New sterically-hindered catechols/o-benzoquinones. Reduction of 4,6-di-tert-butyl-2,3-dihydroxybenzaldehyde

Article abstract of DOI:10.1016/j.mencom.2016.11.032

Reduction of aldehyde group in 4,6-di-tert-butyl-2,3-dihydroxybenzaldehyde leads to methoxymethyl (NaBH₄, MeOH) or methyl (Zn, MeOH) analogues, which were further oxidized into the corresponding o-benzoquinones. Their photostability in benzene

Full text of DOI:10.1016/j.mencom.2016.11.032

Mendeleev
Communications
Mendeleev Commun., 2016, 26, 552–554
New sterically-hindered catechols/o-benzoquinones.
Reduction of 4,6-di-tert-butyl-2,3-dihydroxybenzaldehyde
Maxim V. Arsenyev,* Eugene V. Baranov, Margarita P. Shurygina,
Sergey A. Chesnokov and Gleb A. Abakumov
G. A. Razuvaev Institute of Organometallic Chemistry, Russian Academy of Sciences, 603950 Nizhnii Novgorod,
Russian Federation. Fax: +7 831 462 7497; e-mail: mars@iomc.ras.ru
DOI: 10.1016/j.mencom.2016.11.032
But
But
OH
OH
O
O
Reduction of aldehyde group in 4,6-di-tert-butyl-2,3-dihydroxy-
benzaldehyde leads to methoxymethyl (NaBH4, MeOH) or
methyl (Zn, MeOH) analogues, which were further oxidized
into the corresponding o-benzoquinones. Their photostability
in benzene increases substantially on replacement of the 6-posi-
tioned Me substituent with the CH2OMe group.
NaBH4
[O]
[O]
MeOH, H+
But
But
But
OH
OH
OMe
OMe
But
But
But
OH
OH
O
O
Zn
MeOH, H+
But
But
O
Me
Me
Alkylated catechols and their oxidized analogues, o-quinones,
have been of interest for past few decades. The most explored
o-quinones, 3,5- and 3,6-di-tert-butyl-o-benzoquinones (3,5-Q
and 3,6-Q, respectively), are used as non-innocent ligands1 and
inhibitors or photoinitiators of free-radical polymerization.2 The
usual access to substituted sterically hindered catechols/o-quinones
Apparently, ‘catecholbenzyl’ alcohol A is initially formed
(Scheme 2). This compound, by the analogy with well known
o-hydroxybenzyl alcohol, can be a precursor for o-quinomethide
by the thermo- or photoinduced dehydration.12 However, the
formation of free o-quinomethide from A is unlikely under the
reaction conditions (mixture of methanol and 20% aqueous solu-
tion of H2SO4, 25°C). The more likely intermediate is a protonated
form of corresponding o-quinomethide B, whose resonance
structure is carbocation B' (see Scheme 2). Trapping methanol
molecule gives ether 2. The cation B' and its quinomethide, by
analogy with 2,6-di-tert-butyl-p-quinomethide,13 can be consti-
tutive particles for the formation of new derivatives of 3,5-di-
tert-butylcatechol. This synthetic way with the use of cation B'
may be realized without protective groups in contrast to tradi-
tional methods for obtaining new catechols.14
is addition of O-, N-, S-, C-nucleophiles to o-quinones. For example
,
4-C-substituted 3,6-di-tert-butylcatechols were synthesized by the
nucleophilic addition of organometallic compounds3 or C–H
acids4 to 3,6-Q. The corresponding three-alkylated quinones would
transform to p-quinomethide form (for example, 4-methyl-3,6-Q3)
since substituent at 4-position contains a labile hydrogen atom.
The natural three-alkylated quinones (e.g., celastrol,5 taxodione,6
kendomycin7) and the oxidized forms of three-alkylated catechols
(e.g., barbatusol,8 carnosol9) also exist in the p-quinomethide
form.
But
But
Recently, three-alkylated catechols10 were synthesized from
3,5-di-tert-butylcatechol by the Mannich reaction. The relative
o-quinones are stable only in dry solution in the dark.
Previously, we have synthesized 4,6-di-tert-butyl-2,3-di-
hydroxybenzaldehyde 1 which seems to be a universal precursor
for polyfunctional sterically hindered catechols.11 The aldehyde
group in 1 appreciably influences spectral and chemical properties
of both catechol and the corresponding quinone. Reduction of
aldehyde group should lead to localization of catechol p-system,
so this method claims to be a simple synthetic way to three-
alkylated catechols.
Herein, we report on the synthesis of two new three-alkylated
catechols and two o-quinones from aldehyde 1 (Scheme 1).
Treatment of 1 with NaBH4 in methanol leads to a discoloration
of reaction mixture. Instead of expected benzylic alcohol, methyl
ether 2 was isolated after the addition of aqueous H2SO4 solution
to the reaction mixture at room temperature.†
i, NaBH4
ii, H2SO4, H2O
K3Fe(CN)6,
KOH, Et2O
OH
OH
O
O
But
But
But
OH
OH
OMe
2 (95%)
OMe
4 (38%)
But
CHO
1
But
But
OH
OH
O
O
K3Fe(CN)6,
KOH, Et2O
Zn, H2SO4
But
But
Me
Me
3 (65%)
5 (45%)
Scheme 1
was dried in vacuo to give light yellow powder. Yield 5.09 g (95%),
1
mp 56–57°C. H NMR (200 MHz, CDCl3) d: 8.10 (s, 1H, OH), 6.87
†
3,5-Di-tert-butyl-6-(methoxymethyl)catechol 2. 4,6-Di-tert-butyl-2,3-di-
(s, 1H, HAr), 5.97 (s, 1H, OH), 4.95 (s, 2H, CH2), 3.52 (s, 3H, OMe), 1.42
and 1.38 (s, 2×9H, But). 13C NMR (50 MHz, CDCl3) d: 144.55, 142.08,
137.61, 133.96, 118.58, 115.55, 71.17, 58.19, 35.35, 34.92, 32.13, 29.45.
IR (Nujol, n/cm–1): 3528, 3418, 3239 (OH), 1620, 1571 (CAr–C), 1261,
1057 (C–O), 889, 869. Found (%): C, 72.02; H, 9.90. Calc. for C16H26O3
(%): C, 72.14; H, 9.84.
hydroxybenzaldehyde15 1 (5.0 g, 20 mmol) was dissolved in methanol
(200 ml) and NaBH4 (4×0.38 g, 40 mmol) was added to the solution. The
mixture was stirred for 30 min, then the 20% H2SO4 solution (100 ml) was
added. The product was extracted with hexane (3×50 ml). The extract
was dried with Na2SO4. The solvent was evaporated and the product
© 2016 Mendeleev Communications. Published by ELSEVIER B.V.
on behalf of the N. D. Zelinsky Institute of Organic Chemistry of the
Russian Academy of Sciences.
– 552 –

Mendeleev Commun., 2016, 26, 552–554
manifest themselves in 13C NMR spectra: at 182.48, 179.57 and
But
182.99, 180.24 ppm for 4 and 5, respectively. The UV spectra
reveal two characteristic bands with maxima at 403 nm (lge =
= 3.39) and 580 nm (lge = 1.85) for 4 and at 421 nm (lge = 3.27)
and 576 nm (lge = 1.85) for 5, which can be attributed to p–p*
and n–p* transitions, respectively.16
The single crystals of quinones 4 and 5 were grown and
investigated by X-ray analysis (Figure 1).¶ The bond distances
and angles in O(1,2)C(1–6) fragments for 4 and 5 are typical
of o-quinones: the values of C–O distances are characteristic of
a double bond, and the explicit alternation of short and long C–C
distances in six-membered ring C(1–6) is observed.
OH
OH
H+
[H]
1
But
OH
A
But
But
OH
OH
OH
OH
MeOH
– H+
2
But
But
CH2
CH2
The o-quinone fragment in 4 is more planar than that in 5.
Atoms O(1) and O(2) of 4 lie in the C(1–6) plane and these atoms
deviate out from six-membered ring at 0.009(1) and 0.003(1) Å,
respectively. The corresponding values of deviation for 5 are
0.159(1) and 0.102(1) Å, respectively. The torsion angles O(1)–
C(1)–C(2)–O(2) are 0.5° for 4 and 8.3° for 5. The distances of
C(5)–But bonds [1.5456(11) (4) and 1.5473(10) Å (5)] are longer
than C(3)–But [1.5263(11) (4) and 1.5271(10) Å (5)]. Apparently,
this fact represents steric repulsion between tert-butyl group at
C(5) and substituent at C(6). Methoxy group C(16)O(3) in 4 is
located out of the quinone plane and torsion angle C(1)–C(6)–
C(15)–O(3) is 77.8(1)°.
All quinones undergo the photodegradation in aprotic solvents
under both the UV and visible light irradiation. It was demon-
strated on the series of 3,6-Q derivatives that the o-quinones
photorearrange to bicyclo[3.1.0]hex-3-ene-2,6-dione 6 under
light with l > 520 nm [the absorption band corresponding to the
B
B'
Scheme 2
Treatment of 1 with Zn/H2SO4 in methanol causes reduction
of aldehyde group into a methyl one and affords catechol 3 in high
yield (see Scheme 1).‡ Ether 2 also forms under such conditions
in amount less than 5% (NMR data).
Changing C6-positioned aldehyde group in 1 to CH2OMe and
Me groups in catechols 2 and 3, respectively, shifts dramatically
the signals of phenolic protons in 1H NMR spectra. The signals
of O1H1 and O2H2 protons are observed at 10.71 and 5.99 ppm
for 1;15 8.10 and 5.97 ppm for 2; 5.53 and 4.81 ppm for 3. These
values indicate decreasing H-bonding between H1 and O3 for
compounds 1 and 2 and the absence of such H-bond in com-
pound 3.
Oxidation of catechols 2 and 3 with K3Fe(CN)6 at pH > 7
leads to new o-quinones 4 and 5 in 38% and 45% yields, respec-
tively (see Scheme 1).§ The catechol fragment is oxidized easier
than a substituent at C6 under these conditions. Three charac-
teristic vibration bands are observed at 1610–1680 cm–1 in IR
spectra of quinones 4 and 5. Two signals of carbonyl groups
(a)
(b)
O(2)
O(2)
C(2)
C(3)
C(3)
C(5)
C(2)
C(4)
C(4)
C(5)
C(1)
O(3)
‡
3,5-Di-tert-butyl-6-methylcatechol 3. The water solution of H2SO4
C(1)
O(1)
O(1)
(50%, 25 ml) was added dropwise to the methanolic solution (200 ml) of
4,6-di-tert-butyl-2,3-dihydroxybenzaldehyde 1 (5.0 g) with Zn (granular,
13 g, 0.2 mol) with vigorous stirring for 2 h. The colour of the solution
changed from bright yellow to pale brown. Methanol was evaporated and
the product was extracted with hexane. The extract was dried with Na2SO4.
The solvent was evaporated. The crude product (~4.7 g) contained 5% of
compound 2. Pure product was obtained by the recrystallization from
hexane, white crystals, yield 3.2 g (68%), mp 88–90°C. 1H NMR (200 MHz,
CDCl3) d: 6.95 (s, 1H, HAr) , 5.53 (s, 1H, OH), 4.81 (s, 1H, OH), 2.41 (s,
3H, Me), 1.44 and 1.42 (s, 2×9H, But). 13C NMR (50 MHz, CDCl3) d:
142.61, 140.89, 139.08, 132.13, 120.00, 116.12, 35.70, 34.69, 31.40,
29.84, 14.51. IR (Nujol, n/cm–1): 3160–3400 (OH), 1616 (CAr–C), 852.
Found (%): C, 76.12; H, 10.35. Calc. for C15H24O2 (%): C, 76.23; H, 10.24.
C(6)
C(6)
C(16)
C(15)
C(15)
Figure 1 Molecular structures of o-quinones (a) 4 and (b) 5. Thermal
ellipsoids are given with 30% probability. Hydrogen atoms are not shown.
¶
X-ray diffraction studies. The crystals suitable for X-ray analysis were
grown in hexane. The X-ray data were collected on an automatic Agilent
Xcalibur E diffractometer (graphite-monochromated, MoKa-radiation,
w-scan technique, l = 0.71073 Å) at 100 K. The intensity data were
integrated by CrysAlisPro19 program. SADABS20 for 4 and SCALE3
ABSPACK21 for 5 were used to perform area-detector scaling and absorp-
tion corrections. The structures were solved by dual-space method with
SHELXT program22 and were refined on F2 using SHELXTL23 package.
All non-hydrogen atoms were found from Fourier syntheses of electron
density and were refined anisotropically. The hydrogen atoms were placed
in calculated positions and were refined in the riding model.
§
3,5-Di-tert-butyl-6-methoxymethyl-o-benzoquinone 4. The solutions of 2
(2.68 g, 10 mmol) in Et2O and K3Fe(CN)6 (16.45 g, 50 mmol) with KOH
(1.16 g, 20 mmol) in water (200 ml) were vigorously stirred for 30 min.
Then the mixture was washed with water (3×100 ml) and the ethereal
layer was dried with Na2SO4. The solvent was evaporated and the product
was crystallized from hexane solution (5 ml). The powder was filtered,
washed with cold hexane and dried in vacuo. Red-green crystals, yield
1.01 g (38%), mp 43–45°C. 1H NMR (200 MHz, CDCl3) d: 7.08 (s, 1H,
CH), 4.36 (s, 2H, CH2), 3.35 (s, 3H, OMe), 1.38 and 1.23 (s, 2×9H,
But). 13C NMR (50 MHz, CDCl3) d: 182.48, 179.57, 159.71, 148.66,
137.12, 132.59, 63.87, 58.45, 38.70, 35.41, 30.02, 29.10. IR (Nujol, n/cm–1):
1676, 1653, 1614 (C=O and C=C), 1230, 1098 (CH2OMe). Found (%):
C, 72.60; H, 9.19. Calc. for C16H24O3 (%): C, 72.69; H, 9.15.
3,5-Di-tert-butyl-6-methyl-o-benzoquinone 5 was synthesized in the
same manner as compound 4. Green crystals, yield 1.05 g (45%),
mp 47–49°C. 1H NMR (200 MHz, CDCl3) d: 7.06 (s, 1H, CH) , 2.09 (s,
3H, Me), 1.34 and 1.22 (s, 2×9H, But). 13C NMR (50 MHz, CDCl3)
d: 182.99, 180.24, 154.96, 146.41, 137.29, 133.55, 37.32, 35.19, 30.02,
29.17, 13.82. IR (Nujol, n/cm–1): 1674, 1646, 1612 (C=O and C=C).
Found (%): C, 76.78; H, 9.50. Calc. for C15H22O2 (%): C, 76.88; H, 9.46.
Crystal data for 4. C16H24O3, M = 264.35, crystal size 0.5×0.4×0.1 mm,
monoclinic, space group P21/c, at 100 K: a = 10.5604(4), b = 13.2743(4)
and c = 11.6629(4) Å, b = 114.921(4)°, V = 1482.73(10) Å3, Z = 4,
dcalc = 1.184 g cm–3, m = 0.08 mm–1, F(000) = 576, 2q = 60.9°, Rint
=
= 4491 (0.0482), R1 [I > 2s(I)] = 0.0382, wR2 (all data) = 0.1054. Largest
diff. peak and hole, e/Å3 = 0.407/–0.189.
Crystal data for 5. C16H22O2, M = 234.32, crystal size 0.5×0.3×0.3 mm,
monoclinic, space group P21/n, at 100 K: a = 9.5020(2), b = 12.0120(2)
and c = 12.7785(3) Å, b = 109.101(2)°, V = 1378.21(5) Å3, Z = 4, dcalc
=
= 1.129 g cm–3, m = 0.073 mm–1, F(000) = 512, 2q = 60°, Rint = 4004
(0.0377), R1 [I > 2s(I)] = 0.038, wR2 (all data) = 0.110. Largest diff. peak
and hole, e/Å3 = 0.446/–0.168.
CCDC 1479163 and 1479164 contain the supplementary crystallographic
data for this paper. These data can be obtained free of charge from The
Cambridge Crystallographic Data Centre via http://www.ccdc.cam.ac.uk.
– 553 –

Mendeleev Commun., 2016, 26, 552–554
(c) G. A. Abakumov, O. N. Mamysheva, V. A. Muraev, V. D. Tikhonov,
R
O
V. K. Cherkasov and S. A. Chesnokov, RF Patent 2138070, 1999;
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3 (a) G.A.Abakumov,V. K. Cherkasov, L. G.Abakumova, N. O. Druzhkov,
V. I. Nevodchikov, Yu. A. Kurskii and N. P. Makarenko, Metalloorg.
Khim., 1991, 4, 925 (in Russian); (b) G. A. Abakumov, V. K. Cherkasov,
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O
O
– CO
R
O
7
hv
R
Reactions with
3-membered ring
O
6
Scheme 3
S(n®p*) electronic transition of carbonyl groups]. Further the
intermediate 6 undergoes a decarbonylation with a formation
of the corresponding cyclopentadienone 7 (in the case of 3,6-Q
derivatives), or other reactions related to the opening of three-
membered ring (in the case of 3,5-di-tert-butyl-o-benzoquinone)17
(Scheme 3). The irradiation of 4 and 5 in benzene solutions leads
to products which cannot be unambiguously identified by NMR
spectroscopy. The investigation of photodegradation of 3,6-Q and
o-quinones 4 and 5 in benzene†† showed that the effective rate
constants of photolysis of these quinones are 1.24×10–5, 0.48×10–5
and 2.3×10–5 s–1, respectively, and thus, the ratio of these values
is 1:0.4:1.8. In other words, the photostability (l > 520 nm) of
o-quinone 4 in benzene solution is 4.5 times higher as compared
to 5 and two times higher than for 3,6-Q. The reason for such
differences in the o-quinone photostability has not been unambi-
guously established. Earlier, it was demonstrated on the series of
3,6-Q derivatives, that one of photostability criteria is a coplanarity
of o-quinone fragment, in particular, the value of torsion angle
between carbonyl groups:18 increasing value of this torsion angle
results in decreasing photostability of o-quinone. The values of
torsion angle O(1)–C(1)–C(2)–O(2) are 1.1, 0.5 and 8.3° for
3,6-Q, 4 and 5, respectively. Thus, this structural parameter can
be used for prediction of photostability of different o-benzo-
quinones at a qualitative level.
4 (a) G. A. Abakumov, V. I. Nevodchikov, N. V. Zaitova, N. O. Druzhkov,
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In conclusion, two new three-alkylated catechols, 3,5-di-tert-
butylcatechol derivatives, and the corresponding o-quinones have
been synthesized. The proposed mechanism of the reactions
involves the formation of relatively stable catecholbenzylic carbo-
cation as intermediate. Photostability of new o-quinones in benzene
correlates with the value of torsion angle between carbonyl groups.
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The equipment of the center for collective use ‘Analytical
Center IOMC RAS’ (Nizhnii Novgorod) was used in this study.
This work was supported by the Russian Science Foundation
(project no. 15-13-00137).
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†† The kinetic studies of o-benzoquinones photodegradation were carried
out by UV spectroscopy using o-quinone absorption band. The benzene
solutions of o-quinones (5×10–4 mol dm–3) were degassed, saturated with
Ar, and irradiated in air (lamp KGM-24-150, illuminance 16 kLx, filter
ZhS-16, l > 520 nm). The effective rate constants of o-quinone photo-
degradation were determined by the slope of the straight section depending
ln([Q0]/[Qt]) vs. t, where [Q] is the concentration of o-benzoquinones to
exposure and t is the total exposition time of irradiation of the solution.
The values of the effective rate constants were averaged in three dimen-
sions at the convergence of the results within 10%.
20 G. M. Sheldrick, SADABS-2014/2. Bruker/Siemens Area Detector Absorp-
tion Correction Program, Bruker AXS, Madison, USA, 2012.
21 SCALE3 ABSPACK: Empirical Absorption Correction, CrysAlisPro Soft-
ware Package, Agilent Technologies, Oxford, UK, 2012.
22 G. M. Sheldrick, Acta Crystallogr., Sect. C, 2015, 71, 3.
23 G. M. Sheldrick, SHELXTL v. 6.14, Structure Determination Software
Suite, Bruker AXS, Madison, USA, 2003.
Received: 17th May 2016; Com. 16/4938
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