Oxidative couplings phenol-ethers

Oxidative Reactions. The majority of pesticides, or pesticide products, are susceptible to some form of attack by oxidative enzymes. For more persistent pesticides, oxidation is frequently the primary mode of metaboHsm, although there are important exceptions, eg, DDT. For less persistent pesticides, oxidation may play a relatively minor role, or be the first reaction ia a metaboHc pathway. Oxidation generally results ia degradation of the parent molecule. However, attack by certain oxidative enzymes (phenol oxidases) can result ia the condensation or polymerization of the parent molecules this phenomenon is referred to as oxidative coupling (16). Examples of some important oxidative reactions are ether cleavage, alkyl-hydroxylation, aryl-hydroxylation, AJ-dealkylation, and sulfoxidation. 

Various 2,6-di8ubstituted p-benzoquinones have been prepared by oxidation of the corresponding 2,6-disubstituted phenols with potassium nitrosodisulfonate or lead dioxide in formic acid. Oxidative coupling of 2,6-disubstituted phenols to poly-2,6-disubstituted phenylene ethers followed by treatment of the polymers in acetic acid with lead dioxide is reported to give low yields of the corresponding 2,6-disubstituted p-benzoquinones. 

Poly(phenylene oxide) PPO, or poly(phenylene ether) PPE, is an engineering polymer developed by General Electric. It concerns the oxidative coupling of phenols discovered in 1956 by Allan S. Hay [21], Oxidative coupling leads to the formation of carbon-oxygen bonds between carbon atoms 2,4, and 6 and the phenolic oxygen atom. To avoid coupling with carbon atoms 2 and 6, alkyl substituents at these two positions were introduced. In addition to the polymer a 4,4 dimer is formed, named diphenoquinone (DPQ). The... 

Nonphenolic oxidative coupling of phenol ether derivatives using IBTA can also produce seven-membered N-containing heterocyclic compounds as exemplified by Eq. (45) [96JCS(CC)1481],... 

Oxidative coupling of phenols and phenol ethers.2 This reaction can be conducted with ferric chloride supported on silica gel. 

Kita and Tohma found that exposure of p-substituted phenol ethers to [bis(tri-fluoroacetoxy)iodo]benzene 12 in the presence of some nucleophiles in polar, less nucleophilic solvents results in direct nucleophilic aromatic substitution [Eq. (84)] [156]. Involvement of a single-electron transfer (SET) from phenol ethers to A3-iodane 12 generating arene cation radicals was suggested by the detailed UV-vis and ESR studies. SET was involved in the oxidative biaryl coupling of phenol ethers by 12 in the presence of BF3-Et20 [157]. 

The use of hypervalent iodine reagents in carbon-carbon bond forming reactions is summarized with particular emphasis on applications in organic synthesis. The most important recent methods involve the radical decarboxylative alkylation of organic substrates with [bis(acyloxy)iodo]arenes, spirocyclization of para- and ortho-substituted phenols, the intramolecular oxidative coupling of phenol ethers, and the reactions of iodonium salts and ylides. A significant recent research activity is centered in the area of the transition metal-mediated coupling reactions of the alkenyl-, aryl-, and alkynyliodonium salts. 

In the intermolecular mode, this reaction has been utilized for the preparation of products 28 from various nucleophiles, including C-nucleophiles (e. g. (3-dicarbonyl compounds). A similar reaction in the intramolecular mode provides a powerful synthetic tool for the preparation of various polycyclic compounds via oxidative biaryl coupling [21,27 - 30]. Several examples of these C-C bond forming reactions are shown in Schemes 13-15. Specifically, various dibenzoheterocyclic compounds 30 have been prepared by the oxidation of phenol ether derivatives 29 with [bis(trifluoroacetoxy)iodo]benzene in the presence of BF3-etherate in dichloromethane (Scheme 13) [27-29]. 

Under similar conditions, the phenanthro-fused thiazoles, isoxazoles and pyrimidines 32 (Scheme 14) can be prepared by oxidative coupling of the respective phenol ethers 31 [31,32]. 

Organic compounds having labile hydrogen atoms, such as phenols, anilines, and acetylenes, are also oxidatively polymerized by metal-complex catalysts (Eqs. 1-3). The oxidative coupling is a dehydrogenation reaction the polymer chain produced contains the dehydrogenated monomer structure as a repeating unit. As a remarkable example, poly(phenylene ether), one of the... 

Organic compounds having labile hydrogens, such as phenols [41,42], phenylene-diamines [43], and acetylenes [44], can be oxidatively coupled in the presence of specific metal complexes to form polymeric compounds. The oxidative polymerization of 2,6-disubstituted phenols with a copper-amine complex produces poly(2,6-disubstituted phenylene ether) [45-51], Poly(2,6-dimethylphenylene ether) and poly(2,6-diphenylphenylene ether) are commercially produced from 2,6-dimethyl phenol and 2,6-diphenylphenol, respectively (Figure 5). These polymers exhibit excellent performance as engineering plastics. 

By using this mild and versatile methodology, symmetrical diaryl ethers have been synthesized in a one-pot, two-step procedure starting from arylboronic acids and their partial conversion to the corresponding phenols by oxidation with hydrogen peroxide and a subsequent coupling of the formed phenols with the remaining arylboronic acids upon addition of copper(II) acetate, molecular sieves and triethyl amine (Scheme 7) [22],... 

Tab. 9. Effect of phenol ether substitution and solvent variation on the PI FA-mediated oxidative coupling of polyether substrates.

Scheme 11. Mechanistic speculation on the different pathways taken by phenolic and phenyl ether substrates in the PIFA-mediated oxidative coupling reaction.

Bushby has examined the FeCl3-mediated oxidation of hexyl-protected (Hex) phenol ether units in the preparation of triphenylene-based liquid crystals [63]. This strategy allows the formation of unsymmetrically substituted products 75a-l (Table 18) [64]. The use of methanol in the work-up is critical in order to obtain the products in good yield. If the protecting group on the phenol component is isopropyl (74m), the coupling reaction occurs to give the unprotected phenols 76a-c directly (Scheme 17) [65]. 

In a recent paper, the same authors showed that iron(III) chloride can mediate the oxidative coupling of substituted aryl ethers with an observed regioselectivity that depends on the substitution pattern [66] meta-substituted phenol ethers 77 led to polymers (Scheme 18a) whereas para-substituted phenol ether 79 gave predominantly biphenyl structures (Scheme 18b). ortho-Substituted phenol ether 81 provided a dimer with the Ar-Ar bond at a position para to one of the methoxy substituents (Scheme 18c). 

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