Efficient aerobic oxidative synthesis of 2-substituted benzoxazoles, benzothiazoles, and benzimidazoles catalyzed by 4-Methoxy-TEMPO

Article abstract of DOI:10.1002/anie.200803381

(Chemical Equation Presented) Setting the TEMPO: The scheme shows the efficient aerobic synthesis of 2-substituted benzoxazoles, benzothiazoles, and benzimidazoles where 4-methoxy-2,2,6,6-tetramethyl-1-piperidinyloxy free radical (4-Methoxy-TEMPO) is used as the catalyst.

Full text of DOI:10.1002/anie.200803381

Communications
DOI: 10.1002/anie.200803381
Radical Reactions
Efficient Aerobic Oxidative Synthesis of 2-Substituted Benzoxazoles,
Benzothiazoles, and Benzimidazoles Catalyzed by 4-Methoxy-TEMPO**
Yong-Xing Chen, Ling-Feng Qian, Wei Zhang, and Bing Han*
Aerobic oxygenation/oxidation of hydrocarbons, alcohols,
and amines catalyzed by aminoxyl radicals have been
extensively studied.[1] However, its application in the catalytic
oxidative synthesis of heterocycles is rare. Herein we report a
novel and efficient aerobic approach for the synthesis of
2-substitued benzoxazoles, benzothiazoles, and benzimida-
Recently, we reported an efficient approach to the
oxidative dehydrogenation of Hantzsch dihydropyridines
and pyrazolines using N-hydroxyphthalimide (NHPI) as the
catalyst.[16] As an extension of this chemistry, we attempted to
synthesize 2-aryl benzoxazoles 3 by the aerobic oxidation of
Schiff base 2-(4-chlorobenzylideneamino)-phenol (4), which
was obtained from the condensation of 2-aminophenol (1)
and aldehyde 2, using NHPI as the catalyst. The mechanism as
shown in Scheme 1 was assumed to be operative.
zoles by using
a one-pot reaction of aldehydes with
2-aminophenole, 2-aminothiophenol, and o-phenylenedia-
mine, respectively, and an organic aminoxyl radical as the
catalyst.
Five-membered heterocyclic rings, such as benzoxazoles,
benzothiazoles, and benzimidazoles, are present in natural
products, and in synthetic pharmaceutical and agrochemical
compounds.[2] These compounds have been extensively stud-
ied for their biological and therapeutic activities, such as a
cathepsin S inhibitor,[3] a HIV reverse transcriptase inhib-
itor,[4] an anticancer agent,[5] and an orexin-1 receptor
antagonist.[6]
Classical methods for the synthesis of benzoxazoles
involve two approaches. One is the copper-catalyzed intra-
molecular ortho arylation of o-haloanilides or the intermo-
lecular domino annulations of o-arylhalides with acyl-
amides.[7] The second approach is the condensation of
2-aminophenol with either carboxylic acid derivatives under
strong acid/high temperature conditions,[8] or aldehydes with
subsequent oxidation using strong oxidants such as
2,3-dichloro-5,6-dicyano-l,4-benzoquinone (DDQ),[9] PhI-
Scheme 1. A proposed mechanism for the NHPI-catalyzed aerobic
oxidative synthesis of 2-aryl benzoxazole.
(OAc)2,[10]
(PCC),[12] and BaMnO4.
Th+ClO4
,
pyridiniumchlorochromate
À
[11]
[13]
Recently, catalytic oxidative
reactions using oxygen as the terminal oxidant have received
much attention because of their green chemistry and atom
economy aspects. The aerobic catalytic synthesis of benzox-
azoles promoted by activated carbon[14] or copper nano-
particles[15] has recently been reported. However, these
reactions require the use of large amounts of the catalyst
(50 wt.% of special activated carbon) or excess base, and the
yields were not satisfactory. Therefore, a more effective and
environmentally friendly process is needed.
We supposed that if benzoxazoline 5 was in equilibrium
with imine 4, the former could be easily oxidized to
benzoxazole 3 by a hydrogen abstraction from the activated
À
C H bond of the benzoxazoline. This sequence could be
initiated by a phthalimide-N-oxyl radical (PINO) generated
in situ from the oxidation of NHPI by oxygen. Although
several reaction conditions were tried to accomplish this
reaction, only one gave target compound benzoxazole 3 albeit
in unsatisfactory yield at high temperature using PhCN as the
solvent (Table 1, entries 1–10). High temperature and PhCN
as the solvent were needed because the cyclization of imine 4
to benzoxazoline 5 was difficult at low temperature. Notably,
4-chlorobenzaldehyde oxime was obtained as a product at
relatively low temperature using CH3CN as the solvent
(Table 1, entries 3 and 4). To validate our proposal,
o-phenylenediamine (1e) was used instead of 2-aminophenol
(1) and the reaction was complete in acetonitile at 808C to
give 2-(4-chlorophenyl)-1H-benzimidazole in 83% yield. This
yield results from the much higher nucleophilicity of the
ortho-amine group compared to that of the ortho-hydroxy
group for the cyclization to the imine.
[*] Y.-X. Chen, L.-F. Qian, Dr. W. Zhang, Dr. B. Han
State Key Laboratory of Applied Organic Chemistry and Department
of Chemistry, Lanzhou University
222 Tianshui Street S., Lanzhou 730000 (P. R. China)
Fax: (+86)931-891-2582
E-mail: hanb@lzu.edu.cn
[**] We are grateful to the National Natural Science Foundation of China
(Grant No. 20472027) for financial support. We also thank Dr. Wei
Yu for helpful discussions. TEMPO=2,2,6,6-tetramethyl-1-piperidi-
nyloxy free radical.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200803381.
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Table 1: Effect of aminoxyl radicals on the oxidative synthesis of 2-aryl
benzoxazole.[a]
Entry Catalyst [mol%]
Solvent
CH3CN
T [8C] Yield [%][b]
1
NHPI (10)
NHPI/Co(OAc)2 (10/0.5) CH3CN
NHPI/Co(OAc)2 (10/0.5) CH3CN
80
RT
80
80
80
0
0
2[c]
3
0[d]
0[d]
0
4
5
NHPI (100)
NHPI (10)
CH3CN
CH3CO2C2H5
6
NHPI (10)
CH3CO2C4H9-n 120
0
7
8
9
NHPI (10)
NHPI (10)
NHPI (10)
NHPI (10)
CH3CH2OH
PhCN
PhCN
80
120
150
150
80
RT
RT
80
0
trace
15
71
10[e]
11
12[c]
13[c]
14
15
16
17
18
19[f]
20
21[e]
22
23
PhCN
higher yields than polar solvents under the same reaction
TEMPO (5)
TEMPO/CuBr (5/0.5)
TEMPO/CuBr (10/1)
TEMPO (5)
TEMPO (5)
TEMPO (10)
TEMPO (5)
TEMPO (5)
TEMPO (10)
TEMPO (5)
TEMPO (1)
TEMPO (5)
0
CH3CN
CH3CN
CH3CN
CH3CO2C2H5
18
conditions; the solvent polarity affects the stability of the
resonance structure of 4-methoxy-TEMPO radical and is also
responsible for its reactivity with the substrate[18] (Table 1,
entries 11, 14, 16, and 17). In addition, temperature signifi-
cantly affected the reaction; high temperatures accelerated
the rate of the hydrogen abstraction between 4-methoxy-
TEMPO radical and the substrate, and the rate at which
4-methoxy-TEMPO radical was regenerated (Table 1,
entries 17–19 and 22). Finally, we found that 4-methoxy-
TEMPO/xylene catalytic system is best. All the conditions we
optimized are listed in entries 11–23 in Table 1.
trace
trace
13
23
15
21
43
95
95
CH3CO2C4H9-n 120
CH3CH2OH
Benzene
toluene
80
80
110
120
120
120
RT
xylenes[g]
xylenes[g]
xylenes[g]
xylenes[g]
xylenes[g]
96
5
0
120
Notbaly, if 1 mol% of 4-methoxy-TEMPO was used, the
reaction was complete in 15 hours, resulting in benzoxazole 3
in 96% yield. We checked the tunrover number (TON) and
turnover frequency (TOF) values for different amounts of
4-methoxy-TEMPO radical (Table 2). The use of 1 mol% of
4-methoxy-TEMPO radical gave the best TON and TOF
values.
[a] 2-aminophenol (1; 1 mmol) and aldehyde 2 (1 mmol) were dissolved
in solvent (3 mL) in a 25 mL three-necked flask and stirred for 1 h at
different temperatures. The catalyst was then added and the mixture
stirred under an oxygen atmosphere for another 5 h. [b] 1H NMR
analysis. [c] Used 1 mol of Schiff base
4 as the substrate.
[d] 4-Chlorobenzaldehyde oxime was obtained as the product. [e] After
15 h. [f] After 2.5 h. [g] Mixture of o-, m-, and p-xylene.
The use of 5 mol% of 4-methoxy-TEMPO radical in the
following reactions was aimed at reducing the reaction times.
A variety of 2-aminophenol derivatives (1a–1c) and alde-
hydes 2 were used in this oxidative cyclization by the in situ
formation of Schiff bases, leading to the synthesis of
2-substituted benzoxazoles (3a–3o; Table 3).
Aryl aldehydes gave excellent yields of 2-aryl benzox-
azoles, and aliphatic aldehydes participated in the reaction to
give the corresponding 2-alkyl benzoxazoles in good yields.
When heptylaldehyde was used in the reaction, two benzox-
azoles, 3i and 3j, were obtained in 20% and 35% yields,
respectively (based on 2-aminophenol; Table 3, entry 9).We
believe that 4-methoxy-TEMPO radical initially interacts
with 2-hydroxybezoimine[19] and initiates the reaction with a
Next, we turned our attention to another aminoxyl
radical,
4-methoxy-2,2,6,6-tetramethyl-1-piperidinyloxy
(4-methoxy-TEMPO) free radical, which is widely used for
the selective oxidation of primary or secondary alco-
hols.[1c–d,17] We initially used stoichiometric amounts of
TEMPO+ClO4 as the oxidant to perform this reaction.
À
However, the starting material, 2-hydroxy-bezoimine 4, was
oxidatively decomposed to the corresponding aldehyde and
2-aminophenol (1); the strong oxidizing ability of TEMPO+
caused the latter to be oxidized to give polymers, so only a
trace amount of benzoxazole 3 was observed. However, when
a stoichiometric amount of 4-methoxy-TEMPO radical was
used as the oxidant instead of TEMPO+, target molecule
benzoxazole 3 was produced quantitatively in 48 hours at
room temperature, indicating that 4-methoxy-TEMPO radi-
cal is a privileged oxidant in the oxidative dehydrogenation of
Schiff base 4 [Eq. (1) and Eq. (2)].
4-methoxy-TEMPOH can be slowly oxidized to
4-methoxy-TEMPO radical in air at room temperature, so a
strategy for the aerobic catalytic oxidative synthesis of
benzoxazole using 4-methoxy-TEMPO radical as the catalyst
was developed.
À
hydrogen abstraction from the O H bond of the phenol
moiety[20] to produce phenoxyl radical 7 and 4-methoxy-
Table 2: TON and TOF values for 4-methoxy-TEMPO radical catalyzed
oxidative synthesis of 2-aryl benzoxazole.[a]
Entry
4-methoxy-TEMPO [mol%]
t [h]
TON
TOF [hÀ1
]
1
2
3
10
5
1
2.5
5
15
10
20
100
4
4
6.7
To optimize the reaction conditions we used various
solvents and discovered that nonpolar aromatic solvents gave
[a] Calculated values based on Table 1, entries 19–21.
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Table 3: 4-methoxy-TEMPO radical catalyzed aerobic oxidative synthesis
of 2-substituted benzoxazoles.[a]
Entry
R1
R2
t [h]
Yield [%][b]
1
2
3
4
5
6
7
8
H (1a)
H (1a)
H (1a)
H (1a)
H (1a)
H (1a)
H (1a)
H (1a)
H (1a)
Cl (1b)
Cl (1b)
NO2 (1c)
NO2 (1c)
NO2 (1c)
4-ClC6H4
Ph
4-NO2C6H4
2-CH3C6H4
4-CH3OC6H4
E-C6H5CH CH
2-furyl
(CH3)3C
CH3(CH2)5
4-CH3OC6H4
4-NO2C6H4
2-CH3C6H4
4-ClC6H4
(CH3)3C
5
5
6
5
4
9
8
1
3
13
10
11
17
6
95 (3a)
90 (3b)
91[c] (3c)
96 (3d)
95 (3e)
82 (3 f)
88 (3g)
=
88 (3h)
9
20, 35[d] (3i,j)
95 (3k)
10
11
12
13
14
92[c] (3l)
93[c] (3m)
93[c] (3n)
91[c] (3o)
[a] 2-aminophenol (5 mmol) , aldehyde (5 mmol) and xylenes (15 mL;
mixture of o-, m-, and p-xylene) was placed in a 100 mL three-necked flask
and stirred at 1208C for 0.5 h. 4-methoxy-TEMPO radical (5 mol%;
47 mg, 0.25 mmol) was then added to the mixture and stirred for several
hours under an oxygen atmosphere. [b] Yield of isolated product
obtained after purificantion by silica gel column chromatography
unless noted otherwise. [c] Yield of isolated product obtained after
recrystallization. [d] Two compounds were obtained and structures are
shown below.
Scheme 2. A proposed mechanism for 4-methoxy-TEMPO-catalyzed
aerobic oxidative synthesis of 2-substituted benzoxazoles.
Table 4: 4-methoxy-TEMPO radical catalyzed aerobic oxidative synthesis
of 2-substituted benzothiazoles and benzimidazoles.[a]
TEMPOH which can be reoxidized to 4-methoxy-TEMPO
radical by oxygen. Phenoxyl radical 7 is additionally stabilized
Entry
X
R
t [h]
Yield [%][b]
1
2
3
4
S (1d)
S (1d)
S (1d)
S (1d)
NH (1e)
NH (1e)
NH (1e)
NH (1e)
Ph
9
11
7
80 (3p)
4-ClC6H4
4-NO2C6H4
(CH3)3C
Ph
4-ClC6H4
4-CH3OC6H4
2-Furyl
85 (3q)
87[d] (3r)
77 (3s)
3
5[c]
6[c]
7[c]
8[c]
11
10
7
91[d] (3t)
91[d] (3u)
89[d] (3v)
90[d] (3w)
13
[a] 2-aminothiophenol or o-phenylenediamine (5 mmol), aldehydes
(5 mmol), and xylenes (15 mL; mixture of o-, m-, and p-xylene) was
placed in a 100 mL three–necked flask and stirred at 1008C for 0.5 h.
4-methoxy-TEMPO radical (5 mol%; 47 mg, 0.25 mmol) was added to
the mixture which was then stirred for several hours under an oxygen
atmosphere.[b] Yield of isolated product after purification by silica gel
column chromatography unless noted otherwise.[c] At 1208C.[d] Yield of
isolated product obtained by recrystallization.
by the imine moiety and then undergoes intramolecular
5-endo cycloaddition to the imine to form the corresponding
aminyl radical 8. The driving force for aromatization renders
the second hydrogen abstraction, between the aminyl radical
8 and 4-methoxy-TEMPO/or oxygen, very effective and
eventually yield target compound benzoxazole 3. A proposed
catalytic cycle is shown in Scheme 2.
Having successfully achieved the aerobic oxidative syn-
thesis of benzoxazoles, we expanded the catalytic system to
the oxidative synthesis of benzothiazoles and benzimidazoles
using 2-amino-thiophenol (1d) or o-phenylenediamine (1e)
respectively with aldehydes as starting materials. As shown in
Table 4, benzothiazoles and benzimidazoles were produced in
high yields.
thiazoles, and benzimidazoles can be obtained by using a one-
pot reaction of aldehydes with 2-aminophenole, 2-amono-
thiophenol and o-phenylenediamine, respectively, with
4-methoxy-TEMPO radical as the catalyst. The extension of
this catalytic system for the preparation of other useful
heterocycles is underway in our laboratory.
In conclusion, the reaction of 4-methoxy-TEMPO radical
with 2-benzylideneaminophenols provides a novel and effi-
cient approach for the aerobic catalytic oxidative synthesis of
various heterocycles. The 2-substituted benzoxazoles, benzo-
Experimental Section
Aerobic synthesis of benzoxazole 3a (Table 3, entry 1): 2-amino-
phenol (546 mg, 5 mmol) and 4-chlorobenzaldehyde (703 mg,
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5 mmol) were placed in a 100 mL three-necked flask containing
xylenes (15 mL; mixture of m-, o-, and p-xylene). The reaction
mixture was heated to 1208C for 0.5 h with stirring. 4-methoxy-
TEMPO (47 mg, 5 mol%) was then added to the mixture which was
then stirred under an oxygen atmosphere for 5 h. When the starting
materials were completely consumed as determined by TLC analysis,
the reaction mixture was concentrated under vacuum and the product
was isolated after purification using silica gel column chromatography
as a white crystalline solid (1.100 g ; 95%). The identity and purity of
the product was confirmed by 1H and 13C NMR spectroscopic
analysis.
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