Electrochemical intramolecular c-h amination: Synthesis of benzoxazoles and benzothiazoles

Article abstract of DOI:10.1002/chem.201406398

A new method for metal-free intramolecular C-H amination has been developed. Electrochemical oxidation of 2-pyrimidyloxybenzenes and 2-pyrimidylthiobenzenes, which can be easily prepared from phenols and thiophenols, respectively, followed by the treatment of the resulting pyrimidinium ions with piperidine gives 2-aminobenzoxazoles and 2-aminobenzothiazoles, respectively.

Full text of DOI:10.1002/chem.201406398

DOI: 10.1002/chem.201406398
Communication
&
CÀH Functionalization
Electrochemical Intramolecular CÀH Amination:
Synthesis of Benzoxazoles and Benzothiazoles
Tatsuya Morofuji, Akihiro Shimizu, and Jun-ichi Yoshida*[a]
avoid overoxidation. However, there is another problem, that
is, regioselectivity. Sometimes a mixture of regioisomers is pro-
duced. To solve this problem we envisaged that an intramole-
cularization approach[10] would be promising (Scheme 1b). In
addition, the resulting cyclized cationic intermediate could be
converted to nitrogen-containing heteroaromatics. The idea
works, and we report here the intramolecular electrochemical
CÀH amination that offers an intriguing way of making ben-
zoxazoles and benzothiazoles from phenols and thiophenols,
respectively.[11]
Abstract: A new method for metal-free intramolecular CÀ
H amination has been developed. Electrochemical oxida-
tion of 2-pyrimidyloxybenzenes and 2-pyrimidylthioben-
zenes, which can be easily prepared from phenols and thi-
ophenols, respectively, followed by the treatment of the
resulting pyrimidinium ions with piperidine gives 2-amino-
benzoxazoles and 2-aminobenzothiazoles, respectively.
CÀH amination[1] serves as a powerful method for synthesizing
nitrogen-containing organic compounds, and a variety of
transformations have been developed based on transition-
metals,[2] hypervalent iodines,[3] and radical species.[4] Electro-
chemical oxidation[5,6] serves as a straightforward method for
functionalizing CÀH bonds of aromatic compounds without
using a metal or chemical oxidant.[7] Despite the usefulness of
the method, it often suffers from overoxidation when the oxi-
dation potential of the product is lower than that of the start-
ing material.[8] Therefore, CÀH amination of aromatic com-
pounds by conventional electrochemical oxidation is usually
difficult to achieve selectively. To solve the problem, we have
developed the electrochemical intermolecular CÀH amination
of aromatic compounds via N-arylpyridinium ions
(Scheme 1a).[9]
We first examined the electrochemical oxidation of 2-phe-
noxypyridine (1), which can be easily prepared from phenol
and a halopyridine in one step[12] (Scheme 2). The anodic oxi-
Scheme 2. Electrochemical oxidation of 2-phenoxypyridine followed by
treatment with piperidine.
The key to the success of the method is the intermediacy of
the electrooxidatively inactive cationic intermediates which
dation led to the formation of cyclized pyridinium ion 2, which
was characterized by NMR spectroscopy. Although treatment
of 2 with piperidine gave the 2-substituted benzoxazole 3 in
60% yield, the synthetic utility of 3 seemed to be limited.
To explore a more useful transformation, we designed
a transformation using a pyrimidine ring instead of a pyridine
ring as shown in Scheme 3. The starting 2-pyrimidyloxyben-
Scheme 1. Electrochemical CÀH amination. a) Intermolecular approach. b) In-
tramolecular approach.
[a] T. Morofuji, Dr. A. Shimizu, Prof. Dr. J. Yoshida
Department of Synthetic Chemistry and Biological Chemistry
Graduate School of Engineering, Kyoto University
Nishikyo-ku, Kyoto 615-8510 (Japan)
E-mail: yoshida@sbchem.kyoto-u.ac.jp
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201406398.
Scheme 3. Electrochemical oxidation of 2-pyrimidyloxybenzene followed by
treatment with piperidine.
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Table 1. Synthesis of 2-aminobenzoxazoles by electrochemical intramo-
lecular CÀH amination.[a]
Entry Starting material
Product
Electricity Yield
[F]
[%][b]
85
1
2.5
92[c]
74
2
2.5
Figure 1. 1H NMR spectrum of 5a.
3
2.5
3.0
89
80
zene (4a) was prepared from phenol and 2-bromopyrimidine
in one step.[12] The anodic oxidation of 4a gave the cyclized
pyrimidinium ion 5a, which was characterized by NMR spec-
troscopy (Figure 1). Treatment of 5a with piperidine gave 2-
aminobenzoxazole (6a), which constitutes a key scaffold in
therapeutically important molecules[13,14] in 85% yield. The
present transformation can be performed on 2.0 mmol scale
(Table 1, entry 1).
(a/b=1/1.0)
4[d]
5[d,e]
2.5
48
The reaction mechanism outlined in Scheme 4 seems to be
reasonable. One-electron oxidation of 4a gives the corre-
sponding radical cation. The subsequent intramolecular attack
of the nitrogen atom of the pyrimidine ring followed by one-
electron oxidation and extrusion of a proton gives cyclized cat-
ionic intermediate 5a. In the next step, the attack of piperidine
at the carbon atom next to the positively charged nitrogen
atom of 5a followed by the ring opening and the attack of an-
other molecule of piperidine on the resulting imine gives 2-
aminobenzoxazole (6a). Because the oxidation potential of 6a
(0.88 V vs. Ag/AgNO3) is much lower than that of 4a (1.57 V vs.
Ag/AgNO3), intermediacy of the cationic species 5, which is
electrooxidatively inactive under the conditions, would be criti-
cal for the success of the reaction.
6
2.5
2.5
99
82
7[d]
8[d]
9[d]
10
4.0
2.5
2.5
68
76
78
11
12
2.5
2.5
88
98
As shown in Table 1, the present method is applicable to
various substituted 2-pyrimidyloxybenzenes to give the corre-
sponding 2-aminobenzoxazoles in good yields. o- and p-Ethox-
ycarbonyl-substituted phenol derivatives 4b and 4c gave the
corresponding 2-aminobenzoxazoles 6b and 6c, respectively
(entries 2 and 3). The reaction of m-methoxycarbonyl-substitut-
ed phenol derivative 4d gave a mixture of two regioisomers
6da and 6db (entry 4). The benzylic CÀH group was not af-
fected as shown in entry 5. The transformation is compatible
with various functional groups such as halogen, trifluorometh-
yl, cyano, and ketone carbonyl groups (entries 5–12).
[a] Compound 4 (0.2 mmol) was oxidized electrochemically in the pres-
ence of 0.6 mmol of K2CO3 in a 0.3m solution of LiClO4 in CH3CN in an H-
type divided cell under constant current conditions at room temperature
unless otherwise stated, and the resulting solution was treated with pi-
peridine (2.0 mmol) at 708C. [b] Isolated yields. [c] The transformation
was performed on a 2.0 mmol scale. [d] 1.0m solution of LiClO4 was used.
[e] The electrolysis was carried out at 508C.
Next, we examined the reaction of 2-pyrimidylthiobenzenes
7, which were prepared from thiophenols and a halopyrimidine
or from aryl halides and 2-pyrimidinethiol in one step.[15] Elec-
trochemical oxidation of 7 and the subsequent chemical reac-
tion with piperidine gave 2-aminobenzothiazoles 8, which also
serve as an intriguing motif in medicinal chemistry.[16] The re-
sults are summarized in Table 2. Notably, 2-aminobenzothia-
zoles are often used as precursors of 2-aminothiophenols,
which are important intermediates for the synthesis of bioac-
tive molecules.[17]
The most popular protocols for synthesizing benzoxazoles
and benzothiazoles involve the condensation of 2-aminophe-
nol and 2-aminothiophenol, respectively, with either a carboxyl-
ic acid or aldehyde followed by intramolecular cyclization.[18]
Although protocols based on cyclization using CÀH functionali-
zation have also been developed,[19] most of them involve in-
tramolecular CÀO or CÀS coupling, and therefore aniline deriv-
atives are required as starting materials.
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thiazoles. The present electrochemical transformation serves as
a simple and powerful route to benzoxazoles and benzothia-
zoles via intramolecular CÀH amination.
In conclusion, we have developed a powerful method for
the intramolecular CÀH amination of aromatic compounds
using an electrochemical method. The present method pro-
vides metal- and chemical-oxidant-free routes to benzoxazoles
and benzothiazoles having a variety of functionality. Currently,
we are working to expand the scope of the present method
for the synthesis of other nitrogen-containing heterocyclic
compounds.
Scheme 4. A mechanism for the electrochemical intramolecular CÀH amina-
tion and the subsequent chemical reaction with piperidine.
Experimental Section
Table 2. Synthesis of 2-aminobenzothiazoles by electrochemical intramo-
lecular CÀH amination.[a]
The anodic oxidation was carried out using an H-type divided cell
(4G glass filter) equipped with a carbon felt anode (Nippon Carbon
JF-20-P7, ca. 160 mg, dried at 3008C/1 mmHg for 4 h before use)
and a platinum plate cathode (20ꢁ20 mm). In the anodic chamber
was placed a solution of 4 (0.20 mmol) and K2CO3 (0.60 mmol) in
LiClO4/CH3CN (0.3m or 1.0m, 10.0 mL). In the cathodic chamber
were placed trifluoromethanesulfonic acid (150 mL) and LiClO4/
CH3CN (0.3m or 1.0m, 10.0 mL). The constant current electrolysis
(8.0 mA) was carried out at room temperature or at 508C with
magnetic stirring. After the electrolysis (2.5–4.0 F), piperidine
(200 mL, 2.0 mmol) was added to the anodic solution. The resulting
solution in the anodic chamber was transferred to a round-bottom
flask and was heated at 708C with stirring for 3 h. After removal of
the solvent under reduced pressure, H2O (20 mL) and ethyl acetate
(10 mL) were added. The mixture was extracted with ethyl acetate/
hexane (2/1) (20 mLꢁ3), and the combined extracts were washed
with water (20 mL) and brine (20 mL), and were dried over Na2SO4.
After removal of the solvent under reduced pressure, the crude
product was purified by flash chromatography or preparative GPC
to obtain 6.
Entry Starting material
Product
Electricity [F] Yield
[%][b]
1[c,d]
2.5
4.0
73
68
2
3
2.5
2.5
2.5
72
84
75
4[c,d]
5[c]
Acknowledgements
We thank the Grant-in-Aid for Scientific Research for financial
support.
6
2.5
80
[a] Compound 7 (0.2 mmol) was oxidized electrochemically in the pres-
ence of 0.6 mmol of K2CO3 in a 0.3m solution of LiClO4 in CH3CN in an H-
type divided cell under constant current conditions at room temperature
unless otherwise stated, and the resulting solution was treated with pi-
peridines (2.0 mmol) at 708C. [b] Isolated yields. [c] 1.0m solution of
LiClO4 was used. [d] The electrolysis was carried out at 508C.
Keywords: CÀH amination
·
CÀH functionalization
·
electrochemistry · oxidation · radical ions
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Received: December 8, 2014
Published online on && &&, 0000
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COMMUNICATION
&
CÀH Functionalization
T. Morofuji, A. Shimizu, J. Yoshida*
&& – &&
Electrochemical Intramolecular CÀH
Amination: Synthesis of Benzoxazoles
and Benzothiazoles
Integration of electrochemical and
chemical reactions: A new method for
metal-free intramolecular CÀH amina-
tion has been developed. Electrochemi-
cal oxidation of 2-pyrimidyloxybenzenes
and 2-pyrimidylthiobenzenes, which can
be easily prepared from phenols and
thiophenols, respectively, followed by
the treatment of the resulting pyrimidi-
nium ions with piperidine gives 2-ami-
nobenzoxazoles and 2-aminobenzo-
thiazoles, respectively.
Chem. Eur. J. 2015, 21, 1 – 5
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ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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These are not the final page numbers! ÞÞ