Synthesis and characterization of 2,6-di-tert-butyl-1-thio-1,4-benzoquinone, the first isolable monothio-1,4-benzoquinone

Article abstract of DOI:10.1039/b003330h

The title compound, a labile but isolable substance owing to appreciable steric protection, shows substantially higher electron affinity than the corresponding 1,4-benzoquinone and is reduced by water.

Full text of DOI:10.1039/b003330h

Synthesis and characterization of 2,6-di-tert-butyl-1-thio-1,4-benzoquinone, the
first isolable monothio-1,4-benzoquinone
Riho Suzuki, Kouzou Matsumoto, Hiroyuki Kurata and Masaji Oda*
Department of Chemistry, Graduate School of Science, Osaka University, Toyonaka, Osaka 560-0043, Japan.
E-mail: moda@chem.sci.osaka-u.ac.jp
Received (in Cambridge, UK) 26th April 2000, Accepted 14th June 2000
Published on the Web 30th June 2000
The title compound, a labile but isolable substance owing to
appreciable steric protection, shows substantially higher
electron affinity than the corresponding 1,4-benzoquinone
and is reduced by water.
in deaerated benzene-d6 in a sealed tube at room temperature
gave rise to new proton signals as singlets at d 1.14 and 6.48 and
a
13C signal at d 233.45 (CNS) which are reasonable for the
desired 6. However, in spite of the use of excess DDQ, the
reaction seemed to form a 2 1 equilibrium mixture of 6 and 8
probably by mediation of DDQ and its hydroquinone. Although
the signals of 6 remained almost unchanged in the solution for
at least two months, the attempted isolation of 6 from the
mixture failed because of its lability. This failure suggests that
the isolation of 6 from solutions should be difficult. In the end,
the isolation of 6 in almost pure form was achieved in 70% yield
by its sublimation to a water-cooled cold finger from a solid-
state mixture of 8 and PbO2 (1 30 w/w) heated at 140 °C under
vacuum.
While p-benzoquinones have been extensively studied,1 little
has been known about the corresponding sulfur analogues. A
major reason may be the general lability of thiocarbonyl
compounds that makes their handling and isolation in pure form
difficult.2 Bock and co-workers have reported the pyrolytic
generation and spectroscopic characterization of the parent
dithio- and monothio-1,4-benzoquinones, 1 and 2, in low
temperature matrices.3 Recently 2,6-dialkyl-4-thio-1,4-benzo-
quinones 3 have been generated and trapped by cycloaddition
with 1,3-dienes.4 The only isolable, related monothioquinone so
far known is monothioanthraquinone 4.5 We have recently
reported the synthesis and properties of thioquinone methides 5
in which tert-butyl groups give steric protection to a consider-
able extent.6 Here we report the synthesis and characterization
of 2,6-di-tert-butyl-1-thio-1,4-benzoquinone 6, the first isol-
able, monothio-1,4-benzoquinone.
We have already described the generation of lithium 2,6-di-
tert-butyl-4-lithiothiophenoxide 7 as the key synthon for 5.6
The exposure of a THF solution of 7 to O2 at 270 °C followed
by the usual work-up afforded bisphenol disulfide 8† in 50%
yield. A major byproduct was 2,6-di-tert-butylthiophenol, a
reduction product. Obviously, the two tert-butyl groups at the
ortho-positions are not bulky enough to hinder the coupling of
the intermediate thiophenoxy radical as reported for the 2,6-di-
tert-butylthiophenoxy radical itself.7 The initially expected
mercaptophenol 9† was obtained by reduction of 8 with zinc
powder in acetic acid (60%). The attempted oxidation of 9 to 6
with DDQ in benzene or acetone resulted, however, in the
formation of 8. We therefore expected that oxidation of 8 to its
bis(phenoxy radical) 10 would cause cleavage of the rather
weak disulfide bond, leading to the generation of 6. In fact,
NMR monitoring of a mixture of 8 and excess DDQ (3.5 equiv.)
The monothiobenzoquinone 6† thus obtained is a greenish
yellow solid. Although relatively stable in the solid state as well
as in neutral aprotic solvents such as benzene and acetone at
room temperature, 6 is sensitive to moisture and other protic
substances. The IR spectrum of 6 shows an intense carbonyl
absorption at 1639 cm21 and a less intense thiocarbonyl
absorption (intense in the Raman spectrum) at 1141 cm21. This
thiocarbonyl stretching frequency is considerably lower than
that of 4 (1212 cm21)5 and even lower than the significantly
dipolar 4H-pyran-4-thione (1168 cm21).8 The electronic spec-
trum of 6 exhibits a weak absorption (e = 32) at 758 nm, which
is assignable to the n ? p* transition of the thiocarbonyl group,
as well as a strong absorption at 321 nm. The visible absorption
of 6 is at an appreciably longer wavelength than those of 2 (500
nm)3 and 4 (697 nm).5 The IR and visible absorption spectral
DOI: 10.1039/b003330h
Chem. Commun., 2000, 1357–1358
This journal is © The Royal Society of Chemistry 2000
1357

results suggest a substantial polarization of the thiocarbonyl
group of 6. In this context, a semiempirical theoretical
calculation (PM3) predicts that 6 should take the boat form, due
to steric congestion, with bow and stern angles of 50° and 27°,
respectively. This molecular deformation from planarity could
be responsible for the enhanced polarization (elongation) of the
thiocarbonyl bond.
This work was supported by a Grant-in-Aid for Scientific
Research of Special Field (No.10146102) from the Ministry of
Education, Science, Sports and Culture, Japan.
Notes and references
† Selected physical and spectroscopic data; 6: mp 87–88 °C; MS (EI) m/z
238 (M+ + 2H, 100%), 236 (M+, 20), 221 (M+ 2 CH3, 27), 180 (M+ 2 C4H8,
25), 165 (M+ 2 71, 69); IR: n (KBr)/cm–1 2963 (s), 1639 (s, CNO), 1556
(m), 1456 (m), 1366 (m), 1302 (m), 1261 (m), 1232 (m), 1200 (m), 1141 (m,
CNS), 1096 (m), 1072 (m), 1022 (m), 910 (m), 801 (m); dH (400 MHz,
C6D6) 1.13 (s, 18H), 6.49 (s, 2H); dC(100 MHz, C6D6) 30.98, 36.87, 121.94,
163.58, 189.94 (CNO), 233.45 (CNS); UV–Vis (cyclohexane) lmax/nm (e)
321 (14 300), 456 sh, 758 (32); 8: mp 210–213 °C; MS (EI) m/z 474 (M+,
12%), 238 (M+/2 + 1, 100), 181 (M+/2 2 C4H9, 16); dH(270 MHz, CD2Cl2)
1.04 (s, 18H), 1.60 (s, 18H), 4.84 (br s, 2H, OH), 6.65 (s, 2H), 6.86 (s, 2H);
The NMR spectra show restricted rotation of the S–S or C–S bond with an
estimated energy barrier of DG‡ = 16.5 ± 0.2 kcal mol21 (Tc = 75 °C in
benzene); see also ref. 10; 9: mp 124–127 °C; MS (EI) m/z 238 (M+, 100%),
181 (M+ 2 C4H9, 17); dH(270 MHz, CDCl3) 1.58 (s, 18H), 3.24 (s, 1H),
4.57 (s, 1H), 6.89 (s, 2H).
Thiocarbonyl compounds are usually more easily reduced
than the corresponding carbonyl compounds.9 Upon cyclic
voltammetry, 6 shows two irreversible reduction waves at
20.60 and 21.33 V (both peak potentials vs. Ag/Ag+, in 0.1 M
n-Bu4NClO4–CH3CN, ferrocene/ferrocene+ = 0.10 V). These
reduction potentials are about 0.5 V lower than those of 2,6-di-
tert-butyl-1,4-benzoquinone 11 (the corresponding peak poten-
tials: 1.07 and 1.88 V) measured under the same conditions,
indicating that 6 is a substantially strong electron acceptor.
Surprisingly, 6 was found to be reduced rather than hydrolysed
by water: when a small amount of D2O was added into an
acetone solution of 6 either in the dark or light at room
temperature, the signals of 6 at d 1.37 (s) and 6.46 (s)
disappeared within 1 h with the concurrent appearance of new
signals at d 7.00 (s), 6.93 (br s), 6.73 (br s) and 6.52 (s) in the
olefinic to aromatic region. The products were identified to be
monothiohydroquinone 9, disulfide 8 and benzoquinone 11,
formed in a 45 50 5 ratio (1H NMR and TLC comparison with
those of the authentic samples). The formation of 8 (probably
also 9) points to the intermediate formation of the correspond-
ing thiophenoxy radical. In addition, the ferricyanide test as a
preliminary qualitative test afforded a positive result, conform-
ing the formation of hydrogen peroxide in the solution. Thus,
hydrolysis of 6 to 11 is a very minor process, different from the
usually easy hydrolysis of thioketones to ketones.
1 The Chemistry of the Quinonoid Compounds, ed. S. Patai and Z.
Rappoport, Wiley, 2nd edn., 1988.
2 M. V. Lakshmikantham, M. Levinson, M. Menachery and M. P. Cava,
J. Org. Chem., 1986, 51, 411; M. V. Lakshmikantham, M. S. Raasch,
M. P. Cava, S. G. Bott and J. L. Atwood, J. Org. Chem., 1987, 52,
1874.
3 H. Bock, S. Mohmand, T. Hirabayashi, G. Maier and H. P. Reisenauer,
Chem. Ber., 1983, 116, 273.
4 B. Cantini, G. Capozzi, S. Menichetti and C. Nativi, Synthesis, 1999,
1046.
5 M. S. Raasch, J. Org. Chem., 1979, 44, 632.
6 R. Suzuki, H. Kurata, T. Kawase and M. Oda, Chem. Lett., 1999,
571.
7 W. Rundel, Chem. Ber., 1968, 101, 2956.
8 Á. Somogyi, G. Jalsovszky, C. Fülop, J. Stark and J. E. Boggs,
Spectrochim. Acta, Part A, 1989, 45, 679.
In conclusion, although the two tert-butyl groups at the ortho
positions are not bulky enough for steric protection, 2,6-di-tert-
butyl-1-thio-1,4-benzoquinone 6 was synthesized as a labile but
isolable compound at around room temperature and was found
to show a considerably high electron affinity and to undergo an
unusual reaction with water. We are now investigating the
detailed chemical properties of 6.
9 J. Simonet, in The Chemistry of Sulfur-Containing Functional Groups,
ed. S. Patai and Z. Rappoport, Wiley, 1993, pp. 440–448.
10 H. Kessler and W. Rundel, Chem. Ber., 1968, 101, 3350; N. Ueyama, T.
Okamura, Y. Yamada and A. Nakamura, J. Org. Chem., 1995, 60,
4893.
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Chem. Commun., 2000, 1357–1358