Nano-copper catalysed highly regioselective synthesis of 2,4-disubstituted pyrroles from terminal alkynes and isocyanides

Article abstract of DOI:10.1039/c5cc04166j

Nano-copper(0) stabilized on alumina prepared from Cu-Al hydrotalcite has been reported for completely regioselective synthesis of 2,4-disubstituted pyrroles from unactivated terminal aromatic/aliphatic alkynes and isocyanides. The reaction is operational

Full text of DOI:10.1039/c5cc04166j

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ARTICLE TYPE
Nano copper catalysed highly regioselective synthesis of 2,4-
disubstituted pyrroles from terminal alkynes and isocyanides
Dipak Kumar Tiwaria, Jaya Pogulaa, B. Sridharb, Dharmendra Kumar Tiwaria* and Pravin R. Likhara*
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
5
DOI: 10.1039/b000000x
reduction, Cu (II) gets selectively reduced to Cu(0) with high
50 dispersion and stability. Herein, we describe the regioselective
route for the synthesis of 2,4ꢀdisubstituted pyrroles using
isocyanides and unactivated aromatic terminal alkynes with
Cunano/Al2O3. In addition, the catalytic activity of the
Cunano/Al2O3 was extended to the regioselective synthesis of
55 2,4ꢀdisubstituted pyrroles using isocyanides with unactivated
aliphatic terminal alkynes in which the substituted methylene
group at C4 undergoes oxidation to alkanoyl pyrroles (Figureꢀ
1, eq. 3).
Nano copper(0) stabilized on alumina prepared from Cu-Al
hydrotalcite has been reported for completely regioselective
synthesis of 2,4-disubstituted pyrroles from unactivated
terminal aromatic/ aliphatic alkynes and isocyanides. The
10 reaction is operationally simple, involves ligand-free
inexpensive nano copper, and affords high yielded products.
Oligofunctional pyrroles are an important nitrogen containing
heterocyclic motif often observed in a plethora of biologically
significant natural products and potent pharmaceuticals.1 In
15 addition, pyrrole derivatives are widely used as the building
blocks in the preparation of agrochemicals, flavours, dyes,
molecular sensors and other devices.2,3 Therefore, substantial
attention has been paid to develop mild and efficient methods
for pyrroles synthesis and in recent past, numerous new
20 synthetic protocols have been reported.4 Among them, the
metal catalysed [3+2] cycloaddition of isocyanides and
alkynes has emerged as one of the most reliable and promising
route to access substituted pyrroles due to its atom economical
nature. However, the majority of reactions are reported only
25 with activated alkynes to afford either oligo (2,3ꢀ
disubstituted) or polysubstituted pyrroles.5 Recently, Lei and
Bi coꢀworkers have reported the first transition metal
catalysed regioselective synthesis of 2,3ꢀdisubstituted pyrroles
by using unactivated alkynes (Figureꢀ1, eq. 1).6 However, the
30 challenges for synthesizing regioselective 2,4ꢀdisubstituted
pyrroles from unactivated alkynes and isocyanides remain
elusive. To the best of our knowledge, only Bi group has
described the silver carbonate catalysed regioselective
synthesis of 2,4 disubstituted pyrroles in cycloaddition of
35 isocyanides with 2ꢀpyridyl alkynyl carbinols which needs to
be synthesized prior to use. (Figureꢀ1, eq. 2).7 Therefore, the
development of simple and efficient route to access 2,4ꢀ
disubstituted pyrroles from easily available unactivated
alkynes remains an important research objective. We have
40 investigated an operationally simple methodology, using
inexpensive copper catalyst which can catalyse the
cycloaddition of isocyanides with electron deficient terminal
alkynes to give regioselectively 2,4ꢀdisubstituted pyrroles.
Recently, we directed our efforts on the synthesis of mono
45 dispersed and highly stable Cu(0) from copper aluminium
hydrotalicite.8 One of the important characteristics of nano
Cu(0) on alumina (Cunano/Al2O3) is that it is prepared from
single precursor, CuꢀAl HT Brucite like structures and upon
60 Figure 1. Scheme depicting the regioselective synthesis of 2,3ꢀ and 2,4ꢀ
disubstituted pyrroles along with present study.
At the onset, the readily available phenyl acetylene (1a)
and methyl 2ꢀisocyanoacetate (2a) were selected as model
substrates. Our initial efforts by treating 1a with 2a in the
65 presence of freshly prepared catalyst (Cunano/Al2O3; 30 mg,
3.75%) in DMSO at room temperature under nitrogen
atmosphere were unsuccessful (Table 1, entry 1). No
improvements were observed even after changing the solvents
and varying the temperatures. However, when the mixture was
70 heated at 85 °C in the presence of base (Na2CO3, 1.5 equiv)
under nitrogen atmosphere for 6 h, we indeed obtained 2,4ꢀ
disubstituted pyrrole 3a albeit in very low yield (5%). The
major product obtained was disubstituted imidazole 4a (90%)
resulting from dimerization of methyl 2ꢀisocyanoacetate 2a
75 (entry 2).9 To our delight, a higher yield (65%) of 3a and trace
amount of imidazole 4a were obtained when Na2CO3 (1.5
equiv.) was added in portions over a period of 2 h at 85 °C
(entry 3). This interesting result encouraged us to continue our
investigations to optimize the reaction conditions.
80
After several optimization efforts, the use of 3.75 mol%
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catalyst (30 mg) and 1.5 equiv. of K2CO3 as a base in DMSO 20 we examined different solvents such as CH3CN, toluene, 1,4ꢀ
at 85 °C under nitrogen atmosphere turned out to be the best
result (entryꢀ6). A further increase in catalyst loading (7.5
mol%) resulted in lower yield (58%) of desired product 3a
(entryꢀ5), due to dimerization of 1a. The use of lower amount
dioxane, NMP and DMF for this reaction but all of them
produced the corresponding 2,4ꢀdisubstituted pyrrole 3a in
low yields (entries 13ꢀ17). However, no product formation
was observed in chlorinated solvents such as DCM and EDC
5
of catalyst (2.5 mol%) furnished inferior yield of 3a along 25 (entries 18 and 19). During our investigations, various copper
with unreacted starting material (entryꢀ7). Furthermore, the
screening of other bases such as triethylamine (Et3N), 1,8ꢀ
diazabicyclo[5.4.0]undecꢀ7ꢀene (DBU) and potassium tertꢀ
10 butoxide (tꢀBuOK) produces the desired product in poor
yields (entries 8ꢀ10).
salts such as CuI, Cu(OTf)2, (Cu(OAc)2, CuCl, Cu2O and Cu
powder (nano Cu, 60ꢀ80 nm Aldrich) were also screened, but
most of them led to the formation of only undesired imidazole
4a in (12ꢀ45%) yields, except in the case of Cu powder, which
30 afforded 23% yield of 3a along with 4a in 45% yield (entries
20ꢀ25). The structure of 2,4ꢀdisubstituted pyrrole (3a) was
well characterized using all spectroscopic techniques and
confirmed by 1H NMR, 13C NMR, LRꢀMS, HRꢀMS and a
single Xꢀray diffraction analysis.10
Table 1 Optimization of the reaction conditions for aromatic
alkyne.
35
With the optimized reaction conditions, the scope of the
reaction was further extended to various aromatic terminal
alkynes and isocyanides. We were pleased to see that various
substrates underwent the cycloaddition to afford desired 2,4ꢀ
disubstituted pyrroles (3a-a'''-3j-j') in moderate to good
Entry
Catalyst
Base
(equiv)
Solvent
T
[°C]
rt
Yield (%)
3a
4a
40
1
2
3
4
5
6
7
8
9
Cu(0)/Al2O3 without base
Cu(0)/Al2O3 Na2CO3 (1.5)
Cu(0)/Al2O3 Na2CO3 (1.5)
Cu(0)/Al2O3 K2CO3 (1.5)
Cu(0)/Al2O3 K2CO3 (1.5)
Cu(0)/Al2O3 K2CO3 (1.5)
Cu(0)/Al2O3 K2CO3 (1.5)
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
CH3CN
toluene
0
0
90a
85
85
85
85
85
85
85
85
85
100
60
85
85
5
65 traceb
72 traceb
58 tracec
75 traceb
50 traced
42 traceb
52 traceb
50 traceb
55 traceb
30 traceb
63 traceb
41 traceb
43 traceb
Scheme 1a Scope of the Cunano catalyzed [3+2] cycloaddition
reaction of aromatic alkynes (1a-1j) with isocyanides (2a-2a''')
Cu(0)/Al2O3
Cu(0)/Al2O3
Et3N (1.5)
DBU (1.5)
10 Cu(0)/Al2O3 tꢀBuOK (1.5)
11 Cu(0)/Al2O3 K2CO3 (1.5)
12 Cu(0)/Al2O3 K2CO3 (1.5)
13 Cu(0)/Al2O3 K2CO3 (1.5)
14 Cu(0)/Al2O3 K2CO3 (1.5)
COOR
COOR
COOR
NH
NH
NH
R= Me, Yield, 70%, 3b
R= Et, Yield, 68%, 3b'
R= Me, Yield, 69%, 3c
R= Et, Yield, 73%, 3c'
R= Me, Yield, 75%, 3a
R= Et, Yield, 71%, 3a'
R= t-Bu, Yield, 69%, 3a''
R= P(O)OEt2, Yield, 65%, 3a'''
C4H9
O
O
15 Cu(0)/Al2O3 K2CO3 (1.5) 1,4ꢀdioxane 85
COOR
COOR
NH
COOR
16 Cu(0)/Al2O3 K2CO3 (1.5)
17 Cu(0)/Al2O3 K2CO3 (1.5)
18 Cu(0)/Al2O3 K2CO3 (1.5)
19 Cu(0)/Al2O3 K2CO3 (1.5)
NMP
DMF
85
85
60
80
85
85
85
85
85
85
46 traceb
30 traceb
NH
NH
R = Me, Yield, 72%, 3d
R = Et, Yield, 68%, 3d'
R= Me, Yield, 69%, 3e
R= Et, Yield, 73%, 3e'
R= Me, Yield, 71%, 3f
R= Et, Yield, 67%, 3f'
DCM
0
0
traceb
traceb
37b
Br
EDC
F3C
O
COOR
NH
20
21
22
23
24
25
CuI
Cu(OTf)2
Cu(OAc)2
CuCl
K2CO3 (1.5)
K2CO3 (1.5)
K2CO3 (1.5)
K2CO3 (1.5)
K2CO3 (1.5)
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
0
COOR
NH
COOR
NH
0
41b
R= Me, Yield, 73%, 3g
R= Et, Yield, 72%, 3g'
R= Me, Yield, 70%, 3h
R= Et, Yield, 72%, 3h'
R= Me, Yield, 69%, 3i
R= Et, Yield, 71%, 3i'
0
38b
0
12b
Cu2O
0
45b
COOCH3
NH
Cu(0)/nano K2CO3 (1.5)
60ꢀ80 nm
23
45b
COOR
NH
R= Me, Yield, 68%, 3j
R= Et, Yield, 64%, 3j'
(Aldrich)
3a
3a
a Reaction was performed using 1a (1.0 mmol), 2a (1.0 mmol), Cat. (Cu
b
8 wt%, 30 mg, 3.75 mol%) and K2CO3 (1.5 mmol) at 85 °C for 6 h;
a
Reaction was performed using 1 (1.0 mmol) with 2 (1.0 mmol) in the
Cat. (Cu, 3.75 mol%) and gradual addition of K2CO3 (1.5 mmol) over a
period of 2 h at 85 °C and stirred for another 4 h under nitrogen
atmosphere; Cat. (Cu, 60 mg, 7.5 mol%); Cat. (Cu, 20 mg, 2.5
mol%). Yield indicates yields of isolated product.
presence of Cunano/Al2O3 ( Cu 8 wt%, 30 mg, 3.75 mol%) and gradual
addition of K2CO3 (1.5 mmol) over a period of 2 h at 85 °C and stirred
for another 4 h under nitrogen atmosphere. The reaction time for all the
reactions is 6 h including addition of base. Yield indicates yields of
isolated product.
c
d
The reaction temperature also noticeably influenced the
15 product formation in this cycloaddition reaction. When the
temperature was increased to 100 °C, only 55% yield of
desired product could be achieved (entry 11). On the other
hand, a decrease in the temperature from 85° to 60 °C
afforded the desired pyrrole 3a in 30% yield (entry 12). Next,
yields (Scheme 1). Various isocyanides such as methyl 2ꢀ
isocyanoacetate (2a), ethyl 2ꢀisocyanoacetate (2a'), tertꢀbutyl
isocyanoacetate (2a'') and diethylphosphono acetonitrile
(2a''') were employed in the reactions and comparable yields
2
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of corresponding products were obtained. In addition, the 45 improve the yield (entries 4ꢀ6) except tꢀBuOK (1.5 equiv),
reaction was readily extended to a variety of aryl substituted
terminal alkynes. Both electron donating and electronꢀ
withdrawing groups on the phenyl ring were found to be
compatible with optimized reaction conditions (3b-b'-3h-h').
It is important to note that, there was no apparent steric effect
on the phenyl ring since moderate to good yields of the
desired products were obtained for substrate bearing groups at
which resulted 3k in 46% yield (entry 7). We then screened
different solvents but all of them furnished 3k in poor yield
(entries 8ꢀ11).
Next, the cycloaddition reactions of pentyneꢀ1 (1l) with both
50 methyl 2ꢀisocyanoacetate (2a) and ethyl 2ꢀisocyanoacetate
(2a') were performed and moderate yields of corresponding
alkanoyl pyrroles (3l, 3l') were obtained. On the other hand,
when activated acetylene such as propꢀ2ꢀynꢀ1ꢀyl benzene
(1m) was employed along with the isocyanides (2a & 2a'), a
55 better yield of 3m and 3m' (67% and 65%) were obtained
respectively. The spectral data of alkanoyl pyrroles were in
good agreement with the reported values.12
5
meta, and para positions (3j-j'
& 3b-b'-3h-h'). The
10 cycloaddition reaction were also performed with aromatic
alkynes substituted with CF3 (1g) and Br (1h) which resulted
in good yield of the corresponding products (3g-g' & 3h-h').
Moreover, bicyclic and tricyclic aryl alkynes such as 6ꢀ
methoxyꢀ2ꢀethynylnaphthalene
(1i)
and
9ꢀethynylꢀ
15 phenanthrene (1j) were also found to be suitable reacting
partners and produced the corresponding substituted pyrroles
3i, 3i', 3j and 3j' in moderate yields.
With the above encouraging results, we focused our
attention on aliphatic terminal alkynes. The reaction of
20 hexyneꢀ1(1k) with methyl 2ꢀisocyanoacetate (2a) in the
presence of nitrogen atmosphere afforded selectively methyl
4ꢀbutyrylꢀ1Hꢀpyrroleꢀ2ꢀcarboxylate 3k (12%) instead of the
desired product 5 (Table 2, entry 1). The formation of 3k was
plausibly due to the aerobic oxidation of benzylic methylene
25 group of 5 obtained from the cycloaddition of hexyneꢀ1 (1k)
and methyl 2ꢀisocyanoacetate (2a) in the presence of copper
catalyst.11 All our efforts to isolate the intermeidate 5 were
unsuccesful because it probably oxidised during work up and
purifications. Therefore, we attempted the cycloaddition
30 reaction of 1k under air and oxygen atmosphere. Interestingly,
in both the reactions, the yield of oxidized product 3k was
imrpoved to 25% and 28% respectively (entries 2, 3).
60
Scheme 2a Scope of the Cunano catalyzed [3+2] cycloaddition
reaction of aliphatic acetylenes (1k-m) with isocyanides (2a-a’)
O
O
O
C2H5
C3H7
COOR
COOR
COOR
NH
NH
65
NH
R= Me, Yield, 46%, 3k
3k'
R= Me, Yield, 44%, 3l
3l'
R= Me, Yield, 67%, 3m
3m'
R= Et, Yield, 51%,
R= Et, Yield, 56%,
R= Et, Yield, 65%,
a
Reaction was performed using 1k-m (1.0 mmol) with 2a-a’ (1.0
mmol) in the presence of Cunano/Al2O3 (Cu 8wt%, 30 mg, 3.75 mol%)
70 and tꢀBuOK (1.5 mmol) in DMSO (4 mL) at 85 °C for 8 h under O2.
Yield indicates yields of isolated product.
The plausible mechanism for the formation of 2,4
disubstituted pyrroles
3 from terminal alkynes 1 and
isocyanoacetate 2 can be rationalized as depicted in Scheme 3.
75 Phenyl acetylene gets converted into copper acetylenide 6 (via
Cu insertion), which then rearranges to copper vinylidine
intermediate 7.13 The deprotonation of isocyanoacetate 2
Table 2 Optimization of the reaction conditions for aliphatic
35 alkyne 1k.
furnishes anionic species
cycloaddition with copper vinylidine
8
which undergoes [3+2]
resulting into
7
1k
+
Cunano/Al2O3
COOMe
O
80 intermediate 9. The latter then undergoes proton shift and
rearrangement of double bond leading to 12. The release of
copper and final protonation gives the desired product (3a). In
case of aliphatic alkynes, the benzylic methylene group
undergoes air oxidation to form the alkanoyl pyrroles 3k.11
85
COOMe
base
85 °C, 8 hrs
NH
NH
N
COOMe
2a
5
3k
Not isolated
Entry
Catalyst
Base
Solvent
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
CH3CN
Toluene
Yield(%)
12a
25b
28c
10c
7c
1
2
3
4
5
6
7
8
9
Cunano/Al2O3
Cunano/Al2O3
Cunano/Al2O3
Cunano/Al2O3
Cunano/Al2O3
Cunano/Al2O3
Cunano/Al2O3
Cunano/Al2O3
Cunano/Al2O3
Cunano/Al2O3
Cunano/Al2O3
K2CO3
K2CO3
K2CO3
Et3N
CuNP
R'
R'
1
H
[Cu]
CO2R
R'
R'
CO2R
Cu
H
N
H
3a-j
N
12
H
R' = Aryl
6
DBU
BH
aerial Oxn
Na2CO3
tꢀBuOK
tꢀBuOK
tꢀBuOK
9c
for aliphatic
46c
11c
8c
R'
alkynes
O
•
[Cu]
BH
7
[Cu]
R'
CO2R
H
C
N
R'
CO2R
8
COOR
N
11
N
H
3k-m
10
11
tꢀBuOK 1,4ꢀDioxane
tꢀBuOK DMF
15c
12c
H
[3+2]
additio
Base
-H+
B
n
cyclo
R'= alkyl/aryl
CN
COOR
[Cu]
a Reaction was performed using 1k (1.0 mmol), 2a (1.0 mmol), Cat.
[Cu]
COOR
R'
H
2
R'
CuNP = Cu nanoparticle
COOR
(Cu 8 wt%, 30 mg, 3.75 mol%) and base (1.5 mmol) heated at 85 °C
40 for 8 h; under nitrogen atmosphere;
Yield indicates yields of isolated product.
N
9
N
b
c
H
under air;
under oxygen;
10
H-transfer
Scheme 3. Plausible mechanism for the formation for pyrroles 3a-j
and 3k-m
Further, we screened various bases such as Et3N, DBU,
Na2CO3 but most of them were found less effective to
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2008, 20, 2556 – 2560.
65
In summary, highly efficient regioselective [3+2]
cycloaddition of aromatic alkynes with isocyanides is
achieved by heterogeneous copper nanoparticles stabilised on
alumina. The yield of regioselective 2,4ꢀdisubstituted pyrroles
is optimized by the use of appropriate base and solvent
without using external ligands. Further, the application of
copper nanoparticles is extended in regioselective
cycloddition of aliphatic alkynes with isocyanides to 2,4ꢀ
10 disubstituted alkanoyl pyrroles in moderate yields.
Considering, the readily availabile starting materials, broad
substrate scope, operationally simple, and highly
functionalized products, the present method makes an
attractive option for the synthesis of the 2,4ꢀdisubstituted
15 pyrroles. Further detailed mechanistic studies and the
reusability of catalyst are currently under way in our
laboratory and findings will be reported in due course.
5
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Acknowledgement
The authors thank the Department of Science and Technology
20 (DST) New Delhi, India, for the financial support. The authors
are grateful to the Director, CSIRꢀIICT for providing the
necessary infrastructure.
Notes and references
25 aInorganic and Physical Division CSIR-Indian Institute of Chemical
Technology Uppal road, Tarnaka, Hyderabad 500607(INDIA);
Fax:(+)91-040-2716092; E-mail: dktiwari@iict.res.in/plikhar@iict.res.in
Homepage: www.iict.org.net
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