The facile oxidative coupling of a hindered phenol, 2,6-di-t-butylphenol, driven by N,N′-bis(ethoxy-carbonyl)-1,4-benzoquinone diimine; The reaction pattern traced by 1H NMR spectroscopy

Article abstract of DOI:10.1246/bcsj.75.857

Good yields of 3,3′,5,5′-tetra-t-butyl-4,4′-dipheno- quinone were obtained by the reaction of 2,6-di-t-butylphenol with N,N′-bis(ethoxycarbonyl)-1,4-benzoquinone diimine, which had been shown to undergo addition with aniline derivatives and phenols. The reaction was followed with ¹H NMR spectroscopy to deduce the reaction intermediates.

Full text of DOI:10.1246/bcsj.75.857

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Bull. Chem. Soc. Jpn., 75, 857–858 (2002)
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Short Articles
Table 1. Yields of 2 and 4 in the Reaction of 1 with 3a)
The Facile Oxidative Coupling of a
Hindered Phenol, 2,6-Di-t-butyl-
phenol, Driven by N,Nꢀ-Bis(ethoxy-
carbonyl)-1,4-benzoquinone Diim-
ine; The Reaction Pattern Traced by
1H NMR Spectroscopy
Solvent
C6H6 CHCl3 MeCOMe MeCN AcOEt THF
Reaction timeb) 15d 15d
10h
10h
10h 10h
2
4
72
60
65
56
89
85
94
75
92
86
89
76
Yield/%
02
7
8
SJA8
09-2673
1040
a) [1]0 = [3]0 = 0.1 mol dm−3. b) Required for consumption
of 3.
The peaks at 1.36 (peak a) and 4.93 ppm (the latter is not
shown) due to the t-butyl and hydroxy protons of 1 gradually
disappeared in 3 hours, and were substituted by a new singlet
at 1.33 (peak b) and a multiplet at 2.74 (peak c) ppm. Peaks b
and c had the same chemical shifts as those of t-butyl and me-
thine protons of an authentic sample of 3,3ꢀ,5,5ꢀ-tetra-t-butyl-
1,1ꢀ-bi(2,5-cyclohexadienyl)-4,4ꢀ-dione (5). At this stage, the
signals due to quinone diimine 3 were still observable, while
phenylenediamine 4 started to crystallize in the mixture. Also,
at this stage the signal due to the t-butyl group of 2 began to
appear at 1.43 (peak d) ppm (Fig. 1C), and then slowly but
steadily grew within the next 40–100 hours at the cost of peak
b of 5 ( Fig. 1D). Peaks b and c eventually disappeared within
several days. The overall picture showed that the reaction was
slow in benzene, and that the formation of 5 was faster than its
conversion to 2. It was reported before2b that 5 is stable in non-
polar dry solvents.
Y. Miyagi et al.
Yo Miyagi,* Noriko Banjyo, and Emiko Yamamura
Faculty of Education, Kanazawa University,
Kanazawa 920-1192
01
02
(Received November 5, 2001)
Good yields of 3,3ꢀ,5,5ꢀ-tetra-t-butyl-4,4ꢀ-dipheno-
quinone were obtained by the reaction of 2,6-di-t-butylphenol
with N,Nꢀ-bis(ethoxycarbonyl)-1,4-benzoquinone diimine,
which had been shown to undergo addition with aniline deriv-
atives and phenols. The reaction was followed with 1H NMR
spectroscopy to deduce the reaction intermediates.
A similar 1H NMR monitorimg of the reaction in acetone-d6
showed drastically different dynamics (Fig. 2). The reaction
was much faster and was completed within several hours
wherein the t-butyl singlet of 2 at 1.37 ppm (peak c) increased
at the expense of that of 1 at 1.43 ppm (peak a), as shown in
Figs. 2A and 2D. In this region, the triplet methyl protons of
the ethyl side chain of 3 and 4 also showed concurrent changes
from 1.33 ppm (peak d for 3) to 1.24 ppm (peak e for 4). Fig-
ures 2B and 2C clearly show the “come and go” of these peaks.
A singlet at 1.21 ppm (peak b) appeared immediately upon
mixing and persisted at a low intensity, but eventually disap-
peared on completion of the reaction; this peak b was con-
firmed to be the t-butyl signal of 5 as follows. Upon the disso-
lution of an authentic sample of 5, it showed a singlet at 1.24
ppm (peak a) for the t-butyl group, which rapidly disappeared
within ca. 10 min (Fig. 3). For this recording, a C6D6-acetone-
d6 (3:7) mixture was used as solvent to dampen the fast trans-
There have been reported for the oxidative coupling of a
hindered phenol, 2,6-di-t-butylphenol (1) to 3,3ꢀ,5,5ꢀ-tetra-t-
butyl-4,4ꢀ-diphenoquinone (2) by oxidation with nitric acid,1a
base-catalyzed oxygenation,1b catalytic oxygenation with me-
tallic compounds1c or 1,4-benzoquinone diimine derivatives,1d
and reactions with 2,4,6-tri-t-butylphenoxyl,1e tellurium com-
pounds,1f or a 1,2-benzoquinone monoimine derivative.1g
We reported that N,Nꢀ-bis(ethoxycarbonyl)-1,4-benzoqui-
none diimine (3) undergoes addition with unhindered aniline
derivatives and phenols although their reaction site are differ-
ent.2 It has been discovered now that the mixing of 3 with 1 in
equimolar amounts in solution under aerobic condition at the
room temperature causes a reaction to give 2 in 65–94% de-
pending on the solvents, diethyl 1,4-phenylenediamine-N,Nꢀ-
dicarboxylate (4) being isolated in comparable amounts
(Scheme 1). The reaction was faster in polar solvents than in
1
nonpolar ones as shown in Table 1. H NMR spectroscopy was
used to trace the reaction pattern to deduce the reaction inter-
mediates.
A 1:1 mixture of 1 and 3 in C6D6 was placed in the cavity of
a 400 MHz NMR spectrometer and the spectra were recorded
at intervals. The pertinent spectra are shown in Fig. 1A–1D.
Fig. 1. 1H NMR spectral changes of a solution of 1 (0.05 mol
dm−3) and 3 (0.05 mol dm−3) in C6D6 at room temperature
at 6 min (A), 1 h (B), 3 h (C), and 40 h (D) after mixing;
see text for the assignment of peaks.
Scheme 1.

858 Bull. Chem. Soc. Jpn., 75, No. 4 (2002)
© 2002 The Chemical Society of Japan
Scheme 2.
complex as shown in Scheme 2. In a polar solvent, such as ac-
etone, this rate should be much faster than in benzene. Qui-
none diimine 3 shows a broad tail hitching to the strong 282
nm peak stretching over to 470 nm in acetonitrile. Even in
benzene, its solution developed a light red color upon the addi-
tion of 1, which indicates the formation of a CT complex. At
present it is not certain whether radicals 8 and 9 are directly
formed by Scheme 2 or via the radical scission of a coupling
product between 8 and 9. Although there are many possible
venues for 8 to reach 4, we do not have any hint to suggest a
preference.
Fig. 2. 1H NMR spectral changes of a solution of 1 (0.05 mol
dm−3) and 3 (0.05 mol dm−3) in acetone-d6 at room tem-
perature at 6 min (A), 25min (B), 45 min (C), and 8 h (D)
after mixing; see text for the assignment of peaks.
Experimental
1H NMR spectra were recorded with JEOL JNM-LA 400FT
and JEOL JNM-FX90Q spectrometers. Compound 3 was pre-
pared by a reported method.2a Authentic samples of 2,1b 4,2a 5,3
and 71b were prepared according to the literature.
Fig. 3. 1H NMR spectral changes of a solution of 5 (0.05 mol
dm−3) in C6D6–acetone-d6 (3:7) at room temperature at 2.5
min (A), 7.5 min (B), and 18 min (C) after mixing; see text
for the assignment of peaks.
Procedure of the Reaction of 3 with 2,6-Di-t-butylphenol
(1). A reaction solution was made up by mixing 5 cm3 of a 0.2
mol dm−3 solution of 1 with 5 cm3 of a 0.2 mol dm−3 solution of
3. The proceeding of the reaction was monitored by TLC analysis
(eluent, hexane–ethyl acetate (4:1)). After confirming the con-
sumption of 3, the supernatant solution was separated from red
crystals deposited and concentrated to a residue, from which 2 and
4 were isolated by flush column chromatography (eluent, hexane–
ethyl acetate). Combined red crystals of 2 were recrystallized
from CHC13–hexane. Colorless crystals of 4 were recrystallized
from THF–hexane.
formation in neat acetone. Also, a 90 MHz spectrometer was
used for rapid tuning so that the recording could be started im-
mediately after mixing. Figures 3A and 3B also show a singlet
at 1.51 ppm (peak d) for t-butyl protons of 7, which grow
stronger quickly and dominate all at the end (Fig. 3C). They
also show a pair of singlets with equal intensity at 1.26 and
1.43 ppm (peaks b and c); because they varied the intensity to-
gether and eventually disappeared to give peak d, they are as-
signed to the two t-butyl groups of intermediate 6. This dem-
onstrates that the proton migration in 5 occurs by a stepwise
process, and very rapidly, to give 7. Further, the mixing of a
colorless solution of 7 and a faintly yellow solution of 3 in ace-
tone-d6 immediately developed a red solution which exhibited
a singlet at 1.37 ppm due to t-butyl of 2 (peak c in Fig. 2).
Thus, the dehydrogenation of 7 by 3 is also very rapid, and
even faster than the formation of 7 in acetone, which explains
the fact that in Fig. 2 the t-butyl signal of 5 appears (albeit with
a low intensity) but the t-butyl signal of 7 does not. One can
conclude that in acetone, acetonitrile, ethyl acetate and THF
the proton migration of 5 to 7 and the subsequent oxidation to
2 are much faster than the formation of 5. The fast conversion
must arise from the fact that these solvents have hetero-atoms
that assist a proton migration and oxidation. It follows that
benzene and chloroform lack such a proton coordinating cen-
ter, and do not facilitate the migration. This leads to the accu-
mulation of 5 in benzene (Fig. 1).
We wish to express our sincere thanks to Prof. Y. L. Chow
and K. Maruyama for their encouragement and discussions.
We are also indebted to Prof. M. Suzuki for measuring the cy-
clic voltammogram.
References
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Turning now to the formation of 5, the most plausible route
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which has been widely assumed as the intermediate in such ox-
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versible wave with Eox = +0.8 V vs SCE for 1 and a quasi-re-
versible wave with Ered = −0.3 V vs SCE for 3. Electron
transfer between them is not efficient, but is possible within a
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