Phenol radical cation

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. 

Phenols show a two-electron oxidation wave on cyclic voltammetry in acetonitrile at a less positive potential than for the con-esponding methyl ether (Table 6.5) or a related hydrocarbon. Phenol radical-cation is a strong acid with pKg ca. -5 in water [93], so the two-electron oxidation wave for phenols is due to formation of a phenoxonium ion such as 13, where the complete oxidation process is illustrated for 2,4,6-tri-tt rf-butylphenol. Phenoxide ions are oxidised at considerably less positive potentials than the conesponding phenol. They give rise to a one-electron wave on cyclic voltammetry in aqueous acetonitrile or in aqueous ethanol containing potassium hydroxide. 2,4,6-Tri-/ert-butyiphenoxide ion is reversibly oxidised to the radical in a one-electron proces.s with E° = -0.09 V V5. see. The radical undergoes a further irreversible one-electron oxidation with Ep = 1.05 V vs. see on cyclic voltammetry forming the phenoxonium ion which reacts with water [94J. 

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. 

Phenol radical cations exist only in strong acidic solutions (pKa -1) [1, 2]. However, in non-polar media phenol radical cations with lifetimes up to some hundred nanoseconds were obtained by pulse radiolysis [3], The free electron transfer from phenols (ArOH) to primary parent solvent radical cations (RX +) (1) resulted in the parallel and synchroneous generation of phenol radical cations as well as phenoxyl radicals in equal amounts, caused by an extremely rapid electron jump in the time scale of molecule oscillations since the rotation of the hydroxyl groups around the C - OH is strongly connected with pulsations in the electron distribution of the highest molecular orbitals [4-6]. 

From pulse radiolysis lifetimes of phenol radical cations between 300 and 500 ns are known [4, 9]. Laser photolysis (3 ns, 266nm up to 15 mJ) ofN2-purged solutions of up to 10 3 mol dm 3 phenols yields phenoxyl radicals as dominating products (Figure 2). In the spectrum only a little hint for the phenol radical cations exists. The inset shows that the phenoxyl radical formation does not depend linearly from the energy but appears by biphotonic absorption contradictory to the fs-experiments described above. 

Dixon WT, Murphy D (1976) Determination of the acidity constants of some phenol radical cations by means of electron spin resonance. J Chem Soc Faraday Trans 272 1221-1230 Dizdaroglu M, Scherz H, von Sonntag C (1972) y-Radiolyse von meso-Erythrit in wassriger Losung. Z Naturforsch 27b 29-41... 

Whereas little is known about ketone and enol radical cations in solution, the related one-electron oxidation of phenols has been extensively studied [72]. Nowadays, anodic oxidation of phenols constitutes a valuable synthetic access to phenoxenium ions [73] which are important intermediates for carbon-carbon bond formation processes [74-76] and to various natural products [77]. In light of the biological relevance of phenol oxidation [78,79] redox potentials of phenols [72,80] and phenolates [80-85] as well as pK values of phenol radical cations [80,86,87] are documented in various solvents. Some of the data will be quoted later in comparison with enol systems. 

TABLE 5. The B3LYP data of phenol and phenol radical cation" ... 

The phenol radical cation have recentiy been studied theoretically ... 

The energyygrt of phenol radical cation is determined at the corresponding geometry of the parent phenol. 

Owing to their relatively low ionization energies (IE) of ca 8.0-8.5 eV, phenols are also good electron donor solutes. Recent experimental studies of phenols in non-protic solvents showed that ionized solvent molecules react with phenol to yield not only phenol radical cations by electron transfer, but also phenoxy radicals by hydrogen transfer. An obvious question is whether, under these conditions, the latter radicals were formed from ionized phenols rather than by direct hydrogen abstraction, because proton transfer reactions could be facilitated upon ionization. This also raises a question about the influence of solvent properties, both by specific and non-specific interactions, on the mechanism and kinetics of deprotonation processes ... 

General and theoretical aspects of phenols 1. Molecular and electronic structure of phenol radical cation... 

Table 33 lists the hfcc values calculated at the UB3LYP/6-311-I— -G(d,p) level for both phenol radical cation and phenoxy radical. A few points are noteworthy. 

Atom Phenoxyl radical Isotropic Fermi Contact Couplings Phenol radical cation 21 Isotropic Fermi Contact Couplings... 

FIGURE 30. Relative energies of selected isomers of phenol radical cation containing a six-membered ring. Values given in kJ moU relative to 21 were obtained from UB3LYP/ 6-311++G(d,p)+ZPE calculations. Adapted from Reference 452 with permission... 

It may be noted that the energy amount involved in the CO-loss process is by far smaller than that needed for a deprotonation of phenol cation as mentioned above, namely 857 kJmoC. This suggests that the ease with which a deprotonation of phenol radical cations occurs in different solutions was likely to arise from either a specific participation of the solvent molecules in the supermolecule or a strong continuum effect. 

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 . 

UNIMOLECULAR FRAGMENTATION REACTIONS OF PHENOL RADICAL CATIONS. 263... 

A. Loss of CO from Simple Phenol Radical Cations... 

---