Indium-catalyzed denitrogenative transannulation of pyridotriazoles: Synthesis of pyrido[1,2- A[indoles

Article abstract of DOI:10.1021/acs.orglett.9b00180

Pyrido[1,2-a]indoles are known for medicinally and pharmaceutically important compounds; however, the efficient synthetic routes are scarce in the literature. We report herein a convenient and efficient route to synthesize these molecules through indium-catalyzed transannulation of pyridotriazoles with arenes. A library of compounds have been synthesized employing the method developed with various substituted pyrido[1,2-a]indole derivatives in moderate to good yields. The density functional theory study using SMD_DCB-M06/6-31++G(d,p)/LANL2DZ//B3LYP/6-31G(d)/LANL2DZ method suggests that the reactions proceed via indium-carbenoid complex.

Full text of DOI:10.1021/acs.orglett.9b00180

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Indium-Catalyzed Denitrogenative Transannulation of
Pyridotriazoles: Synthesis of Pyrido[1,2‑a]indoles
Deepa Rawat, Chitrakar Ravi, Abhisek Joshi, Eringathodi Suresh,* Kalyanashis Jana,
Bishwajit Ganguly,* and Subbarayappa Adimurthy*
Academy of Scientific & Innovative Research, CSIR−Central Salt & Marine Chemicals Research Institute, G. B. Marg,
Bhavnagar-364002, Gujarat, India
S
* Supporting Information
ABSTRACT: Pyrido[1,2-a]indoles are known for medicinally and pharmaceutically important compounds; however, the
efficient synthetic routes are scarce in the literature. We report herein a convenient and efficient route to synthesize these
molecules through indium-catalyzed transannulation of pyridotriazoles with arenes. A library of compounds have been
synthesized employing the method developed with various substituted pyrido[1,2-a]indole derivatives in moderate to good
yields. The density functional theory study using SMDDCB-M06/6-31++G(d,p)/LANL2DZ//B3LYP/6-31G(d)/LANL2DZ
method suggests that the reactions proceed via indium-carbenoid complex.
itrogen-fused heterocycles are the key structural motifs
Gevorgyan et al. reported the Rh(II)-catalyzed denitrogenative
transannulations with desired substrates.8
N
predominantly found in the field of medicinal chemistry
and material chemistry.1−3 Pyridine-fused heterocycles such as
imidazo[1,2-a]pyridines, imidazo[1,5-a]pyridines, imidazo[1,2-
a]pyrazines, indolizines, imidazo[2,1-a]isoquinolines, pyrazolo-
[1,5-a]pyridines, and pyrido[1,2-a]indoles are important inter-
mediates in both medicinal chemistry and drug development.
Particularly, pyrido[1,2-a]indoles were known to exhibit a wide
range of biological activities (Figure 1). Due to the importance of
Given the prevalence of denitrogenative transannulations to
generate bioactive molecules, alternative synthetic routes are
important to explore. In this regard, catalytic transformations are
particularly promising. It has been recently reported that Cu and
Rh catalysts can generate metal carbene intermediates to
enhance the electrophilicity for the transannulations with other
nucleophiles (Scheme 1).9
Based on the recent literature10−15 for the generation of metal
carbene intermediates throughthe ringopening ofpyridotriazole
1a (Scheme 2), we hypothesized to employ silver and indium as
metal catalysts to generate electrophilic carbene intermediate A,
and its subsequent reaction with 2-naphthol 2a as nucleophile to
generate tetracyclic heterocycle 3a′(5-phenylnaphtho[2,1-b]-
pyrido[1,2-d][1,4]oxazine) in line with our interest in the
synthesis of fused heterocycles16 and transannulation of
pyridotriazoles.17 Instead, we obtained an interesting product
3a 1-(pyrido[1,2-a]indol-10-yl)naphthalen-2-ol in 20% isolated
yield, when Ag(OTf) was used as catalyst (20 mol %) in
dichlorobenzene (DCB) as solvent at 120 °C, after 5 h (Table 1,
entry 1). With this reaction, we supposed to enhance the
electrophilicity of metallocarbene by harnessing different metals
and conditions. To this end, we screened different metal triflates
and solvents, under these conditions either no reaction or low
yield was observed (entries 2−6). With In(OTf)3 as a catalyst,
little improvement in yield of 3a was observed (entry 7).
Figure 1.
these molecules in various fields, few elegant synthetic routes
have been developed to access pyrido[1,2-a]indoles.4−7 Despite
the significance of these methods, it is highly desirable to develop
an efficient approach utilizing easily available catalysts, with
broad substrate scope, high atom economy, and an environment-
friendly oxidants.
Alternately, transannulation methods are extraordinarily
efficient and show excellent selectivity through in situ generation
of metal-carbenoid species and their reactions with different
coupling partners to transform into a desired products.
Received: January 16, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.9b00180
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
In addition to the In(OTf)3, when trifluoroacetic acid (TFA)
was used as additive up to two equivalents (w.r.t. 1a), the yield of
the desired product 3a was increased to 75% (entries 8−10).
When the reaction was performed by reducing the amount of 2a
(1.5 equiv w.r.t. 1a) and increasing the temperature to 130 °C,
the yield of the product was further increased to 93% (entry 11).
Further, altering the catalyst load and temperature of the
reaction, the yield was decreased (entries 12−15). Without
indiumcatalyst(entry16)andwiththereactioninothersolvents,
no reactions were observed (entries 17−19). When the reaction
was performed under oxygen atmosphere, the desired product
was isolated only in 16% yield along with oxidized product 2-
benzoylpyridine as the major product (entry 20).
Scheme 1. Transannulation Reactions of Pyridotriazoles
With the optimized conditions (Table 1, entry 11) for the
synthesis of 1-(pyrido[1,2-a]indol-10-yl)naphthalen-2-ol, the
scope and limitations of the present transformation has been
studied (Scheme 3). The results in Scheme 3 demonstrate that
Scheme 3. Scope of Indium-Catalyzed Transannulation of
a
Pyridotriazoles
Scheme 2. Transannulation of Pyridotriazole with 2-
Naphthol
a
Table 1. Optimization of Reaction Conditions
2a
additive
(equiv)
solvent
(1 mL)
temp
(°C)
yield
(%)
(equiv) catalyst (mol %)
1
2
3
4
5
6
7
8
2
2
2
2
2
2
2
2
Ag(OTf)(20)
Ag(OTf)(20)
Ag(OTf)(20)
Cu(OTf)2(20)
Ni(OTf)2 (20)
Sc(OTf)3(20)
In(OTf)3(20)
In(OTf)3(20)
In(OTf)3(20)
In(OTf)3(20)
In(OTf)3(20)
In(OTf)3(20)
In(OTf)3(20)
In(OTf)3(20)
In(OTf)3(20)
DCB
toluene
DCE
DCB
DCB
DCB
DCB
120
120
120
120
120
120
120
120
120
120
130
rt
100
130
130
130
130
20
trace
nr
nr
17
trace
24
39
42
75
93
nr
50
68
35
TFA (1) DCB
TFA (2) DCB
TFA (2) DCB
TFA (2) DCB
TFA (2) DCB
TFA (2) DCB
TFA (2) DCB
TFA (2) DCB
TFA (2) DCB
TFA (2) DCB
9
2
2
a
Reaction conditions: 1 (0.2 mmol), 2 (0.3 mmol), In(OTf)3 (20 mol
10
11
12
13
14
15
16
17
%), TFA (0.4 mmol), DCB (1.0 mL), 130 °C, 5 h, isolated yield.
1.5
1.5
1.5
1.5
1.5
1.5
1.5
b
Reaction yield at 1.0 mmol scale.
the reaction holds a high degree of functional group tolerance
with broad substrate scope. Initially, the reaction of 1a with 2-
naphthols 2 bearing electron-donating and withdrawing
substituents (H, OH, OMe, Br, CN, CHO, CO2Me, and Ph) at
C-6 position of 2, gave the corresponding annulated products
(3a−3h) in good yields (41−93%). One of the products 3f was
further confirmed by single crystal XRD. The reaction of 3-(4-
chlorophenyl)-[1,2,3]triazolo[1,5-a]pyridine 1b was reacted
with the above 2-naphthol derivatives and afforded the desired
products (3i−3o) in good yields (40−84%). The methoxy and
bromo substituents at C-7 of 2-naphthols were also reacted with
1a under the optimized conditions to give the desired products
(3p−3r) in 60−87% yields. We turned our attention to other
nr
trace
In(OTf)3(20)
+DCE
18
19
20
1.5
1.5
1.5
In(OTf)3(20)
In(OTf)3(20)
In(OTf)3(20)
TFA (2) EtOH
TFA (2) MeOH
TFA (2) DCB
130
130
130
nr
nr
16
b
a
Reaction conditions: 1a (0.2 mmol), 2a (0.3 mmol), In(OTf)3 (20
mol %), TFA (0.4 mmol), DCB (1.0 mL), 130 °C, 5 h, isolated yield.
O2 balloon; in this case, 2-benzoylpyridine was observed as major
b
product. nr = no reaction
B
DOI: 10.1021/acs.orglett.9b00180
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
arenes such as 2-methoxy naphthalene, anthracene, 9-methylan-
thracene, and 1,3,5-trimethoxybenzene as nucleophiles to the
present conditions. Delightfully, these substrates reacted
smoothly with 1a under the optimized conditions and delivered
the corresponding annulated products (3s−3v) in 55−80%
yields. As evident from the yields of products (3b−3v), the steric
and electronic effects do not affect the efficiency of the present
reaction.
Scheme 5. Plausible Reaction Pathway for the Formation of
Pyrido[1,2-a]indoles
We then focused on the reaction of 3-(4-chlorophenyl)-
[1,2,3]triazolo[1,5-a]pyridine 1b, with different naphthols and
anthracenederivatives, andobtainedthecorrespondingproducts
(3w−3ab) in moderate yields. Further, to enhance the substrate
scope, substituted pyridotriazoles such as 3-phenyl-7-(m-tolyl)-
[1,2,3]triazolo[1,5-a]pyridines, 3,7-diphenyl-[1,2,3]triazolo-
[1,5-a]pyridine, and 7-(4-(tert-butyl)phenyl)-3-(4-chlorophen-
yl)-[1,2,3]triazolo[1,5-a]pyridines were reacted with different
arenes (2-methoxy naphthalene, 1,3,5-trimethoxybenzene, and
anthracene) under the optimized conditions. These substrates
afford the corresponding annulated products (3ad−3ah) in 59%
to 85% yields, respectively. Unfortunately, the arene substrates
(I−IV) and ester substituted pyridotriazoles (1A−1C) were not
reactive under the present conditions, whereas the reaction of 2-
(naphthalen-2-yloxy)aceticacid (v) with 1a under the same
conditions gave the unexpected transannulated product 12-
phenylbenzo[e]pyrido[1,2-a]indole 3ai in 60% isolated yield.
To establish the reaction mechanism, some selective and
control experiments were performed (Scheme 4). Under the
We have examined this mechanistic pathway computationally
at SMDDCB-M06/6-31++G(d,p)/LANL2DZ//B3LYP/6-
31G(d)/LANL2DZ level of theory in o-dichlorobenzene phase
(ε = 10.0) (Scheme 5 and Figure 2).18 The computational results
Scheme 4. Control Experiments
Figure 2. SMDDCB-M06/6-31++G(d,p)/LANL2DZ//B3LYP/6-
31G(d)/LANL2DZ level of theory DFT calculated free energy profile
of the formation of 3. The free energies are given in kcal/mol and
distances (red color) are given in Å.
optimized conditions, the reaction of 1a and 2a was subjected to
two equivalent of TEMPO as a radical scavenger; under these
conditions, the desired product 3a was obtained in 59% yield
(Scheme 4, eq i). This reaction indicates that the present
transformation does not proceed through radical path. Further,
the reaction of 2-benzoylpyridine 4 and 2-benzylpyridine 5 were
subjected with 2a under the present conditions instead of 1a, to
know the possible intermediate in the present transformation,
but these reactions failed to afford the desired product 3a (eqs ii
and iii). These reactions designate that substrates 4 and 5 are not
intermediates in the present transformation (Scheme 4).
suggest that In(OTf)3 can form stable intermediate, A, with 1a,
and the intermediate A is stable by 10.3 kcal/mol; however, the
intermediate A′ formed by AgOTf and 1a is unstable by 13.1
kcal/mol (Figure 2).19 We have further examined the formation
of pyrido[1,2-a]indole at the SMDDCB-M06/6-31++G(d,p)/
LANL2DZ level of theory in o-dichlorobenzene. The naphthol
anion attacks the indium-carbenoid carbon and forms the
intermediate B. The activation free energy barrier is 36.6 kcal/
mol, and the intermediate B is energetically stable by 20.5 kcal/
mol (Figure 2). The carbocation C is generated via a barrier less
process of the elimination of In(OTf)3 at higher temperature
(130 °C). The carbocation C promotes stepwise cyclization
followed by oxidation leads the desired product 3. The pyridine
nitrogen attacks the ortho position of the phenyl ring and form
the cyclic intermediate D. The activation free energy barrier in
this step is 16.9 kcal/mol, and the intermediate D is stable by 6.0
kcal/mol at the SMDDCB-M06/6-31++G(d,p)/LANL2DZ level
of theory. Furthermore, the aromatization of intermediate D is
assisted by the triflate anion via an 8.4 kcal/mol activation free
energy barrier compared to the preceding intermediate. The
desired product 3 is stable by 11.2 kcal/mol compared to C. The
Based on the literature reports6,7,9−15 and with the above
observations, a plausible reaction mechanism has been proposed
(Scheme 5). Denitrogenation of 1 with indiumtriflate (In-
(OTf)3) may generate the indium carbenoid A. The insertion of
2 to the intermediate A may generate metalated pyridinium Aza-
Nazarov type intermediate B,6b,c which upon elimination of
In(OTf)3 underacidicconditionsgivethebiscationicpyridinium
tertiary carbocation C.6b,d High temperature and steric
hindrance of two bulky aryl groups of C may drive the phenyl
group close to the pyridine nitrogen and lead to the formation of
another intermediate D via intramolecular C−H amination. The
subsequent oxidation of D will deliver the desired product 3.
C
DOI: 10.1021/acs.orglett.9b00180
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
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DFT results further suggest that the formation of the
intermediate A′ from AgOTf and is thermodynamically less
preferred compared to that of In(OTf)3 and is corroborated by
the experimentally observed product of 20% yield.
To conclude, we have revealed an expeditious indium-
catalyzed denitrogenative transannulation of pyridotriazoles
with β-naphthols to obtain pyrido[1,2-a]indole derivatives.
The methodology also works very well with a variety of other
substrates such as methoxy naphthalenes, trimethoxybenzenes,
and anthracene derivatives. The method shows very good
functional group tolerance with broad substrate scope and with
good yields. The DFT studies indicates that the reaction
proceeds through an indium-carbenoid intermediate.
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.orglett.9b00180.
■
S
Copies of NMR spectra for all compounds and HRMS
spectrafornewcompounds;computationaldetailsandthe
B3LYP/6-31G(d)/LANL2DZ level of theory optimized
Cartesian coordinates (PDF)
Accession Codes
CCDC 1842992 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge via
www.ccdc.cam.ac.uk/data_request/cif, or by emailing data_
request@ccdc.cam.ac.uk, or by contacting The Cambridge
Crystallographic Data Centre, 12 Union Road, Cambridge
CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
Corresponding Authors
■
́
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6, 6687. (e) Ma, X.; Pan, S.; Wang, H.; Chen, W. Org. Lett. 2014, 16,
4554. (f) Xing, Y.; Sheng, G.; Wang, J.; Lu, P.; Wang, Y. Org. Lett. 2014,
16, 1244. (g) Chuprakov, S.; Kwok, S. W.; Fokin, V. V. J. Am. Chem. Soc.
2013, 135, 4652.
*E-mail: esuresh@csmcri.res.in.
*E-mail: ganguly@csmcri.res.in.
*E-mail: adimurthy@csmcri.res.in.
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ORCID
Eringathodi Suresh: 0000-0002-1934-6832
Kalyanashis Jana: 0000-0001-9792-8195
Bishwajit Ganguly: 0000-0002-9858-3165
Subbarayappa Adimurthy: 0000-0001-5320-4961
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
■
S.A. is thankful to DST, Government of India (EMR/2016/
000010), and CSIR-CSMCRI (MLP-027 and OLP-088) for
financial support. CSIR-CSMCRI Communication No. 102/
2018.
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DOI: 10.1021/acs.orglett.9b00180
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