Synthesis of 1,2,4-Triazoles, N-Fused 1,2,4-Triazoles and 1,2,4-Oxadiazoles via Molybdenum Hexacarbonyl-Mediated Carbonylation of Aryl Iodides

Article abstract of DOI:10.1002/adsc.201500703

A convenient and efficient protocol has been developed for the synthesis of 1,2,4-triazoles, N-fused 1,2,4-triazoles and 1,2,4-oxadiazoles using molybdenum hexacarbonyl where, for the first time, molybdenum hexacarbonyl acts as a convenient and reliable s

Full text of DOI:10.1002/adsc.201500703

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DOI: 10.1002/adsc.201500703
Synthesis of 1,2,4-Triazoles, N-Fused 1,2,4-Triazoles and
1,2,4-Oxadiazoles via Molybdenum Hexacarbonyl-Mediated
Carbonylation of Aryl Iodides
Mangarao Nakka,a Ramu Tadikonda,a Srinivas Nakka,a and Siddaiah Vidavalura,*
a
Department of Organic Chemistry & FDW, Andhra University, Visakhapatnam – 530 003, India
Fax: +08912544683; e-mail: sidduchem@gmail.com or vsiddaih.ocfd@auvsp.edu.in
Received: July 24, 2015; Revised: November 13, 2015; Published online: January 21, 2016
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201500703.
Abstract: A convenient and efficient protocol has
been developed for the synthesis of 1,2,4-triazoles,
N-fused 1,2,4-triazoles and 1,2,4-oxadiazoles using
molybdenum hexacarbonyl where, for the first time,
molybdenum hexacarbonyl acts as a convenient and
reliable solid source of carbon monoxide. This pro-
cedure provides an easy access to a library of 1,2,4-
triazole, N-fused 1,2,4-triazole and 1,2,4-oxadiazole
derivatives in fair to good yields without the need
of gaseous carbon monoxide and palladium cata-
lysts.
zones and amidoximes followed by cyclodehydration
are the most commonly explored strategies.[8] Recent-
ly, carbonylation with a subsequent intramolecular
cyclization reaction has been shown to be an efficient
process for the straightforward synthesis of heterocy-
cles.[9] 1,2,4-Triazoles and 1,2,4-oxadiazoles are synthe-
sized via carbamoylation of aryl iodides using a palla-
dium catalyst and carbon monoxide (CO).[10]
However, this method has some drawbacks such as
the requirement of expensive palladium reagents and
the use of the toxic and cumbersome to handle CO
gas. Therefore, there is a need to develop more eco-
nomical, eco-friendly, palladium and CO gas free al-
Keywords: carbonylation; molybdenum hexacar-
bonyl; 1,2,4-oxadiazoles; 1,2,4-triazoles
Heterocyclic compounds are privileged scaffolds that
have been found in a wide variety of natural products
and biologically active molecules. Particularly, five-
membered ring heterocyclic compounds such as 1,2,4-
triazoles, N-fused 1,2,4-triazoles and 1,2,4-oxadiazoles
have recently received significant attention in the
field of chemistry, biological and material sciences.
These scaffolds are known to exhibit a wide range of
biological properties including antibacterial,[1] anti-in-
flammatory,[2] antiviral,[3] antitumour[4] and antiasth-
matic activities.[5] They have also been used as bioisos-
ters of esters, amides and as dipeptidomimetics in
a number of pharmacologically important molecules.[6]
Moreover, these five-membered ring scaffolds act as
intermediates in the synthesis of many drugs such as
maraviroc, sitagliptin, triazolam, penipanoid A and
ataluren (Figure 1).[7]
Owing to their important applications, a number of
methods have been developed for the synthesis of
1,2,4-triazole, N-fused 1,2,4-triazole and 1,2,4-oxadi-
azole derivatives. Among them, condensation of car-
Figure 1. Selected examples of bioactive and natural com-
boxylic acid derivatives or aldehydes with amidra- pounds containing 1,2,4-triazole and 1,2,4-oxadiazole cores.
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Synthesis of 1,2,4-Triazoles, N-Fused 1,2,4-Triazoles
Table 1. Optimization of the reaction conditions.[a]
Entry
M
(equiv.)
Base
Solvent
Time Yield
[h][b]
[%][c]
1
2
3
4
5
6
7
8
Mo (0.2)
Mo (0.2)
Mo (0.2)
Mo (0.2)
Mo (0.2)
Mo (0.2)
Mo (0.2)
Mo (0.2)
W (0.2)
Cr (0.2)
Mo (0.167) Bu3N
Mo (0.3)
Mo (0.2)
Mo (0.2)
Et3N
Et3N
Et3N
Et3N
DMAP DMF
Cs2CO3 DMF
K2CO3
Bu3N
Bu3N
Bu3N
diglyme
DME
DMF
23
19
15
38
21
61
29
1,4-dioxane 22
17
23
23
10
16
17
15
10
10
10
21
trace
trace
91
31
27
79
91
90
91
DMF
DMF
DMF
DMF
DMF
DMF
NMP
DMF
9
Scheme 1. Synthesis of 1,2,4-triazoles, N-fused 1,2,4-triazoles
and 1,2,4-oxadiazoles.
10
11
12
13
14[d]
Bu3N
Bu3N
Bu3N
ternative methods for the synthesis of 1,2,4-triazoles
and 1,2,4-oxadiazoles. Recently, group VI metal car-
bonyl complexes such as Cr(CO)6, W(CO)6,
Co2(CO)8, Mo(CO)6 etc., have been used as solid
sources of CO. Among them, Mo(CO)6 has emerged
as an ideal candidate and has been used in several or-
ganic transformations.[11] Inspired by these advances,
recently we have described the use of Mo(CO)6 as
a convenient and reliable solid source of carbon mon-
oxide for the synthesis of benzimidazoles and benzox-
azoles.[12] In continuation of our work on the develop-
ment of efficient synthetic methods for various bio-
[a]
Reaction conditions: 1 (1.0 equiv.), 2 (1.0 equiv.), M(CO)6
(x equiv.), base (1.2 equiv.), Et4NCl (0.2 equiv.), solvent
(2 vol) and temperature 1508C.
Time at which TLC (EtOAc:hexane, 1:1) indicated com-
plete disappearance of the starting materials.
Yields reported are isolated yields.
Reaction conditions: 1 (1.0 equiv.), 2 (1.0 equiv.), M(CO)6
(0.2 equiv.), Bu3N (1.2 equiv.), Et4NCl (0.2 equiv.), DBU
(1.0 equiv.) DMF (2 vol) at 1508C.
[b]
[c]
[d]
logically active heterocycles,[13] herein we report for 1,2,4-triazoles, we then focused on the quantity of
the first time a palladium and CO gas free synthesis Mo(CO)6. By decreasing the quantity of Mo(CO)6
of 1,2,4-triazoles, N-fused 1,2,4-triazoles and 1,2,4- from 0.2 equiv. to 0.167 equiv. (stoichiometric amount
oxadiazoles from aryl halides using Mo(CO)6 of CO) the yield of the product dropped to 79% even
(Scheme 1).
after prolonged reaction time (Table 1, entry 11). In
The optimization studies were carried out using io- contrast, no improvement of the yield was observed
dobenzene (1a) and N-phenylbenzamidrazone (2a) to by increasing the quantity of Mo(CO)6 (Table 1,
the catalytic system of Mo(CO)6, Et4NCl and Et3N in entry 12). Next, we carried out the same reaction
diglyme at 1508C for 23 h, which gave the target using NMP (a less likely CO source comparable to
product 3a in 38% yield (Table 1, entry 1). Different DMF) as a solvent under the same reaction condi-
solvents were examined and the results are summar- tions which affords the desired product 3a in 90%
ized in Table 1. As shown, the solvent exerts an im- yield (Table 1, entry 13). This indicates that the DMF
portant effect on the yield of product 3a. However, does not lose CO in this reaction conditions (W.
we found that DMF had a superior solvent effect in Yiqian et al. used DMF as a liquid source of CO).[14]
terms of reaction time as well as yield of the product And also the use of DBU[15] as additive does not
(Table 1, entry 3). Next, various bases such as DMAP, affect the yield of the reaction significantly (Table 1,
Cs2CO3, K2CO3, Bu3N were examined for the reaction entry 14). Finally, based on the optimization study,
(entries 5–8). Based on this information, it was con- the reaction conditions for the preparation of 1,2,4-
cluded that Bu3N was the preferred base for this triazole were set to be 0.2 equiv. of Mo(CO)6,
transformation (Table 1, entry 8). The screening of 0.2 equiv. of Et4NCl and 1.2 equiv. of Bu3N in DMF at
different metal carbonyls demonstrated that Mo(CO)6 1508C under a nitrogen atmosphere. We believe that
was the best for this reaction, and the corresponding this reaction, although not attempted in a microwave,
product was obtained in good yields (Table 1, en- might benefit from reaction acceleration.
tries 8–10). Once we had established the suitable
With this optimized catalytic system in hand, the
metal carbonyl for the synthesis of 3,4,5-trisubstituted scope of the reaction was investigated and the results
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Table 2. Synthesis of various 3,4,5-trisubstituted 1,2,4-tria-
Table 3. Synthesis of various [1,2,4]triazolo[4,3-a]pyridines
and [1,2,4]triazolo[4,3-a]pyrazines[a]
zoles from the corresponding aryl iodides and amidrazones[a]
[a]
Reaction conditions: 1 (1.0 equiv.), 4 or 6 (1.0 equiv.),
Mo(CO)6 (0.2 equiv.), Bu3N (1.2 equiv.), Et4NCl
(0.2 equiv.) and temperature 1508C.
Yields reported are isolated yields.
[a]
Reaction conditions:
Mo(CO)6 (0.2 equiv.), Bu3N (1.2 equiv.), Et4NCl
(0.2 equiv.) and temperature 1508C.
1
(1.0 equiv.),
2
(1.0 equiv.),
[b]
[b]
Yields reported are isolated yields.
tion smoothly to give the corresponding product 3i in
79% yield (Table 2, 3i).
are summarized in Table 2. As expected, all of the io-
After successfully synthesizing a wide range of
dides formed the corresponding 3,4,5-trisubstituted 1,2,4-triazoles in good yields, we turned our attention
1,2,4-triazoles in good to excellent yields. Iodoben- towards the synthesis of N-fused 1,2,4-triazoles such
zene gave the desired product in 91% yield (Table 2, as [1,2,4]triazolo[4,3-a]pyridines and [1,2,4]triazo-
3a). Aryl iodides with electron-donating groups such lo[4,3-a]pyrazines under the optimized reaction condi-
as 4-methyl (2c), 2-methyl (2d), 4-methoxy (2e) and tions. The obtained results are presented in Table 3. It
3,4-dimethoxy (2f) gave the desired products in very is evident from the Table 3 that this methodology can
good yields (Table 2, 3c–3f). Aryl iodides with an be easily applied to the synthesis of various N-fused
electron-withdrawing group, 4-chloro (2g), gave the 1, 2, 4-triazole derivatives. This sequential synthesis
corresponding triazole 3g in 86% yield (Table 2, 3g). protocol also tolerates various aryl and heteroaryl io-
However, the use of 4-nitroiodobenzene does not pro- dides to provide corresponding [1,2,4]triazolo[4,3-
vide the intended product. This may be attributed to a]pyridine and [1,2,4]triazolo[4,3-a]pyrazine deriva-
the formation of a complex mixture of reduced prod- tives in good to excellent yields (Table 3).
ucts.[16] Interestingly, the steric bulk of the aryl iodides
Encouraged by the successful synthesis of 1,2,4-tri-
did not affect the reactivity and an 84% yield of the azoles and N-fused 1,2,4-triazoles, we sought to fur-
product was achieved with 2-methyliodobenzene ther extend the scope of this practical approach by re-
(Table 2, 3d). The heteroaryl iodide such as 3-iodopyr- placing amidrazone with amidoxime to prepare 3,5-
idine (2h) reacted smoothly, affording the desired disubstituted-1,2,4-oxadiazoles. Fortunately, following
product 3h in good yield (Table 2, 3h). Alicyclic the above protocol, we were able to prepare 3,5-di-
iodide, iodocyclohexane (2i) also underwent the reac- substituted 1,2,4-oxadiazoles very efficiently. As
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Synthesis of 1,2,4-Triazoles, N-Fused 1,2,4-Triazoles
Table 4. Synthesis of various 3,5-disubstituted-1,2,4-oxadia-
zoles from the corresponding aryl iodides and amidoximes[a]
[a]
Reaction conditions:
1
(1.0 equiv.),
8
(1.0 equiv.),
Mo(CO)6 (0.2 equiv.), Bu3N (1.2 equiv.), Et4NCl
(0.2 equiv.) and temperature 1508C.
Yields reported are isolated yields.
[b]
shown in Table 4, this protocol tolerates a variety of
iodides such as aromatic ones having electron-donat-
ing and electron-withdrawing substituents, heteroaryl
and alicyclic iodides to provide a series of 3,5-disub-
stituted-1,2,4-oxadiazoles in moderate to good yields.
On the basis of the above results and previous re-
ports,[17] a plausible mechanism was proposed and is
Scheme 2. A plausible mechanism for the preparation of
3,4,5-trisubstituted 1,2,4-triazoles.
shown in Scheme 2. Ren and Yamane suggested two azoles, N-fused 1,2,4-triazoles and 1,2,4-oxadiazoles
possible pathways: (i) Generation of acyl metal inter- via molybdenum-mediated carbonylation of aryl hal-
mediate (C). This would be generated by the oxida- ides. In this transformation, Mo(CO)6 used as a solid
tive addition of aryl halide to molybdenum complex source of carbon monoxide. Moreover, the wide
(A, not isolated) followed by CO insertion to the range of substrate tolerance and satisfactory product
phenyl-molybdenum bond. (ii) Generation of carbon- yields of our method should reasonably make it
yl metal intermediate (D). This would be generated a useful supplement for the known strategies.
from the aryl metal halide (A) or from carbonyl inter-
mediate (B) by the oxidative addition of aryl halide.
The formed intermediates (C and D) gave the N-acyl-
1
amidrazone E, which was confirmed by H, 13C NMR
and HR-MS [in the case of 1,2,4-oxadiazoles the inter-
mediate O-acylamidoxime (G) also isolated and char-
acterized]. Finally, intramolecular nucleophilic attack
of amine on the carbonyl of E affords the correspond-
ing 1,2,4-triazole (3).
Experimental Section
General Experimental Procedure for Synthesis of 3,
5, 7 and 9
A mixture of aryl halide 1 (0.49 mmol), amidrazone (2, 4 or
6) or amidoxime (8) (0.49 mmol), Mo(CO)6 (0.098 mmol),
NEt4Cl (0.098 mmol), and NBu3 (0.58 mmol) in DMF was
In summary, we have successfully developed an ef-
ficient and CO gas free method to access 1,2,4-tri- heated at 1508C under an N2 atmosphere. After completion
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of the reaction as monitored by TLC, the reaction mixture
was cooled to room temperature and partitioned between
water and ethyl acetate. The organic and aqueous layers
were then separated and the aqueous layer was extracted
with ethyl acetate twice. The combined organic layers were
dried over anhydrous Na2SO4 and concentrated under re-
duced pressure to get crude. The crude was purified by silica
gel column chromatography using EtOAc:hexane as eluents
to afford corresponding product.
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Acknowledgements
The authors thank DAE-BRNS, Mumbai for financial assis-
tance (through project 37(2)/14/02/2014-BRNS) and the
UGC, New Delhi for the award of SRF to the authors N. M
and T. R.
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