Copper-catalyzed denitrogenative transannulation reaction of pyridotriazoles: Synthesis of imidazo[1,5-a]pyridines with amines and amino acids

Article abstract of DOI:10.1021/acs.orglett.5b03509

The copper-catalyzed aerobic oxidative synthesis of imidazo[1,5-a]pyridines via cascade denitrogenative transannulation reaction of pyridotriazoles with benzylamines with good functional group tolerance is developed. The present methodology is also applicable to amino acids to obtain imidazo[1,5-a]pyridines via decarboxylative oxidative cyclization.

Full text of DOI:10.1021/acs.orglett.5b03509

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Letter
pubs.acs.org/OrgLett
Copper-Catalyzed Denitrogenative Transannulation Reaction of
Pyridotriazoles: Synthesis of Imidazo[1,5‑a]pyridines with Amines
and Amino Acids
Abhisek Joshi, Darapaneni Chandra Mohan, and Subbarayappa Adimurthy*
Academy of Scientific & Innovative Research, CSIR−Central Salt & Marine Chemicals Research Institute, G. B. Marg, Bhavnagar-364
002, Gujarat, India
S
* Supporting Information
ABSTRACT: The copper-catalyzed aerobic oxidative syn-
thesis of imidazo[1,5-a]pyridines via cascade denitrogenative
transannulation reaction of pyridotriazoles with benzylamines
with good functional group tolerance is developed. The
present methodology is also applicable to amino acids to
obtain imidazo[1,5-a]pyridines via decarboxylative oxidative
cyclization.
Scheme 1
ransition-metal-catalyzed C−H bond amination repre-
Tsents a powerful strategy for the synthesis of azahetero-
cycles.1 Particularly, imidazo[1,5-a]pyridines are highly valuable
organic scaffolds and are useful in many areas of research
including pharmaceuticals and material science.2−4 Therefore, it
is not surprising that, in spite of the existing methods,5−8 the
development of new versatile and efficient protocols for their
synthesis is of continuing interest. There are limited methods to
obtain imidazo[1,5-a]pyridines, despite their wide range of
biological activities.2−4 The newly recognized transition-metal-
catalyzed formation of diazo compounds from 1,2,3-triazoles
via a ring opening pathway, which could be used for the
generation of metallocarbene intermediates, has led to the
further subjection of a wide range of synthetically useful
transformations such as direct heterocycles synthesis, cyclo-
additions, ring expansions, and ylide formation,9,10 and recently
copper-catalyzed denitrogenative transannulations of pyrido-
triazoles for the synthesis of substituted indolizines have been
reported by Gevorgyan et al.11 (Scheme 1, eqs 1 and 2).
Encouraged by these previous successes and our recent work
on copper-catalyzed aerobic oxidative amination of C(sp3)−H
bonds for the syntheses of fused nitrogen-containing hetero-
cycles12 and imidazo[1,5-a]pyridines13 under mild conditions,
we hypothesized that subjecting the pyridotriazole and
benzylamine would obtain the imidazo[1,5-a]pyridines under
copper catalysis. To the best of our knowledge, these later
transformations have been scarcely investigated to date. Herein,
we report the denitrogenative transannulation of pyridotriazoles
with benzylamines and amino acids (decarboxylation) for the
synthesis of imidazo[1,5-a]pyridines using copper catalysis
(Scheme 1). The synthetic scope encompasses pyridotriazole
with a variety of benzylamines and amino acids.
reaction was observed up to 24 h (Table 1, entry 1). When the
same reaction was performed at 80 °C, a trace of 3a was
isolated (Table 1, entry 2). Upon further raising of the reaction
temperature to 120 and 150 °C, the desired product 3a was
isolated in 24% and 68% yield respectively (Table 1, entries 3
and 4). In the latter entry, the reaction time was 6 h; at high
temperature with a long reaction time the product decom-
position was observed.14 With other copper sources (i.e., CuBr,
CuCl, Cu(OAc)2, and Cu(OTf)2), no improvement in yield
was observed (Table 1, entries 5−8). However, with CuBr2, a
comparable but relatively low yield for 3a was observed (Table
1, entry 9). By increasing the catalyst (CuI) loading to 20 mol
%, 3a was obtained in 82% yield (Table 1, entry 10). Upon
further screening of different solvents (DMSO, DMF, DMA,
NMP, and toluene), no reaction was observed (Table 1, entries
To test our initial hypothesis, we first examined the
denitrogenative transannulation of pyridotriazole 1a with
benzyl amine 2a in the presence of 10 mol % of CuI in
dichlorobenzene at room temperature in a closed tube, and no
Received: December 10, 2015
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.5b03509
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Letter
a
Table 1. Optimization Conditions for 3a
Scheme 2. Substrate scope of transannulation of
triazolo[1,5-a]pyridines with amines
a
entry
catalyst (mol %)
solvent
temp
time (h)
yield (%)
1
CuI (10)
CuI (10)
CuI (10)
CuI (10)
CuBr (10)
CuCl (10)
Cu(OTf)2 (10)
Cu(OAc)2 (10)
CuBr2 (10)
CuI (20)
CuI (20)
CuI (20)
CuI (20)
CuI (20)
CuI (20)
CuI (20)
−
DCB
DCB
DCB
DCB
DCB
DCB
DCB
DCB
DCB
DCB
DMSO
DMF
DMA
DMF
NMP
toluene
DCB
−
rt
24
24
24
6
nr
trace
24
68
32
37
23
47
65
82
0
2
80
3
120
150
150
150
150
150
150
150
150
150
150
150
150
150
150
150
150
150
4
5
8
6
8
7
8
8
8
9
8
10
11
12
13
14
15
16
17
18
6
6
6
0
6
0
6
0
6
nr
nr
nr
74
37
10
6
6
CuI (20)
CuI (20)
CuI (20)
6
b
19
DCB
6
c
20
DCB
6
a
Conditions: 1a (0.2 mmol), 2a (0.6 mmol), catalyst, solvent (1.0
a
b
Conditions: 1a (0.2 mmol), 2a (0.6 mmol), catalyst, solvent (1 mL),
mL), in an oil bath, closed tube, isolated yield. Oxygen atmosphere.
c
in an oil bath in a closed tube, 6 h, isolated yield.
Argon atmosphere. nr: no reaction.
11−16). Without a copper iodide catalyst no reaction was
observed (Table 1, entry 17), and the yield decreased under
neat conditions (Table 1, entry 18). To further study the effect
of atmosphere, reactions have been performed under nitrogen
and oxygen atmospheres instead of within a closed tube (Table
1, entries 19 and 20); the yield decreased drastically. The best
3a yield was obtained under the conditions of entry 10; these
parameters were set as optimal for further transannulation of
pyridotriazole with different benzylamines (Scheme 2).
The reaction of 3-phenyl[1,2,3]triazolo[1,5-a]pyridine (1a),
with a neutral, electron-rich substituent at the para-/meta-/
ortho- position of benzyl amines such as −Me, −tBu, −OMe,
reacted very smoothly and gave good to excellent yields (63−
92%) of the desired products 3a−3g. However, 2-MeO benzyl
amine gave a comparatively low yield (45%) of desired product
3h; this may be due to the steric effect of the ortho-methoxy
position. Next we have checked the substrate scope of partially
electron-withdrawing substituents such as −Cl (para, meta, and
ortho), −F (para, meta and ortho), and disubstituted −F (2, 6-
position), they also reacted efficiently and gave moderate to
excellent yields of products 3j−3o. It may be noted that, halide
substituted imidazopyridine derivatives were well tolerated and
which could be further applied in traditional cross-coupling
reactions. Pleasingly we found that the present system was also
applicable to a strong electron-withdrawing substituent under
typical conditions, to afford the desired product 3p in moderate
yield (67%). Additionally the present system is applicable to
heteroamines, and aliphatic cyclic amines such as furfurylamine
and cyclopropyl methyl amine reacted smoothly and produced
the corresponding imidazo[1,5-a]pyridines (3q and 3s) in
moderate yield. However, the present system is not applicable
for azaheterocyclic amines (2-picolyl amine) and simple
aliphatic amines; in both cases, starting substrate decom-
position was observed (3r and 3t). Moreover, we performed
the reaction of triazolo[1,5-a]pyridinecarboxylate with different
benzyl amines, and the corresponding products (3u, 3v, and
3w) were obtained in good to moderate yields (47−95%). With
a methyl substituted substrate, decomposition of starting
substrates was observed. Finally, we also tried using a heteroaryl
such as quinoline triazole under the optimized conditions; the
ring opening is not observed (starting substrate recovered).
Although this route to the formation of a double C−N bond
was only realized for a particular class of substrates,
triazolo[1,5-a]pyridine, it represents a major developmental
strategy to obtain imidazo[1,5-a]pyridine derivatives via a
denitrogenative transannulation reaction of pyridotriazoles with
copper catalysis by a ring opening mechanism.
α-Amino acids are more readily available and more stable
than other starting materials in nature. Therefore, the
decarboxylative reaction of α-amino acids provides a very
efficient synthetic method for synthesis of azaheterocycles.
Recently utilization of α-amino acids for synthesis of
heterocyclic compounds are increasing drastically.15 Further,
we have been evaluating the present strategy of denitrogenative
transannulation reaction of pyridotriazoles with some amino
acids and their derivatives under the optimized conditions of
Scheme 2, and the results are included in Scheme 3.
Triazolo[1,5-a]pyridine 1a reacts with substituted phenyl
glycine derivatives (4-Cl, 2-Cl, 4-F, and 4-OH), alanine, and
leucin, producing the corresponding imidazo[1,5-a]pyridines in
moderate to excellent yields via decarboxylation followed by
transannulation.
B
DOI: 10.1021/acs.orglett.5b03509
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Letter
Based on these results, existing literature,11,16 and our
previous findings on imidazo[1,5-a]pyridines,13 we have
postulated a working mechanism for this transformation
(Scheme 5). First, the α-imino diazo compound A generates
Scheme 3. Transannulation of Triazolo[1,5-a]pyridine with
Amino Acids
a
Scheme 5. Plausible Mechanism
the Cu−carbene complex B, which is generated from
triazolo[1,5-a]pyridine 1a. Migratory insertion of benzylamine
at the carbene C atom of B forms the intermediate C,11,16
which further undergoes oxidative dehydrogenation to provide
imine intermediate D. Such intermediate formation was further
confirmed by the control experiments (Scheme 4). Sub-
sequently D further undergoes single electron transfer in the
process (intermediates E−G).17 Attacking the ring nitrogen
lone pair of electrons from G leads to a cyclic intermediate H.
Finally, aromatization of H will give the desired product 3a.
We have developed an efficient copper-catalyzed aerobic
oxidation followed by denitrogenative transannulation of
pyridotriazoles with benzyl amines and amino acids to obtain
imidazo[1,5-a]pyridine derivatives. In this system, we utilized
molecular oxygen (O2) from air as the sole oxidant,
commercially available benzylamines, and amino acids, which
make the present transformation more sustainable.
a
Conditions: 1a (0.2 mmol), 4 (0.6 mmol), catalyst, solvent (1 mL),
in an oil bath, closed tube, 24 h, isolated yield.
To establish the reaction mechanism, control experiments
were performed (Scheme 4). When the reaction of triazolo[1,5-
Scheme 4. Controlled Experiments
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.5b03509.
1H and 13C NMR spectra for all compounds (PDF)
a]pyridine 1a reacted with N-protected phenyl glycine (R = Ac,
BOC) under the optimized conditions, BOC containing phenyl
glycine gave a moderate yield of 5a and in the case of Ac
containing phenyl glycine a trace of 5a was obtained. With p-
toluidine 6a under the optimized conditions, no imine product
7a formation was observed. But we have isolated 32% of
phenyl(pyridin-2-yl)methanone 7b; it might be formed by the
hydrolysis of 7a at high temperature. Further study of the
reaction was conducted by the addition of radical scavenger
TEMPO under optimized conditions to determine whether the
reaction proceeds via a radical pathway or an ionic path; under
these conditions no adduct formation was observed. However,
the yield of the product decreased drastically. This experiment
clearly indicates that the reaction may proceed via copper−
carbene formation followed by oxidation to give imidazo[1,5-
a]pyridines.
AUTHOR INFORMATION
Corresponding Author
■
*E-mail: adimurthy@csmcri.org.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
CSIR-CSMCRI Communication No. 204/2015. A.J. and
D.C.M. are thankful to AcSIR for their Ph.D. enrollment and
the “Analytical Discipline and Centralized Instrumental
Facilities” for providing instrumentation facilities. A.J. and
D.C.M. are also thankful to CSIR and UGC, New Delhi for
their fellowships. We thank DST, Government of India (SR/
C
DOI: 10.1021/acs.orglett.5b03509
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Letter
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S1/OC-13/2011), for financial support. We also thank CSIR-
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