The oxidation of 2-alkoxy-3,6-di-tert-butylphenols. The reversible dimerization of 2-alkoxy-3,6-di-tert-butylphenoxy radicals

Article abstract of DOI:10.1016/j.tetlet.2005.04.012

The oxidation of 2-alkoxy-3,6-di-tert-butylphenols has been studied. It was found that 2-RO-3,6-di-tert-butylphenoxy radicals (R = Et, Pr, Ph) undergo dimerization by C-O coupling. The X-ray structure of 2-ethoxy-3,6-di-tert- butylphenoxy dimer has been d

Full text of DOI:10.1016/j.tetlet.2005.04.012

Tetrahedron
Letters
Tetrahedron Letters 46 (2005) 4095–4097
The oxidation of 2-alkoxy-3,6-di-tert-butylphenols. The
reversible dimerization of 2-alkoxy-3,6-di-tert-butylphenoxy
radicals
Gleb A. Abakumov, Vladimir K. Cherkasov, Vladimir I. Nevodchikov,
Nikolai O. Druzhkov, Georgii K. Fukin, Yurii A. Kurskyand Alexandr V. Piskunov
*
G.A. Razuvaev Institute of Organometallic Chemistry, Tropinina str. 49, 603950 Nizhny Novgorod, Russia
Received 21 January2005; revised 25 March 2005; accepted 5 April 2005
Available online 19 April 2005
Abstract—The oxidation of 2-alkoxy-3,6-di-tert-butylphenols has been studied. It was found that 2-RO-3,6-di-tert-butylphenoxy
radicals (R = Et, Pr, Ph) undergo dimerization byC–O coupling. The X-raystructure of 2-ethoxy-3,6-di- tert-butylphenoxy dimer
has been determined. Radical dimers dissociate reversiblyin solution to give two phenoxyradicals at 200–350 K. Byusing EPR
spectroscopythe equilibrium concentration of the phenoxyradicals, equilibrium constants and DH and DS of dissociation have been
determined.
Ó 2005 Elsevier Ltd. All rights reserved.
Phenoxyradicals are often employed as reagents in or-
ganic, organometallic and biochemistry. The most com-
monlyused methods for the preparation of phenoxy
radicals are bimolecular oxidation or photolysis of the
corresponding phenols. The formation of phenoxyrad-
icals byoxidation of 2-alkoxy-3,6-di- tert-butylphenols
has been reported.1 Recentlyit was shown that 2-alk-
oxy-3,6-di-tert-butylphenolate complexes are the main
products of the reaction between 3,6-di-tert-butylbenzo-
quinone and alkyl zinc or cadmium derivatives.2 Herein
we report the reversible dimerization of 2-alkoxy-3,6-
di-tert-butylphenoxy radicals by C–O coupling to form
the corresponding quinol ethers of alkoxyphenols.
tion of phenoxyradicals in solutions. Williams and
Kreilick4 observed the formation a 2,6-di-tert-butyl-4-
acetylphenol dimer and its dissociation to phenoxy
1
radicals from the broadening of the H NMR signal.
5
The phenoxyradicals can be prepared byoxidation of
diethyl ether solutions of 1a, b and e using a basic solu-
tion of potassium ferrocyanide (Scheme 2). Compounds
3–5 are stable solids. The structures of 3–5 were con-
6–8
firmed byIR and NMR spectroscopy.
proof of the structure of 3 was obtained byX-raycrys-
Unequivocal
tallography( Fig. 1).
The reversible dissociation of dimers 3–5 takes place in
the temperature range 200–350 K. Investigation of the
temperature dependence on the dissociation equilibrium
The oxidation of 2-alkoxy-3,6-di-tert-butylphenols 1a–e
with PbO2 (Scheme 1) in toluene solution gives phenoxy
radicals as was evident byEPR-spectroscopy( Table 1).
A drop in temperature leads to a decrease in the inten-
sityof the EPR signal which disappears at 200 K.
Increasing the temperature renews the initial spectra.
Similar transformations of EPR spectra have been ob-
served for 3,6-di-tert-butyl-2-methoxyphenoxy radicals.3
These observations are evidence of reversible dimeriza-
Crystal data for 3: Light yellow crystals were grown from hexane,
˚
triclinic, P2(1)/n, Z = 4 in a cell of dimensions: a = 11.7595(4) A,
˚
˚
b = 16.2885(6) A, c = 15.9171(6) A, a = 90°, b = 103.8860(10)°,
c = 90°. Z = 4, q = 1.119 mg/m3 l = 0.072 mmꢀ1, F(000) = 1096. A
total of 23,174 reflections were measured, 5200 unique (Rint = 0.0236).
F2 refinement, R1 = 0.0350, wR2 = 0.0909.
CCDC 261258 contains the supplementarycrystallographic data for
this paper. These data can be obtained free of charge via
www.ccdc.cam.ac.uk/conts/retrieving.html (or from the CCDC, 12
Union Road, Cambridge CB2 1EZ, UK. Fax: +44 1223 336033;
E-mail address: deposit@ccdc.cam.ac.uk).
Keywords: Phenol; Oxidation; Phenoxyradical; Reversible disso-
ciation.
*
Corresponding author. Tel.: +7 8312 127682; fax: +7 8312 127497;
e-mail: pial@imoc.sinn.ru
0040-4039/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2005.04.012

4096
G. A. Abakumov et al. / Tetrahedron Letters 46 (2005) 4095–4097
But
But
PbO2
O
OH
OR
R = Et(a), Pr(b), Pri(c), But(d), Ph(e)
OR
But
2a-e
But
1a-e
Scheme 1.
Table 1. The EPR spectral parameters of phenoxyradicals 2a–e
(toluene, 290 K)
Phenoxyradical Ai (H4), G Ai (H5), G Ai (HR), G gi
2a
2b
2c
2d
2e
8.0
8.0
7.8
8.0
9.2
0.8
0.8
0.7
0.9
1.6
1.7 (2H)
1.8 (2H)
1.2 (1H)
—
2.0050
2.0051
2.0050
2.0050
2.0049
—
for compounds 3 and 5 byEPR spectroscopyallowed
determination of the phenoxyradical concentration (a
solution of TEMPO radical was used as the standard).
On the basis of these measurements the equilibrium con-
stants could be calculated. Theyare approximately
10ꢀ6 mol/L for 3 and 10ꢀ9 mol/L for 5, respectively
(T = 298 K). From the dependence ꢀlnK = f(1/T), the
dissociation enthalpyand entropyhave been calculated
and are as follows: DH = 15.5 0.5 kcal/mol; DS =
23.6 0.5 cal/mol K for 3 and DH = 17.1 0.5 kcal/
mol; DS = 16.5 0.5 cal/mol K for 5.
Figure 1. ORTEP diagram of 3 showing the X-raymolecular structure
at the 30% probabilitylevel. All bond lengths and angles are in the
same range as expected for comparable aromatic or cyclohexadienone
systems, respectively.
But
But
But
But
Oxidation of 1c under analogous conditions did not lead
to a corresponding dimer. The presence of the labile
O
O
OH
K3[Fe(CN)6]
C(CH3)2
i
hydrogen atom in the Pr group of 2c resulted in over-
OPri
oxidation of the latter leading to the formation of the
known9 cyclic ether of 3,6-di-tert-butylcatechol
6
6
1c
(Scheme 3). The phenoxyradical 2d did not form a
dimeric product either and underwent further trans-
formations in solution; we were unable to isolate
anyof these reaction products.
Scheme 3.
Acknowledgements
In summaryit should be noted that compounds 3–5 are
veryconvenient sources of phenoxyradicals in solution.
Theyare easyto handle and the radical concentration
can be adjusted bysimple dissolution in the most aprotic
organic solvent.
We are grateful to the Russian Foundation for Basic
Research (grant 04-03-32413) and Russian President
Grant supporting scientific schools (grant 1649.2003.3)
for financial support of this work.
But
6
1
O
But
But
But
But
5
But
2
4
O
OH
OR
6'
K3[Fe(CN)6]
O
OR
1'
2'
3
5'
4'
H
But
OR
OR
3'
But
2a, 2b, 2e
1a, 1b, 1e
R = Et(3), Pr(4), Ph(5)
Scheme 2.

G. A. Abakumov et al. / Tetrahedron Letters 46 (2005) 4095–4097
4097
(60)); 1.22 (s, 9H, But (6)); 1.39 (t, 3H, CH3, J = 6.9); 1.39 (s,
9H, But (30)); 1.47 (t, 3H, CH3, J = 7.1); 1.49 (s, 9H, But
(3)); 3.66 and 4.49 (m, 1H each, O(20)CHAHB); 3.94 (m, 2H,
O(2)CHAHB); 5.93 (d, 1H, H(4), J = 4.5); 6.47 (d, 1H, H(5),
J = 4.5); 6.89 and 6.93 (d, 1H each, H(40) and H(50),
J = 8.5).
References and notes
1. ProkofÕeva, T. I.; ProkofÕev, A. I.; VolÕeva, V. B.; Ershov,
V. V.; Kabachnik, M. I. Bull. Acad. Sci. USSR, Div. Chem.
Sci. (Engl. Transl.) 1985, 1324–1331.
2. Abakumov, G. A.; Cherkasov, V. K.; Abakumova, L. G.;
Nevodchikov, V. I.; Druzhkov, N. O.; Makarenko, N. P.;
Kursky, Yu. A. J. Organomet. Chem. 1995, 491, 127–
132.
3. ProkofÕev, A. I.; ProkofÕeva, T. I.; Bubnov, N. N.;
Solodovnikov, S. P.; Belostotskaya, I. S.; Ershov, V. V.;
Kabachnik, M. I. Dokl. Chem. (Engl. Transl.) 1978, 239,
1367–1389.
4. Williams, D. J.; Kreilick, R. J. Am. Chem. Soc. 1967, 89,
3408–3412.
5. Details of the experiment are as follows: To 100 mL of an
aqueous solution of 20 g of K3[Fe(CN)6] and 3 g of KOH
under continuous stirring, 5 g of phenol 1a, b, e in 50 mL of
diethyl ether was added. After 30 min of stirring, the
organic layer was separated and washed with water. The
ether was evaporated and the residue was recrystallized
from hexane.
7. 3,6-Di-tert-butyl-4-(3,6-di-tert-butyl-2-propoxyphenoxy)-2-
propoxycyclohexa-2,5-dienone (4): Yield 83%; light yellow
oil; Found (%): C 77.72; H 10.20. Calculated for C34H54O4
(%): C 77.57; H 10.27. IR (Nujol): 1655, 1670 (C@O) cmꢀ1
.
1H NMR (CDCl3, 200 MHz) d/ppm J /Hz: 0.91 (s, 9H, But
(60)); 1.01 (t, 6H, CH3, J = 7.5); 1.22 (s, 9H, But (6)); 1.39 (s,
9H, But (30)); 1.49 (s, 9H, But (3)); 1.61–2.10 (m, 4H,
CH2CH3); 3.54 and 4.35 (m, 1H each, O(20)CHAHB); 3.82
(t, 2H, O(2)CH2, J = 7.0); 5.91 (d, 1H, H(4), J = 4.7); 6.47
(d, 1H, H(5), J = 4.7); 6.91 (s, 2H, H(40) and H(50)).
8. 3,6-Di-tert-butyl-4-(3,6-di-tert-butyl-2-phenoxyphenoxy)-2-
phenoxycyclohexa-2,5-dienone (5): Yield 92%; mp = 151 °C;
Found (%): C 80.70; H 8.45. Calculated for C40H50O4 (%):
1
C 80.81; H 8.41. IR (Nujol): 1650, 1670 (C@O) cmꢀ1. H
NMR (CDCl3, 200 MHz) d/ppm J/Hz: 0.94 (s, 18H, But
(30) and But (60)); 1.30 and 1.32 (s, 9H each, But); 5.83 (d,
1H, H(4), J = 4.7); 6.75 (d, 1H, H(5), J = 4.7); 7.11 (s, 2H,
H(40) and H(50)); 6.70–7.41 (m, 10H, OC6H5).
9. Belostockaya, I. S.; Dzhuaryan, E. V.; Ershov, V. V. Bull.
Acad. Sci. USSR, Div. Chem. Sci. (Engl. Transl.) 1973,
2808–2809.
6. 3,6-Di-tert-butyl-4-(3,6-di-tert-butyl-2-ethoxyphenoxy)-2-
ethoxycyclohexa-2,5-dienone (3): Yield 89%; mp = 116 °C;
Found (%): C 77.24; H 10.13. Calculated for C32H50O4 (%):
C 77.11; H 10.04. IR (Nujol): 1655, 1670 (C@O) cmꢀ1. H
1
NMR (CDCl3, 200 MHz) d/ppm J/Hz: 0.91 (s, 9H, But