Oxidant-switchable selective synthesis of 2-aminobenzimidazoles via c-h amination/acetoxylation of guanidines

Article abstract of DOI:10.1021/ol502815p

The iodine(III) compound promoted C-H amination and tandem C-H amination/acetoxylation of guanidines are achieved for the first time to provide efficiently 2-aminobenzimidazoles and acetoxyl-substituted 2-aminobenzimidazoles, respectively. The amount and

Full text of DOI:10.1021/ol502815p

Letter
pubs.acs.org/OrgLett
Oxidant-Switchable Selective Synthesis of 2‑Aminobenzimidazoles
via C−H Amination/Acetoxylation of Guanidines
†
,†,‡
†
Yue Chi, Wen-Xiong Zhang,* and Zhenfeng Xi
†Beijing National Laboratory for Molecular Sciences, and Key Laboratory of Bioorganic Chemistry and Molecular Engineering of
Ministry of Education, College of Chemistry, Peking University, Beijing 100871, China
‡
State Key Laboratory of Elemento-Organic Chemistry, Nankai University, Tianjin 300071, China
*
S Supporting Information
ABSTRACT: The iodine(III) compound promoted C−H
amination and tandem C−H amination/acetoxylation of
guanidines are achieved for the first time to provide efficiently
2
-aminobenzimidazoles and acetoxyl-substituted 2-aminoben-
zimidazoles, respectively. The amount and type of iodine(III)
compounds control the selective syntheses of two types of 2-aminobenzimidazoles. This reaction shows good regioselectivity
when unsymmetrical substrates are used.
he 2-aminobenzimidazole scaffold is found in a wide range
Scheme 1. Synthesis of 2-Aminobenzimidazole via C−X or
C−H Amination
T
of drug molecules for their unique biological activities
1
,2
(
Figure 1), so the construction of a substituted 2-amino-
10
have also been developed. Hypervalent iodine(III) com-
pounds, owing to their useful oxidizing properties, environ-
11
mental character, and commercial availability, are widely used
1
2
13b
in the C−N bond formation. The Zhu group and Long
13d
group developed the intramolecular C−N bond formation of
amidine via iodine(III) compounds as oxidant to prepare
benzimidazoles. As far as we are aware, intramolecular metal-
Figure 1. Representative examples of 2-aminobenzimidazoles with
biological activities.
13
free aryl C−H amination and tandem C−H amination/benzene-
ring acetoxylation of guanidines have not been reported.
We are interested in the synthesis and reactivity of
guanidines. Here, we report the iodine(III) compound
promoted C−H amination and tandem C−H amination/
acetoxylation of guanidines to prepare 2-aminobenzimidazoles
and acetoxyl-substituted 2-aminobenzimidazoles, respectively.
The amount and type of iodine(III) compounds control the
selective syntheses of two types of 2-aminobenzimidazoles.
benzimidazole has received much attention. There are two
classical methods to synthesize substituted 2-aminobenzimida-
1
4
zoles. The first one is the ring-closed condensation of (2-
3
aminophenyl)thiourea between the NH group and CS bond.
2
4
The second one is the amination of benzimidazoles. Recently,
Cu- or Pd-catalyzed intramolecular C−X amination has been
5
developed to construct 2-aminobenzimidazoles (Scheme 1). In
this strategy, transition metals are required, and halogen waste is
produced. In addition, the N-substituent is usually an aryl and
acyl group. The use of an alkyl group has rarely been reported.
The oxidative C−H bond amination to construct a new C−N
bond has experienced a rapid development because of their virtue
of step-efficiency and atom-economy. Various transition metals
1
,3-Diisopropyl-2-phenylguanidine 1a was chosen as a model
to perform this reaction. When 1.1 equiv of PhI(OAc) was
2
utilized as oxidant, benzimidazole 3a was obtained in 31% yield.
Gratifyingly, another product was an acetoxyl-substituted
6
7
8
9
including Pd, Cu, Ag, and Rh could catalyze this process.
Received: September 23, 2014
Recently, alternative metal-free C−H bond amination methods
©
XXXX American Chemical Society
A
dx.doi.org/10.1021/ol502815p | Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
compound 2a in 26% yield (Table 1, entry 1). The structure of 2a
was confirmed by X-ray crystallographic analysis (see the
Scheme 2. Synthesis of Acetoxyl-Substituted 2-
Aminobenzimidazoles 2a−k
Table 1. Condition Screening of C−H Amination/
Acetoxylation of Guanidine 1a via PhI(OAc)
2
a
a
entry
solvent
n
temp (°C) yield (%) (2a) yield (%) (3a)
1
2
3
4
5
6
7
8
9
MeCN
C H
1.1
2.2
2.2
2.2
2.2
2.2
3.3
2.2
2.2
80
80
80
80
50
80
80
80
80
26
48
31
19
6
6
toluene
THF
16
12
67
trace
15
MeCN
MeCN
MeCN
EtOH
AcOH
59
74
<5
75
<5
25
<5
trace
trace
a
GC yield.
Supporting Information for the X-ray structure of 2a). It clearly
shows the acetoxyl group is selectively connected at the para
position of the benzene ring on 1,3-diisopropyl-2-phenyl-
guanidine 1a. This result drew our interest because it
simultaneously achieved the C−N bond formation and direct
benzene acetoxylation. This acetoxylation process has not been
reported in other benzimidazole synthesis from amidines via
iodine(III) oxidant even though the excess of oxidant was
utilized, and it reflects the different reactivity between amidine
and guanidine compounds. What is more, there are mature
strategies to change acetoxyl group to hydroxyl, alkoxyl, and aryl
incorporated in the final products. Scheme 3 summarized the
representative PhI(OCOCF ) -promoted synthesis of 2-amino-
3
2
benzimidazoles. Various guanidines, such as 1,3-dialkyl-2-
arylguanidines, 1-alkyl-2,3-diarylguanidines, and 1,2,3-triarylgua-
nidines, were tested in this method to afford the corresponding 2-
aminobenzimidazoles 3a−p in medium to high yields. Various
EWG and EDG group could be tolerated, including methoxyl,
trifluoromethyl, cyano, and halogen groups (3c−g). The ortho-
or para-substituents on the benzene ring could be utilized, while
the meta-substituents on the benzene ring led to two isomers as
products (3d and d′). Similar to regioselectivity of 2i in Scheme
2, 3h could be exclusively obtained by the oxidative C−H
amination. When 1-alkyl-2,3-diarylguanidines were applied in
the reaction, C−H amination also occurred on the nitrogen atom
adjoining to the aryl groups to regioselectively provide the
corresponding products 3m,n. The structure of the regioselective
isomer 3n was confirmed by single-crystal X-ray diffraction
analysis (see the Supporting Information for the X-ray structure
of 3n). However, the guanidines without an N-substituent group
could not be tolerated; for example, 1,2-diphenylguanidine
1
5
groups. It can be used in the synthesis of more complex
heterocyclic compounds.
Condition screening of C−H amination/acetoxylation of
guanidine 1a via PhI(OAc) was carried out (Table 1). We found
2
that 2a could be obtained in 74% yield when the reaction was
performed in MeCN at 80 °C for 4 h with 2.2 equiv of PhI(OAc)2
(
entry 6).
As summarized in Scheme 2, this C−H amination/
acetoxylation method could be applied in various guanidines.
1,3-Dialkyl-2-arylguanidines were suitable substrates to proceed
via C−H amination/acetoxylation to give the corresponding
acetoxyl-substituted 2-aminobenzimidazoles 2a−k. The ortho-
substituents on the benzene ring could be tolerated under the
present conditions (2b−d and 2g,h). As for meta-substituents on
the benzene ring, the products were two isomers (2e and 2e′).
When 1-tert-butyl-3-ethyl-2-phenylguanidine was utilized, the
regioselective isomer 2i was exclusively produced, probably
owing to the steric hindrance of the tert-butyl group. When 1-
butyl-3-(2,6-diisopropylphenyl)-2-phenylguanidine was applied
in the reaction, the acetoxyl-substituted 2-aminobenzimidazole
PhNC(NHPh)NH was tested to give the oxidized azo
2
compound. Thus, acyl chloride was utilized to protect the amino
t
group to give guanidine PhNC(NHPh)(NHCO Bu), which
2
j was formed as the major product with the acetoxyl-free
was further oxidized to provide the corresponding 3p. When 1-
aryl-2,3-dibenzylguanidines were utilized in this method, a
complicated mixture was observed. Then 1,2-diaryl-3-dibenzyl-
guanidine was tested to give Bn-substituted 2-aminobenzimida-
zole 3o. The compounds 3o and 3p have the potential
application to give 2-aminobenzimidazoles with free NH2.
Only 3l free of acetoxylation product was obtained when 1.1
byproduct 3m. Interestingly, C−H amination occurred on the
nitrogen atom adjoining to the bulky 2,6-diisopropylphenyl
group (Dipp). In this process, this Dipp-N−H acidity probably
played a vital part in the oxidative C−N formation.
Unexpectedly, even if the excess of phenyliodine(III)
bis(trifloroacetate) (PhI(OCOCF ) ) was utilized as an oxidant
3
2
instead of PhI(OAc) , only 2-aminobenzimidazoles via the C−H
equiv of PhI(OAc) was used as oxidant, probably because the
PhN−H acidity played an important role in this process.
2
2
amination were observed, but no trifluoroacetoxyl group was
B
dx.doi.org/10.1021/ol502815p | Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
1
Scheme 3. Synthesis of 2-Aminobenzimidazoles 3a−p
PhI(OAc) with the release of one molecule of HOAc. When R
=
2
2
aryl and R = alkyl, the oxidation of N−H bond regioselectively
takes place the nitrogen atom attached the aryl group. Then the
electron-deficient N atom in A is attacked by the benzene ring to
obtain the cationic intermediate B. Intermediate B undergoes the
aromatization to yield 2-aminobenimidazole intermediate C,
which remains one N−H bond. The next step is the acetoxylation
16
process. Intermediate C can be further oxidized by the second
molecule of PhI(OAc) to provide intermediate D. The cationic
2
−
intermediate E can accept the nucleophilic attack of OAc and
rearomatize to give the final product 2.
In order to gain information on this mechanism, the isolated 2-
aminobenzimidazole 3m was allowed to react with PhI(OAc) in
2
MeCN at 80 °C with the addition of 4 equiv of HOAc (eq 1).
The yield of 2j was 54%. This result clearly showed that acetoxyl-
substituted 2-aminobenimidazoles 2 could be obtained from the
intermediate C in Scheme 4. Furthermore, the deuterium
experiment was carried out. The reaction of 1a-D under the
standard conditions could provide a mixture of the products 3a-
D and 3a in 66% combined yields, and the ratio of 3a:3a-D was
1
.3 (eq 2). This KIE value reveals that the C−H bond cleavage is
not the rate-determining step in this reaction.
In summary, we have developed the efficient, metal-free
methods to prepare 2-aminobenzimidazoles and acetoxyl-
substituted 2-aminobenimidazoles by the iodine(III) compound
promoted C−H amination and tandem C−H amination/
acetoxylation of guanidines, respectively. The amount and type
of iodine(III) compounds control the selective syntheses of two
types of 2-aminobenzimidazoles. This reaction shows good
regioselectivity when unsymmetrical substrates are used.
a
b
Heated for 8 h. 1.1 equiv of PhI(OAc) used as oxidant.
2
A tentative mechanism for the formation of acetoxyl-
substituted 2-aminobenimidazoles 2 is proposed in Scheme 4.
The first step is the oxidation of one N−H bond in guanidines by
Scheme 4. Proposed Mechanism for the Synthesis of
Compound 2
ASSOCIATED CONTENT
■
*
S
Supporting Information
Experimental details, X-ray data for 2a and 3n (CIF), and
scanned NMR spectra of all new products. This material is
available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
■
*
E-mail: wx_zhang@pku.edu.cn.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the Natural Science Foundation of
China and the “973” program from National Basic Research
Program of China (2011CB808601).
C
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Organic Letters
Letter
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