t-butyl hydroperoxide oxidative dealkylation of hydroquinone ethers to 1,4-quinones

Article abstract of DOI:10.1080/00397910600634175

Various organoselenium compounds (diselenides, benzisoselenazol-3(2H)-ones, and 1,3,2-thiaselenazolones) are effective catalysts for oxidative dealkylation of hydroquinone ethers with t -butyl hydroperoxide to 1,4-quinones. The reactions is most efficient when t -butyl hydroperoxide in the presence of poly(bis-1,2-phenyl) diselenide is used as an oxidizing agent. In this way 1,4-benzo and 1,4-naphthoquinones are produced in good to high yields. Copyright Taylor & Francis Group, LLC.

Full text of DOI:10.1080/00397910600634175

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t‐Butyl Hydroperoxide Oxidative Dealkylation
of Hydroquinone Ethers to 1,4‐Quinones
Halina Wójtowicz a , Jacek Młochowski a , Ludwik Syper a & Hardeo S. Yadav
b
a Department of Organic Chemistry, Faculty of Chemistry, Wrocław
University of Technology, Wrocław, Poland
b North Eastern Regional Institute of Science and Technology, Niriuli, India
Version of record first published: 20 Aug 2006
To cite this article: Halina Wójtowicz, Jacek Młochowski, Ludwik Syper & Hardeo S. Yadav (2006): t‐Butyl
Hydroperoxide Oxidative Dealkylation of Hydroquinone Ethers to 1,4‐Quinones, Synthetic Communications: An
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Synthetic Communicationsw, 36: 1991–2000, 2006
Copyright # Taylor & Francis Group, LLC
ISSN 0039-7911 print/1532-2432 online
DOI: 10.1080/00397910600634175
t-Butyl Hydroperoxide Oxidative
Dealkylation of Hydroquinone Ethers
to 1,4-Quinones
´
Halina Wojtowicz, Jacek Młochowski, and Ludwik Syper
Department of Organic Chemistry, Faculty of Chemistry, Wrocław
University of Technology, Wrocław, Poland
Hardeo S. Yadav
North Eastern Regional Institute of Science and Technology,
Niriuli, India
Abstract: Various organoselenium compounds (diselenides, benzisoselenazol-3(2H)-
ones, and 1,3,2-thiaselenazolones) are effective catalysts for oxidative dealkylation
of hydroquinone ethers with t-butyl hydroperoxide to 1,4-quinones. The reactions is
most efficient when t-butyl hydroperoxide in the presence of poly(bis-1,2-phenyl) dis-
elenide is used as an oxidizing agent. In this way 1,4-benzo and 1,4-naphthoquinones
are produced in good to high yields.
Keywords: t-Butyl hydroperoxide, 1,4-dimethoxyarenes, hydroquinone ethers,
organoselenium compounds, oxidation
Oxidative dealkylation of hydroquinone ethers (1,4-dialkoxyarenes) is a con-
venient way to synthesize biologically important mena-, plasto-, and ubiqui-
nones and their analogues.[1] In our laboratory some useful methods for
oxidation of 1,4-dialkoxybenzenes and 1,4-dialkoxynaphthalenes, having
different substituents in the aromatic ring, to 1,4-benzo- and 1,4-naphthoqui-
nones have been elaborated. The oxidants were silver(II) oxide, its complexes
with pyridinecarboxylic acids, or ammonium cerium(IV) nitrate (CAN).[2] In
Received in Poland December 16, 2005
Address correspondence to Jacek Młochowski, Department of Organic Chemistry,
´
Faculty of Chemistry, Wrocław University of Technology Wyb. Wyspianskiego 27,
50-370, Wrocław, Poland. E-mail: jacek.mlochowski@pwr.wroc.pl
1991

´
H. Wojtowicz et al.
1992
the modern, practical organic synthesis the reagents of this type are replaced
by other, less expensive and environmentally neutral ones. The most prospec-
tive oxidants, employed in both laboratory and industrial scale, are hydroper-
oxides, particularly aqueous 30% hydrogen peroxide and tert-butyl
hydroperoxide (TBHP). They contain a high proportion of active oxygen
and are commercially available, relatively cheap, and environmentaly
neutral because their reduction products are water or tert-butanol.[3] Neverthe-
less, both of these reagents are inactive or only moderately active toward most
organic substrates, and different promoters such as carboxylic acids, transition
metal oxoacid salts, and complexes as well as selenium(IV) oxide and organo-
selenium compounds should be used.[4] The mechanisms of their action
involve formation of the active intermediates that have hydroperoxy group
at the carbon, metal, or selenium atom.[5]
During the past three decades different organoselenium compounds,
particularly areneseleninic acids, diaryl diselenides, and benzisoselenazol-
3(2H)-ones, have been used as catalysts for hydroperoxide oxidation of a
broad spectrum of different organic substrates.[4] They are odorless, their
toxicity is low, and they may be used in laboratory practice without any
special precautions.[6]
In this work we report that TBHP in the presence of selenium catalysts is a
good reagent for oxidative dealkylation of 1,4-dialkoxyarenes (1). When
alkoxyl is a methyl or methoxymethyl group, the 1,4-quinones (2) are
produced and the reaction has a synthetic value. TBHP was selected as a
oxidant because it is known that in some reactions, it is milder reagent
acting more selectively than hydrogen peroxide.[4]
The catalytic effectiveness of various selenium compounds was evaluated
in the TBHP oxidation of 2-methyl-1,4-dimethoxynaphthalene (1a) to
2-methylnaphthoquinone (2a) as is shown in Scheme 1. TBHP was used in
stoichiometric amounts and the catalyst was used in 5% molar (3, SeO2,
CAN) or 2.5% molar amount (diselenides 4–8). The reaction was carried
out in t-butanol at 80 8C for 24 h. After this period, the solvent was evaporated
1
1
and the residue was analyzed by H NMR. In the H NMR spectra of the
mixture, two different singlet signals of substrate Ar-CH3 group (2.48 ppm)
and product methyl group (2.18 ppm) were observed. Based on their
intensity, the ratio of product–substrate was established and then conversion
1a into 2a, given in Table 1, calculated.
One group of catalysts was benzisoselenazol-3(2H)-ones 3a,e and their
analogues 1,3,2-thiaselenazoles (3b–d), having a sulfonyl group instead of
a carbonyl group in the heterocyclic ring (3b–d). Other catalysts were
diaryl diselenides 4–6 with different substituents in ortho positions,
poly(bis-1,2-phenylene) diselenide (7) cyclic diselenide 8, SeO2, and
(NH4)2Ce(NO2)6. The most effective catalysts were 1,3,2-thiaselenazoles 3b
and 3c, their open-chain analogues 2,20bis(sulfamoylphenyl) diselenides
4b and 4c, 2,20-bis(carbamoylphenyl) diselenide 5a, and particularly
poly(bis-1,2-phenylene) diselenide 7. In the presence of 7, the substrate was

t-Butyl Hydroperoxide Oxidative Dealkylation
1993
Scheme 1.
converted into quinone 2a quantitatively, and after recrystallization from
acetone, pure product was obtained in 90% yield. Well-known selenium
catalysts such as SeO2, ebselen (4a), 2,20bis(2-nitrophenyl) diselenide (6c),
and ammonium cerium(IV) nitrate gave substantially worse results, and
without a catalyst no reaction progress was observed.
It should be noted that poly(bis-1,2-phenylene) diselenide (7) was
employed earlier as a catalyst for hydrogen peroxide oxidation of azomethine
compounds into arenecarboxylic acids[7] and for oxidative transformations of
the aromatic ring in naphthalene.[8] It is easy to obtain by selenenylation
of 1,2-diiodobenzene with dilithium diselenide according to the procedure
elaborated in our laboratory.[7]
The elaborated reagent TBHP/7 was successfully used for preparative
oxidation of six different 1,4-dimethoxyarenes (1a–f) to corresponding 1,4-
naphtho- and benzoquinones (2a–f) (Scheme 2, Table 2). The reaction was
carried out at 80 8C in tert-butanol for 12–72 h, depending on the substrate,
until oxidation was completed and its progress was controlled by TLC. For
2b the reaction progress was very slow and it was stopped after 72 h. The
products were separated directly from the reaction mixture by filtration or
were isolated by recrystallization of the solid residue obtained after evapo-
ration of the solvent from the reaction mixture. The reaction conditions
were optimized. It was found that the catalyst could be used in 1.25%

´
H. Wojtowicz et al.
1994
Table 1. Results of oxidative conversion of
1a into 2a with t-BuOOH/catalyst system
Cat.
Conversion, %
3a
58
88
81
45
53
50
84
90
51
39
91
13
42
29
18
15
11
6
3b
3c
3d
3e
4a
4b
4c
4d
4e
5a
5b
5c
5d
5e
6a
6b
6c
7
100
22
20
51
0
8
SeO2
CAN
None
molar in relation to the substrate, and that the reaction was faster when the
molar ration of TBHP to the substrate was 4.25 : 1.
In this way 1,4-dimethoxynaphthalenes 1a and 1b were oxidized to
corresponding quinones 2a and 2b. From 1,2,4,6-tetramethoxybenzene (1c)
both isomeric 1,2- and 1,4-quinone could be produced but the reaction
proceeded to 1,4-quinone 2c. Oxidation of the compound 1d with two
methoxy and two hydroxymethyl group resulted in demethylation and
formation of 1,4-quinone 2d while the hydroxymethyl groups remained
unchanged. On the other hand, oxidative dehydrogenation of hydroxy
phenolic group proceeded easier than demethylation of methoxy group, and
from 2,4,5-trimethoxyphenol (1e), 2,5-dimethoxy-1,4-benzoquinone (2e)
was formed in high yield. Also, the methoxymethyl group was eliminated
easier than the methyl group, and in consequence 1,4-dimethoxy-2,5-
di(methoxymethyloxy)benzene (1f) gave the quinone 2e, the same as
formed from 1e.
The results presented in this work are close to these obtained when hydro-
quinone ethers are oxidized with Ag(II) or Ce(IV) cations although the

t-Butyl Hydroperoxide Oxidative Dealkylation
1995
Scheme 2.
reactions are slower. Because transition metals on higher valency states were
the oxidants, the reaction proceeds via intermediate cation-radicals[9,11] and
supports our earlier hypothesis that the selenium catalyst may be involved
in the reactions of radical mechanism.[4b,12,13] The plausible mechanism of
Table 2. Results of oxidative conversion of 1,4-dimethoxyarenes 1 into 1,4-quinones
2 with t-BuOOH/7
Reaction
time (h)
Substrate
Producta
Yield (%)
Mp (8C)
Lit. mp (8C)
1a
1b
1c
1d
1e
1f
12
72
24
24
24
24
2a
2b
2c
2d
2e
2e
90
48b
67
103–105
126–127
251 (decomp.)
149–151
299
104–105[2a]
127[9]
251–252[2f]
151–152[1d]
298[10]
84
53
64
299
298[10]
aIR and 1H NMR Spectra were compared to the authentic samples.
bUnreacted substrate 49%.

´
H. Wojtowicz et al.
1996
Scheme 3.
conversion of hydroquinone ethers into quinones involves homolytic cleavage
of hydroperoxy group of peroxyseleninic acid 9 and formation of the radical
10 being a strong electron acceptor (Scheme 3). Taking an electron from the
substrate 12, the radical 10 is converted into anion 11, which oxidized with
TBHP regenerate peroxyacid 9. Removal of one electron from 12 leads to
cation-radical 13 and then to radical 14. The one-electron process is
repeated, and the reaction proceeds via intermediates 15 and 16. In the final
step, two molecules of methanol are eliminated from 16 and quinone 17 is
formed. The analogous mechanism is possible when the hydroxy or meth-
oxymethyloxy group is present instead of the methoxy group.
EXPERIMENTAL
Melting points were determined on Digital Melting-Point Apparatus
Electrothermal IA 9100. 1H NMR were measured with a Bruker 300-
MHz spectrometer. IR (KBr pellets) was measured with a Perkin-Elmer
2000 FT.

t-Butyl Hydroperoxide Oxidative Dealkylation
1997
tert-Butyl hydroperoxide (80% in di-tert-butylperoxide/water 3 : 2) and
other commercial reagents and solvents were purchased from Aldrich Chem.
Co and Fluka and used as received without additional purifications. The
organoselenium catalysts were obtained using the procedures described in
our previous works. 2-Substituted benzisoselenazol-3(2H)-ones 2a,e were
prepared by treatment of 2-(chloroseleno)benzoyl chloride with aniline or
2,20-diaminobiphenyl.[14] In the similar manner 1,3,2-benzothiaselenazoles
3b–d were obtained from 2-(bromoseleno)benzenesulfonyl chloride.[15] Dis-
elenides with carbamoyl group 4a,e or sulfamoyl groups 4b,c, were obtained
in the reaction of di[(chlorocarbonyl)phenyl] diselenide or di[(2-chlorosulfo-
nyl)phenyl] diselenide with amines,[14,15] and acid 4d was obtained from
anthranilic acid.[14b] Di(2-aminophenyl) diselenide 5e was obtained by
reduction of nitro group of di(2-nitrophenyl) diselenide 6c and then
converted into derivatives 5a–d following the procedures reported in
Ref.[16] 2-Chloroanizole treated with dilithium diselenide in THF/HMPT
gave diselenide 6a.[16a] For synthesis of diselenide 6b, the methoxymethyl-
oxybenzene was converted into di(2-methoxymethyloxyphenyl) diselenide
and finally the methoxymethyloxy groups were hydrolyzed to hydroxyl
groups.[17] Poly(bis-1,2-phenylene) diselenide (7) was obtained by
treatment of 1,2-diiodobenzene with dilithium diselenide in aprotic
medium.[7] The 9,10-diselenaphenanthrene (8) was obtained in the reaction
of 2,20-dilithiumbiphenyl[18] with elemental selenium and subsequent
oxidation with air.
Hydroquinone ethers 1 were synthesized following the procedures
reported in the literature: 1a,[19] 1b,[9] 1c,[2f] 1d,[20] 1e,[21] and 1f.[22]
tert-Butyl Oxidation of 2-Methyl-1,4-dimethoxynaphthalene (1a) to
2-Methyl-1,4-naphthoquinone (2a) Catalyzed by Organoselenium
Compounds
To
a
solution of 2-methyl-1,4-dimethoxynaphthalene (1a, 0.404 g,
2 mmol) and the catalyst 3, SeO2, CAN (0.1 mmol) or 4–8 (0.05 mmol)
in tert-butanol (5 ml) 80% tert-butyl hydroperoxide (0.55 ml, 4.25 mmol)
was added in one portion. The reaction mixture was magnetically stirred
under heating to 80 8C for 24 h. After this period the reaction was
stopped by the addition of a pinch of 20% Pd/C, and the mixture was
stirred, at room temperature, for 24 h to decompose unreacted hydroperox-
ide. Then Pd/C was filtered off, and from the filtrate solvent was evapor-
ated in vacuo. The sample of solid residue was taken and dissolved in
CDCl3, and 1H NMR spectrum was measured. The conversion of 1a
to 2a was established from the ratio of the intensivity of two singlet
signals 2.48 ppm (Ar-CH3 of the substrate) and 2.18 ppm (ArCH3 of the
product).

´
H. Wojtowicz et al.
1998
Oxidation of Hydroquinone Ethers (1) to 1,4-Quinones (2) with
tert-Butyl Hydroperoxide Catalyzed by poly(Bis-1,2-phenylene)
Diselenide (7)
The solution of hydroquinone ether 1a–f (10 mmol), in tert-butanol (25 ml),
containing tert-butyl hydroperoxide (5.5 ml, 42.5 mmol) and catalyst 7
(0.028 g, 0.0125 mmol), was magnetically stirred under heating for 24–72 h.
The reaction progress was monitored by TLC using Kieselgel 60 F254
(Merck) plates and hexane–ethylacetate 2 : 1 as an eluent. The spots were
visualized by UV l ¼ 254 nm. After the reaction was finished, crystalline
crude product (2c, 2e) was filtered off and recrystallized from acetone.
When the reaction mixture was clear (2a, 2b, 2d), a pinch of 20% Pd/C
was added, and it was stirred for 24 h at room temperature, filtered, and
from the filtrate solvent was evaporated in vacuo. The solid residue recrystal-
lized from acetone gave pure quinones 2a, 2b, and 2d. The purity of the
1
products was confirmed by comparison of their melting points and H NMR
and IR spectra with data given in literature.
ACKNOWLEDGMENTS
This work was supported by the Polish State Committee for Scientific
Research (Grant No. 3T09A 003 27).
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