Chemoselective acylation of benzimidazoles with phenylacetic acids under different Cu catalysts to give fused five-membered N-heterocycles or tertiary amides

Article abstract of DOI:10.1039/c6ob01167e

C-N bond formation via a copper-catalyzed aerobic oxidative decarboxylative tandem protocol was realized. The phenylacetic acids which contain ortho-X (X = F or Br) on the aromatic ring will render a fused five-membered heterocycle via a tandem aromatic nucleophilic substitution and aerobic oxidative decarboxylative acylation at the C(sp²)-H bond of benzimidazoles under the Cu(OAc)₂/K₂CO₃/BF₃·Et₂O catalytic system, while with CuBr as the catalyst and pyridine as the base, N-acylation occurred and tertiary amides were obtained.

Full text of DOI:10.1039/c6ob01167e

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PAPER
Chemoselective Acylation of Benzimidazoles with Phenylacetic
Acids under Different Cu Catalysts to Fused Five-Membered N-
Heterocycles or Tertiary Amides
Received 00th January 20xx,
Accepted 00th January 20xx
Shaoyu Maia, b, Yingwei Zhaoa and Qiuling Songa, c
*
DOI: 10.1039/x0xx00000x
www.rsc.org/
C-N bonds formation via a copper-catalyzed aerobic oxidative decarboxylative tandem protocols were realized. The
phenylacetic acids which contain ortho-X (X = F or Br) on the aromatic ring will render fused five-membered heterocycle
via a tandem aromatic nucleophilic substitution and aerobic oxidative decarboxylative acylation at C(sp2)-H bond of
benzimidazoles under Cu(OAc)2/K2CO3/BF3·Et2O catalytic system. While with CuBr as catalyst and pyridine as base, N-
acylation occurred and tertiary amides were obtained.
attracted widespread attentions due to readily accessible
starting materials, simple operation, and clean by-product (CO2
only). Catalytic decarboxylative couplings of C(sp2) (aromatic
Introduction
N-Heterocycles are ubiquitous structure motifs in bioactive
natural products, pharmaceuticals, organic materials and dyes.
The construction of heterocycles is in the central of organic
synthesis.1 Along with the rapid development of
organometallic chemistry, many efforts have been devoted to
the synthesis and functionalizations of N-heterocycles via
transition-metal-catalyzed coupling reactions. Also, within the
application of this strategy, many complex fused heterocycles
can be achieved via tandem reactions with simple and efficient
synthetic operation accesses.2
and alkenyl) and C(sp) (alkynyl) acids have been well studied,
and decarboxylative coupling at an sp3-hybridized carbon were
also emerged as a hot topic recently.8, 9 During our own efforts
on oxidative decarboxylation coupling of phenylacetic acid
with nucleophiles under Cu/O2 conditions,6c-f we envisioned
that the C(CO)-C bond can be formed via this strategy with the
carbonyl-M at 2-position of benzimidazole as the
nucleophile.10 In conjunction with the tandem aromatic
nucleophilic substitution11 or Cu-catalyzed Ullmann coupling
for the formation of a C-N bond,12 the construction of the
fused heterocycle can be expected (Figure 1).
Fused benzimidazoles, a special kind of N-heterocylces,
represent a class of important compounds that display a broad
range of biological functions,3 therefore many synthetic
methods have been developed toward them.4 Among the
fused benzimidazoles, the imidazoindanone-type structure
such as 11H-benzo[4,5]imidazo[1,2-a]indol-11-one aroused
our great interest because of their potential biological
aitivities.5, 6a-b To our knowledge, there is only one example
referred to the synthesis of such fused compounds through
the reaction of o-fluorobenzaldehyde with benzimidazole,
affording 11H-benzo[4,5]imidazo[1,2-a]indol-11-one only in
33% yield.7
Cuꢀcatalyzed
oxidative acylation
O
C
N
N
R1
R2
SNAr
or
Ullmann coupling
11H-benzo[4,5]imidazo[1,2-a]indol-11-one
Figure 1. Retrosynthetic analysis of the fused benzimidazole
Results and discussion
Transition-metal-catalyzed decarboxylative couplings have
To test our hypothesis, we firstly examined the Ullmann
coupling-oxidative acylation tandem reaction using 2-Br phenyl
acetic acid
(1a) and benzimidazole (2a) as the model
substrates (Table 1). When the reaction was conducted in
DMSO in the presence of MeONa for 18 h at 120 °C under
oxygen and catalyzed by 10 mol % of Cu(OAc)2 and proline as
the ligand, the desired fused heterocycle product 3aa was
obtained in 16% yield (entry 1). The catalytic system containing
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withdrawing groups such as halogen and CF3-, lower reaction
temperature of 100°C was needed to achieve acceptable results
(3e'a-3i'a). We then examined the reaction of 2-fluorophenylacetic
acid (1a') with a variety of benzimidazoles. The derivatives 2a-2d
with electron-donating groups on the benzene ring gave
corresponding products in good yields. The structure of 3a'c was
confirmed by X-ray crystallographic analysis (See supporting
information for details).13 The electron-withdrawing Cl led to a
moderate result. It is noteworthy that imidazole was also tolerated
for this reaction, giving 9H-imidazo[1,2-a]indol-9-one (3a'f) in 56%
yield.
CuI, K3PO4 and DMEDA led to a moderate yield (entry 3).
However, further screening of the catalyst, base and the ligand
could not improve the reaction any more. We considered that
the oxygen atmosphere might suppress the Ullmann process.
Then we used 2-F phenylacetic acid (1a’) instead of 1a, trying
to construct the C-N bond via aromatic nucleophilic
substitution (SNAr). We were pleased to find that when
Cu(OAc)2 was used as the catalyst and K2CO3 as the base, 3aa
was obtained in 60% yield. The choice of base is crucial for this
reaction. Except for K3PO4 and K2CO3, other bases tested didn’t
work well (entries 5-10). On the other hand, it was found that
the Lewis acid additive BF3
affording 3aa in 65% isolated yield (entry 13). Finally, when the
ratio of 1a' 2a was changed to 1:1.4, a satisfied yield of 80%
·Et2O could promote this reaction,
Scheme 1. Substrate scope
O
N
:
COOH
Cu(OAc)2 (10 mol%)
N
R'
R
+
K2CO3 (2 equiv)
BF3—Et2O (20 mol%)
DMF (1 mL)
N
F
can be achieved. This result clearly indicated that our proposal
for the tandem reaction to form fused N-heterocycles indeed
works.
N
H
1
2
R
R'
3
O2, 120 oC, 20 h
0.35 mmol
0.25 mmol
Ph
MeO
Table 1. Optimization of the reaction conditions for the
N
N
N
N
N
N
N
cyclization of benzimidazole with ortho-halogenated
phenylacetic acid a
N
O
O
O
O
3d'a, 45%a
3a'a, 80%
3b'a, 54%
3c'a, 50%
Br
Br
F
Cl
N
N
N
N
N
N
N
N
O
O
O
O
3e'a, 40%b
3f'a, 45%b
3g'a, 62%b
3h'a, 48%b
Entry X catalyst
1c Br Cu(OAc)2
Base
MeONa
MeONa
K3PO4
Additive/Ligand Yield (%)b
F3C
N
N
N
N
proline
16
24
N
N
2c Br
3c Br
CuI
proline
O
O
O
3i'a, 35%b
3a'c
3a'b, 76%
3a'c, 60%
Xꢀray crystal of
CuI
DMEDA
40
4
5
Br
F
F
F
F
F
F
F
F
F
F
F
F
CuI
K3PO4
DMEDA
trace
56
OMe
Cl
N
N
N
N
N
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
K3PO4
-
N
O
O
6
K2CO3
-
60
O
3a'd, 84%
3a'e, 50%
3a'f, 56%
7
Cs2CO3
NaHCO3
t-BuOK
-
-
0
a DMSO as the solvent; b These reactions were carried out at 100 °C
8
trace
trace
23
During our investigation of the parameters on the tandem
reaction, we discovered that CuBr/pyridine system led to the direct
oxidative N-acylation, affording imidazolium amide, which is also an
important structural motif in a myriad of bioactive natural products,
agrochemicals and pharmaceuticals.14 The direct acylation from free
carboxylate acid in the absence of an activating reagent is of great
interest for environmentally benign synthesis. Thus we decided to
expand this divergent pathway in detail by using phenyl acetic acid
(1f) and benzimidazole (2a) as the standard substrates15 (Scheme 2).
It can be seen that the existence of both CuBr and pyridine was
essential for the success of the reaction.
9
-
10
11
12
13d
14
15
16f
Cu(OAc)2 K3PO4·3H2O
-
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
Fe(acac)3
FeCl3
BF3·Et2O
AcOH
CuO
trace
trace
(65) e
0
40
BF3·Et2O
95(80)
a
Conditions: 2-bromophenylacetic acid (1a) or 2-fluorophenylacetic
acid (1a’) (0.5 mmol), benzimidazole (2a) (0.25 mmol), base (0.5 mmol),
solvent (1 mL), additive (0.05 mmol), O2 (1 atm), 120 °C, 20 h; b GC
c
d
e
yield. DMSO as the solvent. Under air. Yields of the isolated
product in parenthesis. f 0.25 mmol of 1a’ and 0.35 mmol of 2a
.
Next, we explored the substrate scope with respect to 2-
fluorophenylacetic acids (1a') and benzimidazoles (2) under the
optimized reaction conditions (Scheme 1). It can be seen that a
broad range of 2-fluorophenylacetic acids bearing an electron-
donating group such as CH3- and CH3O- are good substrates for the
tandem reaction with benzimidazole (2a) under the optimized
conditions to give 11H-benzo[4,5]imidazo[1,2-a]indol-11-ones
(3a'a) in moderate to good yields (3b'a and 3c'a). For electron-
Scheme 2. Oxidative benzoylation of benzimidazole by phenyl
acetic acid
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We next investigated the scope of this new reaction (Scheme 3). obtained while no aimed amide product 4a'a was detected. In order
For 2-fluorophenylacetic acid (1a') and 2-bromophenylacetic acid to confirm that this is a tandem process, compound 6 was prepared
(1a), the model substrates of the tandem annulation, corresponding according to the literature.16 When it was subjected to condition A,
amides 4a'a and 4aa were afforded in moderate yields. The SNAr almost quantitative amount of product 3aa was obtained, which
reaction and Ullmann coupling were both highly suppressed under strongly support that aldehyde was one of the key intermediates
this condition and no 3a'a was ever detected under this for the annulation reaction, but not for the amidation. To get more
CuBr/Pyridine/O2 system, neither under Condition A as well. Ortho- information about the possible intermediates, compound 7 was
Cl phenylacetic acids also led to the amide products smoothly (4ba), exposed to the condition A as well, 38% of compound 3aa was
providing the feasibility for further transformations. Both electron- obtained, compared to ortho-bromo-phenylacetic acid 1a, we could
donating and electron-withdrawing group substituted phenylacetic not rule it out as a possible intermediate. For decarboxylative
acids led to the products in good yields (4ca-4ea, 4ga-4la, 4pa-4qa, amidation under condition B, the control experiments as in eq. 5
4sa-4va). Phenyl and polyphenylene substrates were all tolerated and eq. 6 indicated that the oxidation of phenylacetic acid led to
under the standard conditions to afford the corresponding desired the benzoic acid17 or peroxybenzoic acid18 which were efficient
products in good yields (4ma-4oa). Moreover, 3-thiophene acetic benzoylation reagents for this catalytic system. In order to figure
acid was compatible in this reaction as well (4ra). Finally, out which step, SNAr or aerobic oxidative decarboxylative acylation,
substitutions on the aromatic ring of benzimidazole were also occurred first in this tandem process, both compounds 10 and 11
proven to be compatible and the corresponding products were were prepared19 and subjected to Condition A, desired product
obtained in good yields (4fc and 4fg). However, it is unlucky that were obtained in 68% and 85% yield. However, compound 11 was
under the CuBr/Pyridine/O2 catalytic system, no reaction occurred not observed or detected under either Condition A or Condition B,
for imidazole.
so we have reasons to believe that SNAr reaction occurred first to
lead to C-N bond formation in the tandem process.
Scheme 3. Substrate scope of N-acylation of benzimidazole a
Scheme 4. Control experiments
To gain insight into the mechanism of this divergent reaction,
some control experiments were carried out to find out the potential
reaction intermediates (Scheme 4). In our previous work, we
recognized that phenylacetic acids can be decarboxylatively
transformed to benzaldehydes under [Cu]/O2 conditions.6 As shown
in eq. 1 and 2, when 2-fluorobenzaldehyde was subjected to the
reaction with benzimidazole (2a) under the standard “annulation
condition” (A), a high yield of the fused heterocycle (3aa) was
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heterocycles. Further extension of this methodology and the
mechanistic study are underway in our laboratory.
Experimental section
General
All experiments were conducted with a sealed pressure vessel.
Flash column chromatography was performed over silica gel
1
(200-300 mesh). H NMR spectra were recorded on a Bruker
AVIII-500M spectrometers, Chemical shifts (in ppm) were
referenced to CDCl3 (δ = 7.26 ppm) or DMSO-d6 (δ = 2.54 ppm)
as an internal standard. 13C NMR spectra were obtained by
using the same NMR spectrometers and were calibrated with
CDCl3 (δ = 77.0 ppm) or DMSO-d6 (δ = 40.45 ppm). Unless
otherwise noted, materials obtained from commercial
suppliers were used without further purification.
General procedure for cyclization of benzimidazoles with ortho-
flouro phenyl acetic acids
A
sealed pressure vessel was charged with 2-
fluorophenylacetic acids (0.25 mmol), benzimidazoles
(0.35mmol), Cu (OAc)2 (3.6 mg, 0.025 mmol), BF3 Et2O (7.1 mg,
Scheme 5. Plausible mechanisms
·
0.05mmol), K2CO3 (69mg, 0.5mmol) and DMF (1 mL). The
resulting solution was stirred at 120 oC under O2 (O2 was
blowing into the system several times) monitored by TLC and
GC for 20 hours. Upon completion of the reaction, the solvents
were removed via rotary evaporator and the residue was
purified with flash chromatography (silica gel, ethyl acetate:
dichloromethane: petroleum ether: =1:1:4-1:1:10) to give the
desired product 11H-benzo[4,5]imidazo[1,2-a]indol-11-one (3aa)
as a yellow solid (44 mg, 80%). 1H NMR (500 MHz, CDCl3),c 7.86
(d, J = 8.2 Hz, 1H), 7.65 (d, J = 7.4 Hz, 1H), 7.58 (t, J = 7.7 Hz,
1H), 7.54 (d, J = 8.1 Hz, 1H), 7.46 (t, J = 7.6 Hz, 1H), 7.30 (t, J =
8.1 Hz, 2H), 7.19 (t, J = 7.5 Hz, 1H). 13C NMR (126 MHz, CDCl3),
δ 179.8, 148.9, 148.7, 143.2, 136.7, 129.8, 127.9, 127.3, 125.91,
125.8, 124.4, 123.9, 111.6, 111.1.
Based on all of above control experiments, we gave a
putative mechanism for these two divergent acylations: under
condition A, C-N bond formation is occurred first to lead to
intermediate
intermediate
intermediate
A
B
(
10), which is further oxidized into
in the presence of Cu/O2 system. If
is -oxyphenylacetic acid, activated
B
α
benzimidazole moiety will attack the newly formed carbonyl
group to render product 3aa by the release of CO2, if
intermediate
the carbonyl group, intermediate
oxidation eventually leads to the product 3aa
decarboxylative amidation (condition in Scheme 5),
B
is aldehyde, after intramolecular attacking on
is formed, further
For
D
.
B
phenylacetic acid 1a is firstly oxidized into 2-oxo-2-
phenylacetic acid, which might be further converted into
benzoic acid via oxidative decarboxylation. Cu(II)-superoxide
species was formed under O2 atmosphere in the presence of
pyridine as ligand, according to the literature,18 this type of
Cu(II)-superoxide species can react as a nucleophile (although
General procedure for benzoylation of benzimidazole by phenyl
acetic acid
A sealed pressure vessel was charged with phenylacetic acids
(0.25 mmol), benzimidazoles (0.5 mmol), CuBr (3.6 mg, 0.025
mmol), pyridine (60 mg, 0.75 mmol), and o-xylene (1 mL). The
resulting solution was stirred at 130 oC under O2 (O2 was
blowing into the system several times) monitored by TLC and
GC for 24 hours. Upon completion of the reaction, the solvents
were removed via rotary evaporator and the residue was
purified with flash chromatography (silica gel, ethyl acetate:
petroleum ether=1:4-1:10) to give the desired product (1H-
very rare) to attack compound
E to afford Cu(III)-peroxide
intermediate F, which is very susceptible upon attacking by an
electrophile, such as benzimidazole to give intermediate
G.
After elimination, desired product 4fa is obtained.
Conclusions
In summary, we have developed a divergent reaction of o-
halogenated phenylacetic acid with benzimidazole. Under
different conditions, fused 11H-benzo[4,5]imidazo[1,2-a]indol-
11-one or N-benzoylated benzimidazole can be formed
respectively. The annulation process is based upon a tandem
pathway through an aromatic nucleophilic substitution
followed by an oxidative acylation. In both cases phenylacetic
acids played as a novel benzoylation reagent. These reactions
provide new synthetic routes to the fused N-acylated
benzo[d]imidazol-1-yl)(phenyl)methanone
(4fa) as a white
1
solid (44 mg, 79%). H NMR: (500 MHz, CDCl3), δ 8.22(s, 1 H),
8.21-8.19 (m, 1 H), 7.85-7.83 (m, 1 H), 7.82-7.80(m, 2 H), 7.71-
7.68 (m, 1 H), 7.61-7.58 (m, 2 H), 7.47-7.42 (m, 2 H); 13C NMR:
(125 MHz, CDCl3 ) δ 167.1, 144.0, 143.1, 133.2,132.8, 132.1,
129.5, 129.0, 125.8, 125.3, 120.5, 115.4.
Acknowledgements
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Financial support from the National Natural Science
Foundation of China (21202049), the Recruitment Program of
Global Experts (1000 Talents Plan), the Natural Science
Foundation of Fujian Province (2016J01064), Fujian Hundred
Talents Plan and Program of Innovative Research Team of
Huaqiao University (Z14X0047) are gratefully acknowledged.
We also thank Instrumental Analysis Center of Huaqiao
University for analysis support.
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