Phenols radical substitution

Photo-induced electron-transfer decarboxylation reactions have been reviewed. A variety of methyl- and methoxy-substituted phenol radical cations have been generated by either photo-induced electron transfer or photo-ionization in dry solvents such as acetonitrile. In the presence of small amounts of water the radical cations are not detected and the phenoxyl radical is the only transient species observed. The 2-methoxyphenol radical cation was found to be more reactive than the 4-methoxy radical cation. 

Figure 9 shows plots of Hammett fr+ values versus E j2 for the 8-p-X-Ph-dG adducts. In Fig. 9A, the OH (—0.92 ) fr+ value was used and the regression deviated from linearity. However, Fig. 9B shows that the regression is improved to almost unity when the O (2.30 ) fr+ value is used. These results suggested that the oxidation of 8-p-PhOH-dG may be coupled with phenol deprotonation. As shown in Scheme 12, resonance structures for the radical cation of 8-p-PhOH-dG create a p-substituted phenol radical cation, which possess negative pAa values (pifa for phenol radical cation ). Phenolic radical cations undergo deprotonation rapidly in the presence of water (0.6-6 x to yield neutral phenolic radicals. In the anhydrous DMF solvent used for electrochemical measurements, an N-7 adduct atom or adventitious water in the solvent could serve as base to facilitate phenolic radical production. 

This diazotization reaction is compatible with the presence of a wide variety of substituents on the benzene ring. Arenediazonium salts are extremely important in synthetic chemistry, because the diazonio group (N=N) can be replaced by a nucleophile in a radical substitution reaction, e.g. preparation of phenol, chlorobenzene and bromobenzene. Under proper conditions, arenediazonium salts react with certain aromatic compounds to yield products of the general formula Ar-N=N-Ar, called azo compounds. In this coupling reaction, the nitrogen of the diazonium group is retained in the product. 

Because the p value is positive, negatively charged carbon ions are considered to be the primary transition state complex (TSC). The TSC will dissociate to a substituted phenol radical and a stable anion. It may also be neutralized by the toluene, resulting in the addition of a proton, H+ (Arai and Dorfman, 1964) therefore, eaq is considered to interact with the ir-orbital of the ring as in electrophilic substitution rather than to affect electron distribution and polarizability of a certain substituent. 

Fig. 2. Re-establishment of Hammett correlation for halogen-substituted phenols in the reaction with polyvinyl acetate radical. The straight line represents the Hammett equation found for 26 compounds (No. 12 in Table 3). Key o non-halogen substituted phenols including 2,6-dimethylphenols, excluding 2,6-t-Bu2-4-substituted-phenols halogen-substituted phenols.

Contrary to the easy synthesis of alkylated phenols, e.g. via the electrophilic alkylations of phenols using alkyl halides, alkenes or other alkyl derivatives, there is no universal method for the preparation of arylated phenols. Radical or other arylation procedures usually lead to a mixture of difficult to separate isomers. Aryl-substituted phenols have, therefore, to be prepared by special routes. 

The reduction of phenols and phenolic esters and ethers is discussed in Chapter 19 (see 19-38 and 19-35). The reaction ArX ArH is treated in Chapter 11 (reaction 11-39), although, depending on reagent and conditions, it can be nucleophilic or free-radical substitution, as well as electrophilic. 

We also mention that the ionized phenol-water complex has been observed and examined in depth" . Complexes of phenol radical cation with ammonia" and molecular nitrogen have also been produced. The existence of an intramolecular hydrogen bond in orf/zo-substituted phenol radical cations has also been demonstrated . 

Alkylperoxyl radicals substituted at the a-position by halogens show a higher reactivity with respect to oxidation of phenolates (equation 8). 

Phenol-ketone novolacs 1487, 1488 Phenol-nitrile complexes 377 Phenol radical cations 1101 fragmentation of 289-291 Phenols—see also Biphenols, Bis-phenols, Hydroxybenzenes, Polyphenols acidities of, gas-phase 310-312 acylation of 629-632, 933, 934 Lewis acid catalyzed 631 montmoriUonite-catalyzed 632 pyridine-catalyzed 631 adsorption of 944 alkylation of 606-629, 941 Brdnsted acid catalyzed 612 Lewis acid catalyzed 607-611 solid acid catalyzed 612-621 stereoselective 621-626 under supercritical conditions 621 as antioxidants 139-143, 840-901 ort/io-substituted 845 thermochemistry of 139, 140, 179 autoxidation of 1118, 1119 bromination of 649-651 jr-cation interaction of 322 chlorination of 649 comparison with isoelectronic methyl, amino and fluoro aromatic derivatives 226... 

Optimum protection of the substrate towards oxidation can be achieved when both 2- and 6-positions of the phenol are substituted with tertiary butyl groups and the 4-position is substituted with an n-alkyl group [28]. Replacement of a tertiary butyl group by a methyl group in the ortho position reduces the antioxidant activity considerably. Table 4.1. Steric hindrance in both ortho positions beneficially prevents hydrogen abstraction by the phenoxy radical. Reaction (4.34) ... 

However, phenols are not active as antioxidants unless substitution at either the ortho or para position has increased the electron density at the hydroxy group and lowered the oxygen-hydrogen bond energy, in effect increasing the reactivity towards the lipid free radicals. Substitution in phenolic compounds at the meta position has a rather limited effect, and compounds like resorcinol with a hydroxyl group in the meta... 

The pKj of monosubstituted benzene radical cations vary from 7 for CjHjNHj " to -2 for C5H5OH. CgHgCHj is even more acidic but the p has not been determined experimentally. In table 4, the pK s of a number of substituted phenol- and aniline radical cations are given. The pK s of aniline radical cations have been determined by pulse radiolysis as previously described[12,13,57,58] and the pK,s of phenol radical cations have been measured by ESR in mixtures of sulphuric acid and water. [59,60]... 

In principle, the pK of the toluene radical cation can be estimated from the one-electron reduction potential of the radical cation and the C-H bond dissociation enthalpy for toluene (368 kj mor )[62] using equation (5). The resulting pR is ca -10, i.e., considerably more acidic than the phenol radical cation. Nicholas and Arnold have estimated the pfC of the toluene radical cation to between -9 and -13 in acetonitrile which is weU in line with the estimate given here. [32] Since the C-H bond dissociation enthalpies of substituted toluenes seem to be almost invariant with substituent,[63-66] the substituent effect on the pK of... 

For a number of different substituted benzenes, the substituents are involved themselves in the redox chemistry. Among these substituent-active compounds, we find phenolic substances, aromatic amines, chalcogenide substituted benzenes, benzyUc substances, benzoic adds, thiobenzoic adds and benzyl alcohols. As has already been discussed, phenol radical cations, aniline radical cations and toluene radical cations are more or less acidic and can deprotonate to form the corresponding neutral radical. Chedcogenide substituted benzenes (S, Se and Te) are usually characterized by the fact that their redox properties are determined by the chalcogenides rather than by the substituent pattern. 

Typically, peroxidase-catalyzed polymerization of phenol is carried out in the presence of H2O2, which acts as an oxidizing agent. The free radicals of monomers (substrates) formed initially undergo coupling to produce dimers, and successive oxidation and coupling eventually results in the formation of polymers. The peroxidase-catalyzed polymerization of phenols and substituted phenols usually produce the polymer with complicated structures. The main structure was estimated to be of phenylene units or a mixture of phenylene and oxyphenylene units (5). 

---