Copper-Catalyzed Aerobic Oxidative C(aryl)-OH Bond Functionalization of Catechols with Amines Affording Benzoxazoles

Article abstract of DOI:10.1002/adsc.201500515

The first copper-catalyzed aerobic oxidative C(aryl)-OH bond functionalization of catechols is reported. Under air, at room temperature, in the presence of a copper catalyst, catechols react with amines to produce the corresponding benzoxazoles in high yields. Mechanistic studies suggest that the reaction proceeds via an intriguing competitive oxidation and a synergetic catalysis between catechol and amine.

Full text of DOI:10.1002/adsc.201500515

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DOI: 10.1002/adsc.201500515
À
Copper-Catalyzed Aerobic Oxidative C(aryl) OH Bond Func-
tionalization of Catechols with Amines Affording Benzoxazoles
Xiuling Chen,+a,b Fangyan Ji,+a Yalei Zhao,a Yanxi Liu,a Yongbo Zhou,a,*
Tieqiao Chen,a and Shuang-Feng Yina,*
a
State Key Laboratory of Chemo/Biosensing and Chemometrics, College of Chemistry and Chemical Engineering, Hunan
University, Changsha 410081, Peopleꢀs Republic of China
Fax: (+86)-731-8882-1171; e-mail: zhouyb@hnu.edu.cn or sf_yin@hnu.edu.cn
College of Chemistry and Chemical Engineering, Jinzhong University, Jingzhong, 030619, Peopleꢀs Republic of China
b
+
These authors contributed equally to this work.
Received: May 28, 2015; Published online: September 10, 2015
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201500515.
Abstract: The first copper-catalyzed aerobic oxida- reaction proceeds via an intriguing competitive oxi-
À
tive C(aryl) OH bond functionalization of catechols dation and a synergetic catalysis between catechol
is reported. Under air, at room temperature, in the and amine.
presence of a copper catalyst, catechols react with
amines to produce the corresponding benzoxazoles Keywords: aerobic oxidative; benzoxazoles; C(aryl)
À
in high yields. Mechanistic studies suggest that the OH bond functionalization; copper catalysis
Introduction
structures in many natural products,[9] biologically,
therapeutically active molecules[10] and functional ma-
The preparation of a complex molecule via the effi- terials.[11] Although many efficient methods for the
cient functionalization of an aryl group is a hot topic synthesis of functionalized benzoxazoles have been
in organic synthesis. For this purpose, transition developed,[5a,b,12,13,14] in most cases, they suffer from
metal-catalyzed cross-couplings of haloarenes are one or more drawbacks, such as requirement of stoi-
most frequently used.[1] Phenols are readily available chiometric or excess amounts of strong oxidants,
materials.[2] Coupling reactions using phenols to re- strong acids or bases, or high temperatures, thus have
place the haloarenes are of current interest. However, narrow substrate scope and poor compatibility with
despite extensive studies,[3] all such reactions have to sensitive functional groups.
be conducted by using phenol derivatives,[4–8] rather
than phenols themselves, i.e., the direct cross-coupling
À
of the C(aryl) OH bond has not been achieved be- Results and Discussion
À
cause the large activation barrier for the C(aryl) OH
bond prevents it from undergoing oxidative addition In a typical experiment, 3,5-di-tert-butylcatechol 1a
to a metal catalyst.[3a]
and isobutylamine 2a were stirred in the presence of
Herein, we disclose the first copper-catalyzed aero- Cu(OAc)2 under air at 258C for 2 h to produce 5,7-di-
bic direct coupling of catechols with amines affording tert-butyl-2-isopropylbenzoxazole 3a in 64% yield
benzoxazoles with a high generality and tolerance for (Table 1, entry 1). Preliminary screening of the cata-
a broad range of functional groups [Eq. (1)]. Benzox- lysts indicated that CuBr was the best catalyst for this
azoles, especially 2-substituted benzoxazoles, are key reaction (Table 1, entries 2–5). The effect of the sol-
vents was also investigated (Table 1, entries 6–8), and
DCE was found to be the best choice. A copper cata-
lyst was crucial for the current reaction and only
a trace amount of 3a was detected in the absence of
the catalyst (Table 1, entry 9). By prolonging the reac-
tion time a higher yield of the product (90%, Table 1,
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Copper-Catalyzed Aerobic Oxidative C(aryl) OH Bond Functionalization
Table 1. Optimization of the reaction conditions.[a]
Table 2. Different amines with 3,5-di-tert-butylcatechol.[a]
Entry
Catalyst
Solvent
Yield [%][b]
1
2
3
4
5
6
7
8
Cu(OAc)2
CuBr2
CuBr
CuCl
CuI
CuBr
CuBr
CuBr
–
DCE
DCE
DCE
DCE
DCE
CHCl3
EA
toluene
DCE
DCE
64
75
85
79
76
47
72
61
9
trace
90
10[c]
CuBr
[a]
Reaction conditions: 1a (0.2 mmol), 2a (0.24 mmol), cata-
lyst (0.02 mmol, 10 mol%), solvent (2.0 mL), under air,
258C, 2 h.
Isolated yield based on 1a.
6 h.
[b]
[c]
entry 10) was obtained. It was noted that the other
possible isomer 4,6-di-tert-butyl-2-isopropylbenzoxa-
zole 3a’ was not detected at all from the reaction (for
details, see the Supporting Information).
The substrate scope of the copper-catalyzed aerobic
couplings of catechols and amines was investigated
with the optimal reaction conditions. As shown in
Table 2, aliphatic amines 2a–2d reacted smoothly with
1a to produce the corresponding alkyl-substituted
benzoxazoles 3a–3d in high yields (81–90%; Table 2,
entries 1–4). Phenylethylamine 2e also worked well as
the substrate, producing benzyl-substituted benzoxa-
zole 3e in 78% yield (Table 2, entry 5). On treatment
of 1a with methylamine (water solution), 2f could
give 5,7-di-tert-butylbenzoxazole 3f in 60% yield
(Table 2, entry 6). Primary amines 2g–2n bearing vari-
ous functional groups, such as trifluoromethyl, hy-
droxy, ester, alkenyl, alkynyl, amino groups, were all
tolerated in this catalytic oxidation system, generating
the 2-functionalized benzoxazoles 3g–3n in satisfacto-
ry yields (61–90%; Table 2, entries 7–14). Remarka-
bly, when propargylamine 2k was employed as the
substrate, 2-ethynyl-substituted benzoxazole 3k was
selectively elaborated in 66% yield with the alkynyl
group untouched (Table 2, entry 11). When diamino
compounds 2l–2n were treated with 1a, only the pri-
mary amine participated in the amination and annula-
tion reaction to furnish the corresponding benzoxa-
zoles 3l–3n in good yields (71–80%; Table 2, en-
tries 12–14).
In addition to alkyl-substituted benzoxazoles, 2-
aryl-substituted benzoxazoles 3o–3v, with a variety of
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Xiuling Chen et al.
[a]
Reaction conditions: catechol 1a (0.2 mmol), primary
amine 2 (0.24 mmol), CuBr (0.02 mmol, 10 mol%), DCE
(2.0 mL), air, 258C, 6 h. Isolated yield.
Amine (0.4 mmol), 408C for 16 h.
[b]
[c]
12 h.
functional groups on the phenyl rings, could be readi-
ly achieved in high yields from benzylamines 2o–2v
(77–91%; Table 2, entries 15–22). Excitingly, heteroar-
yl methanamines 2w and 2x were also applicable to
the present transformation, and the corresponding 2-
heteroaryl-substituted benzoxazoles 3w and 3x were
obtained in 88% and 70% yields respectively
(Table 2, entries 23 and 24). Note that the more steri-
cally hindered naphthyl group was smoothly intro-
duced to the benzoxazole structure, giving the naph-
thyl-substituted benzoxazole 3y in 89% yield (Table 2,
entry 25). The reaction of 1a with isopropylamine,
À
with only one C H bond a to the nitrogen atom
being involved, provided 5,7-di-tert-butyl-2,2-dimeth-
yl-2,3-dihydrobenzoxazole 3z in 63% yield (Table 2,
entry 26). When tert-butylamine was used as substrate,
no desired product was observed. The structure of 3x
was unambiguously confirmed by single-crystal X-ray
crystallography (Figure 1; for details, see the Support-
ing Information).[19] X-ray analysis showed that the
oxidative amination occurred selectively at the meta-
position of tert-butyl groups, due to its higher electro-
philicity in the reaction intermediate this bond was
À
transformed into a C(aryl) N bond.
Substituted catechols also worked well under the
reaction conditions as shown in Table 3. On treating
diisopropylcatechol 1b with 2a, the desired product
3z1 was produced in 82% yield (Table 3, entry 1).
When 4-tert-butylcatechol 1c was used as the sub-
strate, the phenolic hydroxy group in its meta position
was also selectively dehydroxylated, and the corre-
sponding products 3z2–3z4 were furnished in high
yields (80–90%, Table 3, entries 2–4). In contrast to
bulky substituents, when 4-methyl- and 3-methoxy-
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Copper-Catalyzed Aerobic Oxidative C(aryl) OH Bond Functionalization
took place concomitantly. Thus, benzoxazoles 3z5 and
3z6, with the amino group being incorporated into the
phenyl ring, were obtained in 61% and 65% yields,
respectively (Table 3 entries 5 and 6). Such a transfor-
mation could produce amino group-functionalized
benzoxazoles from simple substrates in one pot,
which was not observed previously. However, when
the electron-withdrawing group substituted catechol
À
1h and 2a were used as substrates, C O bond cleav-
age was not observed under the present reaction con-
ditions (Table 3, entry 7).
To get an insight on the reaction mechanism, sever-
al control experiments were performed as shown
below. The reaction of 3,5-di-tert-butyl-o-benzoqui-
none Ia with 2o was conducted under the standard re-
action conditions, a 90% yield of 3o was obtained in
only 3 h [Eq. (2)]. In absence of the copper catalyst,
the reaction of 3,5-di-tert-butyl-o-benzoquinone Ia
with benzylamine 2o was conducted, yielding 3o and
3o’ in 42% and 55% yields, respectively [Eq. (3)].
Indeed, Ia could be observed in the presence of other
amines [Eq. (4)].[15] The above results indicated that
oxidation of catechol to o-benzoquinone was the key
step in this reaction system. The in situ-generated Ia
was trapped by benzylamine 2o, and then transformed
into benzoxazole 3o under the standard reaction con-
ditions. Reaction of 2o with 20 mol% 1a generated 4a
in 75% yield (based on 2o), whereas, in the absence
of catechol derivative 1a, the oxidation of 2o to 4a
did not proceed [Eq. (5)]. This result indicated that
the catechol derivatives 1 worked as organic cofactors
for the catalytic oxidation of amines instead.[16] Final-
ly, the reaction of 4a with 2 equiv. of 1a was per-
formed, which gave 3o in 44% yield, with a nearly
stoichiometric amount of benzaldehyde being re-
leased [42% yield, Eq. (2)]. Imine 4a was probably
hydrolyzed to amine 2o and aldehyde, then 2o reacted
with catechol 1a to give benzoxazole 3a. Whereas,
under the standard reaction conditions, only a trace
amount of benzaldehyde was detected, suggesting
that the oxidation of primary amine to imine 4a was
suppressed in the reaction system.
Figure 1. Single-crystal X-ray structure of compound 3x.[19]
Table 3. Different catechols with different amines.[a]
According to literature reports[16b,17] and the experi-
mental results above, a plausible reaction process for
the synthesis of benzoxazoles is proposed as shown in
Scheme 1 (solid line). First, with the aid of amine 2,
oxidation of catechol 1 catalyzed by copper salt gives
the key intermediate ortho-quinone I, followed by
a nucleophilic addition of primary amine to the car-
bonyl group on the meta-position of I to generate in-
termediate II. Subsequently, II is dehydrated to give
imine III. Then imine III tautomerizes to the Schiff
base IV. Next, the Schiff base is converted to benzox-
azoline V via an addition–cyclization reaction. Finally,
[a]
Reaction conditions: substituted catechol 1 (0.2 mmol),
primary amine
2
(0.24 mmol), CuBr (0.02 mmol,
10 mol%), DCE (2.0 mL), air, 258C, 6 h. Isolated yield.
2a (0.6 mmol).
[b]
substituted catechols 1d and 1e were subjected to the aerobic oxidative dehydrogenation of benzoxazoline
reaction system, a dehydrogenative cross-coupling re- V leads to the corresponding benzoxazole 3. The oxi-
À
À
action of the C(aryl) H bond and the N H bond dation system involves a competitive oxidation and
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Xiuling Chen et al.
Scheme 1. Proposed mechanism.
vide a general and concise method for the preparation
of benzoxazoles but also open an avenue for catalytic
À
transformations of the C(aryl) OH bond.
Experimental Section
General Experimental Procedure for the Synthesis of
Benzoxazoles
A 10-mL Schlenk-type tube equipped with a magnetic stir
bar was charged with catechol 1 (0.2 mmol), primary amine
2 (0.24 mmol) and CuBr (10 mol%), under an air atmos-
pheres, DCE (2 mL) was added at room temperature, and
then the reaction mixture was stirred at 258C for 6 h. The
reaction was monitored by GC or GC-MS. After completion
of the reaction, the resulting solution was neutralized with
saturated NH4Cl solution. The product was extracted with
EtOAc or CHCl3, dried over anhydrous Na2SO4 and concen-
trated under vacuum. The crude product was purified by
flash column chromatography on silica gel to give the ana-
lytically pure product.
a synergetic catalysis between catechol and amine. An
easier oxidation of substituted catechol derivative
1 relative to primary amine is required, which makes
the oxidation of 1 to the ortho-quinone preferentially,
followed by a quick nucleophilic attack by the pri-
mary amine to give benzoxazole 3. Otherwise, the
imine 4 is produced under the cooperative catalysis
by copper and catechol.
3,5-Di-tert-butylcyclohexa-3,5-diene-1,2-dione (Ia):[18] The
title compound was prepared according to the general pro-
cedure and purified by column chromatography on silica gel
and eluted with ethyl acetate/petroleum ether (1/100) to
afford the desired product as a colorless liquid. 1H NMR
(CDCl3, 400 MHz): d=6.89 (s, 1H), 6.18 (s, 1H), 1.22 (s,
9H), 1.18 (s, 9H); 13C NMR (CDCl3, 100 MHz): d=181.0,
179.9, 163.2, 149.8, 133.4, 121.9, 35.9, 35.3, 29.1, 27.7.
Conclusions
In summary, we have discovered the first copper-cata-
lyzed aerobic oxidative functionalization of the
À
C(aryl) OH bond. A large number of 2-functional-
ized benzoxazoles has been produced via selective
Acknowledgements
À
C(aryl) OH bond cleavage of catechols under mild
Financial support by National Natural Science Foundation of
reaction conditions. The present findings not only pro- China (Grant Nos. 21172062, 21273066, 21273067, J1210040)
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Copper-Catalyzed Aerobic Oxidative C(aryl) OH Bond Functionalization
and Fund of Ministry of Education of China
(20110161120008) is gratefully appreciated.
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[19] CCDC 1059859 (3x) contains the supplementary crys-
tallographic data for this paper. These data can be ob-
tained free of charge from The Cambridge Crystallo-
graphic Data Centre via www.ccdc.cam.ac.uk/data_
request/cif.
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Adv. Synth. Catal. 2015, 357, 2924 – 2930