Alkynylation of heterocyclic compounds using hypervalent iodine reagent

Article abstract of DOI:10.1039/c4ob02625j

The alkynylation of various nitrogen- and/or sulphur-containing heterocyclic compounds using hypervalent iodine TMS-EBX by utilization of tertiary amines under mild conditions is described. The developed metal-free methodology furnishes the corresponding alkynylated heterocycles bearing quaternary carbon in high yields.

Full text of DOI:10.1039/c4ob02625j

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Alkynylation of heterocyclic compounds using
hypervalent iodine reagent
Cite this: DOI: 10.1039/x0xx00000x
M. Kamlar,a I. Císařováb and J. Veselýa
Received 00th January 2012,
Accepted 00th January 2012
DOI: 10.1039/x0xx00000x
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Alkynylation of various nitrogen- and/or sulphur-containing
heterocyclic comounds using hypervalent iodine TMS-EBX
by utilization of tertiary amines under mild conditions is
described. Developed metal-free methodology enables to
obtain corresponding alkynylated heterocycles bearing
quaternary carbon in high yields.
Chemistry of heterocyclic compounds is diverse and varies based on
the structural properties of corresponding heterocycle.
a
Alkynylation reaction of heterocycles containing acidic CꢀH bond
using electrophilic acetylene is quite scarce, although there were
developed several alkynylation methods for various nucleophiles
containing tertiary acidic CꢀH bond either with hypervalent iodine
reagents19 or with another electrophilic acetylene source20.
Specifically, in case of heterocycles there are only two references
regarding the alkynylation of oxindoles21 and azlactones22.
Alkynylation of oxindoles was carried out using electronꢀdeficient
alkynes by catalysis of chiral scandium complexes and azlactones
were alkynylated using hypervalent iodine reagents. Especially, in
case of hypervalent iodine chemistry, which is nowdays wellꢀ
estabilished field in organic synthesis23, there were also a few reports
related to the alkynylation of “nonꢀacidic” heterocyclic
compounds.24 For example Waser and coꢀworkers reported goldꢀ
catalyzed alkynylation of indoles24a, pyrroles23b and thiophenes24c
Heterocyclic compounds play an important role in biological
processes. Heterocyclic moieties are often part of a biologically
active natural and synthetic compounds.1 Majority of synthetic drugs
available on the market have a structural heterocyclic core2 and
therefore the new synthetic approaches leading to their preparation
are of particular interests in organic synthesis.
Fiveꢀmembered heterocyclic compounds such as pyrazolone,
oxindole, azalactone or rhodanine derivatives are deeply studied due
to their significant biological activity. For example pyrazolones
belong to the group of compound which are well studied due to their
broad application in drug chemistry3, agrochemistry4 or material
science5. In the literature there are many examples of pyrazolone
motif containing compounds possesing interesting biological
properties such as metamizole which is used as pain reliever6 and
shows also antipyretic properties7 or edavarone used in treatment of
brain ischemia8. So far only few methodologies related to the
synthesis of disubstituted pyrazolones are published.9 Rhodanines,
fiveꢀmembered heterocyclic compounds containing thiazole nucleus,
are known for antimicrobial10, antifugal11 or antiꢀinflammatory12
properties. Another interesting group containing fiveꢀmembered
heterocyclic ring are oxinodoles, which can be used as tumor
supressors.13 Azlactones, the cyclic Nꢀacylꢀαꢀamino acids, which are
also known to exhibit antifungal14, antibacterial15 and antiꢀ
inflammatory16 activities. Some of them are used as synthones for
preparation of immunomodulators17 and biosensors18.
using
1ꢀ((triisopropylsilyl)ethynyl)ꢀ1,2ꢀbenziodaoxolꢀ3(1H)ꢀone
(TIPSꢀEBX). Recently TIPSꢀEBX, as an efficient acetylene source,
was also used for alkynylation of thiols.25
In accordance with our interest in hypervalent iodine chemistry19b,26
and our previous experience with functionalization of pyrazolones27
we wish to report efficient, metal free electrophilic alkynylation of
heterocyclic compounds containing acidic CꢀH bond such as 4ꢀ
substituted pyrazolones, oxindoles, rhodanines and azlactones using
hypervalent iodine reagents. This methodology allows to obtain
corresponding products under mild condition with high yields
tolerating multitude of substrates.
Our preliminary studies have been carried out between benzylated
pyrazolon 7a as a model nucleophile and 1ꢀ((trimethylsilyl)ethynyl)ꢀ
1,2ꢀbenziodoxolꢀ3(1H)ꢀone (TMSꢀEBX) 3a as alkynylation agent in
presence of tertiary amine Et3N. Due to our previous experiences in
alkynylation of fluorinated sulfones19b we chose toluene as a default
solvent. Unfortunately, formation of a desired product 13a wasn´t
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observed after 5 days of stirring even after heating up to 50 °C (table dihydroꢀ1Hꢀpyrazolꢀ5ꢀone (7g) led to formation of alkynylated
1, entries 1ꢀ2). Switching to the polar solvents was essential for the product 13g in 95% yield (table 2, entry 9). Also the sterically
formation of the corresponding alkynylated product 13a. When hindered pyrazolon 7h substituted with 1ꢀnaphthyl group gave the
DMF as the polar aprotic solvent was used, the reaction reached full corresponding alkynylated product 13h in 90% yield after 8 hours
conversion in 8 hours and desired product 13a was isolated in good (table 2, entry 10).
yield (77 %) (table 1, entry 3). The best results with respect to the
yield and the reaction time were obtained in case of polar protic Table 2 Scope of of alkynylation reactiona
solvents such as ethanol and trifluoroethanol (table 1, entry 4ꢀ5). In
both examples full conversion of the reaction was achieved up to 3
hours and the desired product was isolated in the yield exceeding 80
%. Besides, it was shown that other tertiary amines such as Dabco or
N,Nꢀdiisopropylethylamine instead of Et3N can be used at the cost of
lowered yield (73ꢀ76 %) even with prolonged reaction time (table 1,
entries 6ꢀ7). In summary, the best result (84 % of yield) was
obtained using ethanol as a solvent in presence of Et3N at room
temperature. Under these conditions was full conversion reached
after 3 hours.
Entry
R1
R2
Product
Time
(h)
3
24
48
6
4
4
3
5
Conversion
(%)
100
Yield
(%)
84
52
ꢀ*
70
85
76
82
1
2
3
4
5
6
7
8
9
CH2C6H4
CH2C6H4
CH2C6H4
TMS
TES
13a
13a
13a
13b
13c
13d
13e
13f
Table 1 Solvent and temperature screening of alkynylation reactiona
100
0
100
100
100
100
100
100
TIPS
TMS
TMS
TMS
TMS
TMS
TMS
TMS
CH2(pꢀBrC6H4)
CH2(pꢀNO2 C6H4)
CH2(mꢀNO2 C6H4)
CH2(oꢀNO2 C6H4)
CH2(pꢀCN C6H4)
Me
94
95
90
13g
13h
5
8
10
CH2(1ꢀnaphthyl)
100
a
In a vial pyrazolone (1.0 equiv.), base (20 %) and TMSꢀEBX (1.5 equiv.) were added in
solvent (1 mL).* recovery of starting pyrazolone 7a and TIPSꢀEBX higher than 80 %.
Entry
Base
Solvent
Temperature
Time
(h)
120
120
8
3
3
5
10
Conversion
Yield
(%)
ꢀ
To show the versatility of designed alkynylation procedure we
decided to broaden the scope from pyrazolones to other heterocyclic
compounds. With respect to the nucleophilicity, oxindole (9),
rhodanine (11) and azlactone (12) derivatives were chosen as
convenient candidates (figure 1).
(°C)
25
50
25
25
25
25
25
(%)
0
0
100
100
100
100
100
1
2
3
4
5
6
Et3N
Et3N
Et3N
Et3N
Et3N
toluene
toluene
DMF
EtOH
CF3CH2OH
EtOH
ꢀ
77
84
81
76
73
DIPEA
Dabco
7
EtOH
a
In a vial pyrazolone 7a (1.0 equiv.), base (20 %) and TMSꢀEBX (1.5 equiv.) were
added in solvent (1 mL).
Next, we screened scope of the alkynylation reaction for diverse
substituted pyrazolones and hypervalent iodine derivatives (table 2).
First we turned our attention on hypervalent iodine reagent and apart
from the model TMSꢀEBX (3a) we also examined
Figure 1 Nucleophilic heterocyclic compounds used in alkynylation
reaction.
1ꢀ((triethylsilyl)ethynyl)ꢀ1,2ꢀbenziodoxolꢀ3(1H)ꢀone
(TESꢀEBX)
(3b) and 1ꢀ((triisopropylsilyl)ethynyl)ꢀ1,2ꢀbenziodoxolꢀ3(1H)ꢀone
(TIPSꢀEBX) (3c). As we expected, TMSꢀEBX (3a) showed best At the beginning we took over the conditions used in alkynylation of
activity and desired product was obtained in 84 % yield after three pyrazolone derivatives. To our delight, in case of oxindole (9) and
hours (table 2, entry 1). Lower activity toward alkynylation reaction rhodanine (11) derivatives we observed formation of alkynylated
was observed in case of sterically more hindered TESꢀEBX (3b) and products 14 and 15, nevertheless, the yield was in both cases
TIPSꢀEBX (3c). Comparing to TMSꢀEBX, use of TESꢀEBX led to
unsatisfactory (table 3, entries 1ꢀ2). Unfortunately, these conditions
the formation of corresponding alkynylated pyrazolon 13a in showed to be totally inefficient for azlactone derivative 12, which
moderate yield of 52 % after 24 hours of stirring (table 2, entry 2). decomposed without a trace of alkynylated product 16 (table 3, entry
Employing the bulkiest TIPSꢀEBX (3c) didn´t lead to the formation 3). Switching to polar aprotic solvent such as DMF showed to be
of the corresponding product 13a at all (table 2, entry 3) even under
suitable solvent in alkynylation of all three heterocycles. In the
prolonged time. Instead, complete recovery of both starting presence of DMF as a solvent and Et3N reaction proceeded well and
materials, i.e. 7a and 3c, was observed. This observation was in reached full conversion within an hour. Isolated yields of 1,3ꢀ
accordance with our previous experiences, which revealed superior dibenzylꢀ3ꢀethynylindolinꢀ2ꢀone (14) and 5ꢀbenzylꢀ5ꢀethynylꢀ3ꢀ
activity of 3a over the other alkynylating agents.
phenylꢀ2ꢀthioxothiazolidinꢀ4ꢀone (15) exceeded 80 % (table 3,
entries 10ꢀ11). In case of alkynylated azlactone 16 was isolated yield
lower and reached 63 % (table 3, entry 12). Replacement of DMF for
CHCl3 in case of azlactone derivative 12 led to dramatic
improvement of the yield up to 85 % (table 3, entry 9). It is also
apparent that substrates 9, 11 and 12 in contrast to pyrazolone
derivatives tolerate nonpolar solvents such as toluene, though the
isolated yield of corresponding products are moderate (table 3,
entries 9ꢀ12). As well as in case of pyrazolones, Dabco was found to
Next was examined the scope of differently substituted pyrazolones.
To our delight, reaction proceeded well in all cases. Formation of
alkynylated product 13 was independent on substitution of starting
pyrazolon derivatives and full conversion was reached in short
reaction time. For example 3ꢀmethylꢀ4ꢀ(2ꢀnitrobenzyl)ꢀ1ꢀphenylꢀ1,4ꢀ
dihydroꢀ3Hꢀpyrazolꢀ5ꢀone (7e) gave corresponding product 13e in
82% yield and full conversion was reached after 3 hours (table 2,
entry 7). The less sterically hindered 3,5ꢀdimethylꢀ1ꢀphenylꢀ1,4ꢀ
2 | J. Name., 2012, 00, 1-3
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be a suitable base in acetlylene transfer reaction of oxindol, Scheme 1 Transformation of 13a to compounds 17 and 18 via
rhodanine and azlactone derivatives (table 3, entries 13ꢀ15).
Sonogashira and Huisgen reaction.
Table
3 Alkynylation of oxindol, rhodanine and azlactone
derivativesa
Bn
Bn
O
O
N
N
Bn
Bn
14
9
Bn
Bn
TMS-EBX (3a)
S
base (20 mol%)
S
O
O
N
solvent, rt
N
S
S
Ph
Ph
11
15
Figure 2 View on the one of two symmetrically independent
molecules 18 with atom numbering scheme.
Bn
Bn
N
N
O
We have also made an investigation regarding to asymmetric version
of designed alkynylation reaction employing various chiral tertiary
amines as catalysts. In that context several of cinchona alkaloids
were tested in order to induce enantioselectivity in our alkynylated
products (table 4). In our preliminary reaction we tested quinine as a
catalyst of reaction between pyrazolone 7a and TMSꢀEBX (3a). We
observed formation of corresponding alkynylated product 13a in
good yield in ethanol but with unsatisfactory enantiomeric excess.
(table 4, entry 2). We have also examined other cinchona based
alkaloids but with similar results in terms of enantioselectivity
(entries 3ꢀ6). When toluene was used to slow down the reaction, no
formation of 13a was observed (entry 1). On the other hand, the
reaction utilized with bifunctional Takemoto catalyst (VI) in toluene
afforded the corresponding product 13a in low yield (38%) with
unsatisfactory enantioselectivity (14%, entry 7).
O
O
O
Ph
Ph
12
16
Time
Yield
(%)
30
47
ꢀb
38
69
59
41
85
85
85
94
63
81
75
78
Entry
Substrate
Base
Solvent
(h)
96
48
48
96
15
15
96
1
1
1
1
1
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
9
Et3N
Et3N
Et3N
Et3N
Et3N
Et3N
Et3N
Et3N
Et3N
Et3N
Et3N
Et3N
Dabco
Dabco
Dabco
EtOH
EtOH
EtOH
Toluene
Toluene
Toluene
CHCl3
CHCl3
CHCl3
DMF
11
12
9
11
12
9
11
12
9
11
12
9
DMF
DMF
DMF
DMF
3
2
2
11
12
CHCl3
a In a vial substrate 9, 11, 12 (1.0 equiv.), catalyst (20 %) and TMSꢀEBX
(1.5 equiv.) were added in solvent (1 mL)b degradation of a starting
azlactone
Table 4 Attempts in enantioselective alkynylation of pyrazolone 7a a
The utility of acetylene functionalized heterocycles has been
demonstrated by Sonogashira coupling of 13a with iodobenzene
affording compound 17 in good yield 61 %. Alkynylated pyrazolone
7a was also subjected to “click” reaction with 1ꢀ(azidomethyl)ꢀ4ꢀ
bromobenzene leading to corresponding triazole derivative 18 in
high yield (92 %). Structure of corresponding triazole derivative 18
was confirmed according Xꢀray diffraction analysis (figure 2).
Br
Ph
Ph
O
H3C
a)
b)
N
O
Ph
Bn
O
N
N
(92 %)
(61 %)
N
N
Ph
N
N
N
Ph
Bn
H3C
N
13a
H3C
17
a) C6H5I, Pd(PPh3)2Cl2, CuI, Et3N, DMF;
18
.
b) 1-(azidomethyl)-4-bromobenzene, sodium ascorbate, CuSO4 7H2O,
-BuOH/H O.
t
2
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We are grateful for the financial support from the Czech Science
Foundation (Grant No. P207/10/0428) and the Charles University
Grant Agency (grant no. 427011). The authors would like to thank to
Dr. Zdeněk Tošner for low temperature NMR experiments.
Notes and references
a
Department of Organic Chemistry, Faculty of Science, Charles
University in Prague, Hlavova 2030, 128 43, Praha 2, Czech Republic, eꢀ
mail: jxvesely@natur.cuni.cz, fax: +42ꢀ221951226.
b
Department of Inorganic Chemistry, Faculty of Science, Charles
University in Prague, Hlavova 2030, 128 43, Praha 2, Czech Republic.
†
Electronic Supplementary Information (ESI) available: 1H and 13C
NMR spectra of 11 13a-h 14 18. See DOI: 10.1039/c000000x/
,
,
ꢀ
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Temperature
Yield
(%)
72
n.d.
69
77
76
32
71
Ee
(%)
8
ꢀ
0
0
4
0
ꢀ14
ꢀ11
Entry
Catalyst
Solvent
Time (h)
2
3
a) A. R. Katritzky and A. F. Pozharskii, Handbook of Heterocyclic
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(°C)
25
25
25
25
25
25
25
1
2
3
4
5
6
7
I
I
II
III
IV
V
VI
VI
Ethanol
Toluene
Ethanol
Ethanol
Ethanol
Ethanol
Toluene
Toluene
15
120
15
15
15
120
10
96
a) K. L. Kees, John J. Fitzgerald, Jr., K. E. Steiner, J. F. Mattes, B.
Mihan, T. Tosi, D. Mondoro and Michael L. McCaleb, J. Med.
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8
ꢀ10
38
a
In a vial pyrazolone 7a (1.0 equiv.), catalyst (10 %) and TMSꢀEBX (1.5 equiv.) were
added in solvent (1 mL). b Determined by HPLC (Chiral AD 90:10 Hept:iPrOH, 1mL).
In order to understand the mechanism of above mentioned
alkynylation, NMR experiments were performed. When TMSꢀEBX
(3a) was subjected to trimethylamine in CD2Cl2 without appropriate
nucleophile, no formation of ethynylꢀ1,2ꢀbenziodoxolꢀ3(1ꢀH)ꢀone
(EBX) at ꢀ40 °C and lower temperatures took place. Slow formation
of EBX in equilibrium with products of decomposition was observed
at – 40 °C. Full conversion of TMSꢀEBX (3a) to EBX and products
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19a
of EBX is in accordance with data reported by Waser et al.
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additionꢀelimination mechanism.28
Conclusions
4
5
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In conclusion, we report metalꢀfree organocatalytic alkynylation of
heterocyclic compounds containing acidic CꢀH bond employing
hypervalent iodine reagents. Our methodology gives access to
alkynylated heterocyclic compounds containing quaternary centres.
Alkynylated pyrazolone, oxindole, rhodanine or azlactone
derivatives were obtained in good to excellent yields. Acetylene
bearing pyrazolone derivative 13a was used in Sonogashira coupling
and “click” reaction leading to corresponding products 17 and 18 in
good to high yield. Further studies and synthetic applications based
on this methodology are currently ongoing in our group.
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Acknowledgements
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