way of reducing alkylbenzenes and polyaromatics to benzene which, of course, is of considerable significance in aromatic cracking processes. 

Numerous papers have appeared discussing the oxidation of hydrocarbons. For example, the effects of pressure on product yields in the nitrogen oxide (NO ) photo-oxidations of aromatic hydrocarbons such as toluene and o-xylene have been described and a study has been reported of the mechanism of the photo-oxidation of /3,/3-dimethylstyrene. Photo-oxidation of cis-a,a -dimethylstilbene using [Ru(bipy)3] or tetraphenylporphine as sensitizer leads to the dioxetane (19), whereas with Rose Bengal or Methylene Blue the 

10- epoxide and 9-hydroxyphenanthrene by treatment with the carbonyl oxide Ph2C=6—O, obtained photolytically from Ph2CN2 and O2.  

Murray and S. Kumar, Polynucl. Aromat. Hydrocarbons Phys. Biol. Chem., Int. Symp., 6th, 1981, 575, ed. M. Cooke. A. J. EDennis, and G. L. Fisher. 

A simple general method has been described for the synthesis of functionalized cis-l,2-diacylethylenes. Thus low-temperature irradiation of chloroform solutions of (20 = R = Me, R = R = COjMe) in the presence 

2-Amino-3-(4 -nitrophenyl)propan-l,3-diol in its optically active form has been selectively photooxidised to the corresponding ketone,and a study of the oxidation of podophyllotoxin (27) by sodium persulfate using laser flash photolysis has enabled rate constants to be determined for the formation and decay of transients. Photoinduced electrochemical oxidation of benzyl alcohol to benz-aldehyde has been achieved with 100% product selectivity and 100% current efficiency using visible radiation in the presence of riboflavin 2, 3, 4, 5 -tetraace-tate.  

Photoelectron transfer oxidation of various phenols in the presence of 2-nitrofluorene has been examined in both acetonitrile and cyclohexane solution. Although no charge transfer donor-acceptor pairs are present in the ground state, a contact exciplex is apparent in cyclohexane, and in acetonitrile the anion radical of 2,6-dimethylphenol has been observed as the final product. Irradiation of 

The photokinetics of electron transfer within the exciplex acenaphthenone/ 9,10-dicyanoanthracene in which the ketone acts as donor have been measured, and show that the forward rate exceeds the rate of the reverse process and other decay processes. 

Some substituted dibenzo-7-silabicyclo[2,2,l]hepta-2,5-dienes are reported to phototransfer an electron to TCNE using 2,4,6-triphenylpyrylium tetrafluoro-borate as sensitizer to give difluorosilane and anthracene as products. The MOP AC MP3 method has been used to examine the electronic features of the radical cation of the parent silane. 

Furfural can be photooxidised in methanol using uranyl salts as sensitiser to give furan-2-carboxylic acid in a reaction which proceeds via U(V) as intermediate, and photooxidation of the alditol derivative of furan [(5 )-29 = 

A heterosupramolecule, defined by the authors as an artificial microdevice that carries out advanced functions by cooperating with inorganic solids and organic molecules has been constructed from Ti02 and a cationic surfactant. The surfactant, trimethylstearylammonium chloride, forms a bilayer on the surface of the semiconductor, and the substrate, in this case 2-naphthol, is incorporated into hydrophobic nanospaces in the adsorbed bilayer. A very high level of activity for 2-naphthol oxidation to phthalic acid is obtained. The activity is attributed to the concentration of the substrate near the Ti02 surface and an increase in the effective surface area due to the improved dispersibility of the particles. 

1-diphenylethene, cis- and tmns-stilbenes and triphenylethene, in the zeolite nanocavities under O2 produces benzaldehyde and benzophenone as the major oxidation products. It is suggested that the formation of these products involves the alkene radical cations and superoxide produced by excitation of the contact charge-transfer complex formed by stilbenes and styrene in zeolites, or by PET from the excited alkenes to O2 for 1,1-diphenylethene and triphenylethene. The concept that the reaction is controlled by electrostatic interactions between the guest molecule and metal cations in zeolite cavities is again invoked in respect of this system. 

The photoisomerization of cis-and trans-stilbenes has been studied in ionic liquids and evidence is presented that the mechanism depends on the particular ionic liquid used. In basic N-butylpyridinium chloride/AlCb the photoequilibration involves stilbene radical cations, whereas in basic l-ethyl-3-methyl-imidazolium chloride/AlCb the process occurs via the standard singlet state photoisomerization mechanism. It has been found that the tetra-O-acetylribo-flavin (126) sensitized dehydrogenation of substituted benzyl alcohols, giving the 

Some new photo-oxidation chemistry of aromatic systems in conventional solvents has also been described. Thus the ET photochemistry of cis- and trans- 

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Attenlion should be drawn to ihe use of tin oxide systems as heterogeneous catalysts. The oldest and mosi extensively patented systems are the mixed lin-vanadium oxide catalysis for the oxidation of aromatic compounds such as benzene, toluene, xylenes and naphthalene in the. synthesis of organic acids and acid anhydride.s. More recenily mixed lin-aniimony oxides have been applied lo the selective oxidaiion and ammoxidaiion of propylene to acrolein, acrylic acid and acrylonilrile. 

Oxepin and its derivatives have attracted attention for several reasons. Oxepin is closely related to cycloheptatriene and its aza analog azepine and it is a potential antiaromatic system with 871-elcctrons. Oxepin can undergo valence isomerization to benzene oxide, and the isomeric benzene oxide is the first step in the metabolic oxidation of aromatic compounds by the enzyme monooxygenase. 

Hardisson C, JM Sala-Trapat, RY Stanier (1969) Pathways for the oxidation of aromatic compounds by Azotobacter. J Gen Microbiol 59 1-11. 

Reactions of partial electrochemical oxidation are of considerable interest in the electrosynthesis of various organic compounds. Thus, at gold electrodes in acidic solutions, olefins can be oxidized to aldehydes, acids, oxides, and other compounds. A good deal of work was invested in the oxidation of aromatic compounds (benzene, anthracene, etc.) to the corresponding quinones. To this end, various mediating redox systems (e.g., the Ce /Ce system) are employed (see Section 13.6). 

Scheme 12.22. Side Chain Oxidation of Aromatic Compounds... 

Intermediates generated at an electrode surface may react while still near the electrode. If so, one side of the intermediate may be wholly or partly shielded from attack by other reactants by the electrode itself. Such behavior is particularly common in the electrochemical oxidation of aromatic compounds since, as we have already seen with coumarin, aromatic compounds are generally tightly adsorbed parallel to the electrode surface at potentials positive of the p.z.c. For example, electrochemical oxidation of the stilbenes in alkaline methanol affords a mixture of dl and meso-1,2 dimethoxy-1,2-diphenylethane (1) 10>. It is found that c/s-stilbene affords a mixture of isomers of 1 in which the... 

Palmisano, G., Addamo, M., Augugliaro, V., Caronna, T., Garci a-Lopez, E., Loddo, V., and Palmisano, L. (2006) Influence of the substituent on selective photocatalytic oxidation of aromatic compounds in aqueous Ti02 suspensions. Chemical Communications (9), 1012-1014. 

Vazquez-Duhalt, R. Westlake, D. W. S., and Fedorak, P. M., Lignin Peroxidase Oxidation of Aromatic-Compounds in Systems Containing Organic-Solvents. Applied and Environmental Microbiology, 1994. 60(2) pp. 459-466. 

Fujihira, M. Satoh, Y. Osa, T. 1981. Heterogeneous photocatalytic oxidation of aromatic-compounds on Ti02. Nature 293 206-208. 

Stereocontrolled oxidation of aromatic compounds (hydroxylation or dihy-... 

Boncz, M.A., et al., Substituent effects in the Advanced Oxidation of Aromatic Compounds, in Proceedings of the International Regional Conference of the IOA, Poitiers, 1998, France. 

Scheme 38 Side-chain oxidation of aromatic compounds mediated by Fe(ll).

A broad spectrum of chemical reactions can be catalyzed by enzymes Hydrolysis, esterification, isomerization, addition and elimination, alkylation and dealkylation, halogenation and dehalogenation, and oxidation and reduction. The last reactions are catalyzed by redox enzymes, which are classified as oxidoreductases and divided into four categories according to the oxidant they utilize and the reactions they catalyze 1) dehydrogenases (reductases), 2) oxidases, 3) oxygenases (mono- and dioxygenases), and 4) peroxidases. The latter enzymes have received extensive attention in the last years as bio catalysts for synthetic applications. Peroxidases catalyze the oxidation of aromatic compounds, oxidation of heteroatom compounds, epoxidation, and the enantio-selective reduction of racemic hydroperoxides. In this article, a short overview... 

More drastic conditions of temperature, pressure and time gave carbon as the product from both aromatic and aliphatic compounds. This carbon is an activated char with decolorizing properties. Tars, tarry substances, the oxides of carbon, and the usual products of the oxidation of aromatic compounds (dicarboxylic acids) were all conspicuous by their absence. Benzotrifluoride oxidized to benzoyl fluoride. The aliphatic compounds cyclohexane, methycyclohexane, n-heptane, etc., could be oxidized to carbon and water. 

Besides a variety of other methods, phenols can be prepared by metal-catalyzed oxidation of aromatic compounds with hydrogen peroxide. Often, however, the selectivity of this reaction is rather poor since phenol is more reactive toward oxidation than benzene itself, and substantial overoxidation occurs. In 1990/91 Kumar and coworkers reported on the hydroxylation of some aromatic compounds using titanium silicate TS-2 as catalyst and hydrogen peroxide as oxygen donor (equation 72) . Conversions ranged from 54% to 81% with substituted aromatic compounds being mainly transformed into the ortho-and para-products. With benzene as substrate, phenol as the monohydroxylated product... 

SCHEME 142. Vanadium-catalyzed oxidation of aromatic compounds using H2O2... 

SCHEME 143. Oxidation of aromatic compounds using Re oxides as catalyst in the presence of H2O2... 

Some reactions of 2,2 -bipyridine /V-oxides have been reported. The l,T-dioxide is nitrated readily to 4,4 -dinitro-2,2 -bipyridine 1,T-dioxide. ° ° °" 2,2 -Bipyridine 1-oxide is also nitrated in the 4 position. The nitro groups in 4,4 -dinitro-2,2 -bipyridine l,T-dioxide are reactive, being replaced by chlorine with concentrated hydrochloric acid," by bromine with acetyl bromide, by hydroxyl with dilute sulfuric acid, and by alkoxy groups with sodium alkoxides. Some of the dialkoxy derivatives are useful catalysts for the oxidation of aromatic compounds. The dinitro dioxide is deoxygenated to 4,4 -dinitro-2,2 -bipyridine with phosphorus trichloride in chloroform, and other substituted l,T-dioxides behave similarly, but with phosphorus trichloride alone, 4,4 -dichloro-2,2 -bipyridine results. The dinitro dioxide is reduced by iron powder in acetic acid to 4,4 -diamino-2,2 -bipyridine, whereas 4,4 -dichloro-2,2 -bipyridine l,T-dioxide is converted to its 4,4 -diamino analogs with amines. Related reactions have been described. ... 

F. Effenberger, Acc. Chem. Res. 22,27-35 (1989) l,3,5-Tris(dialkylamino) benzenes Model Compounds for the Electrophilic Substitution and Oxidation of Aromatic Compounds". 

The electrochemical oxidations of aromatic compounds in the presence of a fluoride ion sources have been widely studied by a number of workers to produce a range of partially fluorinated compounds [9-12]. 

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