Metal Coordinated Phenoxyl Radicals - Stable Radicals: Fundamentals and Applied Aspects of Odd-Electron Compounds - page 283

H 3 CO t-Bu H 3 CO t-Bu

t-Bu t-Bu OCH 3 OCH 3

O O O O

N N N N

Cu Cu Cu Cu

Py Py Py Py

N N N N

N N N N

(a) (b) (c) (d)

Figure 8.2 Relative stabilities of copper(II)-radical complexes (Py denotes pyridine). (See Table 8.6 for
referencesandnomenclature).

σ + Hammet

donating properties of the methoxy compared to the tert -butyl group ( 0.256).

The same would hold for (b) relative to (d), but the opposite is experimentally observed. This paradox is
explained by the increased bulk provided by the tert -butyl group around the oxygen atom, thus decreasing
its reactivity, and enhancing the overall stability of the molecule.

=− 0 . 778 vs −

8.2.2 Electrochemistry of phenoxyl radicals
Phenoxyl radicals are usually obtained by oxidation of a phenol precursor. In the absence of a metal,
the oxidation has to be performed under alkaline conditions for two reasons. The first reason is that the
oxidation potential of phenols is high. For example, the oxidation potential of one of the most easily oxi-
dizable phenols, the tri- tert -butylphenol, is 1.07 V vs Fc + /Fc. 24,25 When electrochemical measurements

are performed in the presence of a strong base, such as sodium hydroxide, this value falls to

+

0.68 V 26 ;

−

12.2 for the tri- tert -butylphenol in water), 27

the base deprotonates the phenol (pKa ≈ thus affording a

phenolate that is electron enriched and, consequently, easier to oxidize.
The second reason is mechanistic. Oxidation of a phenolate is a very simple process from an elec-
trochemical point of view, as it is a simple electron transfer. In contrast, oxidation of phenols involves
several elemental steps (except under certain particular conditions, as noted below). 19 The first step is
electron removal, which affords a phenoxyl radical cation. Its pKa has been estimated to be

9.5 in ace-
tonitrile for the tri- tert -butylphenoxyl, 24 meaning that such species could only be detected in highly acidic
media (H 2 SO 4 12 M). In moderate acids, the radical cation is deprotonated to give a phenoxyl radical
(proton transfer coupled to electron transfer). Since the phenoxonium/phenoxyl redox potential is lower
than the oxidation potential of the initial phenol, the phenoxyl is again one-electron oxidized giving rise
to a phenoxonium ion. The overall mechanism is thus a two-electron oxidation, affording a phenoxonium
and a released proton. A particular case often encountered in biological systems concerns phenols that are
hydrogen bonded to an adjacent weak base (amine for instance). 28-32 The overall mechanism also implies
a proton coupled electron transfer but it is, in these cases, a single kinetic step. As a result the oxidation
potential is much lower and the oxidation product in this case is a phenoxyl radical hydrogen bonded to
a neighboring ammonium proton and not a phenoxonium ion.
The electrochemistry of coordinated phenols has not been well studied, presumably because of their
high oxidation potentials due to the positive charge of the metal located in the vicinity of the oxygen atom.
Oxidation of coordinated phenolates is more interesting, as this takes place at potentials intermediate
between those of coordinated (or not) phenols and free phenolates, and the mechanism involves a single-
electron transfer process, affording phenoxyl radicals.

−