2,6-Dimethylphenol oxidation

The oxidative dehydrogenation polymerization of 2,6-dialkylphenoles has been known for many years to be catalyzed by copper amine complexes [1] see figure for 2,6-dimethylphenol oxidation. The mechanism of action of this industrially very important reaction has been studied for some time by several groups. 

Keywords mixed oxide, Ti02-Ce02, Ti02-V20s, 2,6-dimethylphenol oxidation, H2O2 1. Introduction... 

The selectivity of the oxidation of 2,6-disubstituted phenols depends on the type of oxidizing agent. For example, with a series of cobalt-containing catalysts of the salcomine type, oxidation of 2,6-dimethylphenol produces three products the poly(phenylene oxide), the diphenoquinone, and... 

Polymerization Mechanism. The mechanism that accounts for the experimental observations of oxidative coupling of 2,6-disubstituted phenols involves an initial formation of aryloxy radicals from oxidation of the phenol with the oxidized form of the copper—amine complex or other catalytic agent. The aryloxy radicals couple to form cyclohexadienones, which undergo enolization and redistribution steps (32). The initial steps of the polymerization scheme for 2,6-dimethylphenol are as in equation 6. 

Halogen Displacement. Poly(phenylene oxide)s can also be prepared from 4-halo-2,6-disubstituted phenols by displacement of the halogen to form the ether linkage (48). A trace of an oxidizing agent or free radical initiates the displacement reaction. With 4-bromo-2,6-dimethylphenol, the reaction can be represented as in equation 10 ... 

Alkylated phenol derivatives are used as raw materials for the production of resins, novolaks (alcohol-soluble resins of the phenol—formaldehyde type), herbicides, insecticides, antioxidants, and other chemicals. The synthesis of 2,6-xylenol [576-26-1] h.a.s become commercially important since PPO resin, poly(2,6-dimethyl phenylene oxide), an engineering thermoplastic, was developed (114,115). The demand for (9-cresol and 2,6-xylenol (2,6-dimethylphenol) increased further in the 1980s along with the growing use of epoxy cresol novolak (ECN) in the electronics industries and poly(phenylene ether) resin in the automobile industries. The ECN is derived from o-cresol, and poly(phenylene ether) resin is derived from 2,6-xylenol. 

The oxidative coupling of 2,6-dimethylphenol to yield poly(phenylene oxide) represents 90—95% of the consumption of 2,6-dimethylphenol (68). The oxidation with air is catalyzed by a copper—amine complex. The poly(phenylene oxide) derived from 2,6-dimethylphenol is blended with other polymers, primarily high impact polystyrene, and the resulting alloy is widely used in housings for business machines, electronic equipment and in the manufacture of automobiles (see Polyethers, aromatic). A minor use of 2,6-dimethylphenol involves its oxidative coupling to... 

Poly(phenylene ether). The only commercially available thermoplastic poly(phenylene oxide) PPO is the polyether poly(2,6-dimethylphenol-l,4-phenylene ether) [24938-67-8]. PPO is prepared by the oxidative coupling of 2,6-dimethylphenol with a copper amine catalyst (25). Usually PPO is blended with other polymers such as polystyrene (see PoLYETPiERS, Aromatic). However, thermoplastic composites containing randomly oriented glass fibers are available. 

Poly(phenylene ether) Alloys. Poly(phenylene ether) resins (91), composed of phenoHc monomers, have a very high T. The commercial resins are based on 2,6-dimethylphenol. The resin is produced by oxidative polymerization in toluene solution over an amine catalyst (see also PoLYETPiERS, aromatic). 

Triazine 4-oxides 55 react with phenols (phenol, 2,6-dimethylphenol, resorcinol, 4-hexyh esorcinol) in trifluoroacetic acid in a similar way, yielding intermediate (T -adducts 5-hydroxyphenyl-4-hydroxy-4,5-dihydro-l,2,4-triazines 61. Subsequent oxidation leads to the corresponding 5-hydroxyphenyl-l,2,4-triazine 4-oxides 62 (97MC116). 

Polyphenylene oxide (PPO) is produced by the condensation of 2,6-dimethylphenol. The reaction occurs by passing oxygen in the phenol solution in presence of CU2CI2 and pyridine ... 

Rapid oxidation by acidic hexachloriridate of phenol and 2,6-dimethylphenol takes place to give the corresponding phenoxyl radical . At low Ir(iri) concen-... 

Poly(2,6-dimethyl-l,4-oxyphenylene) (poly(phenylene oxide), PPG) is a material widely used as high-performance engineering plastics, thanks to its excellent chemical and physical properties, e.g., a high 7 (ca. 210°C) and mechanically tough property. PPO was first prepared from 2,6-dimethylphenol monomer using a copper/amine catalyst system. 2,6-Dimethylphenol was also polymerized via HRP catalysis to give a polymer exclusively consisting of 1,4-oxyphenylene unit, while small amounts of Mannich-base and 3,5,3, 5 -tetramethyl-4,4 -diphenoquinone units are always contained in the chemically prepared PPO. 

Figure 15.18. Oxidative coupling of dimethylphenol dimers to tetramers...

It is well known that 2,6-dimethylphenol is oxidatively polymerized to poly(2,6-dimethyl-l,4-phenyleneoxide) with a copper amine complex as catalyst in the presence of oxygen at room temperature (Eq. 1)... 

The formed thin and uniform poly(phenyleneoxide) films on electrode are interesting because of their electric and electrochemical properties. Figure 1 shows a typical cyclic voltammogram for the oxidation of 2,6-dimethylphenol at a platinum electrode in... 

Electro-oxidative polymerization of 2,6-disubstituted phenols is listed in Table I, with the polymerizations catalyzed by the copper-pyridine complex and oxidized by lead dioxide. 2,6-Dimethylphenol was electro-oxidatively polymerized to yield poly(2,6-dimethylphen-yleneoxide) with a molecular weight of 10000, as was attained by other polymerization methods. The NMR and IR spectra were in complete agreement with those measured for the other polymerization... 

Figure 1. Cyclic voltammogram for the oxidation of 10 mM 2,6-dimethylphenol at a pltinum electrode in alkaline methanol.

The following questions on the electro-oxidative polymerization arose. First, why various phenol derivatives were smoothly polymerized which could not occur by the oxidation with the copper catalyst or lead dioxide. Secondly, why the activated phenol was reacted preferentially through C-0 coupling to form the poly(phenyleneoxide). The mechanism of the electro-oxidative polymerization is discussed below by using the example of 2,6-dimethylphenol. 

The study of the molecular weight of the intermediate course is an effective method for the classification of polymerization as chain or stepwise reaction. In Figure 3, the molecular weight of the obtained polymer is plotted against the yield, for the oxidative polymerization of dimethylphenol with the copper catalyst and for the electro-oxidative polymerization. The molecular weight rises sharply in the last stage of the reaction for the copper-catalyzed polymerization. This behavior is explained by a stepwise growth mechanism. 

Table H. Electro-oxidative Polymerization of 2,6-Dimethylphenol and its Dimer...

Figure 4a. 200 MHz H-NMR spectrum of -(2,6-dimethylphenol) poly(2,6-di-methyl-1,4-phenylene oxide) (PRO) (M 2,235, CCl, IMS).

Chemical/Physical. Wet oxidation of 2,4-dimethylphenol at 320 °C yielded formic and acetic acids (Randall and Knopp, 1980). 2,4-Dimethylphenol will not hydrolyze because there is no hydrolyzable functional group (Kollig, 1993). 

Chemical/Physical. Under atmospheric conditions, the gas-phase reaction of o-xylene with OH radicals and nitrogen oxides resulted in the formation of o-tolualdehyde, o-methylbenzyl nitrate, nitro-o-xylenes, 2,3-and 3,4-dimethylphenol (Atkinson, 1990). Kanno et al. (1982) studied the aqueous reaction of o-xylene and other aromatic hydrocarbons (benzene, toluene, w and p-xylene, and naphthalene) with hypochlorous acid in the presence of ammonium ion. They reported that the aromatic ring was not chlorinated as expected but was cleaved by chloramine forming cyanogen chloride. The amount of cyanogen chloride formed increased at lower pHs (Kanno et al., 1982). In the gas phase, o-xylene reacted with nitrate radicals in purified air forming the following products 5-nitro-2-methyltoluene and 6-nitro-2-methyltoluene, o-methylbenzaldehyde, and an aryl nitrate (Chiodini et ah, 1993). 

The arylamine 780b required for the total synthesis of carbazomycin B (261) was obtained by catalytic hydrogenation, using 10% palladium on activated carbon, of the nitroaryl derivative 784 which was obtained in six steps and 33% overall yield starting from 2,3-dimethylphenol 781 (see Scheme 5.85). Electrophilic substitution of the arylamine 780b with the iron-complex salt 602 provided the iron complex 787 in quantitative yield. The direct, one-pot transformation of the iron complex 787 to carbazomycin B 261 by an iron-mediated arylamine cyclization was unsuccessful, probably because the unprotected hydroxyarylamine moiety is too sensitive towards the oxidizing reaction conditions. However, the corresponding 0-acetyl derivative... 

---