Copper-catalyzed aerobic oxidative amination of C(sp3)-H bonds: Synthesis of imidazo[1,5-a]pyridines

Article abstract of DOI:10.1039/c5ob00381d

Copper-catalyzed synthesis of imidazo[1,5-a]pyridine-1-carboxylates through oxidative amination of C(sp³)-H bonds under mild aerobic conditions with broad substrate scope is described. Use of naturally abundant air as the sole oxidant was found

Full text of DOI:10.1039/c5ob00381d

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Copper-Catalyzed Aerobic Oxidative Amination of C(sp3)-
H bonds: Synthesis of Imidazo[1, 5-a]pyridines
Cite this: DOI: 10.1039/x0xx00000x
Darapaneni Chandra Mohan,a Sadu Nageswara Rao,a Chitrakar Ravi,a and
Subbarayappa Adimurthy*a
Received 00th January 2012,
Accepted 00th January 2012
DOI: 10.1039/x0xx00000x
www.rsc.org/
generates only water as byꢀproduct in the system, which makes this
transformation more atomꢀeconomical and importantly sustainable
than the reported methods (Scheme 1)
Copper-catalyzed synthesis of imidazo[1, 5-a]pyridine-1-
carboxylates through oxidative amination of C(sp3)-H bonds
under mild aerobic conditions with broad substrate scope is
described. Use of naturally abundant air as a sole oxidant
found to be efficient and selective. The present protocol is also
applicable for direct synthesis of functionalized imidazo[1, 5-
a
]pyridines from amino acid derivatives.
Transition metal catalyzed C−H bond amination via a stepꢀ
economic and environmental methods are significant strategies for
the synthesis of azaheterocycles.1 Recently, various transition metal
catalysts have been developed for C−H bond amination which
including ruthenium2 rhodium3 and palladium based4 systems, where
Scheme 1.
We started our investigation on the oxidative amination of C
(sp3)ꢀH bonds by choosing tertꢀbutyl 2ꢀ(pyridinꢀ2ꢀyl) acetate 1a and
stoichiometric or excess amount of oxidants were employed. The use 4ꢀchlorobenzylamine 2a as model substrates (Table 1). Initially, the
of molecular oxygen for copperꢀcatalyzed aerobic oxidative CꢀH reaction of 1a and 2a was performed employing CuI (10 mol %) as
bond activation for CꢀN bond formations are highly attractive5 due to catalyst, in DMSO at room temperature in open air, trace amount of
the large abundance of oxygen, low cost and avoids the formation of
tertꢀbutyl 3ꢀ(4ꢀchlorophenyl) imidazo [1, 5ꢀa] pyridineꢀ1ꢀcarboxylate
3a was observed (Table 1, entry 1). The same reaction was carried
toxic byꢀproducts.6
Imidazo[1, 5ꢀa]pyridine (IP) scaffolds are found in many out with 0.1 equivalent of acetic acid as additive; 3a was isolated in
pharmacologically important compounds.7 Other applications of 15% yield (Table 1, entry 2). The product 3a was further conformed
these derivatives also have been actively probed in the context of
organic lightꢀemitting diodes (OLED)8 and organic thin layer field
effect transistors (FET).9 In addition, 2ꢀazaindolizines have been
examined as pharmaceuticals (eg, HIVꢀprotease inhibitors)10 as
precursors of Nꢀheterocyclic carbenes11 and additionally, several
metal complexes have been reported.12 However, only limited
synthetic routes are available to access these molecules. The reported
methods mainly relied on the traditional Vilsmeierꢀtype cyclizations
of Nꢀ2ꢀpyridylmethylamides.13 Recent reports available for synthesis
of imidazo[1,5ꢀa]pyridines with stoichiometric amount of iodine
source in presence of excess of oxidant14&15 and other methods.16
Therefore, developing of more practical and efficient synthetic
routes for imidazo[1,5ꢀa]pyridines is highly desirable. In
continuation of our efforts on the development of green and
sustainable methods for the synthesis of azaheterocycles,17 we report
herein an efficient copper catalyzed aerobic oxidative amination of
C(sp3)ꢀH bonds for the synthesis imidazo[1,5ꢀa]pyridine derivatives
utilising molecular oxygen (O2) from air as sole oxidant which
by single crystal XRD (fig.S1).
Figure S1
.
To uphold the reaction yield, it was conducted at 45 ᵒC and
65 ᵒC; the yield was also increased (Table 1, entries 3 & 4). Further
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increasing the amount of additive to 1.0 equivalent at 65 ᵒC, 3a was
isolated in 80% yield (Table 1, entry 5). Use of trifluoroacetic acid
(TFA) as additive the yield was decreased (Table 1, entry 6). With
pivalicacid (PivOH) as additive, 3a was isolated in 91% yield (Table
1, entry 7). When the temperature of the reaction was further
increased to 80 ᵒC, yet, same the yield (91%) was observed (Table 1,
entry 8). At the optimum temperature of 65 ᵒC, it was performed
without additive to check the efficiency of the reaction, but yield was
reduced to 38 % (Table 1, entry 9). The reaction was also performed
with other copper salts (CuBr, CuCl, CuCl2, Cu(OAc)2 and
Cu(OTf)2) (Table 1, entries 10ꢀ14) as well as solvents (DMF,
dioxan, ACN and chlorobenzene) (Table1, entries 15ꢀ18) they found
to be less effective. Under these conditions reactions were either
poorly effective or entirely ineffective (Table1, entries 10ꢀ18).
Further, under the conditions of entry 7, reaction was performed by
using oxygen atmosphere (balloon) instead of open air; the yield of
3a was decreased to 76 % (Table 1, entry 19). It may be due to the
Table 2. Scope of pyridyl estersa
Table 1. Optimization of reaction conditions for 3aa
aReaction conditions; 1 (0.25 mmol), 2a (0.5 mmol), Additive (0.25 mmol),
catalyst (0.025 mmol) Solvent (1 mL), in an oil bath under air, isolated yield,
nr: no reaction .
Under the optimised conditions, the scope of the copperꢀ
catalysed (sp3) CꢀH amination followed by annulation of 2a with
various pyridyl esters 1 was investigated. The results can be seen in
Table 2; ester groups such as –COOtBu, ꢀCOOiPr, ꢀCOOnBu, ꢀ
COOEt, and ꢀCOOCy, were well tolerated. The reaction proceeded
very well and provided the desired products 3aꢀ3e in good to high
yields. However, 2ꢀ(pyridinꢀ2ꢀyl) acetonitrile did not give the
desired product (3f).
To extend the synthetic scope of this method, a variety of pyridyl
esters 1 and substituted benzylamines 2 were subjected to these
optimized conditions. As can be seen from the results of Table 3;
electronꢀwithdrawing groups as well as electronꢀdonating groups
were well tolerated. In general, the reaction of 1a with benzylamine
bearing both electronꢀdonating and withdrawing substituents on the
aromatic ring proceeded very well and gave the desired products 4bꢀ
4m in good to high yields (74ꢀ95%). It may be noted that, halide (Cl,
Br and F) substituted imidazo[1,5ꢀa]pyridine derivatives were well
tolerated, and provided the corresponding products 4iꢀ4m in
excellent yields, which could be further applied in traditional crossꢀ
coupling reactions. The presence of
a variety of electronꢀ
donating/withdrawing groups in benzylamine moieties at either
(o/m/p) position does not affect the efficiency of the process.
However, steric hindrance was observed with 2ꢀmethoxy
benzylamine, in which case trace amount of product 4n was
observed. The present catalytic system is also applicable to hetero
and aliphatic amines under typical conditions, to afford the desired
products 4oꢀ4x in moderate to excellent yield. However, with 2ꢀ
picolyamine no desired product formation was observed.
Interestingly, when reaction was performed with amino acid
derivatives like ethyl glycinate (5a) and 1a under standard condition,
(Scheme 2) 53% of 1ꢀtertꢀbutyl 3ꢀethyl imidazo[1,5ꢀa]pyridineꢀ1,3ꢀ
dicarboxylate (6a) was isolated. This method is useful for straight
forward synthesis of functionalized imidazo [1, 5ꢀa] pyridines.
Under the same conditions secondary amines like dibenzyl amine
and nꢀmethyl benzyl amines were not proceeded efficiently (Scheme
2).
aReaction conditions: 1a (0.25 mmol), 2a (0.50 mmol), Additive (0.25
mmol), catalyst (0.025 mmol), solvent (1 mL), in an oil bath under open air,
15 h. bIsolated yield. c 0.1 Equiv. of additive is used. [nr = no reaction].
conversion of benzyl amine to imine under aerobic conditions.18
Reaction under argon atmosphere, only 15% of 3a was isolated
(Table 1, entry 20). Entries 19 and 20 indicate that, oxygen from
open air was found to be effective and selective oxidant for the
present transformation. Also, without catalyst, no reaction under the
same experimental conditions (Table 1, entry 21). Hence, the
optimized reaction conditions identified for the present investigation
was 10 mol% CuI, 1.0 equivalents of PivOH as additive, at 65 ºC, in
DMSO under open air (Table 1, entry 7). Hear
Then, to ascertain the reaction mechanism, some control
experiments were performed (Scheme 3). Reaction of 1a with pꢀ
toluidine 8a under optimised conditions, we isolated 89% of imine
product 9a (eqn. 1) and with αꢀmethyl benzyl amines 8b, a different
product 10a was isolated (eqn. 2). Initially nitrogen of benzylamine
forms Cꢀ N bond with pyridyl carbon, the presence of methyl
2 | J. Name., 2012, 00, 1-3
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of equations 1ꢀ3 without PivOH, in all the cases considerably low
yield of products (9a,10a and 11a) were observed. Based on these
experiments, it indicates that, PivOH plays a crucial role to generate
imine intermediate under the present conditions. Further by
subjecting 11a under the optimised conditions with 2a, trace amount
of product formation was observed (eqn. 4). To confirm the reaction
whether ionic or radical pathway, 1a and 2a were subjected to the
optimized conditions using TEMPO as radical scavenger (eqn. 5), as
expected no desired product formation was observed, instead an
adduct 12a was isolated in 92 % yield, it indicates the radical
reaction mechanism (Scheme 3).
Table 3. Substrate Scope of various amines a
COOR1
CuI (10 mol%), PivOH
DMSO, 65°C
N
+
R2
NH2
COOR1
N
N
1
4
2
R1
COOtBu
COOtBu
N
COOtBu
N
COOtBu
N
N
N
N
N
N
4d; 83%
4b; 84%
4c; 86%
4a; 85%
COOtBu
NH2
CuI (10 mol)
N
COOtBu
N
COOtBu
N
+
COOtBu
N
(1)
COOtBu
COOtBu
N
N
DMSO, 65 °C
9a
8a
with PivOH, 89%
with out PivOH, 40%
1a
N
N
N
N
N
COOtBu
N
4e; 82%
4h; 74%
4f; 83%
4g; 71%
NH2
CuI (10 mol)
(2)
1a
+
OMe
N
10a.
Ph
DMSO, 65 °C
8b
CF3
OMe
with PivOH, 85%
with out PivOH, 19%
COOtBu
N
COOtBu
N
COOtBu
COOtBu
O
O
N
N
N
N
CuI (10 mol)
N
(3)
N
N
O
N
O
DMSO, 65 °C, Air
with PivOH, 65%
1a
4k; 89%
4i; 87%
O
11a
4l; 89%
4j; 88%
Cl
with out PivOH, 9%
F
Br
F
COOtBu
N
COOCyhex
N
COOtBu
COOtBu
N
CuI (10 mol%), PivOH
DMSO, 65 °C
(4)
11a
2a
+
3a; trace
N
N
N
N
N
Cl
OMe
4p; 71%
COOtBu
O
4o; 80%
O
4n; trace
4m; 82%
N
CuI (10 mol%), PivOH
NH2
(5)
1a
+
O
DMSO, 65 °C
Cl
TEMPO
N
COOiPr
N
COOtBu
N
COOtBu
N
COOnBu
N
2a
12a, 92%
N
N
Scheme 3. Additional reactions
N
N
4t; nr
4q; 93%
N
4r; 65%
4s
; 65%
Based on the above control experiments and literature
reports,6c&19 we proposed a plausible reaction mechanism as
described in Scheme 4. Initially CuI coordinates either with
pyridines or amines to form LnCu(I) complex, which is
N
N
COOtBu
N
N
COOtBu
COOtBu
COOtBu
N
N
N
N
N
oxidized to Cu(II) species by O2.6a&20 The reaction of
Cu(II) generates a radical intermediate
benzylic carbocation via a singleꢀelectronꢀtransfer process in
the presence of Cu(II).21 Addition of benzylic amine to
gives
1
with
N
N
4w; 69%
4v; 57%
4u; 68%
4x; 39%
A
, which generates
Cl
B
N
aReaction conditions: 1 (0.25 mmol), 2 (0.5 mmol), additive (0.25 mmol),
catalyst (0.025 mmol), solvent (1 mL), in an oil bath under open air, isolated
yield, and nr: no reaction
B
intermediate
intermediate
C
D
, it further undergoes oxidation to provide imine
, which is further confirmed by the additional
experiments (Scheme 3). Subsequently D further undergoes
singleꢀelectronꢀtransfer process (intermediates E-G) in the
presence of Cu(II) catalyst.6a&21 Attacking nitrogen lone pair of
electrons from G, leads to cyclic intermediate H. Finally,
aromatization of H will give the desired product (3 or 4).
In conclusion, we demonstrated a copperꢀcatalyzed aerobic
oxidative amination of C (sp3)ꢀH bonds for the syntheses of imidazo
[1, 5ꢀa] pyridineꢀ1ꢀcarboxylates under mild conditions. The method
has the following advantages: i) use of atmospheric air as efficient
and selective oxidant (oxygen source), ii) employing commercially
available starting substrates and catalyst, iii) broad range of substrate
scope with good to excellent yields, iv) activation of C(sp3)ꢀH bonds
under mild reaction temperature (65 °C).
Scheme 2.
group at αꢀ position of benzylamine prevented the cyclization; hence
formation product 10a was obtained (eqn. 2). When the reaction of
1a was subjected to these conditions without amine, the oxidised
product tertꢀbutyl 2ꢀoxoꢀ2ꢀ(pyridinꢀ2ꢀyl)acetate 11a was isolated
(eqn. 3). To understand the role of PivOH, we performed reactions
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CuI
O2
Ln
air
CuI
II
I
Cu
II
Cu
Cu
COOR
COOR
N
1
COOR
SET
N
N
B
A
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air
R1
NH2
CuI
II
Cu
COOR
E
COOR
N
N
COOR
Oxidation
R1
N
N
R1
N
R1
NH
D
II
Cu
C
air
SET
I
Cu
COOR
N
4
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COOR
COOR
N
N
COOR
N
+
H
N
+
N
R1
N
N
3 or 4
R1
R1
+
G
H
R1
F
Scheme 4. Plausible mechanism
ACKNOWLEDGMENT
CSIRꢀCSMCRI Communication No. 137/2014. D.C.M, C. R &
S. N. R. are thankful to AcSIR for their Ph.D. enrollment and
the “Analytical Discipline and Centralized Instrumental
Facilities” for providing instrumentation facilities. D.C.M. and
S. N. R. are also thankful to UGC & CSIR, New Delhi for their
fellowships. We thank DST, Government of India (SR/S1/OCꢀ
13/2011), for financial support. We also thank CSIRꢀCSMCRI
(OLPꢀ0076) for partial assistance.
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Notes and references
aAcademy of Scientific & Innovative Research, CSIR–Central Salt & Marine
Chemicals Research Institute, G.B. Marg, Bhavnagarꢀ364 002. Gujarat
(INDIA).* Eꢀmail: adimurthy@csmcri.org.
1H and 13C NMR spectra for all compounds and HRMS spectra for new
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Centre
via
www.ccdc.cam.ac.uk/data_request/cif.
Electronic
Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
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