Isopropyl-substituted o-benzoquinones and oxanthrenequinones. Effect of steric shielding of alkyl substituents on reactivity

Article abstract of DOI:10.1007/s11172-021-3167-6

A synthetic procedure was developed for the preparation of sterically hindered di- and trisubstituted catechols and o-quinones by the reaction of catechol with isopropanol. Compared to sterically more hindered analogs, o-quinones bearing isopropyl substituents exhibit a higher reactivity toward the formation of oxanthrenequinone derivatives.

Full text of DOI:10.1007/s11172-021-3167-6

Russian Chemical Bulletin, International Edition, Vol. 70, No. 5, pp. 916—924, May, 2021
916
Isopropyl-substituted o-benzoquinones and oxanthrenequinones.
Effect of steric shielding of alkyl substituents on reactivity*
T. N. Kocherova, N. O. Druzhkov, A. S. Shavyrin, M. V. Arsenyev, E. V. Baranov,
V. A. Kuropatov, and V. K. Cherkasov
G. A. Razuvaev Institute of Organometallic Chemistry, Russian Academy of Sciences,
49 ul. Tropinina, 603950 Nizhny Novgorod, Russian Federation.
Fax: +7 (831) 462 7497. E-mail: tanya@iomc.ras.ru
A synthetic procedure was developed for the preparation of sterically hindered di- and
trisubstituted catechols and o-quinones by the reaction of catechol with isopropanol. Compared
to sterically more hindered analogs, o-quinones bearing isopropyl substituents exhibit a higher
reactivity toward the formation of oxanthrenequinone derivatives.
Key words: o-quinones, catechols, redox-active ligands, alkylation, isopropanol.
The chemistry of metal dioxolene complexes has at-
unsubstituted catechol with propylene in the presence of
aluminum isopropoxide in toluene at 265 C afforded
3,5- and 3,6-diisopropylcatechols21 (1 and 2, Scheme 1).
The heating of catechol in isopropanol in the presence of
concentrated sulfuric acid under reflux resulted in the
isolation of only 3,5-diisopropylcatechol22 (2). Previously,
3,4,6-triisopropylcatechol (3) was prepared by the slow
addition of concentrated sulfuric acid to a refluxing mix-
ture of catechol with an excess of isopropanol in n-hept-
ane.23 Since sterically hindered dihydric phenols have
received research attention solely as industrial antioxi-
dants, the oxidation of only catechol 3 to o-quinone 6 was
reported in the literature. Previously, 3,5-diisopropyl-o-
quinone (5) was synthesized by the oxidation of 2-nitroso-
4,6-diisopropylphenol,24 whereas 3,6-diisopropyl-o-quin-
one (4) was not described. Based on the results of the
earlier studies, it can be concluded that variations of the
conditions for the alkylation of catechol lead to the forma-
tion of products with different structures.
A synthetic procedure was developed for the prepara-
tion of sterically hindered catechols and o-quinones by
the perchloric acid-catalyzed reaction of catechol with
cyclohexene.25 It was shown that the reaction using a par-
ticular reagent molar ratio affords simultaneously three
compounds: 3,6-di-, 3,5-di-, and 3,4,6-trialkylated prod-
ucts. The oxidation of these compounds with an alkaline
solution of potassium hexacyanoferrate gives o-quinones.
When applying this procedure to the synthesis of de-
rivatives containing isopropyl substituents (see Scheme 1),
we initially used the appropriate alcohol as the alkylating
agent. However, the heating to 120 C proved to be insuf-
ficient for the reaction of catechol with isopropanol to
occur, and the reaction temperature was increased to
140 C. The further increase in the reaction temperature
to 160 C resulted in the formation of a large amount of
tracted research attention due to interesting properties of
these compounds. First of all these are reversible redox
processes,1 in which the oxidation state of the metal com-
plex can change through changes in the oxidation state of
either the metal or the ligand. Of particular interest are
intramolecular redox transformations2—8 of such com-
plexes characterized by the unique phenomenon of redox
isomerism via reversible metal—ligand electron transfer,
which is manifested as the coexistence of two isomers with
different location of the unpaired electron. This property
is responsible for the ability of molecular crystals of some
dioxolene complexes to exhibit reversible deformation
upon thermal or photoactivation.9—12 The most important
and interesting results in this area were obtained using
3,6-di-tert-butyl-1,2-benzoquinone and its derivatives as
dioxolene ligands. A series of rhodium(I) dioxolene com-
plexes were synthesized, and their molecular crystals were
shown to form 1D coordination polymers with Rh—Rh
bonds.13—20 Since the steric factors play a significant role
in intermolecular interactions, it seemed important and
interesting to reveal the effect of these factors on the for-
mation of molecular associates (coordination polymers)
as analogs of the rhodium(I) complexes mentioned above.
With this goal in mind, we developed a procedure for the
synthesis of 3,6-diisopropyl-1,2-benzoquinone, the steri-
cally less hindered analog of 3,6-di-tert-butyl-1,2-benzo-
quinone.
Several examples of the introduction of isopropyl
substituents into the catechol ring, the reduced form of
o-quinone, using propylene or isopropanol as the alkylating
agent were reported in the literature. The reaction of
* Dedicated to Academician of the Russian Academy of Sciences
V. N. Charushin on the occasion of his 70th birthday.
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 5, pp. 916—924, May, 2021.
1066-5285/21/7005-0916 © 2021 Springer Science+Business Media LLC

Substituted o-benzoquinones and oxanthrenequinones
Russ. Chem. Bull., Int. Ed., Vol. 70, No. 5, May, 2021
917
Scheme 1
resinification products. The optimal amount of isopropanol
in the reaction mixture (the catechol to isopropanol ratio
was 1 : 2.5) was determined in an effort to diminish the
formation of trisubstituted catechol 3 and to avoid the
formation of a large amount of monoalkylated products.
We used 57% perchloric acid as the catalyst. The reaction
affords a mixture of products. Scheme 1 shows only the
di- and triisopropyl-substituted catechol derivatives, which
were isolated in the individual state.
Monoalkylation products are also present in the reac-
tion mixture, but they are more soluble in water compared
to catechols 1—3, and they are mainly removed from the
organic layer during washing from catalyst residues.
Catechols 1—3 were isolated in the individual state by
column chromatography and recrystallized from hexane.
These compounds are colorless, air-stable crystalline
substances. Compounds 1—3 are difficult to separate by
chromatography. Hence, it is advantageous to perform the
quantitative isolation of reaction products after the oxid-
ation of the reaction mixture with potassium ferricyanide
in an alkaline medium, i.e., in the step of the formation
of o-quinones. The mixture containing compounds 4—6
was separated by column chromatography. All these com-
pounds are crystalline, deeply colored substances. Com-
pounds 1—6 were characterized by physicochemical
methods, such as IR and NMR spectroscopy and elemen-
tal analysis.
The structure of 3,6-diisopropyl-o-quinone 4 was
determined by X-ray diffraction. According to the X-ray
diffraction data, compound 4 is 3,6-diisopropyl-substituted
o-quinone (Fig. 1, Table 1). The C—O distances corre-
spond to double bonds (~1.215(2) Å). The C—C bond
lengths in the six-membered ring are not equalized and
alternate, the C(3)—C(4) and C(5)—C(6) bonds being the
shortest (~1.342(2) Å). The quinone moiety of molecule 4
is planar; the mean deviation of the O(1,2)C(1—6) atoms
from the plane is 0.02 Å. The O(1)—C(1)—C(2)—O(2)
torsion angle is 2.1. It should be noted that the o-quinone
moiety in the related 3,6-di-tert-butyl compound is also
planar.6 The corresponding parameters are 0.005 Å and
1.1, respectively. The isopropyl substituents in compound
4 adopt an eclipsed conformation, like tert-butyl substitu-
ents in o-quinone.26,27 One methyl group lies nearly in
the plane of o-quinone and points toward the hydrogen
atoms at the C(4) and C(5) atoms, while the second methyl
group and the hydrogen atom of the isopropyl group are
directed toward the oxygen atoms O(1) and O(2).
The crystal packing of compound 4 is composed of
centrosymmetric pairs of chains of o-quinone molecules
running along the a axis. One pair of chains is shown in
Fig. 2. In each chain, the molecules are arranged in a head-
to-tail fashion via intermolecular O(1)⋅⋅⋅H(5A) and
C(7)
C(3)
C(4)
C(2)
O(1)
C(5)
O(2)
C(1)
C(6)
C(10)
Fig. 1. Molecular structure of compound 4.

Kocherova et al.
918
Russ. Chem. Bull., Int. Ed., Vol. 70, No. 5, May, 2021
O(2)⋅⋅⋅H(4A) contacts. The lengths of these contacts are
2.57 and 2.48 Å, respectively, which are similar to the
average O⋅⋅⋅H van der Waals length (2.45 Å).28 The chains
in the pair are linked by intermolecular O(1)⋅⋅⋅C(2) con-
tacts (2.97 Å) corresponding to the O⋅⋅⋅C van der Waals
length.28 These contacts are the shortest interactions found
in the crystal of compound 4. Adjacent molecules 4 in two
different chains of the pair are shifted with respect to each
other along the a axis by approximately one-half of the
distance between the centers of the quinone molecules in the
chain, i.e., by one-half of the translation along the a axis.
It is worth noting that only the sterically most hindered
quinone 6 is a stable compound and can be stored under
ambient conditions for a long period of time. Unlike
compound 6, diisopropyl-substituted quinones 4 and 5
are highly reactive and undergo a further transformation
in concentrated solutions and even in the solid state.
During the storage of crystalline samples of quinone 4 at
room temperature, this compound is gradually transformed
into oxanthrene-o-quinone 7. This behavior is well con-
sistent with the X-ray diffraction data (see Fig. 2). The
crystal of 4 contains chains of o-quinone molecules ar-
ranged in a head-to-tail fashion, in which the distances
between the carbonyl oxygen atoms of one quinone mol-
Table 1. Selected bond lengths (d) and bond
angles () of compound 4
Parameter
Bond
Value
d/Å
O(1)—C(1)
O(2)—C(2)
C(1)—C(6)
C(1)—C(2)
C(2)—C(3)
C(3)—C(4)
C(4)—C(5)
C(5)—C(6)
1.2149(14)
1.2145(14)
1.4692(15)
1.5587(16)
1.4712(15)
1.3421(16)
1.4628(16)
1.3420(16)
Angle
/deg
O(1)—C(1)—C(6)
O(1)—C(1)—C(2)
C(6)—C(1)—C(2)
O(2)—C(2)—C(3)
O(2)—C(2)—C(1)
C(3)—C(2)—C(1)
C(4)—C(3)—C(2)
C(3)—C(4)—C(5)
C(6)—C(5)—C(4)
C(5)—C(6)—C(1)
123.49(10)
117.61(10)
118.87(9)
123.63(10)
117.72(10)
118.65(9)
116.77(10)
124.65(10)
123.86(10)
117.07(10)
0
a
H(4A)
H(4A)
H(5A)
O(2)
C(2)
C(2)
O(2) C(2)
H(5A)
O(1)
O(1)
O(1)
O(1)
O(1)
H(5A)
H(4A)
O(1)
H(5A)
H(4A)
C(2)
C(2)
C(2) O(2)
O(2)
b
Fig. 2. Fragment of the crystal packing of compound 4 projected onto the (001) plane with displacement ellipsoids drawn at the 50%
probability level. Hydrogen atoms, except H(4A) and H(5A), are omitted for clarity; the isopropyl substituents are represented by
lines. The atoms involved in intermolecular contacts are numbered.

Substituted o-benzoquinones and oxanthrenequinones
Russ. Chem. Bull., Int. Ed., Vol. 70, No. 5, May, 2021
919
Table 2. Reduction potentials of о-quinones
4—8 measured by cyclic voltammetry*
ecule and the hydrogen atoms at the 4 and 5 positions of
the ring of another molecule are close to the average O⋅⋅⋅H
van der Waals length. The stability of 4 can be increased
by decreasing the storage temperature. Thus, quinone can
be stored up to two months already at –18 C. The labil-
ity of quinone 5 is due to the possibility of the rearrange-
ment to the p-quinone methide derivative10 via the migra-
tion of the methine proton of the 5-isopropyl group to the
carbonyl oxygen atom of the dioxolene site followed by
the transformation of this intermediate. Apart from target
o-quinones 4—6, dibenzo[b,e][1,4]dioxane-2,3-diones
(oxanthrene-o-quinones) 8 and 9, which are the products
of the addition of o-quinone 5 and 4-isopropyl-o-quinone,
respectively, to 3,6-diisopropyl-o-quinone (4), were iso-
lated in the individual state by column chromatography
of the reaction mixture obtained after the oxidation.
o-Quinone
–E1/2(1)
–E1/2(2)
М
4
5
6
7
8
0.41
0.37
0.53
0.59
0.59
0.95
—
0.74
1.00
0.93
* Ag/AgCl/KCl (satur.), MeCN, C =
= 5•10–3 mol L–1
.
properties of o-quinones. Thus, the first wave for both 7
and 8 is observed at –0.59 V. However, the change in the
position of the isopropyl group exerts an effect on the
second reduction wave. Thus, a typical irreversible reduc-
tion wave is observed for compound 7, while the change
in the position of the isopropyl group in quinone 8 leads
to stabilization of the catecholate dianion form, with the
result that the CV curve shows a reversible reduction wave
at 0.93 V.
Except for o-quinone 5, compounds 4, 6, 7, and 8
display stability in different redox forms, which confirms
their potential use as dioxolene-type ligands of variable
valence.
Compound
R
Pri
Pri
H
R´
H
Rʺ
Pri
H
7
8
9
Pri
Pri
H
The cyclic voltammetry (CV) curves for the new
o-quinones vary depending on their structural features.
Sterically hindered o-quinones generally exhibit the first
reversible and second irreversible reduction wave pattern.
The reversibility of the latter is attributed to strong basic-
ity of the resulting catecholate dianion promoting its strong
protonation, as well as disproportionation of radical anion
species into dianions and neutral species.29 This situation
is valid for o-quinones 4 and 6. The first reduction wave
of compound 4 is observed at –0.41 V, and the second one
is at –0.95 V (Table 2). For o-quinone 6, the correspond-
ing parameters are –0.53 and –0.74 V, respectively. For
3,5-disubstituted o-quinone 5, the first reduction wave is
already irreversible. This is indicative of lability of the
resulting radical anion due to its rearrangement to the
p-quinone methide derivative followed by the transforma-
tion of this intermediate.30
The reduction potentials of these o-quinones are shifted
to more anodic values compared to the tert-butyl31 and
cyclohexyl25 analogs described previously, which is con-
sistent with the lower electron-donating ability of iso-
propyl substituents compared to tert-butyl and cyclohexyl
groups.32
The reduction potentials of the oxanthrene derivatives
are shifted to more cathodic values by ~0.15 V compared
to alkyl-substituted o-benzoquinones, the position of the
isopropyl group having no significant effect on the redox
Then we studied the chemical reduction of o-quinones
with alkali metals and evaluated their coordination capa-
bility. All the synthesized o-quinones are easily reduced
with potassium metal. The intensity of the EPR spectrum
of potassium 3,5-diisopropyl-o-benzosemiquinolate in
solution decreases with time. The instability of this com-
pound in solution is apparently associated with insufficient
shielding of the dioxolene site of the ligand and the pos-
sibility of isomerization of the molecule to the quinone
methide form. The intensity of the EPR spectra of singly
reduced o-quinone derivatives 4—8 in degassed solutions
remains unchanged for several hours. Hence, it can be
stated that the stability of radical anions of 3,6-diisopropyl-
substituted o-quinones is comparable with that of the
corresponding 3,6-di-tert-butyl-o-benzoquinones.
The ability to form paramagnetic chelate complexes
with transition metals was demonstrated by the reactions
of the new o-quinones with dimanganese decacarbonyl.
Mixed-ligand manganese semiquinone complexes were
prepared by the treatment of the corresponding carbonyl
adducts with one equivalent of triphenylphosphine33
(Scheme 2).
Table 3 presents the EPR parameters for singly reduced
o-quinone derivatives 4—8 in solution. The hyperfine
coupling (HFC) constants with manganese nuclei in the
tetracarbonyl and phosphine complexes and the splitting
on the phosphorus nucleus in the phosphine adducts are

Kocherova et al.
920
Russ. Chem. Bull., Int. Ed., Vol. 70, No. 5, May, 2021
Scheme 2
i. Toluene, hν.
Table 3. EPR parameters for singly reduced quinone derivatives in solution (293 K)a
Quinone
g-Factor
Fragment
aH/Gb
aM/G
3c
4c
5c
6c
4a
(THF)
2.0047
2.0032
2.0026
2.0047
2.0029
2.0027
2.0045
2.0033
2.0026
2.0048
2.0036
2.0047
2.0036
2.0033
Carom
H
—
3.10
3.10
—
—
—
CHMe2
CHMe2
0.38
—
0.38
—
0.04 (6 H)
—
3.17
—
—
2.90
—
—
3.15
—
—
3.15
—
0.04 (6 H)
—
4b
(toluene)
Carom
H
—
0.15
3.17
—
—
2.90
—
—
—
1.9
—
0.15
7.27 (Mn)
CHMe2
CHMe2
—
—
0.015 (6 H)
0.015 (6 H)
4c
Carom
H
—
0.30
—
0.30
10.57 (Mn)
33.8 (P)
(toluene)
CHMe2
CHMe2
0.03 (6 H)
—
0.64
0.053 (6 H)
—
0.30
0.04 (6 H)
—
0.30
0.03 (6 H)
—
0.03 (6 H)
0.705
—
—
—
—
5а
(THF)
Carom
H
CHMe2
CHMe2
—
—
0.40
0.205 (6 H)
5b
(toluene)
Carom
H
—
2.37
6.70 (Mn)
—
CHMe2
CHMe2
—
—
—
—
0.19 (6 H)
—
5c
Carom
H
3.02
—
2.10
0.19 (6 H)
3.26
—
0.35
9.95 (Mn)
32.1 (P)
(toluene)
CHMe2
CHMe2
—
—
—
—
6a
(THF)
Carom
H
—
—
—
—
—
CHMe2
CHMe2
0.10
—
—
0.10
1.07
0.50
0.45 (6 H)
—
0.05 (6 H)
6b
(toluene)
Carom
H
—
1.15
3.36
—
—
0.39
6.70 (Mn)
CHMe2
CHMe2
0.35 (6 H)
—
0.04 (6 H)
6c
C
aromH
—
0.10
—
—
—
—
—
—
1.20
3.07
—
—
0.40
9.60 (Mn)
33.2 (P)
(toluene)
CHMe2
CHMe2
0.30 (6 H)
—
0.35 (6 H)
7a
(THF)
Carom
H
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
CHMe2
CHMe2
—
—
7b
(toluene)
Carom
H
—
5.30 (Mn)
CHMe2
CHMe2
0.26
—
—
0.26
—
—
—
—
8a
(THF)
C
aromH
—
—
—
—
CHMe2
CHMe2
—
—
—
—
—
—
—
—
8b
(toluene)
Carom
CHMe2
CHMe2
Carom
CHMe2
CHMe2
H
—
—
—
5.33 (Mn)
—
—
—
—
—
—
—
—
8c
H
—
—
—
7.60 (Mn)
27.4 (P)
(toluene)
—
—
—
—
—
—
a The experimental proton HFC constants were refined using the simulated spectra.
b The HFC constants from the aromatic protons at the 3—6 positions and the methine or methyl protons of the isopropyl substituent.
c The positions of aromatic protons that interact with the unpaired electron are given in accordance with the notations in Scheme 1.

Substituted o-benzoquinones and oxanthrenequinones
Russ. Chem. Bull., Int. Ed., Vol. 70, No. 5, May, 2021
921
and manganese semiquinolates were prepared as described pre-
viously.33,35
in the ranges characteristic of analogous derivatives with
other o-quinones described previously.33
Alkylation of catechol. Propan-2-ol (19 mL, 0.25 mol) was
slowly added dropwise with continuous stirring to a melt of
catechol (11 g, 0.1 mol) in the presence of 57% perchloric acid
(2 mL, 0.02 mol) in a flat-bottom flask with a reflux condenser
at 140 C for 2 h. The mixture was stirred for 4 h, cooled, dis-
solved in diethyl ether, and washed with water to a neutral reac-
tion of washing water. The solvent was removed under reduced
pressure using a rotary evaporator. The mixture of catechols was
separated by column chromatography in two steps. Two fractions
were collected using hexane—ethyl acetate (7 : 1) as the eluent:
the first fraction contained a mixture of compounds 1 and 3; the
second one, catechol 2. The repeated column chromatography
of the first fraction using hexane—ethyl acetate (20 : 1) as the
eluent allowed the separation of compounds 1 and 3. All catechols
1—3 were crystallized from hexane.
3,6-Diisopropylbenzene-1,2-diol (1). Colorless crystals, m.p.
73—74 C. IR, /cm–1: 1635 m, 1613 m, 1583 m, 1572 m, 1510
m, 1455 s, 1441 s, 1386 s, 1365 s, 1337 s, 1296 s, 1270 s, 1258 s,
1198 s, 1173 m, 1156 m, 1141 m, 1107 m, 1072 m, 1052 w, 987 s,
958 w, 924 m, 863 m, 806 s, 770 w, 682 m, 640 m, 590 w, 526 m,
497 m. 1H NMR, δ: 1.27 (d, 12 H, Me, J = 6.9 Hz); 3.13 (sept,
2 H, CHMe2, J = 6.9 Hz); 5.13 (s, 2 H, OH); 6.77 (s, 2 H,
CaromH). 13C NMR, δ: 22.6, 27.2, 117.4, 131.9, 140.7. Found (%):
C, 74.02; H, 9.28. C12H18O2. Calculated (%): C, 74.19; H, 9.34.
3,5-Diisopropylbenzene-1,2-diol (2). Colorless crystals, m.p.
90—91 C. IR, /cm–1: 1592 s, 1518 m, 1494 s, 1442 s, 1364 s,
1322 s, 1290 s, 1252 m, 1225 m, 1205 m, 1178 s, 1134 m, 1108 w,
1080 w, 983 s, 945 w, 924 w, 878 w, 859 m, 843 w, 762 m, 747 w,
653 w, 610 w, 540 w, 510 w. 1H NMR, δ: 1.22, 1.28 (both d,
6 H each, Me, J = 6.9 Hz); 2.81, 3.21 (both sept, 1 H each, CHMe2,
J = 6.9 Hz); 5.15, 5.26 (both s, 1 H each, OH); 6.61, 6.66 (both
d, 1 H each, CaromH, J = 1.9 Hz). 13C NMR, δ: 22.7, 24.2, 27.4,
33.7, 110.6, 116.4, 134.8, 139.0, 141.3, 142.8. Found (%):
C, 74.11; H, 9.25. C12H18O2. Calculated (%): C, 74.19; H, 9.34.
3,4,6-Triisopropylbenzene-1,2-diol (3) was described in the
literature.23
The HFC constants on magnetic nuclei of dioxolene
ligands were assigned using the computer simulation of
the EPR spectra. It should be noted that the character of
the spin density distribution in the singly reduced deriva-
tives of 3,5-, 3,6-, and 3,4,6-substituted o-quinones is
similar to that in the cyclohexyl adducts described previ-
ously.25 It is interesting that relatively high HFC constants
on methyl groups of isopropyl substituents are observed
in some cases. For the corresponding cyclohexyl deriva-
tives, the HFC constants on protons at the 2 and 6 positions
of CH2 groups of cyclohexyl rings are not larger than the
EPR linewidth. Most probably, this difference in the
spectra is attributed to the free rotation of methyl groups
in the isopropyl substituent, which increases the possibil-
ity of the interaction between the orbital containing the
unpaired electron and the corresponding protons.
In conclusion, we developed a synthetic procedures
for the preparation of isopropyl-substituted o-quinones,
the dioxolene moiety in which is less shielded compared
to that in the tert-butyl and cyclohexyl analogs. This leads
to high reactivity of the new compounds, making it pos-
sible to produce new o-quinones with the oxanthrene
structure. Despite a decrease in the shielding, the synthe-
sized o-quinones and their reduced derivatives can be used
as redox-active dioxolene ligands for the preparation of
metal complexes.
Experimental
The NMR spectra were recorded on a Bruker Avance III 400
spectrometer operating at 400 MHz (1H) and 100 MHz (13C and
DEPT). The EPR experiments were performed on a Bruker
EMX-8/2,7 spectrometer. The EPR spectra were simulated with
the WinEPR SimFonia v1.25 software (Bruker). The IR spectra
were measured on a Bruker Vertex 70 Fourier-transform infrared
spectrometer equipped with a RAM II Raman module. The el-
emental composition was determined using an Elementar vario
EL cube automated elemental analyzer (Germany).
Synthesis of compounds 4—6. The mixture of alkylated cate-
chols (see above) was dissolved in diethyl ether and then oxidized
with a magnetically stirred solution of K3Fe(CN)6 (100 g), KOH
(5 g), and Na2CO3 (10 g) in water (250 mL). The organic layer
was washed to a neutral reaction and dried. The solvent was
removed. The mixture of o-quinones 4—6 was separated by
column chromatography (hexane—ethyl acetate, 25 : 1, as
the eluent) and crystallization from hexane. The following com-
pounds were isolated: quinone 4 in a yield of 1.54 g (8%), quinone
5 in a yield of 5.74 g (30%), and quinone 6 in a yield of
3.5 g (15%). Crystals of 3,6-diisopropyl-o-quinone (4) suitable
for X-ray diffraction were isolated after the crystallization
from pentane.
The reduction potentials were measured by cyclic volt-
ammetry (CV) in a three-electrode cell using an Elins P-45X
potentiostat under argon. A stationary glassy-carbon electrode
with a diameter of 2 mm was used as the working electrode;
platinum mesh, as the auxiliary electrode; Ag/AgCl/KCl (satu-
rated) with a water-impermeable diaphragm, as the reference
electrode. The potential sweep rate was 100 mV s–1. A 0.1 M
Bu4NClO4 solution (99%, Aldrich), which was twice recrystal-
lized from aqueous EtOH and dried in vacuo at 50 C for 48 h,
served as the background electrolyte. The concentration of
3,6-Diisopropylcyclohexa-3,5-diene-1,2-dione (4). Maroon
crystals, m.p. 49—50 C. IR, /cm–1: 1670 s, 1650 s, 1585 m,
1465 s, 1460 s, 1395 s, 1310 s, 1240 w, 1170 w, 1150 s, 1110 m,
1085 w, 1030 w, 960 m, 945 m, 920 m, 895 s, 870 w, 830 w, 720 w,
o-benzoquinones 4—8 in acetonitrile was 5•10–3 mol L–1
.
1
The solvents were purified and dehydrated according to
standard procedures.34 Commercially available catechol was
used as received. The column chromatography was per-
formed using Silica 60 0.063—0.2 mm Macherey-Nagel adsorb-
ent (Germany). For EPR spectroscopy, samples of potassium
695 w, 675 w, 660 w, 620 m, 465 w. H NMR, δ: 1.08 (d, 6 H,
Me, J = 6.8 Hz); 2.90 (sept, 1 H, CHMe2, J = 6.8 Hz); 6.70
(s, 1 H, CaromH). 13C NMR, δ: 21.5, 27.1, 133.1, 147.2, 180.9.
Found (%): C, 75.03; H, 8.40. C12H16O2. Calculated (%):
C, 74.97; H, 8.39.

Kocherova et al.
922
Russ. Chem. Bull., Int. Ed., Vol. 70, No. 5, May, 2021
3,5-Diisopropylcyclohexa-3,5-diene-1,2-dione (5). Red crys-
Table 4. Crystallographic data and the X-ray diffraction data
collection and refinement statistics for compound 4
tals, m.p. 30—32 C. IR, /cm–1: 1680 s, 1660 s, 1595 m, 1578 s,
1467 s, 1455 s, 1393 s, 1380 s, 1364 m, 1314 s, 1280 m, 1264 s,
1253 s, 1206 m, 1180 m, 1154 m, 1110 m, 1050 s, 1030 m,
1000 w, 968 m, 957 m, 930 m, 914 m, 876 m, 865 s, 816 m, 798 s,
754 w, 736 w, 722 w, 660 w, 648 m, 590 m, 540 m, 505 m, 474 w.
1H NMR, δ: 1.13, 1.19 (both d, 6 H each, Me, J = 6.8 Hz); 2.56,
2.98 (both sept, 1 H each, CHMe2, J = 6.8 Hz); 6.10—6.13,
6.61—6.64 (both m, 1 H each, CaromH). 13C NMR, δ: 20.4,
21.5, 27.4, 34.7, 122.0, 134.5, 148.9, 161.7, 180.3, 180.7.
Found (%): C, 74.90; H, 8.32. C12H16O2. Calculated (%):
C, 74.97; H, 8.39.
Parameter
Value
Molecular formula
Molecular weight
Т/К
Crystal system
Space group
a/Å
C
12H16O2
192.25
100(2)
Monoclinic
P21/n
6.66936(19)
16.9798(5)
9.6860(3)
90
104.229(3)
90
1063.24(6)
4
b/Å
c/Å
3,4,6-Triisopropylcyclohexa-3,5-diene-1,2-dione (6). Maroon
crystals, m.p. 45—47 C. IR, /cm–1: 1680 s, 1650 s, 1625 s,
1562 m, 1465 s, 1400 s, 1387 s, 1367 m, 1360 m, 1328 s, 1305 m,
1290 m, 1252 m, 1200 w, 1165 w, 1148 w, 1124 w, 1095 m,
1079 m, 1028 m, 994 w, 955 w, 943 w, 928 w, 920 w, 906 m,
870 w, 853 w, 838 w, 810 m, 720 w, 683 w, 640 w, 609 w, 584 w,
553 w, 498 w, 482 w, 476 w. 1H NMR, δ: 1.10, 1.12 (both d,
6 H each, Me, J = 6.9 Hz); 1.21 (d, 6 H, Me, J = 7.1 Hz); 2.90
(sept, 1 H, CHMe2, J = 6.9 Hz); 3.08 (sept, 1 H, CHMe2,
J = 7.1 Hz); 3.20 (sept, 1 H, CHMe2, J = 6.9 Hz); 6.72 (s, 1 H,
CaromH). 13C NMR, δ: 20.3, 21.0, 21.5, 26.9, 27.4, 29.3, 133.6,
140.3, 146.9, 153.4, 180.6, 181.4. Found (%): C, 76.88; H, 9.68.
C15H22O2. Calculated (%): C, 76.88; H, 9.46.
1,4,6,9-Tetraisopropyldibenzo[b,e][1,4]dioxane-2,3-dione
(7). o-Quinone 4 (0.4 g, 2 mmol) was kept under temperature
control at 50 C for 24 h. Then the resin was cooled to 5 C,
washed with pentane at this temperature, filtered off, and dried.
The yield of oxanthrene 7 was 0.19 g (48%). Orange crystals,
m.p. 233—234 C (with decomp.). IR, /cm–1: 1670 s, 1655 s,
1636 s, 1588 s, 1317 s, 1295 s, 1260 m, 1230 m, 1138 m, 1126 m,
1105 m, 1077 w, 1015 m, 988 m, 822 m, 786 m, 723 m, 651 w,
643 w, 588 w, 576 w, 532 w. 1H NMR, δ: 1.31 (d, 12 H, Me,
J = 6.9 Hz); 1.34 (d, 12 H, Me, J = 7.1 Hz); 3.44 (sept, 2 H,
CHMe2, J = 6.9 Hz); 3.46 (sept, 2 H, CHMe2, J = 7.1 Hz); 7.08
(s, 2 H, CaromH). 13C NMR, δ: 20.1, 22.7, 24.6, 26.6, 121.4,
124.4, 134.2, 134.9, 148.8, 178.8. Found (%): C, 75.42; H, 7.90.
C24H30O4. Calculated (%): C, 75.36; H, 7.91.
/deg
β/deg
γ/deg
V/Å3
Z
d/g cm–3
1.201
μ/mm–1
0.080
F(000)
416
Crystal size/mm
Scan range, θ/deg
Index ranges, h, k, l
0.57×0.23×0.13
3.24—27.48
–8 ≤ h ≤ 8
–22 ≤ k ≤ 22
–12 ≤ l ≤ 12
Number of reflections
observed
16645
2441
(0.0367)
1.027
unique
(Rint
)
Goodness of fit (F2)
R1 0.0374
wR2 (I > 2σ(I))
0.0931
0.0505
0.1015
R1 (based on all reflections)
wR2 (based on all reflections)
Residual electron density
(ρmax/ρmin)/e Å–3
0.355/–0.190
Synthesis of compounds 8 and 9. A mixture of o-quinones
4—6 was kept under temperature control at 50 °C, and the com-
position was monitored by TLC. After three days, quinone 4 was
not detected in the mixture, but the formation of new products
was observed. Oxanthrenes 8 and 9 were isolated after column
chromatography (hexane—ethyl acetate, 25 : 1, as the eluent)
and crystallization.
1,4,7-Triisopropyldibenzo[b,e][1,4]dioxane-2,3-dione (9).
Orange crystals, m.p. 117—118 C. IR, /cm–1: 1655 s, 1638 s,
1620 s, 1583 s, 1508 s, 1495 m, 1435 m, 1390 s, 1327 s, 1311 s,
1273 s, 1260 s, 1222 m, 1198 m, 1163 m, 1126 m, 1112 s, 1060 w,
985 m, 963 w, 868 m, 848 w, 825 m, 811 w, 773 w, 712 w, 650 m,
629 m, 568 w, 545 w, 456 w. 1H NMR (CDCl3, 400 MHz),
δ: 1.24—1.36 (m, 18 H, Me); 2.95 (sept, 1 H, CHMe2, J = 6.9 Hz);
3.37—3.51 (m, 2 H, CHMe2); 7.05 (dd, 1 H, CaromH, J = 8.3 Hz,
J = 1.7 Hz); 7.11 (d, 1 H, CaromH, J = 1.7 Hz), 7.17 (d, 1 H,
CaromH, J = 8.3 Hz). 13C NMR (CDCl3, 100 MHz), δ: 20.17,
20.21, 24.0, 24.45, 24.49, 33.8, 114.3, 116.4, 123.1, 124.66,
124.73, 135.7, 137.5, 146.6, 149.10, 149.14, 178.6, 178.7.
Found (%): C, 73.95; H, 7.05. C21H24O4. Calculated (%):
C, 74.09; H, 7.11.
X-ray diffraction study of compound 4 was performed on an
Agilent Xcalibur diffractometer (-scanning technique, Mo-K
radiation,  = 0.71073 Å). The X-ray intensity data were inte-
grated using the CrysAlisPro program.36 The structure was solved
by the direct methods and refined by the full-matrix least-squares
method based on F2hkl with anisotropic displacement parameters
for nonhydrogen atoms using the SHELXTL program package.37
1,4,6,8-Tetraisopropyldibenzo[b,e][1,4]dioxane-2,3-dione (8).
Orange crystals, m.p. 122—123 C. IR, /cm–1: 1670 s, 1653 s,
1637 s, 1630 s, 1594 m, 1585 s, 1490 m, 1467 s, 1435 s, 1390 s,
1385 s, 1372 s, 1341 s, 1305 s, 1290 s, 1260 s, 1227 m, 1200 m,
1180 m, 1160 m, 1154 m, 1135 m, 1122 m, 1103 m, 1068 w, 1058 w,
1013 s, 988 w, 974 w, 960 m, 950 w, 934 w, 895 w, 886 w, 855 m,
840 w, 820 w, 808 w, 752 w, 735 w, 695 w, 650 m, 625 w, 595 w,
587 w, 555 w, 517 w, 503 w, 475 w. 1H NMR, δ: 1.27 (d, 6 H,
Me, J = 6.9 Hz); 1.29—1.36 (m, 18 H, Me); 2.92 (sept, 1 H,
CHMe2, J = 6.9 Hz); 3.37—3.51 (m, 3 H, CHMe2); 6.93—6.97
(m, 2 H, CaromH). 13C NMR, δ: 20.1, 20.2, 22.7, 24.0, 24.4,
24.5, 26.9, 34.0, 111.6, 120.2, 124.4, 124.5, 133.1, 136.8, 137.3,
146.0, 149.0, 149.1, 178.69, 178.72. Found (%): C, 75.88; H, 9.78.
C
24H30O4. Calculated (%): C, 75.36; H, 7.91.

Substituted o-benzoquinones and oxanthrenequinones
Russ. Chem. Bull., Int. Ed., Vol. 70, No. 5, May, 2021
923
Hydrogen atoms were positioned geometrically and refined
isotropically. An absorption correction was applied with the
SCALE3 ABSPACK program.38 The crystallographic data and
the X-ray diffraction data collection and refinement statistics are
given in Table 4. The structure was deposited with the Cambridge
Crystallographic Data Centre (CCDC 2049469) and is available
at http://ccdc.cam.ac.uk/getstructures.
14. M. P. Bubnov, V. I. Nevodchikov, G. K. Fukin, V. K.
Cherkasov, G. A. Abakumov, Inorg. Chem. Commun., 2007,
10, 989; DOI: 10.1016/j.inoche.2007.05.011.
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10.1246/cl.2004.970.
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bayashi, T. Yokoyama, H. Tanaka, S.-I. Kuroda, K. A.
Toriumi, Angew. Chem., 2005, 44, 4164; DOI: 10.1002/
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This work was financially supported by the Russian
Science Foundation (Project No. 19-13-00142) using
equipment of the Joint Analytical Center of the G. A.
Razuvaev Institute of Organometallic Chemistry of the
Russian Academy of Sciences.
No human or animal subjects were involved in this
research.
The authors declare no conflict of interest, financial
or otherwise.
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Received March 16, 2021;
in revised form March 29, 2021;
accepted March 31, 2021