Synthesis of naphthalenediols by aerobic oxidation of diisopropylnaphthalenes catalyzed by N-hydroxyphthalimide (NHPI)/α, α′-azobisisobutyronitrile (AIBN)

Article abstract of DOI:10.1002/adsc.200303168

Naphthalenediols were successfully synthesized in a one-pot reaction through the oxidation of diisopropylnaphthalenes with air catalyzed by N-hydroxyphthalimide (NHPI) combined with α,α′- azobisisobutyronitrile (AIBN) followed by decomposition with sulfuric acid. Thus, the oxidation of 2,6-diisopropylnaphthalene with air (20 atm) in the presence of AIBN (3 mol %) and NHPI (10 mol %) in CH₃CN at 75°C for 21 h followed by treatment with 0.3 M H₂SO₄ gave 2,6-naphthalenediol in 92% yield.

Full text of DOI:10.1002/adsc.200303168

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Synthesis of Naphthalenediols by Aerobic Oxidation of
Diisopropylnaphthalenes Catalyzed by N-Hydroxyphthalimide
(NHPI)/a,a'-Azobisisobutyronitrile (AIBN)
Yasuhiro Aoki, Satoshi Sakaguchi, Yasutaka Ishii*
Department of Applied Chemistry, Faculty of Engineering, Kansai University, Suita, Osaka 564 8680, Japan
Fax: ( ‡ 81)-6-6339-4026, e-mail: ishii@ipcku.kansai-u.ac.jp
Received: October 11, 2003; Accepted: January 28, 2004
hydroperoxy-1-methylethyl)naphthalene (4) and 2-iso-
Abstract: Naphthalenediols were successfully syn-
propyl-6-(1-hydroperoxy-1-methylethyl)naphthalene
thesized in a one-pot reaction through the oxidation
(5).[4] In a previous paper, we have reported the aerobic
of diisopropylnaphthalenes with air catalyzed by N-
oxidation of methylbenzenes like toluene and xylene to
hydroxyphthalimide (NHPI) combined with a,a'-
the corresponding carboxylic acids using N-hydroxyph-
azobisisobutyronitrile (AIBN) followed by decom-
thalimide (NHPI) as a key catalyst.[5] In addition, we
position with sulfuric acid. Thus, the oxidation of 2,6-
reported the epoxidation of olefins with ethylbenzene
diisopropylnaphthalene with air (20 atm) in the
hydroperoxide, generated in situ from the aerobic
presence of AIBN (3 mol %) and NHPI (10 mol %)
oxidation of ethylbenzene by NHPI, in the presence of
in CH3CN at 758C for 21 h followed by treatment
[6]
Mo(CO)6. This fact shows that the NHPI-catalyzed
with 0.3 M H2SO4 gave 2,6-naphthalenediol in 92%
oxidation of alkylbenzenes can be applied to the
yield.
synthesis of alkyl hydroperoxides from alkylbenzenes.[7]
ly, Sheldon et al. have reported phenol synthesis by
Keywords: homogeneous catalysis; hydroperoxides,
means of the NHPI-catalyzed aerobic oxidation of
N-hydroxyphthalimide; oxidation; radicals
cyclohexylbenzene.[8]
We now report an efficient synthesis of naphthalene-
diols via aerobic oxidation of diisopropylnaphthalenes
catalyzed by NHPI and AIBN.
Phenols and naphthols are an important class of com-
The aerobic oxidation of 1 was chosen as a model
pounds needed for the syntheses of dyes, pharmaceut- reaction and was carried out in the presence of catalytic
icals and polymers. In particular, naphthalenediols are amounts of AIBN and NHPI under various conditions
essential components of intelligent polymers such as (Table 1).
engineering plastics and liquid crystalline polymers. 2,6-
Naphthalenediol (2) has attracted much attention for its
chemical and physical properties as a liquid crystalline
monomer material. A classical process for the produc-
tion of 2 is based on the sulfonation of naphthalene
followed by alkali fusion of the resulting sulfonates.[1]
However, 2 is difficult to synthesize in high yield by this
method, which results in a complex mixture of several
sulfonated products. It has been reported that the
reaction of b-naphthol with hydrogen peroxide in the
presence of a Lewis acid like SbF5 produces 2, but this
ꢀ1†
method also brings about a mixture of several regioiso- Compound 1 was allowed to react under O2 (1 atm) in
meric naphthalenediols.[2] There have been several the presence of AIBN (3 mol %) and NHPI (10 mol %)
patents on the synthesis of 2 by the oxidation of 2,6- in CH3CN (5 mL) at 758C for 21 h followed by treat-
diisopropylnaphthalene (1) followed by treatment of ment of the reactants with 0.3 M sulfuric acid at room
the resulting 2,6-bis(2-hydroxy-2-propyl)naphthalene temperature for 2 h, leading to 2 in 81% yield along with
with H2O2 and H2SO4.[3]
6-isopropyl-2-naphthol (3) (18%) (run 1). This shows
The phenol synthesis through cumene hydroperoxide that 1 is completely oxidized with O2 by NHPI to give
obtained by the aerobic oxidation of isopropylbenzene selectively 4 upon treatment with sulfuric acid leading to
is well known as the Hock process. Recently, this 2. When the reaction was carried out using 5 mol % or
methodology was applied to the synthesis of 2,6-bis(1- 1 mol % of NHPI, a considerable amount of 3 was
Adv. Synth. Catal. 2004, 346, 199 202
DOI: 10.1002/adsc.200303168
¹ 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
199

Yasuhiro Aoki et al.
Yield [%][b]
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Table 1. Oxidation of 1 with O2 by NHPI under various conditions.[a]
Run
NHPI (mol %)
Additive (mol %)
Solvent
Conv. [%]
2
3
1
2
3
10
5
1
AIBN (3)
AIBN (3)
AIBN (3)
AIBN (3)
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
AcOH
AcOEt
PhCN
CH3CN
CH3CN
> 99
> 99
78
19
No Reaction
81
28
81
67
19
n.d.
18
23
50
19
4
0
5
10
10
10
10
10
10
10
10
6[c]
7
Co(OAc)2 (0.5)
NO2 (0.5)
AIBN (3)
AIBN (3)
AIBN (3)
AIBN (3)
AIBN (3)
n.d.
n.d.
n.d.
58
57
92
6
3
1
19
16
1
8
9
10
11[d]
12[e]
5
> 99
> 99
> 99
93
55
37
[a]
1 (3 mmol) was allowed to react with O2 (1 atm) in the presence of NHPI and additive in CH3CN (5 mL) at 758C for 21 h,
followed by the addition of 0.3 M H2SO4 (1 mL) at 258C for 2 h.
Based on 1 used.
6 (20%) and 7 (10%) were obtained.
Under air (20 atm).
For 3 h.
[b]
[c]
[d]
[e]
formed (runs 2 and 3). Removing of NHPI from the solvent, the employment of acetic acid in the present
oxidation system resulted in lower conversion (run 4). oxidation under these conditions led to a considerable
The oxidation did not take place in the absence of AIBN decrease of the conversion of 1, and the yields of 2 and 3
under these conditions (run 5). Although the oxidation were very low. This is believed to be due to the fact that
of 1 proceeded by adding a very small amount of the resulting hydroperoxides 4 and 5 react with acetic
Co(OAc)2 in place of AIBN, the selectivity to 2 and 3 acid to form 2 and 3 that serve as radical inhibitors in the
was drastically decreased (run 6). This is because the Co aerobic oxidation of 1. The same oxidation in AcOEt
ions promote the redox decomposition of the resulting and PhCN afforded 2 and 3 in somewhat lower yields
hydroperoxides like 5 to 2-(2-hydroxy-2-propyl)-6-iso- than in CH3CN. From an industrial viewpoint, it is
propylnaphthalene (6) (20%) and its dehydrated prod- important to run the oxidation with air instead of pure
uct (7), etc.
oxygen. Thus, the oxidation of 1 under air (20 atm) was
In a previous paper, we showed that NO2 can abstract examined and found to afford 2 in high yield (92%)
the hydrogen atom from the hydroxyimide moiety of the together with a very small amount of 3 (1.4%) (run 11).
NHPI to generate phthalimide N-oxyl (PINO) which When the reaction was stopped after 3 h under these
abstracts the hydrogen atom from the isopropyl moiety conditions, 2 and 3 were obtained in 55% and 37%
of 1.[8] Thus, the oxidation of 1 was examined by a yields, respectively, at 93% conversion of 1 (run 12).
combination of NHPI with NO2 instead of AIBN
In order to obtain information on the reaction course
(run 7). However, the reaction proceeded in low con- of the present reaction, the oxidation of 1 with O2
version to result in undesired products. This may be (1 atm) under the influence of AIBN and NHPI at
attributed to in situ formation of HNO2 and/or HNO3 758C was followed as a function of time (Fig. 1).
from NO2 in the course of the reaction since these acids
induce the decomposition of 5 to 3, which inhibits the converted at 3 h to 4 and 5 which led to 2 and 3 upon
radical chain reaction. treatment with sulfuric acid, respectively. Subsequently,
From Figure 1 it can be seen that about 93% of 1 was
We next examined the effect of several solvents in the 5 was slowly oxidized to 4 and the yield of 2 reached a
present oxidation of 1 (runs 8 10). In contrast to the maximum at 21 h.
NHPI-catalyzed aerobic oxidation of methylbenzenes
An extension of this oxidation to 2,7-diisopropyl-
to benzoic acids by NHPI where acetic acid was the best naphthalene (8) with O2 (1 atm) in the presence of
200
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Adv. Synth. Catal. 2004, 346, 199 202

Synthesis of Naphthalenediols by Aerobic Oxidation
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plate or KBr press disk. A GLC analysis was performed with a
flame ionization detector using a 0.2 mm  25 m capillary
column (OV-17). Mass spectra were determined at an ionizing
voltage of 70 eV. All starting materials, catalysts, and initiators
were purchased from commercial sources and used without
further treatment. The yields of products were estimated from
the peak areas based on the internal standard technique.
General Procedure for the Oxidation of
Diisopropylnaphthalenes to the Naphthalenediols
An acetonitrile (5 mL) solution of the diisopropylnaphthalene
(3 mmol), AIBN (3 mol %), and NHPI (10 mol %) was placed
in a two-necked flask equipped with a balloon filled with O2.
The mixture was stirred at 758C for 21 h. The reaction mixture
was treated with 0.3 M H2SO4 in CH3CN (1 mL) at 258C for
2 h. Removal of the solvent under reduced pressure afforded a
cloudy solution, which was purified by column chromatogra-
phy on silica gel (n-hexane) to give the corresponding product.
The products were characterized by 1H and 13C NMR, IR, and
GC-MS, respectively.
Figure 1. Time-dependence curves for the aerobic oxidation
of 1. Conditions: 1 (3 mmol) was allowed to react under O2
(1 atm) in the presence of AIBN (3 mol %), NHPI
(10 mol %) in CH3CN (5 mL) at 758C, followed by the
treatment with 0.3 M H2SO4.
NHPI (10 mol %) and AIBN (3 mol %) in CH3CN
(5 mL) at 758C for 21 h, followed by treatment with
0.3 M H2SO4 afforded 2,7-naphthalenediol (9) in 67%
yield and 7-isopropyl-2-naphthol (10) (21%)
[Eq. (2)].The same oxidation under pressure of air
(20 atm) led to 9 in high yield (90%) along with a trace
amount of 10.
Procedure for the Oxidation of
Diisopropylnaphthalenes under Air (20 atm)
An acetonitrile (5 mL) solution of the diisopropylnaphthalene
(3 mmol), AIBN (3 mol %), NHPI (10 mol %) was placed in a
50-mL teflon-coated autoclave, and 20 atm of air were
charged. The solution was stirred at 758C for 21 h. The
reaction mixture was treated with 0.3 M H2SO4 in CH3CN
(1 mL) at 258C for 2 h. The work-up was performed by the
same method as described above. The products were identified
by 1H NMR, 13C NMR, IR and GC-MS, respectively.
1
10: H NMR : d ˆ 7.70 7.00 (m, 6H), 4.94 (br, 1H), 3.07
2.97 (m, 1H), 1.32 (d, J ˆ 7.2 Hz, 6H); 13C NMR : d ˆ 153.2,
147.0, 134.7, 129.4, 127.6, 127.5, 123.4, 122.6, 116.7, 109.2, 34.3,
‡
24.0: HRMS (EI): calcd. for C10H8O2 [M À H] : 160.0531;
found: 160.0524.
ꢀ2†
Acknowledgements
In conclusion, diisopropylnaphthalenes were found to
be efficiently converted into the corresponding dihy-
droxynaphthalenes by the aerobic oxidation by NHPI in
the presence of AIBN followed by treatment with
sulfuric acid. The reduction of the amount of the catalyst
employed is required for industrial application of the
present method. Further investigations are underway in
our laboratory.
This work was partially supported by a Grant-Aid for Scientific
Research (KAKENHI) (S) (No. 13853008) from Japan Society
for the Promotion of Science (JSPS), and DAICEL Chemical
Industries, Ltd.
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¹ 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
asc.wiley-vch.de
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