Ethers oxidative coupling

Scheme 8. Hypervalent iodine-mediated phenyl methyl ether oxidative coupling.

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. 

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. 

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. 

On the other hand, the flavan-3-ol units can also be doubly linked by an additional ether bond between C2 07 (A-type). Structural variations occurring in proanthocyanidin oligomers may also occur with the formation of a second interflavanoid bond by C-0 oxidative coupling to form A-type oligomers (Fig. 3) [17,20]. Due to the complexity of this conversion, A-type proanthocyanidins are not as frequently encountered in nature compared to the B-type oligomers. 

An improved synthesis of dithieno[3,2-A2, 3 -<7]thiophene 15a has been achieved from 2,3-dibromothiophene 304 (Scheme 57). Lithiation of 2,3-dibromothiophene 304 using -butyllithium followed by oxidative coupling with cupric chloride provided 3,3 -dibromo-2,2 -bithiophene 305 in 79% yield. Treatment of 305 with 2 equiv of -butyllithium in ether at —78 °C under nitrogen for 40 min and then adding benzenesulfonic acid thioanhydride and leaving the reaction mixture to reach room temperature afforded dithieno[3,2-A2, 3 -<7]thiophene 15a in 70% yield <2002TL1553>. 

Condensation of 2-ethynylcyclohex-l-eneylcarbaldehyde 69 with an excess of acetone gave the ketone 544 Reaction of equimolar amounts of 85 and 86 in ethereal methanolic potassium hydroxide gave the ketone 87. Oxidative coupling of 87 under Glaser conditions afforded two separable isomeric bisdehydro[15]-annulenones 88 and 89. The mono-ds isomer 88 may have the structure 90 in which... 

Six novel fluorinated poly(aryl ether)s containing 1,4-naphthalene moieties were synthesized in high yield using 2,2-bis[4-( 1 -naphthoxy)phenyl]hexafluoro-propane (1). Oxidative coupling ofl yielded a polymer with high 7, low moisture absorption, and low dielectric constant that could be cast into flexible films. The low dielectric constant and low moisture absorption of 6FNE may make it useful as a dielectric insulator in microelectronics applications. 

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... 

Phenoxyacetophenone, 46, 94 Phenylacetylene, oxidative coupling to diphenyldiacetylene, 46,39 reaction with sodium hypobiomite to yield phenjdbromoethyne, 46, 86 Phenylacetyl fluoride, 46, 6 Phenylbrojioethyne, 46, 86 l-Phenyl-l,3-hutadiene, 46, 94 Phenyl (-butyl ether, 46, 89 />-Phenylene diisothiocyanate, 45, 21 Phenylethynyllithium 46, 88 Phenylethynymagnesium Grignard reagent, 46, 88... 

Examples of catalytic formation of C-C bonds from sp C-H bonds are even more scarce than from sp C-H bonds and, in general, are limited to C-H bonds adjacent to heteroatoms. A remarkable iridium-catalyzed example was reported by the group of Lin [116] the intermolecular oxidative coupling of methyl ethers with TBE to form olefin complexes in the presence of (P Pr3)2lrH5 (29). In their proposed mechanism, the reactive 14e species 38 undergoes oxidative addition of the methyl C-H bond in methyl ethers followed by olefin insertion to generate the intermediate 39. p-hydride elimination affords 35, which can isomerize to products 36 and 37 (Scheme 10). The reaction proceeds under mild condition (50°C) but suffers from poor selectivity as well as low yield (TON of 12 after 24 h). 

Since enol silyl ethers are readily accessible by a number of methods in a regioselective manner and since the trialkylsilyl moiety as a potential cationic leaving group facilitates the termination of a cyclization sequence, unsaturated 1-trialkylsilyloxy-1-alkenes represent very promising substrates for radical-cation cyclization reactions. Several methods have been reported on the synthesis of 1,4-diketones by intermolecular oxidative coupling of enol silyl ethers with Cu(II) [76, 77], Ce(IV) [78], Pb(IV) [79], Ag(I) [80] V(V) [81] or iodosoben-zene/BFa-etherate [82] as oxidants without further oxidation of the products. 

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],... 

In 2005, Cahiez and coworkers and Nagano and Hayashi showed that a catalytic amount of iron(III) chloride is efficient when a suitable oxidant is added to the reaction mixture (Figure 3). Nagano and Hayashi used an excess of 1,2-dichloroethane (the stoichiometric amount is 0.5 equivalent) in refluxing diethyl ether to couple various aryl Grignard reagents in high yield (Scheme 46). 

Diketones (8, 126 127). Complete details of the synthesis of 1,4-diketones by oxidative coupling of ketone enolales and trimethylsilyl enol ethers with Cu(OTf)2 are available.1 Use of isobutyronitrile is essential for the coupling it is not only a suitable solvent, but the nitrile group apparently facilitates reduction of the intermediate copper enolate to CuOTf.2 When acetonitrile is used by-products containing a nitrile group are formed. 1,4-Diketones are formed only in traces when DMF, DMSO, or HMPT is used. 

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

Unsubstituted 3-hydroxythiophene is less stable than 2-hydroxythiophene (63AHC(l)l). This instability may be due to oxidative coupling. In order to confirm this, 2,5-dimethyl-3-hydroxythiophene was subjected to ferricyanide oxidation (72ACS31). Racemic and meso forms of 2,2, 5,5 -tetramethyl-bi-4-thiolen-3-one (445) were isolated. When the a-substituent was r-butyl instead of methyl, exposure to air gave 2-(2,5-di-r-butyl-4-thiolen-3-one) 3-(2,5-di-r-butylthienyl) ether (446), formed by carbon-oxygen coupling. 

Table 2 contains some examples of TTN oxidative coupling reactions. These examples show that the reaction can often tolerate unprotected amino add side chains, and that both 14- and 17-membered cycloisodityrosines can be obtained. In addition, the TTN oxidative coupling has been employed in the formation of 16-membered cyclic biaryl ethers of the type found in vancomycinJ20,21 The drawbacks of TTN oxidative coupling are the low yields, the formation of byproducts, and the need for additional transformations to arrive at the cycloisodityrosine target. 

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