Hypervalent iodine(iii) catalyzed oxidative C-N bond formation in water: Synthesis of benzimidazole-fused heterocycles

Article abstract of DOI:10.1039/c4ra02279c

An iodine(iii) catalyzed C(sp²)-H functionalization- intramolecular amination reaction of N-aryl-2-amino-N-heterocycles has been developed in water and under ambient conditions. This metal-free open-flask chemistry is general and successfully applied in synthesizing the benzimidazole-fused heterocycles pyrido[1,2-a]benzimidazole, benzimidazo[1,2-a]quinoline and benzimidazo[2,1-a]isoquinoline derivatives. the Partner Organisations 2014.

Full text of DOI:10.1039/c4ra02279c

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Hypervalent iodine(III) catalyzed oxidative C–N
bond formation in water: synthesis of
benzimidazole-fused heterocycles†
Cite this: RSC Adv., 2014, 4, 25600
b
ab
D. Nageswar Rao,ab Sk. Rasheed,ab Ram A. Vishwakarma and Parthasarathi Das
Received 15th March 2014
Accepted 26th April 2014
*
DOI: 10.1039/c4ra02279c
www.rsc.org/advances
An iodine(III) catalyzed C(sp2)–H functionalization–intramolecular
The benzimidazole-fused heterocyclic scaffold exists in a
amination reaction of N-aryl-2-amino-N-heterocycles has been wide range of biologically active compounds (Fig. 1).7 Conse-
developed in water and under ambient conditions. This metal-free quently, substantial synthetic methods have been developed for
open-flask chemistry is general and successfully applied in synthe- the preparation of this class of molecules.8–10
sizing the benzimidazole-fused heterocycles pyrido[1,2-a]benzimid-
azole, benzimidazo[1,2-a]quinoline and
isoquinoline derivatives.
Synthesis of pyrido[1,2-a]benzimidazoles by copper(II) cata-
benzimidazo[2,1-a]- lyzed intramolecular C–H amination of N-aryl-2-aminopyridines
in acidic media has been reported by Zhu8d and Maes8e inde-
pendently. However, both of the approaches require a high
reaction temperature (120 ꢀC), and the substrate scope is
Direct cross-dehydrogenative amination of inert C(sp2)–H
bonds has become a valuable tool in organic synthesis.1 Over
the years, numerous protocols for intramolecular direct C–N
bond formation have been developed, and most of these
methods involve the catalytic use of Pd complexes2 or Cu salts.3
Generally, transition metal catalyzed carbon–heteroatom bond
formation reactions require elevated temperatures and high
loading of the catalyst and/or metal oxidant. Furthermore, the
presence of heavy metal contaminants in the nal products
restricts their application in the bulk synthesis of active phar-
maceutical ingredients (APIs) for commercial use. Hence,
reaction protocols that enable the preparation of nitrogen
containing compounds through C–N cross-coupling reactions
in the absence of transition metals are attractive and considered
greener.4 In this context, hypervalent iodine reagents that
promote oxidative C–N bond formation have received much
attention due to their low toxicity, comparable reactivity with
transition-metals and availability.5,6 However, in most cases of
iodine(III) mediated cross-coupling chemistry, uorinated sol-
vents6a,e,h,i are still preferred, which severely impedes applica-
tion, particularly in large scale synthesis. Therefore a new, more
efficient and environmentally benign metal-free catalytic system
for oxidative C–H amination is highly desirable.
limited in terms of substituents on the pyridine moiety. An
alternative approach mediated by iodine(III) has also been
reported by Zhu recently.6i However, this methodology failed in
terms of regioselectivity and the use of expensive uorinated
alcohol remains the drawback of this synthesis. To overcome
these issues, herein, we report oxidative C–N bond formation
using a catalytic amount of in situ generated hypervalent iodi-
ne(III) reagent [hydroxy(tosyloxy)iodo]benzene (Koser's reagent,
HTIB)11 in water12,13 and under ambient conditions (Scheme 1).
This protocol is general, regioselective and has been
successfully utilized in synthesizing medicinally important
heterocycles such as pyrido[1,2-a]benzimidazole, benzimi-
dazo[1,2-a]quinoline and benzimidazo[2,1-a]isoquinoline
derivatives.
First, N-phenyl-2-aminopyridine14 1a was selected as a model
compound to explore the optimized reaction conditions with
iodine(III) in water at room temperature (Table 1). It was found
that the reaction of 1a with phenyliodine diacetate (PIDA, 1
aAcademy of Scientic and Innovative Research (AcSIR), India
bMedicinal Chemistry Division, Indian Institute of Integrative Medicine (CSIR), Canal
Road, Jammu-180001, India. E-mail: partha@iiim.ac.in; Fax: +91-191-2569019; Tel:
+91-191-2560000
† Electronic supplementary information (ESI) available: Experimental details and
spectroscopic data for all compounds. See DOI: 10.1039/c4ra02279c
Fig. 1 Medicinally interesting compounds containing benzimidazole-
fused heterocycles.
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water, an annulated product was obtained in a 75% yield (entry
11, Table 1).
When we used Koser's reagent (HTIB) (1.0 equiv.) in the
absence of m-CPBA and PTSA, 2a was isolated in an 85% yield,
which indicates that Koser's reagent is the active species (entry
12, Table 1).
Reactions of a variety of N-arylated-2-aminopyridines14 were
investigated under the optimized reaction conditions (Table 2).
As depicted in Table 2, reactions involving N-aryl-2-amino-
pyridines bearing electron-donating (Me and t-Bu) or electron-
withdrawing groups (F, Cl, Br, CF3) at the para position of the
aniline moiety proceeded to give almost quantitative yields (2b–
g). A high yield was also obtained with ortho substitution as
compound 2h was isolated in an 89% yield. Reactions involving
both electron donating (OMe) and electron withdrawing (F, Cl,
CF3, NO2) substituents at the meta-position of the aniline of the
Scheme 1 Formation of benzimidazole-fused heterocycles.
Table 1 Optimization of the reaction conditionsa
Entry Precat. Amount
Additive
Oxidant t (h) Yieldb (%)
N-aryl-2-aminopyridine derivatives proceeded in
a highly
regioselective manner to afford exclusively C-7 substituted pyr-
ido[1,2-a]benzimidazoles (2i–m).8d,e Interestingly, no other
regioisomers i.e. C-9 substituted pyrido[1,2-a]benzimidazoles
were detected. In this context, a disubstituted derivative has
been tested under these optimized conditions and the product
2n was isolated in an 82% yield with complete regioselectivity.
To further enhance the generality of the reaction, substitution
on the pyridine ring was also investigated. 5-Bromo and 2-
methyl derivatives reacted efficiently to give the corresponding
products 2o and p, and 2q, respectively in excellent yields (90–
94%). Cyclization of N-napthyl-2-aminopyridine was also per-
formed under the optimized conditions and a novel pyridine
annulated naptho imidazo product (2r) was isolated in an 82%
yield. By applying these optimized conditions the benzo[d][1,3]
1
PIDA
PIDA
PIDA
PIDA
PIDA
PIDA
PIDA
PIDA
PIDA
PIDA
PhI
1 equiv.
1 equiv.
1 equiv.
1 equiv.
0.5 equiv. PTSA$H2O
1 equiv.
0.2 equiv. PTSA$H2O
0.2 equiv. PTSA$H2O
0.2 equiv. PTSA$H2O
0.1 equiv. PTSA$H2O
0.2 equiv. PTSA$H2O
—
—
—
—
—
—
—
—
24
24
4
8
8
8
4
8
8
n. r.
n. r.
90
15
30
40
90
n. r.
n. r.
60
2c
3
PTSA$H2O
CH3SO3H
4
5
6
7
8
9
10
11
12
PTSA$H2Od
m-CPBA
TBHPe
f
H2O2
m-CPBA
m-CPBA
—
4
4
4
75
85
HTIB
1 equiv.
—
a
Reaction conditions: 1a (1.0 equiv.), PIDA (0.2 equiv.), PTSA$H2O (2
b
equiv.), m-CPBA (1 equiv.) and H2O (1 mL); rt. Isolated yields.
c
f
d
e
Reaction carried out at 100 ꢀC. 1.0 equiv. used. 70 vol% in water.
30 vol% in water; n. r. ¼ no reaction.
Table 2 Synthesis of pyrido[1,2-a]benzimidazole derivativesa
equiv.) in water at room temperature was unsuccessful (entry 1,
Table 1). The product was not formed even at an elevated
temperature, 100 ꢀC (entry 2, Table 1). To our delight when we
performed the reaction with PIDA (1 equiv.) in the presence of p-
toluenesulphonic acid monohydrate (PTSA$H2O) (2 equiv.) as
an additive, the desired product was isolated in a 90% yield
(entry 3, Table 1).11a When methane sulphonic acid (2 equiv.)
was combined with PIDA a poor yield (15%) was obtained (entry
4, Table 1). When we reduced the amount of PIDA from 1 to 0.5
equiv., the yield of the isolated product dropped to 30% (entry 5,
Table 1). The yield also decreased when the amount of
PTSA$H2O was reduced from 2 equiv. to 1 equiv. (entry 6, Table
1). Next we turned our attention to making the reaction catalytic
and performed the reaction using PIDA with several oxidants
(entries 7–9, Table 1). It was observed that a combination of
PIDA (0.2 equiv.) with PTSA$H2O (2 equiv.) and m-CPBA (1
equiv.) in water gave 2a in the best yield (90%) (entry 7, Table 1).
Oxidants like tert-butyl hydroperoxide (TBHP) (entry 8, Table 1)
and H2O2 (entry 9, Table 1) remained ineffective. The use of 10
mol% PIDA gave the desired product only in a 60% isolated
yield (entry 10, Table 1). When we tried to promote the cycli-
zation by in situ generated HTIB by using PhI (0.2 equiv.) as an
iodine source11b with PTSA$H2O and m-CPBA as an oxidant in
a
Reaction conditions: N-arylpyridine-2-amine (0.29 mmol), Phl(OAc)2
(0.058 mmol), m-CPBA(0.29 mmol), PTSA$H2O (0.58 mmol) and H2O
(1 mL); rt; air.
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dioxole annulated pyridoimidazo compound (2s) has been
synthesized in a 94% yield.
Next we turned our attention to applying the present
protocol for the synthesis of another important complex
heterocyclic system, benzimidazo[1,2-a]quinoline (Table 3).
Under the optimized conditions, N-aryl-2-aminoquinolines14
bearing electron donating (3a) or electron withdrawing groups
(3b–d) produced the desired heterocycles in high yields (92–
95%). Here also the reaction was highly regioselective as
substrates with substituents at the meta-position of the aniline
ring gave exclusively C-9 substituted products (3e–g) in good
yields (72–75%). We have further applied the optimized
conditions in the synthesis of benzimidazo[2,1-a]isoquinolines
(Table 3).
We were pleased to nd that the iodine(III) promoted C–H
amination of N-arylisoquinoline-1-amine derivatives14 was facile
and the cyclic compounds were isolated in excellent yields (84–
85%) (Table 3). The electronic nature of the substituents at the
para-position (3h–j) had no effect on the cycloamination reac-
tion. In this case also the reaction remained completely regio-
selective and only the C-10 substituted product (3k) was isolated
in a 72% yield.
Scheme 3 A plausible reaction mechanism.
Based on these ndings and previous literature reports15,6c,h,i
we put forward a plausible mechanism for the amination of 1a.
We suggest that the operating mechanism for this reaction
starts from an interaction between the in situ generated PhI(OH)
OTs (Koser's reagent) and N-phenyl-2-aminopyridine (1a)
(Scheme 3), to result in the electrophilic N-iodo species A. In the
subsequent steps the electrophilic annulation on the pyridine
nitrogen of A generates intermediate B, which upon deproto-
nation forms 2a (eqn (1), Scheme 3). The eliminated PhI enters
the catalytic cycle upon oxidation by m-CPBA in the presence of
PTSA$H2O to generate reactive iodine(III) PhI(OH)OTs and
complete the catalytic cycle. An explanation for the high regio-
selectivity can be put forward, where the intermediate AA is
favoured over AB due to steric effects (eqn (2), Scheme 3), in
order to give one regioisomer exclusively.
The scalability of this reaction was tested by performing the
reaction of 1a on a gram scale (5.8 mmol) under the optimized
conditions (Scheme 2).
Table 3 Synthesis of benzimidazo[1,2-a]quinoline and benzimidazo
[2,1-a]isoquinoline derivativesa
In conclusion, we have developed a practical method for the
synthesis of benzimidazole-fused heterocycles from readily
available N-aryl-2-amino-N-heterocycles under metal-free condi-
tions. The reaction is catalyzed by in situ generated hypervalent
iodine(III) at room temperature. Use of water as a solvent and
open-ask chemistry makes this process greener and more
attractive for large scale synthesis. To the best of our knowledge,
this is one of the rare examples in which water has been used as a
solvent in hypervalent iodine(III) catalyzed oxidative C–N bond
formation. More signicantly, complete control of the regiose-
lectivity was achieved in this C–H cycloamination process. In view
of the growing understanding of hypervalent iodine C–H activa-
tion–functionalization processes, the reaction described herein
showcased a reactivity prole that is notably different to those
previously reported.6h,i,8c It is believed that the new hypervalent
iodine(III) promoted protocol will add value in developing a
number of efficient and practical methods for C–N bond
construction from unactivated C–H bonds.
a
Reaction conditions: N-arylquinolin-2-amine/N-arylisoquinolin-1-
amine (0.29 mmol), Phl(OAc)2 (0.058 mmol), m-CPBA (0.29 mmol),
PTSA$H2O (0.58 mmol) and H2O (1 mL); rt; air.
Acknowledgements
D. N. R and Sk. R. thank UGC and CSIR-New Delhi for
research fellowship, respectively. This research work is
nancially supported by CSIR-New Delhi (BSC 0108). IIIM
communication no 1491.
Scheme 2 Gram scale synthesis of pyrido[1,2-a]benzimidazole.
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K. Kulikov and J. Lategahn, Angew. Chem., Int. Ed., 2011,
50, 8605; (b) H. J. Kim, J. Kim, S. H. Cho and S. Chang, J.
Am. Chem. Soc., 2011, 133, 16382; (c) S. K. Alla,
R. K. Kumar, P. Sadhu and T. Punniyamurthy, Org. Lett.,
2013, 15, 1334; (d) A. A. Kantak, S. Potawarthi,
R. A. Barham, K. M. Romano and B. DeBoef, J. Am. Chem.
Soc., 2011, 133, 19960; (e) J. Huang, Y. He, Y. Wang and
Q. Zhu, Chem.–Eur. J., 2012, 18, 13964; (f) C. Roben,
Notes and references
1 For recent reviews, see: (a) I. P. Beletskaya and
A. V. Cheprakov, Organometallics, 2012, 31, 7753; (b)
M. Zhang and A. Zhang, Synthesis, 2012, 1; (c) J. Bariwal
and E. Van der Eycken, Chem. Soc. Rev., 2013, 42, 9283; (d)
M.-L. Louillat and F. W. Patureau, Chem. Soc. Rev., 2014,
43, 901.
˜
J. A. Souto, E. C. Escudero-Adam and K. Muniz, Org. Lett.,
2 For Pd-catalyzed intramolecular oxidative C–N bond
formation, see: (a) W. C. P. Tsang, N. Zheng and
S. L. Buchwald, J. Am. Chem. Soc., 2005, 127, 14560; (b)
K. Inamoto, T. Saito, M. Katsuno, T. Sakamoto and
K. Hiroya, Org. Lett., 2007, 9, 2931; (c) W. C. P. Tsang,
R. H. Munday, G. Brasche, N. Zheng and S. L. Buchwald, J.
Org. Chem., 2008, 73, 7603; (d) J. A. Jordan-Hore,
C. C. C. Johansson, M. Gulias, E. M. Beck and M. J. Gaunt,
J. Am. Chem. Soc., 2008, 130, 16184; (e) T.-S. Mei, X. Wang
and J.-Q. Yu, J. Am. Chem. Soc., 2009, 131, 10806; (f) Y. Tan
and J. F. Hartwig, J. Am. Chem. Soc., 2010, 132, 3676; (g)
S. W. Youn, J. H. Bihn and B. S. Kim, Org. Lett., 2011, 13,
3738; (h) R. K. Kumar, Md. A. Ali and T. Punniyamurthy,
Org. Lett., 2011, 13, 2102; (i) T.-S. Mei, D. Leow, H. Xiao,
B. N. Laforteza and J.-Q. Yu, Org. Lett., 2013, 15, 3058.
3 For Cu-catalyzed intramolecular oxidative C–N bond
formation, see: (a) G. Brasche and S. L. Buchwald, Angew.
Chem., Int. Ed., 2008, 47, 1932; (b) M. M. Guru, M. A. Ali
and T. Punniyamurthy, J. Org. Chem., 2011, 76, 5295; (c)
S. H. Cho, J. Yoon and S. Chang, J. Am. Chem. Soc., 2011,
133, 5996; (d) X. Wang, Y. Jin, Y. Zhao, L. Zhu and H. Fu,
Org. Lett., 2012, 14, 452; (e) P. Sang, Y. Xie, J. Zou and
Y. Zhang, Org. Lett., 2012, 14, 3894; (f) G.-R. Qu, L. Liang,
H.-Y. Niu, W.-H. Rao, H.-M. Guo and J. S. Fossey, Org. Lett.,
2012, 14, 4494; (g) H. Chen, S. Sanjaya, Yi.-F. Wang and
S. Chiba, Org. Lett., 2013, 15, 212.
2013, 15, 1008; (g) E. Malamidou-Xenikaki, S. Spyroudis,
M. Tsanakopoulou and D. Hadjipavlou-Litina, J. Org.
Chem., 2009, 74, 7315; (h) D. Liang, Y. He, L. Liu and
Q. Zhu, Org. Lett., 2013, 15, 3476; (i) Y. He, J. Huang,
D. Liang, L. Liu and Q. Zhu, Chem. Commun., 2013, 49, 7352.
7 For medicinal properties of benzimidazole fused
heterocycles see: (a) M. Hranjec, I. Piantanida, M. Kralj,
´
L. Suman, K. Pavelı and G. Karminski-Zamola, J. Med.
Chem., 2008, 51, 4899; (b) M. Hranjec, M. Kralj,
´
´
I. Piantanida, M. Sedı, L. Suman, K. Pavelı and
G. Karminski-Zamola, J. Med. Chem., 2007, 50, 5696; (c)
H. Takeshita, J. Watanabe, Y. Kimura, K. Kawakami,
H. Takahashi, M. Takemura, A. Kitamura, K. Someya and
K. Nakajima, Bioorg. Med. Chem. Lett., 2010, 20, 3893; (d)
N. Perin, L. Uzelac, I. Piantanida, G. Karminski-Zamola,
M. Kralj and M. Hranjec, Bioorg. Med. Chem., 2011, 19,
6329; (e) A. J. Mdakala, R. K. Gessner, P. W. Gitari,
N. October, K. L. White, A. Hudson, F. Fakorede,
D. M. Shackleford, M. Kaiser, C. Yeates, S. A. Charman and
K. J. Chibale, J. Med. Chem., 2011, 54, 4581.
8 For synthesis of pyrido[1,2-a]benzimidazole see: (a)
K. Liubchak, K. Nazarenko and A. Tolmachev, Tetrahedron,
2012, 68, 2993; (b) Z. Wu, Q. Huang, X. Zhou, L. Yu, Z. Li
and D. Wu, Eur. J. Org. Chem., 2011, 5242; (c)
N. Kutsumura, S. Kunimatsu, K. Kagawa, T. Otani and
T. Saito, Synthesis, 2011, 3235; (d) H. Wang, Y. Wang,
C. Peng, J. Zhang and Q. Zhu, J. Am. Chem. Soc., 2010, 132,
13217; (e) K.-S. Masters, T. R. M. Rauws, A. K. Yadav,
W. A. Herrebout, B. Van der Veken and B. U. W. Maes,
Chem.–Eur. J., 2011, 17, 6315.
4 For selected examples of metal-free C–N bond formation,
see: (a) F. Diness and D. P. Fairlie, Angew. Chem., Int. Ed.,
2012, 51, 8136; (b) A. Beyer, C. M. M. Reucher and C. Bolm,
´
Org. Lett., 2011, 13, 2876; (c) I. Thome and C. Bolm, Org.
Lett., 2012, 14, 1892; (d) M. T. Barros, S. S. Dey,
C. D. Maycock and P. Rodrigues, Chem. Commun., 2012, 48,
10901; (e) W.-B. Wu and J.-M. Huang, Org. Lett., 2012, 14,
5832; (f) X. Zhang and L. Wang, Green Chem., 2012, 14,
2141; (g) Y. Yan, Y. Zhang, C. Feng, Z. Zha and Z. Wang,
Angew. Chem., Int. Ed., 2012, 51, 8077; (h) Y. Yan, Y. Zhang,
9 For synthesis of benzimidazo[1,2-a]quinoline see: (a)
J.-Y. Kato, Y. Ito, R. Ijuin, H. Aoyama and T. Yokomatsu,
Org. Lett., 2013, 15, 3794; (b) S. Xu, J. Lu and H. Fu, Chem.
Commun.,
2011,
47,
5596;
(c)
C.
Venkatesh,
G. S. M. Sundaram, H. Ila and H. Junjappa, J. Org. Chem.,
2006, 71, 1280.
´
Z. Zha and Z. Wang, Org. Lett., 2013, 15, 2274; (i) I. Thome,
10 For synthesis of benzimidazo[2,1-a]isoquinoline see: (a)
V. P. Reddy, T. Iwasaki and N. Kambe, Org. Biomol. Chem.,
2013, 11, 2249; (b) V. Rustagi, R. Tiwari and A. K. Verma,
Eur. J. Org. Chem., 2012, 4590; (c) N. Kavitha, G. Sukumar,
V. P. Kumar, P. S. Mainkar and S. Chandrasekhar,
Tetrahedron Lett., 2013, 54, 4198.
11 For synthesis of Koser's reagent see: (a) M. S. Yusubov and
T. Wirth, Org. Lett., 2005, 7, 519; (b) E. A. Merrih,
V. M. T. Carneiro, L. F. Silva Jr and B. Olofsson, J. Org.
Chem., 2010, 75, 7416.
C. Besson, T. Kleine and C. Bolm, Angew. Chem., Int. Ed.,
2013, 52, 7509; (j) C. Zhu, G. Li, D. H. Ess, J. R. Flack and
L. Kurti, J. Am. Chem. Soc., 2013, 134, 18253.
5 For selected reviews on hypervalent iodine see: (a) F. V. Singh
and T. Wirth, Synthesis, 2013, 2499; (b) R. Samanta, K. Matcha
and A. P. Antonchick, Eur. J. Org. Chem., 2013, 5769; (c)
P. Finkbeiner and B. J. Nachtshein, Synthesis, 2013, 979; (d)
R. Samanta and A. P. Antonchick, Synlett, 2012, 809; (e)
T. Dohi and Y. Kita, Chem. Commun., 2009, 45, 2073.
6 For hypervalent iodine mediated oxidative C–N bond
formation, see: (a) A. P. Antonchick, R. Samanta,
12 For oxidative reactions of hypervalent iodine in water see: (a)
T. Dohi, T. Nakae, N. Takenaga, T. Uchiyama,
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K.-I. Fukushima, H. Fujioka and Y. Kita, Synthesis, 2012, 14 For the synthesis and characterization of all N-aryl-2-amino-
1183; (b) T. Dohi, N. Takenaga, A. Goto, H. Fujioka and
Y. Kita, J. Org. Chem., 2008, 73, 6513.
N-heterocycles see: D. N. Rao, Sk. Rasheed, S. Aravinda,
R. A. Vishwakarma and P. Das, RSC Adv., 2013, 3, 11472.
13 Organic reactions in water, for recent reviews see: (a) B. Li 15 (a) R. M. Moriarty, J. Org. Chem., 2005, 70, 2893; (b) M. Ito,
and P. H. Dixneuf, Chem. Soc. Rev., 2013, 42, 5744; (b)
M.-O. Simon and C.-J. Li, Chem. Soc. Rev., 2012, 41, 1415.
C. Ogawa, N. Yamaoka, H. Fujioka, T. Dohi and Y. Kita,
Molecules, 2010, 15, 1918.
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