Metal-Free One-Pot Synthesis of N,N′-Diarylamidines and N-Arylbenzimidazoles from Arenediazonium Salts, Nitriles, and Free Anilines

Article abstract of DOI:10.1021/acs.orglett.6b02994

A highly efficient and facile metal-free, one-pot reaction has been developed to afford diversely substituted N-arylbenzimidazoles through chemoselective in situ generation of N,N′-diarylamidines from arenediazonium salts, nitriles, and free anilines. The advantages of this protocol consist of the operationally easy and simple one-pot procedure under metal-free and mild conditions, the direct use of inexpensive and commercially available chemicals, and thus, a cost-effective and greener process.

Full text of DOI:10.1021/acs.orglett.6b02994

Letter
pubs.acs.org/OrgLett
Metal-Free One-Pot Synthesis of N,N′-Diarylamidines and
N‑Arylbenzimidazoles from Arenediazonium Salts, Nitriles, and Free
Anilines
So Won Youn* and Eun Mi Lee
Center for New Directions in Organic Synthesis, Department of Chemistry and Institute for Material Design, Hanyang University,
Seoul 04763, Korea
S
* Supporting Information
ABSTRACT: A highly efficient and facile metal-free, one-pot
reaction has been developed to afford diversely substituted N-
arylbenzimidazoles through chemoselective in situ generation
of N,N′-diarylamidines from arenediazonium salts, nitriles, and
free anilines. The advantages of this protocol consist of the
operationally easy and simple one-pot procedure under metal-
free and mild conditions, the direct use of inexpensive and commercially available chemicals, and thus, a cost-effective and
greener process.
enzimidazoles are ubiquitous motifs found in a broad
spectrum of biologically active natural products and
N-aryl(thio)amides with anilines (path d, Scheme 1). The former
provides only symmetrical N,N′-diarylamidines, while the latter
requires the use of (thio)phosgene, oxalyl chloride, PCl5, PCl3, or
SOCl2 (for amides) and HgBr2 or HgO (for thioamides), which
are highly toxic and corrosive.8 Transition-metal-catalyzed
domino processes for the in situ formation of N,N′-diary-
lamidines from N-arylamidines (path e, Scheme 1)9 or N,N′-
diarylcarbodiimides (path f, Scheme 1)10 and subsequent
cyclization have also been developed for the synthesis of N-
arylbenzimidazoles.
Although progress has been made for the formation of N-
arylbenzimidazoles, which remains more challenging than N-
unsubstituted benzimidazoles, most of the known methods have
several conspicuous drawbacks, such as requirement of not
readily accessible starting materials, multistep procedures,
narrow substrate scope, use of costly and/or toxic transition
metals and reagents, and harsh reaction conditions, leading to
cost and environmental concerns and limiting their practical
utility. Hence, the development of new synthetic methods with
improved efficiency and generality for the construction of the N-
arylbenzimidazole skeleton is highly desirable.
B
pharmaceuticals as well as materials.1 Among them, N-
arylbenzimidazoles are a class of privileged heterocycles that
are known to possess various biological properties,2 and they
have also been used as the backbone in dyes, polymers, and
ligands.3 Consequently, a diverse range of synthetic strategies
have been reported for the construction of N-arylbenzimida-
zoles.4 Intramolecular cyclization of N,N′-diarylamidines is one
of the straightforward methods for the construction of an N-
arylbenzimidazole moiety; therefore, a variety of transition-
metal-catalyzed cross-coupling reactions of N-(o-haloaryl)-
amidines have been developed (path a, Scheme 1).5
Scheme 1. Synthetic Methods for the Construction of N-
Arylbenzimidazoles
Because of their easy preparation and wide availability from
inexpensive and readily available anilines, arenediazonium salts
have been used as useful and versatile precursors in a variety of
organic transformations.11 The intrinsic electrophilicity of
dizonium salts offers a pathway for the introduction of various
functional groups into an aromatic ring. In general, their
reactions with anilines lead to the formation of triazenes and
azo compounds through the addition of anilines over the
diazonium moiety (Scheme 2, top).12 In biaryl synthesis via an
aryl−aryl coupling between diazonium salts and anilines,
therefore, protection strategies have been developed to utilize
Undoubtedly, a more efficient route toward this scaffold is a
direct C−H amination of N,N′-diarylamidines, obviating the
need for prehalogenation (path b, Scheme 1). Both transition-
metal catalysis6 and metal-free7 reactions have been developed
for this process. The requisite N,N′-diarylamidines for C−H
amination are generally prepared by the reaction of 2 equiv of an
aniline with orthoesters (path c, Scheme 1) or by the reaction of
Received: October 4, 2016
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.6b02994
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Letter
was introduced, triggering the subsequent oxidative cyclization
to afford N-arylbenzimidazoles. In contrast to the stepwise
reaction, the presence of K2CO3 in step 1 was beneficial to the
formation of N-arylbenzimidazoles in step 2 of the one-pot
process; thus, we speculate that the in situ generated acid (i.e.,
HBF4) might have a deleterious effect on the subsequent C−H
amination. Finally, the use of 2 equiv of PhI(OAc)2 was required
to give the optimal results. As shown in Scheme S1, a one-pot
reaction was preferable to the stepwise reaction with regard to
product yield and efficiency, avoiding the tedious and time-
consuming step-by-step isolations and purifications of inter-
mediates and reducing the amount of waste.
Scheme 2. Addition vs Substitution Reactions of
Arenediazonium Salts
free anilines as a reactant.13 On the other hand, substitution
reaction of arenediazonium salts with nitriles is also well-known
to occur along with the loss of N2 and to afford N-arylnitrilium
salts (I), which can further undergo the subsequent reaction with
other nucleophiles under metal-free conditions (Scheme 2,
bottom),14 while acetonitrile (MeCN) has been used as a
favorable solvent in a variety of reactions of arenediazonium
salts.11 Therefore, we were interested in the reactivity of
arenediazonium salts in the presence of both nitriles and anilines,
which are both good nucleophiles for arenediazonium salts. Since
N2 is readily replaced by nucleophilic solvents, such as MeCN,
which exist in large quantities as reaction media, we envisaged
that arenediazonium salts could be attacked preferentially by
nitriles over anilines to afford N-arylnitrilium salts (I), followed
by the addition of anilines as a second nucleophile, leading to
N,N′-diarylamidines (II), which can be further transformed to N-
arylbenzimidazoles. The realization of this proposal would make
significant advances, overcoming the aforementioned deficien-
cies of the precedents for the synthesis of N-arylbenzimidazoles
as well as N,N′-diarylamidines: use of simple, cheap, amenable,
and easily accessible reagents, easy assembly of unsymmetrical
N,N′-diarylamidines from two different aniline precursors via
one-pot reaction, and a good chance of using metal-free
conditions.
With the establishment of a viable one-pot reaction system, we
set out to explore the scope of this domino process. First, we
examined the reaction of various free anilines (1) with
phenyldiazonium salt (2a) in MeCN (Scheme 3). A wide
a
Scheme 3. Substrate Scope: Anilines
Herein, we report a highly effective one-pot reaction to
construct a wide range of diversely substituted N-arylbenzimi-
dazoles (path g, Schemes 1 and 2, bottom). This new protocol
involves the chemoselective in situ formation of N,N′-diary-
lamidines from arenediazonium salts, nitriles, and free anilines.
We began our studies on the proposed reaction using N-
methyl-p-toluidine and phenyldiazonium salt (2a) as the test
substrates in MeCN and first examined the viability of three-
component reaction to form amidines (for details, see the
Supporting Information). Much to our delight, a simple mixture
of these three reagents gave the desired amidine product in 58−
66% yields regardless of the presence of K2CO3. These findings
suggest that, compared to anilines, nitriles are indeed more
favorable to attack arenediazonium salts, leading to N-
arylnitrilium salts (I) rather than triazenes and/or azo
compounds. K2CO3and MeCN were identified as the most
effective base and solvent to give the desired amidine products;
other bases and solvents led to no formation of amidines and/or
lower efficiency. Gratifyingly, primary anilines such as p-toluidine
and aniline also proved to be suitable substrates in this reaction.
With the success of amidine formation, metal-free oxidative C−
H amination of N,N′-diarylamidines (II, R = H) derived from
primary anilines was next investigated under the previously
reported conditions.7 Reliable and reproducible outcomes could
be obtained under the slightly modified conditions, using 1 equiv
of PhI(OAc)2 in MeCN at 25 °C. With the established optimal
conditions for a stepwise process, we proceeded with the
investigation of the one-pot domino reaction. Upon completion
of the three-component amidine-forming reaction, PhI(OAc)2
a
Standard reaction conditions: (1) 1 (1 equiv), 2a (1 equiv), and
K2CO3 (1 equiv) in MeCN (0.1 M) at 90 °C for 2 h; (2) PhI(OAc)2
(2 equiv) at 25 °C for 2 h. Combined isolated yields of two separable
regioisomers (3 and 4) are provided. For 18 h in step 2. For 6 h in
b
c
d
step 2. 3 and 4 are inseparable, and ratios of 3 and 4 were determined
e
f
g
by 1H NMR. For 8 h in step 2. For 42 h in step 2. At 40 °C for 15 h
h
in step 2. At 80 °C in step 2.
range of anilines underwent domino three-component amidine
formation/oxidative cyclization smoothly to afford the corre-
sponding N-arylbenzimidazoles (3 and 4) in good yields
irrespective of their electronic properties and the position of
their substituents. However, anisidine derivatives led to
complicated mixtures along with significant decomposition in
step 1. Consistent with the related hypervalent iodine(III)-
mediated oxidative cyclization,7 two regioisomers could be
formed from the in situ generated unsymmetrical N,N′-
diarylamidines: Oxidative cyclization reaction took place
preferentially on the electron-rich benzene ring over electron-
deficient counterparts through an electrophilic pathway. Note-
worthy is the fact that this process can tolerate various functional
groups such as halogen, nitro, ester, and cyano groups. This
functional group tolerance could be advantageous compared to
other transition-metal-catalyzed syntheses of N-arylbenzimida-
zoles5,6,9,10 and makes this method particularly appealing, since it
should permit further elaboration and enable greater structural
diversity (vide infra).
B
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Next, we proceeded to investigate the scope of arenediazo-
nium salts (2) for this reaction (Scheme 4). Electron-neutral and
arenediazonium salts and leading to a direct and probably
regioselective assembly of N-arylbenzimidazoles from two
different anilines. Indeed, as shown in Scheme 5, a variety of
Scheme 4. Substrate Scope: Arenediazonium Salts and
Nitriles
a
Scheme 5. One-Pot, Three-Step Synthesis of N-
Arylbenzimidazoles
a
a
Reaction conditions: (1) Pre-2 (1 equiv), tBuONO (1 equiv), and aq
HBF4 (1 equiv) in MeCN (0.1 M) at 25 °C for 0.5 h; (2) 1 (1 equiv)
and K2CO3 (1 equiv) at 90 °C for 2 h; (3) PhI(OAc)2 (2 equiv) at 25
b
°C for 2 h. Isolated yields are provided. At 40 °C for 3 h in step 3.
N-arylbenzimidazoles 4 could be synthesized regioselectively via
a one-pot, three-step reaction, albeit in somewhat lower yields
than those in a one-pot, two-step reaction. Again, high functional
group tolerance was observed in this one-pot reaction.
To highlight the synthetic utility of this one-pot reaction,
further elaboration of the halogenated N-arylbenzimidazoles
obtained from this process has been undertaken. As illustrated in
Scheme 6, Suzuki reaction of 3 and 4, which have bromo and/or
a
Standard reaction conditions as described in Scheme 3. Isolated
b
c
d
yields are provided. For 13 h in step 2. Separable isomers. For 9 h
in step 2. Inseparable isomers. Using 3 equiv PhI(OAc)2.
e
a
Scheme 6. Synthetic Application
moderately electron-rich aryl groups were well tolerated
regardless of the position of their substituents. Arenediazonium
salts with a strongly electron-donating substituent, such as MeO,
at the ortho or para positions led to decomposition or low
conversion, respectively, while the reactions with a meta-
substituted salt uneventfully proceeded to give the correspond-
ing N-arylbenzimidazoles (4ffa,b and 4ffa,b′) in good yields. In
contrast, strongly electron-withdrawing substituents, such as
NO2, resulted in the formation of only azo compounds in step 1.
These results suggest the SN1 pathway seems more likely than
SNAr among ionic mechanisms involved in fragmentation of
diazonium salts.11 Oxidative cyclization of the meta-substituted
substrates resulted in an almost 1:1 mixture of two regioisomers
(4ffa,b and 4fga vs 4ffa,b′ and 4fga′). Particularly noteworthy is
the fact that excellent regioselectivity was observed for the
substrates bearing iodo- and bromo-substituted two-benzene
rings with the cyclization taking place only on the iodo-
substituted benzene ring (3jha) as well as with both halogens
remaining intact.
Last, we explored the scope of nitriles. Primary and secondary
alkyl and aryl nitriles proved to be suitable substrates for the
formation of the corresponding 2-alkyl- and 2-arylbenzimida-
zoles. However, the reaction with bulky tert-alkyl nitriles such as
tBuCN failed to afford the desired N-arylbenzimidazole products
even at the higher reaction temperature for a longer reaction time
in step 2 (at 90 °C for 24 h), leading to only the corresponding
N,N′-diarylamidine intermediate along with significant decom-
position in step 2.
Since diazonium salts are generally not commercially available,
the requirement of their preparation may limit the synthetic
utility of this process. Therefore, their in situ preparation from
readily available anilines through diazotization followed by
amidine formation with another anilines and subsequent
oxidative cyclization would significantly improve the practicality
and efficiency of this one-pot protocol, obviating the isolation of
a
For details of the reaction conditions, see the Supporting
Information.
iodo substituents at either the benzimidazole moiety or 1-aryl
substituent, with arylboronic acids proceeded smoothly to give
the highly conjugated, polysubstituted benzimidazoles in good to
high yields. Particularly noteworthy is the regiocontrolled
synthesis of both 5f and 5h from a common, dihalogenated
substrate 3jha.
In the three-component amidine formation, two reaction
pathways, ionic or radical mechanisms, may be proposed.11 To
gain insight into this reaction, the amidine-forming reaction was
performed in the presence of a radical scavenger, such as
C
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TEMPO or 1,1-diphenylethylene (for details, see the SI). Their
inclusion had a deleterious effect on the amidine formation, and
an arylated 1,1-diphenylethylene was obtained as a byproduct,
whereas the corresponding TEMPO adduct was not observed.
These results may suggest a mechanism involving a radical
intermediate during the reaction. However, an alternative SN1-
type ionic pathway cannot be completely excluded.11,14
In summary, we developed a highly efficient and facile one-pot
reaction for the synthesis of diversely substituted N-arylbenzi-
midazoles through chemoselective in situ generation of N,N′-
diarylamidines from arenediazonium salts, nitriles, and free
anilines. The remarkable features of this protocol are the
operationally easy and simple one-pot procedure under metal-
free and mild conditions and the use of inexpensive, readily
available, simple chemical feedstocks: nitriles and free anilines.
The three-component reaction presented herein represents an
attractive and potentially powerful route for a straightforward
access to a diverse range of N,N′-diarylamidines and further
functionalized N-arylbenzimidazoles, offering a new access to N-
heterocycles from arenediazonium salts. Further investigations
to construct other privileged heterocyclic scaffolds with this
strategy are currently underway in our laboratory.
ASSOCIATED CONTENT
* Supporting Information
■
S
(8) (a) Dunn, P. J. Amidines and N-Aryl Amidines. In Comprehensive
Organic Functional Group Transformations II; Katritzky, A., Taylor, R.,
Eds.; Elsevier: Oxford, 2005; Vol. 5, pp 655−699. (b) Shriner, R. L.;
Neumann, F. W. Chem. Rev. 1944, 35, 351. (c) Rauws, T. R. M.; Maes, B.
U. W. Chem. Soc. Rev. 2012, 41, 2463. (d) Katritzky, A. R.; Cai, C.; Singh,
S. K. J. Org. Chem. 2006, 71, 3375 and references cited therein.
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.orglett.6b02994.
Full experimental details and characterization data (PDF)
AUTHOR INFORMATION
Corresponding Author
́
(9) (a) Li, J.; Benard, S.; Neuville, L.; Zhu, J. Org. Lett. 2012, 14, 5980.
■
(b) Sun, M.; Chen, C.; Bao, W. RSC Adv. 2014, 4, 47373.
(10) He, H.-F.; Wang, Z.-F.; Bao, W. Adv. Synth. Catal. 2010, 352,
2905.
*E-mail: sowony73@hanyang.ac.kr.
Notes
(11) (a) Patai, S. The Chemistry of Diazonium and Diazo Groups; John
Wiley & Sons: Chichester, 1978. (b) Saunders, K. H. The Aromatic
Diazonium Compounds and their Technical Applications; Edward Arnold
and Co.: London, 1949. (c) Zollinger, H. Angew. Chem., Int. Ed. Engl.
1978, 17, 141. (d) Galli, C. Chem. Rev. 1988, 88, 765. (e) Roglans, A.;
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by both the Basic Science Research
Program and Nano·Material Technology Department Program
through the National Research Foundation of Korea (NRF)
funded by the Ministry of Education and Ministry of Science,
ICT and future Planning (Nos. 2012M3A7B4049644,
2015R1A2A2A01002559, and 2014-011165).
Pla-Quintana, A.; Moreno-Manas, M. Chem. Rev. 2006, 106, 4622.
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(f) Mahouche-Chergui, S.; Gam-Derouich, S.; Mangeney, C.; Chehimi,
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Wiley−VCH: Weinheim, 2005. (b) Hartman, W. W.; Dickey, J. B. Org.
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