Of oxygenated aromatic compounds

Chlorination is also an important step in the synthesis of oxygenated aromatic compounds. In this case, chlorination takes place at alkyl groups attached to the rings and is conducted in the absence of iron. The use of UV light... 

Until recently, noncatalytic stoichiometric oxidations of arenes with toxic inorganic reagents, such as CrOj and K Cr O., MnO and KMnO, OsO, Pb(OAc), TllNOj), nitric acid, ceric ammonium nitrate, and some others, were the main route for the production of oxygenated aromatic compounds [1, 2, 6, 51]. A classic example is the manufacture ( 1500t/year) of vitamin via stoichiometric oxidation of 2-methylnaphthalene (MN) with carcinogenic CrO in sulfuric acid (Eq. 14.14) [52]. 

The characterization of catalysts after reaction by infrared spectroscopy was carried out in order to determine the nature of the adsorbed species in the pores of the zeolite, which were mainly constituted of oxygenated aromatics compounds (CxHyOz) as shown in the literature during xylene oxidation over Pd- and Pt-based zeolite catalysts (Scheme 5.1). ... 

SCHEME 47 Chlorination and bromination of oxygenated aromatic compounds. 

Koini et al. [125] reported an efficient and mild method for the chlorination and bromination of oxygenated aromatic compounds (phenols, substituted phenols, methoxyarenes, and 1,4-benzoxazines) (178/180) with good regioselectivity and yields (47-68%), using a combination of HCl or HBr/(30%)H2O2/AcOH in petroleum ether as solvent (Scheme 47). The use of ultrasound irradiation did not confer any additional advantage when compared with conventional heating. 

E.N. Koini, N. Avlonitis, T. Calogeropoulou, Simple and efficient method for the halogenation of oxygenated aromatic compounds, Synlett 11 (2011) 1537-1542. 

The operation of the nitronium ion in these media was later proved conclusively. "- The rates of nitration of 2-phenylethanesulphonate anion ([Aromatic] < c. 0-5 mol l i), toluene-(U-sulphonate anion, p-nitrophenol, A(-methyl-2,4-dinitroaniline and A(-methyl-iV,2,4-trinitro-aniline in aqueous solutions of nitric acid depend on the first power of the concentration of the aromatic. The dependence on acidity of the rate of 0-exchange between nitric acid and water was measured, " and formal first-order rate constants for oxygen exchange were defined by dividing the rates of exchange by the concentration of water. Comparison of these constants with the corresponding results for the reactions of the aromatic compounds yielded the scale of relative reactivities sho-wn in table 2.1. 

Nitration at a rate independent of the concentration of the compound being nitrated had previously been observed in reactions in organic solvents ( 3.2.1). Such kinetics would be observed if the bulk reactivity of the aromatic towards the nitrating species exceeded that of water, and the measured rate would then be the rate of production of the nitrating species. The identification of the slow reaction with the formation of the nitronium ion followed from the fact that the initial rate under zeroth-order conditions was the same, to within experimental error, as the rate of 0-exchange in a similar solution. It was inferred that the exchange of oxygen occurred via heterolysis to the nitronium ion, and that it was the rate of this heterolysis which limited the rates of nitration of reactive aromatic compounds. 

The endoperoxides of polynuclear aromatic compounds are crystalline soHds that extmde singlet oxygen when heated, thus forming the patent aromatic hydrocarbon (44,66,80,81). Thus 9,10-diphenyl-9,10-epidioxyanthrancene [15257-17-7] yields singlet oxygen and 9,10-diphenylanthracene. 

Styrene undergoes many reactions of an unsaturated compound, such as addition, and of an aromatic compound, such as substitution (2,8). It reacts with various oxidising agents to form styrene oxide, ben2aldehyde, benzoic acid, and other oxygenated compounds. It reacts with benzene on an acidic catalyst to form diphenylethane. Further dehydrogenation of styrene to phenylacetylene is unfavorable even at the high temperature of 600°C, but a concentration of about 50 ppm of phenylacetylene is usually seen in the commercial styrene product. 

Application of NMR has been made to a restricted range of chlorinated aromatic compounds (Kolehmainen et al. 1992), and has been used to establish the source of oxygen in the metabolites produced from acetate and 02 by Aspergillus melleus (Staunton and Sutkowski 1991). 

In the absence of molecular oxygen, a nnmber of alternative electron acceptors may be used these include nitrate, sulfate, selenate, carbonate, chlorate, Fe(III), Cr(VI), and U(VI), and have already been discussed in Chapter 3, Part 2. In Chapter 14, which deals with applications, attention is directed primarily to the role of nitrate, sulfate, and Fe(III)— with only parenthetical remarks on Cr(VI) and U(VI). The role of nitrate and sulfate as electron acceptors for the degradation of monocyclic aromatic compounds is discnssed, and the particularly broad metabolic versatility of sulfate-reducing bacteria is worthy of notice. 

Still unexplained are the reactions, with periodate, of hydroxy phenols and certain oxygenated aromatic compounds,1 as well as of some a-amino acids not containing the 2-hydroxyamine structure.8... 

Selective hydroxylation of some aromatic compounds can be achieved using HRP C in the presence of oxygen and dihydroxyfumaric acid (270). This process afforded l-DOPA from L-tyrosine, D-(-)-3,4-dihydroxy-phenylglycine from D-(—)-4-hydroxyphenylglycine, and L-epinephrine (adrenalin) from L-(-)-phenylephrine in yields of up to 70%. 

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... 

Kropp and Windsor also examined the effect of oxygen on the fluorescence properties of rare-earth solutions. They found that the lifetimes remained constant to 0.5 per cent for (a) normal air-equilibrated solutions, (b) solutions that had been flushed with helium, and (c) pure-oxygen-flushed solutions. This last effect is in variance with the well-established fact that dissolved oxygen is an efficient quencher of fluorescent aromatic compounds. 

Figure 1. Thermodynamic equilibrium in atmospheres of varying elemental proportions. The ternary diagram shows all compositions of systems containing carbon, hydrogen, and oxygen (each point represents 100% of the three components). Lower curves indicate the potential formation of solid carbon if equilibrium could be attained. Dashed curve holds at 500°K., the continuous one at 700°K. The upper lines indicate the asphalt threshold, the dashed one at 500° K., and the continuous one at 700° K. Above this threshold, thermodynamic equilibrium favors the formation of large proportions of polycyclic aromatic compounds ( asphalt ) ana a lesser increase of most of the other families of compounds. The dots through points A to C indicate the points used in the computations for Figure 2 (6).

Irradiation of powdered titanium dioxide suspended in solutions containing aromatic compounds and water under oxygen has recently been shown to induce hydroxylation of aromatic nuclei giving phenolic compounds and oxidation of side chains of the aromatic compounds (50-55). These reactions have been assumed to proceed through hydroxyl and other radical intermediates, but the mechanism for their generation, whether reactive free radicals result from oxidation of water, from reduction of oxygen, or from oxidation of the substrates on the surfaces of the excited titanium dioxide, has not been clear. 

Six major compounds of the cardamom essential oil have been identified, namely 1,8-cineole, a-terpinyl acetate, linalool, lin-alyl acetate, a-terpineol and terpin-4-ol, in order of importance. These six compounds represent almost 90% of the aromatic compounds of the essential oil from cardamom and all of them are oxygenated compounds. 

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