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2,3,5,6-tetra¯uorophenol [16]) or the stabilization of cation radicals and radicals
(2,6-di-tert-butylphenol) causes a considerable decrease in the yield of 1,4-
benzoquinones and an increase in the yield of the corresponding 4,40-
diphenoquinones (2,6-di-tert-butylphenol [14]) and/or polyphenylene oxides
(2,6-di¯uorophenol, 2,3,5,6-tetra¯uorophenol). The increase in yield can be
lowered by an increase in the acidity of the reaction medium if cation radicals
and radicals are not well-stabilized and if the protonation constant of the oxidized
phenol derivative is lower than 4. This leads to the conclusion that the most
probable reaction pathway will be determined by the acidic properties of the ®rst
intermediates (cation radicals) in the reaction of phenols with one-electron
oxidizing reagents, by the electron densities on active atoms of these cation
radicals (i.e. on the oxygen atom and carbon atom C4 with great spin density), and
by the acidity of the reaction medium [17].
In this paper we describe simple methods of oxidation of some 2,6-
disubstituted phenol derivatives by cerium(IV) ions as well as methods of
synthesis of corresponding 1,4-benzoquinones and/or 4,40-diphenoquinones and/or
oligomeric poly(1,4-phenylene) oxides. We discuss the in¯uence of the reaction
conditions and the properties of cation radicals and radicals resulting from
quantum mechanical calculations.
Results and Discussion
When 2,6-dimethylphenol is oxidized with cerium(IV) ions in aqueous or aqueous-
acetonitrile solutions of perchloric acid (0.5±1.0 M) at room temperature, the
corresponding 4,40-diphenoquinone and 1,4-benzoquinone derivatives are obtained
as the main products [14]. The oxidation of 2,6-diisopropylphenol (1a), 2-tert-
butyl-6-methylphenol (1b), 2,6-diphenylphenol (1c), and 2,6-dichlorophenol (1d)
performed under similar conditions results in the corresponding 4,40-diphenoqui-
nones (2a±d) and oligomeric poly(1,4-phenylene) oxides (3a±d). In the case of
1c, the quantity of the resulting poly(1,4-phenylene) oxides (3c) is low due to the
steric hindrance caused by the phenyl groups in the neighbourhood of the oxygen
atom 1a and 1b, similarly to 2,6-dimethylphenol, are oxidized by cerium(IV) ions
in a two-phase system (CCl4/aqueous-acetonitrile solution of perchloric acid)
and at high concentration of perchloric acid (4.0 M) in the reacting phase to give
the corresponding 1,4-benzoquinones in good yields. This is not observed with
2,6-di-tert-butylphenol, 1c, and 1d. The results are summarized in Scheme 1 and
Table 1.
Such different behaviour of the particular phenol derivatives in reactions with
cerium(IV) ions requires some comment. The ®rst intermediates of the oxidation of
phenols, anisoles, and anilines by cerium(IV) ions are corresponding cation
radicals, whose formation is a result of an electron transfer from the organic
compound to the cerium(IV) ion in the donor-acceptor complex formed by the
substrates. The following facts point to the crucial role of the oxygen and nitrogen
donor atoms in the formation of the complexes:
1) Although in aqueous solution of 0.5 M perchloric acid N,N-diethylaniline reacts
readily with cerium(IV) ions, in 4.0 M perchloric acid the reaction proceeds
very slowly, and when the aniline derivative is added, neither a change of the