Phenol ethers mechanism

Pyridinium chloride ([PyHjCl) has also been used in a number ofcyclization reactions of aryl ethers (Scheme 5.1-4) [4, 18]. Presumably the reaction initially proceeds by deallcylation of the methyl ether groups to produce the corresponding phenol. The mechanism of the cyclization is not well understood, but Pagni and Smith have suggested that it proceeds by nucleophilic attack of an Ar-OH or Ar-0 group on the second aromatic ring (in a protonated form) [4]. 

Ketones can also be obtained by treating phenols or phenolic ethers with a nitrile in the presence of F3CS020H. The mechanism in this case is different. 

Anodic C, C-coupling is a very powerful tool to synthesize cyclic compounds with high regio- and stereoselectivity. It involves inter- and intramolecular coupling of arylolefins, dienes, enolethers, phenol ethers, and aromatic amines and often opens a quick entry into complex natural products in a few steps. Although the mechanism is fully established in only a few cases, it does appear to involve the coupling of two radical cations at the site of their highest radical density and is further controlled by steric constraints. This important type of reaction is reviewed in Chap. 5 and in Refs. [89, 90]. 

Intramolecular oxidative cyclizations in the appropriately substituted phenols and phenol ethers provide a powerful tool for the construction of various practically important polycyclic systems. Especially interesting and synthetically useful is the oxidation of the p-substituted phenols 12 with [bis(acyloxy)iodo]-arenes in the presence of an appropriate external or internal nucleophile (Nu) leading to the respective spiro dienones 15 according to Scheme 6. It is assumed that this reaction proceeds via concerted addition-elimination in the intermediate product 13, or via phenoxenium ions 14 [18 - 21]. The recently reported lack of chirality induction in the phenolic oxidation in the presence of dibenzoyltar-taric acid supports the hypothesis that of mechanism proceeding via phenoxenium ions 14 [18]. The o-substituted phenols can be oxidized similarly with the formation of the respective 2,4-cyclohexadienone derivatives. 

Step 6. The catecholic and phenolic ethers were removed by treatment with hydrobromic acid. Benzyl ethers are frequently removed by reduction (e.g. hydrogenation) but reduction, of course, would not remove the methyl ether. The mechanism of the deprotection is shown below. 

One-electron oxidation of an aryl ether, for example at the anode, gives rise to a radical cation whose fate may be radical coupling (shown as dimerization) or substitution into a neutral phenol ether both paths are shown in Scheme 6 and converge to a biaryl product. Other products are possible if coupling at a quaternary center takes place. Mechanisms of this type must operate for the important couplings of phenol ethers with phenol ethers, and phenols with phenol ethers possibly they should not be neglected for phenol-phenol coupling also, in certain cases. 

The use of vanadium(V) salts in oxidative coupling reactions was prompted by the work of Funk et al. who recognized the ability of vanadium oxytrichloride to form phenoxyvanadium complexes with phenols [llO]. As it was shown by Schwartz et al., such complexes can be isolated and used for oxidative couplings [111]. Vanadium oxytrifluoride, a superior reagent [112] was found to be effective not only with free phenols but also with phenol ethers revealing the non-phenolic mechanism of this oxidation [113]. The method was successfully adapted to the oxidative macrocyclisation of a vancomycin subunit [114]. 

Other phenols and phenol ethers were examined to assess the breadth of activity of this catalyst. Anisole was selected as an electron rich aromatic system though less so than phenol. A cleaner reaction at lower conversion was expected. Under similar conditions employed for phenol hydroxylation, a 20% conversion of anisole was measured with selectivity to 4-methoxy phenol of 15% and to guiacol of 42%. 1-naphthol was also assessed. No conversion was seen, presumably due to the bulky nature of the molecule. These findings are consistent with a mechanism involving an electrophilic oxidant species. 

Scheme 1.5 Simplified single-electron transfer (SET) mechanism for the reaction of p-substituted phenol ethers with [bis(trifluoroacetoxy)iodo]benzene.

Processes involving a single-electron transfer (SET) step and cation-radical intermediates can occur in the reactions of X - or X -iodanes with electron-rich organic substrates in polar, non-nucleophilic solvents. Kita and coworkers first found that the reactions of p-substituted phenol ethers 29 with [bis(trifluoroacetoxy)iodo]benzene in the presence of some nucleophiles in fluoroalcohol solvents afford products of nucleophilic aromatic substitution 31 via a SET mechanism (Scheme 1.5) [212,213]. On the basis of detailed UV and ESR spectroscopic measurements, it was confirmed that this process involves the generation of cation-radicals 30 produced by SET oxidation through the charge-transfer complex of phenyl ethers with the hypervalent iodine reagent [213,214],... 

A similar SET mechanism involving cation-radical intermediates 30 has also been confirmed for the reactions of phenolic ethers with diaryliodonium salts in hexafluoroisopropanol [215], The use of fluoroal-cohols as solvents in these reactions is explained by their unique ability to stabilize the aromatic cation-radicals [107],... 

Thienyl(phenyl)iodonium salts and other heteroaryl(phenyl)iodonium salts can be used as the selective heteroaryl transfer agents in reactions with phenol ethers. These heteroarylations occur at room temperature in the hexafluoroisopropanol solution in the presence of trimethylsilyl triflate via a SET mechanism [876]. 

Matsumoto and Mochida [33], using NMR with a hydrogenated coal tar pitch mesophase carbon fiber, showed that the initial attack by oxygen was on -CH3 and -CH2- groups, with the gradual formation of carboxyls, esters and aryl carbonyls. Fairly stable cross-links were formed via phenols, ethers and esters. The workers also found that slower heating rates (0.5°C/min instead of 2.0°C/min) produced better mechanical properties and that the final choice would be controlled by the desired target properties and economics. 

In 1932, Quelet used the Blanc procedure, replacing formaldehyde with aliphatic aldehydes in the reaction with phenolic ethers. The resulting reaction mixtures were found to contain a-chloroalkyl derivatives. Although the conditions are virtually identical and the reaction proceeds via the same basic mechanism, the Blanc chloromethylation is often referred to as the Quelet reaction. 

In addition to electrostatic colloid stabilization generated by anionic surfactants, liquid dispersions are also made from nonionic surfactants. Stabilization of the emulsion is achieved by electrosteric stabilization or by pure steric stabilization (2,13). Polyoxyethylene dodecyl ethers, polyoxyethylene nonyl phenyl ethers, and polyoxyethylene nonyl phenol ethers are a few surfactants typically used in emulsion polymerization with nonionic surfactants (14-16). Non-ionic emulsion polymerizations are characterized by lower critical micelle concentration than their ionic counterparts. Thus, the emulsion particle sizes are generally much larger than in the ionic polymerizations. The mechanism of radical entry and exit in polymeric surfactant stabilizer systems are different than in anionic systems. With water-soluble initiators, the kinetics depends on initiator concentration. 

A novel intramolecular alkylation using a spiroketal (14) with BFj.OEt, in THF at reflux forms the benzene-fused 8-oxabicyclo[3.2.1]octane ring system (15) in satisfactory yield. IV-Tosylpipecolinic acid (16) in the presence of sulfuric acid in benzene forms the unexpected aromatized derivative (17) in 18% yield and the mechanism is suggested to involve the reaction of intermediate (18) with benzene to form (19). Cumyl and r-butyl hydroperoxides have been used for the electrophilic alkylation of activated aromatic substrates, mainly phenols and phenol ethers.The hydroperoxides behave differently as far as catalysis and regioselectivity are concerned. The latter is believed to be explicable by steric and reactivity/selectivity considerations. Electrophilic r-butylation may be followed by radical reactions due to the r-butyl... 

Q The mechanism of the Claisen rearrangement of other allylic ethers of phenol is analogous to that of allyl phenyl ether What is the product of the Claisen rearrangement of C6H50CH2CH CHCH3 /... 

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