Oxidative Csp3-H functionalization of 2-methylazaarenes: A practical synthesis of 2-azaarenyl-benzimidazoles and benzothiazoles

Article abstract of DOI:10.1016/j.tetlet.2017.04.007

Oxidative C_sp3-H functionalization of 2-methylazaarenes using I₂-DMSO in open flask has been described first time for the synthesis of 2-azaarenyl benzimidazoles and 2-azaarenyl benzothiazoles. Generally, methyl group of 2-methylazaa

Full text of DOI:10.1016/j.tetlet.2017.04.007

Accepted Manuscript
Oxidative C -H Functionalization of 2-Methylazaarenes: A Practical Synthesis
sp3
of 2-Azaarenyl-benzimidazoles and benzothiazoles
Srinivasarao Yaragorla, P. Vijaya Babu
PII:
S0040-4039(17)30427-6
DOI:
http://dx.doi.org/10.1016/j.tetlet.2017.04.007
TETL 48799
Reference:
To appear in:
Tetrahedron Letters
Received Date:
Revised Date:
Accepted Date:
9 February 2017
31 March 2017
2 April 2017
Please cite this article as: Yaragorla, S., Vijaya Babu, P., Oxidative C -H Functionalization of 2-Methylazaarenes:
sp3
A Practical Synthesis of 2-Azaarenyl-benzimidazoles and benzothiazoles, Tetrahedron Letters (2017), doi: http://
dx.doi.org/10.1016/j.tetlet.2017.04.007
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Graphical Abstract`
Oxidative Csp3-H Functionalization of 2-Methylazaarene Ls :eave this area blank for abstract info.
A Practical Synthesis of 2-Azaarenyl-benzimidazoles and benzothiazoles
∗
Srinivasarao Yaragorla and P. Vijaya Babu

1
Tetrahedron Letters
journal homepage: www.els evier.com
Oxidative C -H Functionalization of 2-Methylazaarenes: A Practical Synthesis of 2-
sp3
Azaarenyl-benzimidazoles and benzothiazoles
∗
a, b
Srinivasarao Yaragorla and P.Vijaya Babu
School of Chemistry, University of Hyderabad, Prof. C. R. Rao Road, Gachibowli, Hyderabad, PO Central University, 500046, Telangana, India.
a
b
Department of Chemistry, Central University of Rajasthan, Bandarsindri, NH-8, Ajmer-distt., Rajasthan, 305817, India.
ARTICLE INFO
ABSTRACT
Article history:
Received
2
Oxidative Csp3-H functionalization of 2-Methylazaarenes using I -DMSO in open flask has been
described first time for the synthesis of 2-azaarenyl benzimidazoles and 2-azaarenyl
benzothiazoles. Generally, methyl group of 2-methylazaarenes serves as a carbon nucleophile
and in this work the methyl group served as electrophilic carbon (Umpolung!) and condensed
with o-Phenylenediamine and 2-Aminothiophenol to furnish the corresponding benzimidazoles
and benzothiazoles in high yields with good substrate scope and functional group tolerance.
Received in revised form
Accepted
Available online
Keywords:
Oxidative functionalization
Iodine catalyzed
2
-Methylazaarenes
2
009 Elsevier Ltd. All rights reserved.
Benzimidazoles
Benzothiazoles
Quinolines are one of the most extensively studied structural
motiffs for the drug discovery and hence it is considered as a
2
For example Li et al reported a copper promoted (CuI, Cu(OAc) ,
DMSO, DTBP) oxidative C-H amination for the synthesis of
4a
imidazoquinolines, Yin et al reported another copper catalysed
1
privileged structure”. Owing to its potential structural features
“
and medicinal importance it is always in high demand to generate
quinoline based “new chemical entities” (NCE) to fulfil the
pharmaceutical needs. Among the several synthetic methods
process (CuCl, Ph PO H, PhCl) for the synthesis of azaarenyl-
2 2
4b
quinazolinones. and an iodine catalyzed oxidative benzylic C-
H amination of azaarenes for the synthesis of quinazolinones was
4
c
developed,
C
sp3-H functionalization of 2-methylazaarenes
also reported by Yang et al.
become more challenging and intriguing technique for the
synthetic chemists owing to the large scope of versatile
2
-4
products.
methylazaarene served as a carbon nucleophile and was added to
In most of these methods, methyl group of 2-
2
a suitable electrophile (Figure 1, equation 1). In this context we
had contributed three seminal publications for the synthesis of
azaarenyl based heterocycles through a C-H functionalization of
3
-methylazaarenes. Nevertheless, a limited number of reports
2
are available where the polarity of the methyl group of 2-
methylazaarene has been reversed from nucleophilic carbon to
electrophilic carbon (probably “umppolung” type) as depicted in
4
the Figure 1, equation 2.
Benzimidazoles are known to be potential building blocks for the
development of medicinally important molecules and especially
substituted benzimidazoles have found diverse applications as
therapeutic agents, including antiulcers, antihypertensives,
5
antivirals, antifungals, anticancers, and antihistaminics. Most of
the synthetic methods will consider a o-Phenylenediamine and
6
aryl aldehyde as starting materials (Scheme 1). Looking at the
biological importance of benzimidazoles and quinolones we
designed a new synthetic strategy for the hybrid molecule which
comprises of these two important scaffolds. When looked into the
literature we found that this kind of molecules are known as
selective A1 adenosine receptor antagonists with stimulant
7
activity on human colon motility and metal chelating agents to
8
show oligimerization reactivity. Indeed, these methods also
involved the condensation of quinaldinaldehyde with diamine
—
——
∗
Corresponding author. Tel.: +91-40-23133800; fax: +91-463-238722; e-mail: srinivas.yaragorla@uohyd.ac.in and ysriict@gmail.com

2
Tetrahedron
Condensation of quinoline-2-
7
1
0c
10a
using oxone,
carboxylic acid with diamine was reported with PPA . Iron
NaHSO3.
sulfide mediated benzimidazole synthesis was reported with
Table 2. Substrate scope in the
functionalization of 2-methylazaarenes in the open flask reaction.
2
I -DMSO promoted oxidative
a
1
0b
quinaldine. In continuation of our research interest towards the
C-H functionzalization of 2-methyl azaarenes herein we report a
iodine catalyzed double oxidative functionalization/condensation
cascade for the synthesis of benzimidazol-2-ylazaarenes and
benzothiozol-2-ylazaarenes.
X
I2 (10 mol%), DMSO
N
X
N
H2N
H2N
10 oC, 12-15 h
1
+
N
HN
R
CH3
R
Open Air
3
1
2
X= C or N; R= H or Me
Table 1. Optimization of reaction conditions for the synthesis of 2-(1H-
a
benzimidazol-2-yl)quinoline (3a) from 1a and 2a.
HN
HN
HN
Cl
N
N
N
N
N
N
N
3a, 94%
3b, 95%
3c, 98%
a
b
Entry
Reaction conditions
Yield (%)
HN
HN
HN
N
1
2
3
4
5
6
7
8
9
I
I
I
I
I
I
I
I
2
2
2
2
2
2
2
2
(100 mol%), NMO (100 mol%)
(100 mol%), TBHP (100 mol%)
45
80
15
35
90
90
93
94
N
N
N
N
N
Me
OMe
OEt
(100 mol%),
2 2
H O (100 mol%)
O
O
O
(100 mol%), TEMPO (100 mol%)
(100 mol%), Open air
(50 mol%), Open air
3
d, 82%
3e, 90%
3f, 90%
(20 mol%), Open air
Me
HN
O
HN
(10 mol%), Open air
N
N
Me
N
N
HN
Open air
no reaction
N
o
a]. 1a (0.67 mmol.), 2a (0.84 mmol) in DMSO (0.6 mL) at 110 C for
OMe
OEt
N
[
Cl
Cl
N
N
over night. [b]. Isolated yields reported. NMO:N-Methylmorpholine-N-
oxide; TBHP: tert-butyl hydrogenperoxide; TEMPO: 2,2,6,6-
tetramenthyl-1-peperidinyloxy.
O
N
H
3i, (1:0.8) 92%b
3
g, 89%
3h, 90%
Me
Me
Me
Me
Me
Inspired by I
2
-DMSO catalysed oxidation of active methyl
groups, we commenced our initial experiment by treating 2-
methylquinoline 1a (159 mg, 1.2 equiv.) and
9
HN
HN
HN
N
N
N
N
N
N
N
OMe
orthophenylenediamine 2a (100 mg, 1 equiv.) with molecular
iodine (1 equiv.), NMO (1 equiv.) in 0.6 mL DMSO at 110 C for
overnight. Gratifyngly benzimidazol-2ylquinoline 3a was
isolated in 45% (Table 1, entry 1) and confirmed by the spectral
OEt
o
Cl
Cl
Me
O
N
O
N
N
1
0.
N
H
N
data and matched with reported one. Experiments with other
oxidants such as TBHP (entry 2), H (entry 3) and TEMPO
entry 4) could not furnished the satisfactory yield of 3a.
Interestingly the reaction in the open air gave very good yield
90%) of 3a in the absence of external oxidating agents (Table 1,
entry 5). Efforts to minimize the iodine loadings were successful
and 10 mol% of I in open air gave excellent yield (94%) of 3a
entries 6-8, Table 1) and it is worth to mention here that the
N
N
N
H
OMe
H
2
O
2
OEt
(
O
O
3j, (1:0.6) 90%b
(
k, (1: 0.8)93%b
3
3l, (1:0.5)92%b
2
[
a]. Reaction conditions: 1 (1.0 equiv.), 2 (1.2 equiv.) in DMSO (0.6 mL)
o
at 110 C for over night. Isolated yields reported. [b]. the regioisomeric
(
reaction could not initiated in the absence of iodine (entry 9).
1
ratio was determined by H NMR spectra and combined yields are
reported.
Having established the efficient conditions for the synthesis of 3a
from 1a and 2a, we decided to check the scope of our protocol
for a large variety of 2-methylazaarenes and the results are
summarized in the Table 2. 2-Methyl-7-chloroquinaldine and 2-
methyl quinoxaline showed similar reactivity towards 2a and
yielded the respective benzimidazoles 3b and 3c in nearly
quantitative yields. 2-Methyl, 3-acyl, 4-phenyl quinoline (fully
substituted) underwent a smooth chemoselective oxidation (2-
methyl verses methyl ketone) to furnish the benzimidazole 3d in
Encouraged by the successful demonstration of cascade oxidative
C-H functionalization and bezimidazole formation (Table 2), we
were interested to look at the synthesis of a similar bioisosteric
heterocyclic compound, benzothiazol-2-ylquinoline 5a by simply
replacing orthophenylenediamine with 2-aminobenzenethiol.
When the reaction was performed with 2-methylquinoline 1a and
2
-aminobenzenethiol 4 under standard conditions the expected
product 5a was found in <10% yield. Addition of oxidatng agents
such as NMO and TEMPO could increase the yields of 5a upto
82% after 12 h. The reaction profile was not found clean enough
on TLC) when left for long time (24 h), probably methyl group
(
3
5%. Neverthless addition of TBHP increased the yield of 3a to
of acyl moiety started reacting after longer reaction times. Other
fully substituted 2-mentyl quinolines also reacted well to produce
the respective condensed products 3e, 3f and 3g in excellent
yields (Table 2). In case of 4-methylbenzene-1,2-diamine (2b),
the reaction proceeded with equal ease however it resulted a
almost quantitative (95%). This reaction proceeded with same
ease with other substituted 2-methylazaarenes to result the
benzothiazoles 5b-5f in excellent yields as depicted in the Table
3
.
1
1
mixture of regioisomers 3i-3l in good yields.

3
Table 3. Substrate scope in the
functionalization of 2-methylazaarenes for the synthesis benzothiazol-2-
2
I -DMSO promoted oxidative
In summary, we are successful in demonstrating a simple and
a
ylazaarenes.
2
open-flask oxidative functionalization (promoted by I -DMSO)
X
I2 (10 mol%), DMSO
TBHP (1 equiv.)110 oC,
N
S
of 2-methylazaarene to furnish 2-azaarenyl benzimidazoles and
benzothiazoles for the first time. The broad substrate scope,
functional group tolerance, high yields and open air conditions
made this protocol more potential.
All the compounds were characterized by H NMR, C NMR,
1
Mass and IR.
H2N
HS
+
N
N
N
CH3
Open Air
5
1
4
1
13
5
S
S
S
N
N
N
N
N
Acknowledgements
Me
OMe
SY and PVB are thankful to Council of Scientific and Industrial Research
(
0
O
O
CSIR)-India for the financial support under Extramural Research Grant -No.
2(0200)/14/EMR-II.
5
a, 95%
5
b, 88%
5c, 90%
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S
S
S
1
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5
e, 85%
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at 110 C for over night. Isolated yields reported.
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2
2
.
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enamine 6 which then reacts with I to produce the methyl iodide
. Iodide 7 will undergo a Komblum oxidation to yield the
2
1
3
7
1
4
respective aldehyde 8 which further makes an imine 10.
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3
4
.
.
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N
imine 10. An intramolecular azacyclisation of 10 followed by HI
elimination furnish the benzimidazole 3a. Among the two paths
proposed, path a seems to be more predominant as the aldehyde 8
was isolated and confirmed. Though internmediate 9 was not
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Wang L-X. Aisan J. Org. Chem. 2016;5:494 (d) Liu M, Chen T, Zhou
Y, Yin S-F. Catal. Sci. Technol. 2016, 6, 5792.
4c
o
isolated, path b can not be ruled out. Since at 110 C, the
reaction may be very fast in furnishing the intermediate 9 and an
intramolecular cyclisation aromatization is the driving force to
form 3a.
5.
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4
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1
The ratios of the regioisomers is determined based on the HNMR
1
1
1
1.
2.
3.
spectra.
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113.
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1
4.
5.
Aldehyde 8 was isolated and characterized.
General experimental procedure for the synthesis Benzimidazol-
2
ylquinolines 3: A mixture of o-Phenylenediamine 2a (0.84 mmol),
2
-methylazaarene (0.67 mmol) and Iodine (10 mol%) were heated in
o
DMSO (0.6 mL) in open flask at 110 C for overnight. After
completion of the reaction (monitored by TLC), reaction mixture was
diluted with 10 mL aqueous saturated solution of sodium thiosulphate
and extracted with dichloromethane (3x15 mL). Combined organic
layers were washed with brine solution, dried over anhydrous
2
Na SO4, solvent was removed under reduced pressure and the
resultant crude product was purified by silica gel column
chromatography using hexane and ethyl acetate (10:1) as eluents to
obtain the desired product 3. General experimental procedure for the
synthesis Benzothiazol-2ylquinolines 5:
A
mixture of 2-
aminobenzenethiol 4 (0.84 mmol), 2-methylazaarene (0.67 mmol),
Iodine (10 mol%) and TBHP (2 equiv.) were heated in DMSO (0.6
o
mL) in open flask at 110 C for overnight. After completion of the
reaction (monitored by TLC), reaction mixture was diluted with 10
mL aqueous saturated solution of sodium thiosulphate and extracted
with dichloromethane (3x15 mL). Combined organic layers were
2
washed with brine solution, dried over anhydrous Na SO4, solvent
was removed under reduced pressure and the resultant crude product
was purified by silica gel column chromatography using hexane and
ethyl acetate (10:1) as eluents to obtain the desired product 5. Spectral
data of selected compounds: 2-(1H-benzo[d]imidazol-2-yl)quinoline
8
c
1
(
3a): Yield: 94%; mp 227-229 °C; H NMR (500 MHz, CDCl
.60 (d, J = 8.5 Hz, 1H), 8.35 (d, J = 8.5 Hz, 1H), 8.13 (d, J = 8.5
Hz, 1H), 7.90 (d, J = 8 Hz, 2H), 7.78 – 7.75 (m, 1H), 7.62 – 7.59 (m,
3
): δ
8
1
3
1
H), 7.46 (s, 1H), 7.34 – 7.32 (m, 2H); C NMR (125 MHz, CDCl
δ 150.9, 148.2, 147.5, 144.5, 137.4, 134.0, 130.2, 129.1, 128.6, 127.9,
27.3, 124.3, 122.8, 120.3, 119.1, 111.3; HRMS (ESI) m/z calcd. for
[M+] 245.0953; found 245.0956.; 2-(quinolin-2-
3
):
1
C
16
H
11
N
3
o
yl)benzo[d]thiazole (5a): Yellow solid; mp 198-299 C; Yield: 95%;
1
H NMR (500 MHz, CDCl
3
): δ 8.39 (d, J = 8.5 Hz, 1H), 8.20 (d, J = 9
Hz, 1H), 8.10 (d, J = 8.5 Hz, 1H), 8.04 (d, J = 8 Hz, 1H), 7.89 (d, J =
1
3
8
Hz, 1H), 7.77 (d, J = 8.5 Hz, 1H), 7.67 – 7.35 (m, 4H); C NMR
125 MHz, CDCl ): δ 169.9, 154.3, 151.3, 147.9, 137.0, 130.1, 129.8,
29.7, 129.0, 127.7, 127.6, 126.3, 125.9, 123.8, 122.0, 118.3; IR
(
3
1
−
1
(
KBr): 2930, 2853, 2432, 1590, 1421, 1248 cm ; Mass (m/z):
63.0640; ethyl 2-(benzo[d]thiazol-2-yl)-4-phenylquinoline-3-
carboxylate (5d): Yield: 89%; H NMR (500 MHz, CDCl
2
1
3
): δ 8.01 (s,
H), 8.00 – 7.80 (m, 2H), 7.63 – 7.61 (m, 1H), 7.57 – 7.55 (m, 1H),
.54 – 7.50 (m, 4H), 7.49 – 7.44 (m, 4H), 4.28 (q, J = 7 Hz, 2H), 1.16
1
7
1
3
(t, J = 7.5 Hz, 3H); C NMR (125 MHz, CDCl
3
): δ 168.0, 167.4,
54.1, 147.4, 147.3, 146.5, 136.3, 134.6, 130.7, 129.8, 128.7, 128.2,
27.3, 126.7 (2C), 126.0 (2C), 125.6, 124.1, 121.8, 61.5, 13.8; IR
1
1
−1
(
KBr): 2945, 2820, 1708, 1590, 1418 cm ; Mass (m/z): 411.1158.;
ethyl 2-(benzo[d]thiazol-2-yl)-6-chloro-4-phenylquinoline-3-
carboxylate (5f): Yield: 83%; H NMR (500 MHz, CDCl
1
3
): δ 8.10 (s,
H), 7.80 (d, J = 8 Hz, 1H), 7.74 - 7.71 (m, 1H), 7.57 – 7.55 (m, 5H),
.46 – 7.44 (m, 2H), 7.35 – 7.32 (m, 1H), 7.30 -7.27 (m, 1H), 4.33 (q,
1
7
1
3
J = 14.5 Hz, 2H), 1.17 (t, J = 7 Hz, 3H); C NMR (125 MHz,
CDCl ): δ 167.1, 148.9, 146.6, 145.5, 144.5, 143.9, 133.9, 133.8,
33.5, 131.7, 130.9, 129.5, 129.0, 128.4, 127.7, 127.1, 125.5, 124.5,
22.5. 121.0, 111.1, 61.9, 13.7; IR (KBr): 2980, 2850, 2420, 1698,
3
1
1
1
−
1
2 2
530, 1434, 1205 cm ; HRMS (ESI) m/z calcd. for C25H17ClN O S
[M+] 444.1356; found 444.1359.

5
Highlights of the Work
•
•
Oxidative Csp3-H Functionalization of 2-
Methylazaarenes using Iodine as catalyst
First Report for the Synthesis of
Benzimidazol-2-ylquinolines and
Benzothiazol-2-ylquinolines using this
Concept
•
•
Open Air Reaction
Functional Group Tolerance (-COMe, -
CO
2 2
Me, -CO Et, -Cl)
•
High Yields and broad Substrate Scope