A Benzoquinone Imine Assisted Ring-Opening/Ring-Closing Strategy of the RCOCHN1N2 System: Dinitrogen Extrusion Reaction to Benzimidazoles

Article abstract of DOI:10.1002/ejoc.201700357

A mild, three-component dinitrogen extrusion reaction proceeding at room temperature to give different 1,2-disubstituted benzimidazoles from 2-oxoaldehydes, 4-azidophenol, and benzotriazoles was successfully developed. Mechanistically, this transformation occurs through the benzoquinone imine assisted ring opening/ring closing of a highly unstable RCOCHN¹N² system.

Full text of DOI:10.1002/ejoc.201700357

DOI: 10.1002/ejoc.201700357
Communication
Multicomponent Reactions
A Benzoquinone Imine Assisted Ring-Opening/Ring-Closing
Strategy of the RCOCHN1N2 System: Dinitrogen Extrusion
Reaction to Benzimidazoles[‡]
Atul Kumar[a,b] and Qazi Naveed Ahmed*[a,b]
Abstract: A mild, three-component dinitrogen extrusion reac- cally, this transformation occurs through the benzoquinone im-
tion proceeding at room temperature to give different 1,2-dis-
ubstituted benzimidazoles from 2-oxoaldehydes, 4-azidophen-
ol, and benzotriazoles was successfully developed. Mechanisti-
ine assisted ring opening/ring closing of a highly unstable
RCOCHN1N2 system.
Introduction
1,2-Disubstituted benzimidazoles are an important class of
heterocyclic compounds and are found in a diverse range of
natural products and biologically active compounds.[1] The sig-
nificance of these structures can be demonstrated by the profu-
sion of drug leads/pharmaceutical products, including hepati-
tis C virus polymerase inhibitor 1,[1c,1e,2] agonist 2 against the
γ-aminobutyric acid A receptor (GABAA),[3] telmisartan (the anti-
hypertensive Micardis),[4] candesartan (Atacand),[5] and bilastine
(Bilaxten)[6] (Figure 1). Besides, 1,2-disubstituted benzimidazoles
are represented as intermediates to different dyes and poly-
mers[7] and have been used as ligands.[8] Consequently, ample
efforts have been dedicated to developing efficient methods to
assemble this construct.[9] Whereas the number of procedures
for the preparation of 1- or 2-substituted benzimidazoles has
increased greatly during the last years,[10] the assembly of 1,2-
disubstituted benzimidazoles is still a challenge (Scheme 1).[11]
Reported strategies often require additional steps to prepare
either the precursors or involve the use of expensive substrates/
reagents.[12] Thus, designing new strategies for the construction
of the benzimidazole backbones is highly desirable.
Figure 1. Structures of pharmacologically active benzimidazoles.
has not been well studied.[15] Recently, our group established a
novel route for the synthesis of 6-aminophenanthridines
through a 2-oxo driven N2 elimination induced decarbonylative
cyclization strategy. This reaction presented a creative way to
extrude N2 from benzotriazoles.[16] Later, the same concept
involving the RCOCHN1N2 system was extended to the syn-
thesis of 2-oxoacetamidines.[17] In continuation, we recently
established the divergent behavior of 2-oxo aldehydes (OAs)
towards amino acid alkyl ester hydrochlorides. Therein,
selenium dioxide was found to play a crucial role in promoting
the regioselective synthesis of imidazoles through the in situ
generated RCOCHN1N2 system.[18] An imperative feature of the
RCOCHN1N2 system in this case was the presence of an adjacent
electrophilic carbonyl group, which helped to fix two nitrogen
atoms from the same amino acid. This observation led us to
establish a reaction between 2-oxo aldehdyes, benzotriazoles,
and 4-azidophenols. 4-Azidophenols have the innate tendency
In the past, the Katritzky group produced several important
heterocycles by using benzotriazoles as synthetic auxiliaries.[13]
Benzotriazoles (BTZs), owing to their diverse roles in chemical
transformations and in the discovery of bioactive scaffolds, have
proven to be intriguing substrates.[14] However, owing to their
innate tendency to exist as diazonium salts and to their high
structural stability, the ring-opening chemistry of benzotriazoles
[‡] IIIM communication no. IIIM/2027/2017
[a] Medicinal Chemistry Division, Indian Institute of Integrative Medicine (IIIM),
Canal Road, Jammu, Jammu & Kashmir, 180001, India
E-mail: naqazi@iiim.ac.in
http://www.iiim.res.in/People/CV/cv_naveed.php
[b] Academy of Scientific and Innovative Research (AcSIR), Indian Institute of
Integrative Medicine (IIIM),
Canal Road, Jammu, Jammu & Kashmir, 180001, India
Supporting information and ORCID(s) from the author(s) for this article are
available on the WWW under https://doi.org/10.1002/ejoc.201700357.
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Scheme 1. Summary of this work.
to promote reactions through the benzoquinone imine system studies, we learned that the optimized conditions for the reac-
and have proven helpful in establishing a novel approach to tion involved treatment of 1a (1 mmol) with 2 (1.1 mmol), 3a
1,2-benzimidazoles given that the in situ generated RCOCHN1N2 (1.1 mmol), and Cu(OAc)2 (10 mol-%) in toluene at room tem-
system has an electrophilic nitrogen atom. The present method perature (Table 1, Entry 11).
represents a complementary dinitrogen-extrusion/ring-open-
ing/ring-closing strategy that is perhaps driven by two directing
Table 1. Optimization of the reaction conditions.[a]
groups. This method once again justifies the role of the 2-oxo
group in promoting the ring-opening/N2-elimination reaction
in benzotriazoles through a push–pull mechanism.
Results and Discussion
Entry
2a
[mmol]
3a
[mmol]
Catalyst (mol-%)
Temp.
[°C]
Yield
[%][b]
We started our study by investigating the nature of the product
acquired upon mixing phenylglyoxal (1a; 1 mmol), 4-azido-
phenol (2; 1 mmol), and 1H-benzotriazole (3a; 1 mmol) in tolu-
ene at 80 °C (Table 1, Entry 1). To our delight, the reaction
produced unanticipated product 4a in 32 % yield. The structure
of 4a was established by HRMS, LC–MS (ESI), IR spectroscopy,
and NMR spectroscopy (for details, see the Supporting Informa-
tion). Following this promising outcome, the reaction condi-
tions, particularly the concentrations and temperature along
with the effects of copper salts, were thoroughly investigated
(Table 1, Entries 2–15). Primarily, a survey of the concentrations
of 2 and 3a (Table 1, Entries 2–5) intimated that an improved
yield of 4a could be obtained at loadings of 1.1 mmol for 2
and 1.1 mmol for 3a (Table 1, Entry 4). Later, the same reaction
was performed at different temperatures (Table 1, Entries 6–9).
The best yield was obtained at room temperature (Table 1, En-
try 9). To improve the yield, we screened the reaction in the
presence of different copper salts (Table 1, Entries 10–15).
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
1
1
1
1
–
–
–
–
–
–
–
–
–
80
80
80
80
80
60
50
40
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
32
39
41
56
55
61
64
66
68
71
79
81
63
73
60
1.1
1.2
1.1
1.1
1.1
1.1
1.1
1.1
1.1
1.1
1.1
1.1
1.1
1.1
1.1
1.2
1.1
1.1
1.1
1.1
1.1
1.1
1.1
1.1
1.1
1.1
Cu(OAc)2 (5)
Cu(OAc)2 (10)
Cu(OAc)2 (20)
CuBr (10)
CuBr2 (10)
CuI (10)
[a] Reaction conditions: 2-oxo aldehyde 1a (1 mmol), 4-azidophenol (2;
1.1 mmol), and 1H-benzotriazole (3a; 1.1 mmol) in toluene (3 mL). [b] Yield
of isolated product.
Having observed that 4-azidophenol (2) assisted the addition
Cu(OAc)2 (10 mol-%) was found to be the catalyst of choice for of 1H-benzotriazole (3a) with 2-oxo aldehyde 1a under the
the generation of 4a (79 % yield; Table 1, Entry 11). After these above-optimized conditions, we decided to study the substrate
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scope of this method. As compiled in Table 2, a variety of 2- of the phenyl ring in the OA had a slight effect on the yield
oxo aldehydes 1 were tested against different benzotriazoles 3 of the product. On the basis of our observations, OAs bearing
with diverse electronic and steric properties (see products 4a– halogen/electron-withdrawing groups, for example, F (see 4h),
r). We were pleased to find that the desired products 4 were Cl (see 4i), Br (see 4j), CF3 (see 4l and 4m), and NO2 (see 4n),
produced in good yields in all of the tested reactions (72–84 % afforded slightly higher yields than an unsubstituted 2-oxo
yield). In one set of experiments, different reactions were per- aldehyde (see 4a) and 2-oxo aldehydes containing electron-
formed between diverse 2-oxo aldehydes 1, 4-azidophenol (2), donating groups such as CH3 (see 4b–d) and OMe (see 4e–
and 1H-benzotriazole (3a) (see products 4a–o). Both electron- g). The same reaction with naphthylglyoxal and a heterocyclic
rich and electron-deficient OAs could be smoothly transformed substrate produced good yields of products 4k and 4o. Further-
into the desired products. However, the electronic environment more, the reaction of phenylglyoxal (1a) and 4-azidophenol (2)
Table 2. Scope of the reaction with respect to 1 and 3.[a]
[a] Reaction conditions: 2-oxo aldehyde 1 (1 mmol), 4-azidophenol (2, 1.1 mmol), benzotriazole 3 (1.1 mmol), and Cu(OAc)2 (10 mol-%) in toluene (3 mL) at
room temperature. [b] Yields of the isolated products are given.
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Table 3. General substrate scope of the reaction.[a]
[a] Reaction conditions: 2-oxo aldehyde 1 (1 mmol), 4-azidophenol (2, 1.1 mmol), benzotriazole 3 (1.1 mmol), and Cu(OAc)2 (10 mol-%) in toluene (3 mL) at
room temperature. [b] Yields of the isolated products are given.
with other benzotriazoles, namely, 5,6-dimethylbenzotriazole, 5- Experiment (2)], we isolated the respective keto amides in high
chlorobenzotriazole, and 5-methylbenzotriazole, proceeded yields. This in fact is a new approach to hydroxy-substituted α-
smoothly and gave analogous yields of the desired products keto amides. This experiment indeed pointed towards the in-
4p, 4q, and 4r, respectively. However, as expected, reactions
with 5-substituted BTZs produced regioisomers (see 4q/4q′ and
4r/4r′ in the Supporting Information).
In continuation, another set of experiments was performed
between different OAs, BTZs, and 4-azidophenol to check the
volvement of an α-keto amide as an intermediate for the gener-
ation of product 4. To investigate this possibility, we performed
a reaction between keto amide 5a (1 mmol) and 3a (1.1 mmol)
under the optimized conditions [Experiment (3)]. The absence
of any reaction proved the non-engagement of a keto amide
substrate scope (Table 3). In general, substituents at different in the reaction. Experiments (1)–(3) proved that a reaction inter-
positions of the arene group of the OA and their electronic mediate is first generated from 1 and 2, and it reacts in an
nature did not affect the effectiveness of the reaction (see 4s– unprecedented manner with 3 to give the desired product 4.
ad). However, variations in the BTZ affected the overall yields Furthermore, failure of phenylglyoxal (1a) to react with phenyl
to some extent.
azide and 3a certainly indicated the crucial role of 4-azido-
phenol in facilitating attack of the BTZ [Experiment (4)]. In con-
tinuation, two different reactions [Experiments (5) and (6)] were
performed with 2-/3-azidophenol under the same conditions.
However, in both reactions, no product was generated, which
thus indicated the important role of the para-hydroxy group in
To gain insight into the mechanism of this reaction and to
establish the significance of different groups/reagents, several
control experiments were performed (Figure 2). In Experi-
ment (1), the failure of phenylglyoxal (1a; 1 mmol) to react with
3a (1.1 mmol) under the optimized conditions directly con-
firmed the nonparticipation of the BTZ with the OA in the initial 2 in promoting the reaction through a benzoquinone imine
step. Further, upon performing four different reactions between system. In addition, the reaction of benzaldehyde (1 mmol) with
2-oxo aldehydes 1 (1 mmol) and 4-azidophenol [2; 1.1 mmol; 2 and 3a failed to produce any product, which thereby once
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again proved the role of the 2-oxo group in facilitating the reac-
tion [Experiment (7)]. Furthermore, we performed the optimized
reaction in the presence of BF3·OEt2 [Experiment (8)] and found
a slight increase in the yield of 4a. However, upon performing
the optimized reaction in the presence of Cu(OAc)2 (10 mol-%)
under argon, the yield was not improved [Experiment (9)]. Both
of these experiments indicate that copper, besides acting as a
Lewis acid, increases the nucleophilicity of the BTZ through the
assistance of air, which ultimately enhances the overall yield of
the reaction.
On the basis of our understanding of activation through the
RCOCHN1N2 system,[16–18] literature reports on the activation
of copper,[19] and the above-mentioned results, we propose a
mechanism for this reaction (Figure 3). The preliminary step
involves the reaction of 2-oxo aldehyde 1 with 4-azidophenol
(2) to form intermediate A. Intermediate A reacts at room tem-
perature through the assistance of the para-hydroxy group and
extrudes nitrogen to give intermediate B. On the one hand,
intermediate B can undergo tautomerism followed by rear-
rangement to C, which ultimately generates α-keto amide 5.
On the other hand, in the presence of 3, intermediate B reacts
through an air-assisted, copper-catalyzed cycle to produce a
novel RCOCHN1N2 system (intermediate D) having an electro-
philic benzoquinone imine group. This intermediate perhaps
generates intermediate E, which loses nitrogen (as a result of
conjugation) and undergoes the expected 6,1-addition[20] to
give product 4.
Conclusions
We established a new concept around the benzoquinone imine
assisted ring-opening/ring-closing strategy of the RCOCHN1N2
system and successfully applied it to the generation of different
1,2-disubtituted benzimidazoles from readily available sub-
strates. Further studies related to the applications of this system
in terms of different electrophiles are in progress.
Figure 2. Control experiments.
Figure 3. Plausible mechanism.
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Experimental Section
General Procedure for the Synthesis of 4a–ad: A reaction vessel
was charged with 2-oxo aldehyde 1 (1 mmol), 4-azidophenol (2;
1.1 mmol) and benzotriazole 3 (1.1 mmol) in toluene (3 mL). The
mixture was stirred at room temperature for 12 h. Upon completion
of the reaction, the solvent was removed under reduced pressure.
The crude mixture was purified by column chromatography (100–
200 # silica gel; CH2Cl2/MeOH, 9:1) to afford product 4 in good yield
(72–84 %).
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Acknowledgments
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A. K. thanks the University Grants Commission (UGC) for the
award of a JRF. Q. N. A. also thanks the analytical department
of the Indian Institute of Integrative Medicine (IIIM). Finally, the
authors are grateful to the Council of Scientific and Industrial
Research (CSIR) network project (BSC-0108) for funding this
work.
Keywords: Aldehydes · Copper · Nitrogen · Nitrogen
heterocycles · Multicomponent reactions
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Received: March 10, 2017
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