Phenols, complex formation with

Rates of ligand exchange depend quite strongly on the coordina-tive environment of the metal center. The water exchange rate of Fe(H2O)5(OH)is almost three orders of magnitude higher than that of Fe(H20)g+, and follows a dissociative, rather than an associative exchange mechanism (20). Fe(1120)5(OH)has also been shown to form inner-sphere complexes with phenols (27), catechols (28), and a-hydroxycarboxylic acids (29) much more quickly than Fe(H20) +. The mechanism for complex formation with phenolate anion (A-) is shown below (27) ... 

Stack and co-workers recently reported a related jx-rf / -peroxodi-copper(II) complex 28 with a bulky bidentate amine ligand capable of hydroxylating phenolates at - 80 °C. At - 120 °C, a bis(yu,-oxo)dicopper(III) phenolate complex 29 with a fully cleaved 0-0 bond was spectroscopically detected (Scheme 13) [190]. These observations imply an alternative mechanism for the catalytic hydroxylation of phenols, as carried out by the tyrosinase metalloenzyme, in which 0-0 bond scission precedes C - 0 bond formation. Hence, the hydroxylation of 2,4-di-tert-butylphenolate would proceed via an electrophilic aromatic substitution reaction. 

Mechanism. The ability of S02 to participate in complex formation with unsaturated hydrocarbons and a host of other compounds such as amines, ethers, phenols, and aromatic hydrocarbons is known (2, 6, 11, 22). SO has been found to initiate polymerization of some monomers... 

This reaction, known as the Gatterman-Koch reaction, does not work with phenolic or amino aromatic species due to complex formation with the Lewis acid. It does work well with aromatic hydrocarbons and is used industrially to prepare benzaldehyde and, as here, p-tolualdehyde. 

Annapurna et al. [13] established simple, accurate, and reproducible UV spectrophotometric methods for fhe assay of buclizine based on the formation of precipitation, charge transfer, and redox products. Precipitation/ charge transfer complex formation of fhe buclizine with I2/p-nitro methyl amino phenol sulfate-sulfanilic acid by method A, the precipitation/complex formation with ammonium molybdate/potassium thiocyanate by method B and precipitation/redox reaction of buclizine with phosphomolybdic acid/Co /EDTA by method C were proposed. Determination of buclizine in bulk form and in pharmaceutical formulations was also incorporated. 

Polyoxyethylene stearates are unstable in hot alkaline solutions owing to hydrolysis, and will also saponify with strong acids or bases. Discoloration or precipitation can occur with salicylates, phenolic substances, iodine salts, and salts of bismuth, silver, and tannins.Complex formation with preservatives may also occur. The antimicrobial activity of some materials such as bacitracin, chloramphenicol, phenoxymethylpenicillin, sodium penicillin, and tetracycline may be reduced in the presence of polyoxyethylene stearate concentrations greater than 5% w/w. ... 

Acetal resins are sensitive to oxidation and are generally stabilized by high-molecular-weight phenols. Polyesters and polyurethanes are coitunonly stabilized by phosphites. Polyamides are stabilized by phosphites and also (surprisingly) by copper and manganese salts, presumably through complex formation with the amide groups themselves. 

Some of you may wonder why we used a combination of phenol-chloroform for method I. Codeine phosphate has a solubility of 1 5000 in chloroform, yet when phenol was added the alkaloidal salt readily dissolved. It has been suggested that the codeine phosphate may undergo complex formation with the phenol and that the complex generated is readily soluble in chloroform. It is interesting to point out that neither a visual nor potentiometric titration could be performed until acetonitrile was added to the solvent system. 

Li Relaxation times have been used to study complex formation with nitroxides in the aqueous phase, and the interaction of Li with Mn. Dynamic NMR spectroscopy has been used to study equihbria of chelated lithium phenolates. The formation of a stable betaine Hthium salt adduct has... 

In addition to nonheme iron complexes also heme systems are able to catalyze the oxidation of benzene. For example, porphyrin-like phthalocyanine structures were employed to benzene oxidation (see also alkane hydroxylation) [129], Mechanistic investigations of this t3 pe of reactions were carried out amongst others by Nam and coworkers resulting in similar conclusions like in the nonheme case [130], More recently, Sorokin reported a remarkable biological aromatic oxidation, which occurred via formation of benzene oxide and involves an NIH shift. Here, phenol is obtained with a TON of 11 at r.t. with 0.24 mol% of the catalyst. 

Apart from complex formation involving metal ions (as discussed in Chapter 4), crown ethers have been shown to associate with a variety of both charged and uncharged guest molecules. Typical guests include ammonium salts, the guanidinium ion, diazonium salts, water, alcohols, amines, molecular halogens, substituted hydrazines, p-toluene sulfonic acid, phenols, thiols and nitriles. 

As the reaction temperature increases, the equilibrium constant diminishes, since complex formation is accompanied by heat liberation. Sterically hindered phenols form loose complexes because of the impeding effect of voluminous alkyl substituents in the ortho-position. Hydrogen bonding reduces the activity of phenols, which was first observed in the studies of the effects of cyclohexanol and butanol on the inhibitory activity of a-naphthol in cyclohexane [9]. This phenomenon was investigated in detail with reference to the oxidation of methylethylketone [10]. The k7 values for some inhibitors of the oxidation of ethylbenzene and methylethylketone are given below (333 K) [10,46] ... 

Since inhibitor molecules in a polar solvent may exist in the free (InH) or bound (InH Y) form, and peroxyl radicals attack preferentially the free In—H bonds that are not involved in complex formation, the decrease in kq in such solvents is due to the decreasing concentration of free and, hence, more reactive phenol molecules. The concentrations of phenol and the complex are related as [InH Y] = XuflnHJfY]. An inhibitor occurring in the complex is unlikely to react with the peroxyl radical by virtue of this reaction. Therefore, the empirical rate constant k7cmp is related to [Y] in the following way ... 

The addition of the second methyl group on the phenol ring led to the observation of the consecutive inclusion process with a decrease in the dynamics for complex formation (Table 8, cf. 29 with 28 (R = CH3)). This result supports the previous suggestion190 that small guests can slip into the CD cavity and in one process form the stable host-guest complex. 

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