Transition-Metal-Free Annulation of Enamines and Tosyl Azide toward N-Heterocycle Fused and 5-Amino-1,2,3-Triazoles

Article abstract of DOI:10.1002/ejoc.202000938

The annulation reactions between gem-diamino enaminones (ketene aminals) and tosyl azide providing N-heterocycle fused and 5-amino side chain functionalized 1,2,3-triazoles have been developed. The synthesis of these N-side chain functionalized 1,2,3-tria

Full text of DOI:10.1002/ejoc.202000938

European Journal of Organic Chemistry
Accepted Article
Accepted Article
Title: Transition Metal-Free Annulation of Enamines and Tosyl Azide
Toward N-Heterocycle Fused and 5-Amino-1,2,3-Triazoles
Authors: Leiling Deng, Yunyun Liu, Yanping Zhu, and Jie-Ping Wan
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To be cited as: Eur. J. Org. Chem. 10.1002/ejoc.202000938
Link to VoR: https://doi.org/10.1002/ejoc.202000938
01/2020

10.1002/ejoc.202000938
European Journal of Organic Chemistry
COMMUNICATION
Transition Metal-Free Annulation of Enamines and Tosyl Azide
Toward N-Heterocycle Fused and 5-Amino-1,2,3-Triazoles
Leiling Deng,a Yunyun Liu,a Yanping Zhub* and Jie-Ping Wana*
[a]
L. Deng, Prof. Dr. Y. Liu and Prof. J.-P. Wan
College of Chemistry and Chemical Engineering
Jiangxi Normal University, Nanchang 330022
P. R. China
E-mail: wanjieping@jxnu.edu.cn
Homepage: https://www.x-mol.com/groups/Wan_Jie-Ping
[b]
Prof. Dr. Y. Zhu
School of Pharmacy, Key Laboratory of Molecular Pharmacology and Drug Evaluation, Ministry of Education, Collaborative Innovation Center of Advanced
Drug Delivery System and Biotech Drugs in Universities of Shandong
Yantai University, Yantai, 264005
P. R. China
E-mail: chemzyp@foxmail.com
Supporting information for this article is given via a link at the end of the document.((Please delete this text if not appropriate))
Abstract: The annulation reactions between gem-diamino
enaminones (ketene aminals) and tosyl azide providing N-heterocycle
fused and 5-amino side chain functionalized 1,2,3-triazoles have been
developed. The synthesis of these N-side chain functionalized 1,2,3-
triazoles has been executed with high efficiency and broad synthetic
scope under transition metal-free conditions with the assistance of
NaHCO3 only.
On the other hand, as typical heteroatom functionalized 1,2,3-
triazoles, the amino functionalized 1,2,3-triazoles have been
identified to possess dramatically attractive biological
relevance.[14] The synthesis of amino group substituted 1,2,3-
triazoles has therefore become an important issue in 1,2,3-
triazole chemistry. However, due to the low reactivity of
aminoalkynes in the typical AAC reaction, synthesis of such
heteroatom functionalized 1,2,3-triazoles has remined as a
challenge. Only a few available synthetic methods are hitherto
available. The ruthenium-catalyzed AAC reaction represents one
early breakthrough in the synthesis of 5-amino-1,2,3-triazoles by
employing aminoalkyne as the dipolarophile.[15] Later on, similar
amino-functionalized 1,2,3-triazole synthesis have been also
realized by means of rhodium catalysis[16] and iridium catalysis,[17]
respectively (Scheme 1A). In addition, 5-amino-1,2,3-triazoles
have been synthesized by an interesting interrupted CuAAC via
the reactions of terminal alkynes, organo azides and amines
(Scheme 1B).[18] Despite the success of these methods, their
common feature relying on transition metal catalysis, however,
has posed the urgency of developing transition metal-free
approach to complement the present methodologies for the
synthesis of 1,2,3-triazoles. On the basis of the amazingly diverse
Owing to its ubiquitous presence in molecules with biological
function, organic materials as well as a variety of other application
areas, the 1,2,3-triazole motif has been proven as a heterocyclic
system with extraordinary importance.[1] Since the landmark
works on “click”-like copper-catalyzed alkyne azide cycloaddition
(CuAAC), the interest in the synthesis of 1,2,3-triazoles has
received dramatic advances.[2-3] Over the recent decade, one
specially notable direction in 1,2,3-triazole chemistry is the
transition metal-free 1,2,3-triazole synthesis via either pure
organocatalysis or other non-transition metal reagent catalyzed or
promoted triazole annulation reactions.[4-5] Technically, the
transition metal-free synthesis is not only more atom economical,
but also better promotes application of 1,2,3-triazoles in biological
research because no trace heavy metal contamination is likely to
occur in the 1,2,3-triazole samples.
Among the splendid progress in the transition metal-free
1,2,3-triazole synthesis, the triazole ring formation employing
enaminones or analogous stable enamines as key building blocks
have been identified as particularly useful and attractive toolkit.
Depending on the cleavage of different chemical bonds, highly
diverse 1,2,3-triazoles, including the 1,5-disbustiuted 1,2,3-
triazoles,[6] N1-H 1,2,3-triazoles,[7] ester group functionalized
1,2,3-triazoles,[8] enantiomerically pure 1,2,3-triazoles,[9] 1,4-
substituted 1,2,3-triazoles,[10] fully substituted 1,2,3-triazoles[11]
and sulfur side chain functionalized 1,2,3-triazoles[12] etc have
been independently synthesized with efficiency. The successful
application of enaminones and analogous enamines in 1,2,3-
triazole synthesis, together with the versatile and featured
reactivities of these stable enamines,[13] inspire us that high space
remains in the designation of practical synthetic methods toward
more diverse 1,2,3-triazole compounds.
Scheme 1 Methods for the synthesis of 5-amino-1,2,3-triazoles
1
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10.1002/ejoc.202000938
European Journal of Organic Chemistry
COMMUNICATION
reactivity and synthetic applications of enaminones disclosed by
us and others,[19] we have made extensive efforts in establishing
a practical protocol toward the synthesis of 5-amino-1,2,3-
triazoles via transition metal-free operation. Although some N-
heterocycle fused 1,2,3-triazoles have been previously
synthesized by employing specific fluoroalkyl functionalized
sulfonyl azide or observed as side products from the cycloaddition
reactions of enamines with organoazides,[20] a generally selective
and applicable method for the transition metal-free synthesis of 5-
amino functionalized 1,2,3-triazoles using conventional enamines
is yet unavailable. Herein, we report a new transition metal-free
method for synthesis of both N-heterocycle fused and acyclic 5-
amino side chain functionalized 1,2,3-triazoles by employing
gem-diamino enaminones and tosyl azide, wherein NaHCO3 is
only used to promote the reactions (Scheme 1C).
To start the work, the reaction of enamine substrate 1a and
tosyl azide 2 was investigated under different conditions. First,
performing this reaction in a series of different medium, including
water, DMF, DMSO, MeCN, and toluene by heating at 120 oC or
reflux in the presence of tetramethyl ethylene diamine (TMEDA)
proved that DMSO was amongst the most favourable reaction
medium (entries 1-5, Table 1) in providing N- heterocycle fused
1,2,3-triazole 3a. Afterwards, the species of base additive was
screened by using NaOH, t-BuONa, NaHCO3 and DBACO,
respectively. The results indicated that NaHCO3 was the best
additive by affording target product with evidently higher yield
(entries 6-9, Table 1). While modifying the centration of the
reaction led to slight improvement on the result (entries 10-11,
Table 1), the variation on the base loading led to the observation
that 1.5 equiv base in the reaction provided product 3a with further
enhanced yield (entries 12-13, Table 1). The reaction conducted
at different temperature, however, led to not further improved
result (entries 14-16, Table 1).
14[c,e]
15[c,e]
16[c,e]
NaHCO3
NaHCO3
NaHCO3
DMSO
DMSO
DMSO
130
78
71
66
110
100
[a] General conditions: 1a (0.2 mmol), 2 (0.3 mmol), base (0.4 mmol), in solvent
(2 mL) and stirred for 12 h. [b] Isolated yield. [c] In 2.5 mL DMSO. [d] In 3 mL
DMSO. [e] With NaHCO3 (0.3 mmol). [f] With NaHCO3 (0.6 mmol).
With the optimized parameters, the scope on the synthesis of
the N-heterocycle fused 1,2,3-triazoles 3 was then examined by
using diversely functionalized enamines of type 1 to react with
tosyl azide 2 under the optimized conditions. As outlined in Table
2, the synthesis of fused 1,2,3-triazoles 3 via this transition metal-
free annulation was identified with good application scope and
generally good to excellent product yields. The enamines 1
featuring the five-membered heterocycle reacted with tosyl azide
to provide corresponding products bearing alkyl, alkoxyl, halogen
atom as well as trifluoromethyl substitution in the phenyl ring (3a-
3h, Table 2), and strong electron withdrawing or donating group
in the phenyl ring of 1 led to relatively lower product yield (3c and
3g, Table 2). Analogously, the reactions employing six-membered
heterocycle-based enamines could also be smoothly employed
for the synthesis of six-membered hetero-ring fused 1,2,3-
triazoles (3i, 3k-3q, Table 2). Notably, alongside the fine reactions
using those enamines functionalized with variously substituted
phenyl ring, the fused aryl such naphthyl (3j-3k, Table 2) and
heteroaryl ring (3q, Table 2) functionalized enamines could also
take part in the synthesis of the 1,2,3-triazole products.
Table 2 Scope on the enaminone-based synthesis of N-heterocycle fused 1,2,3-
triazoles[a,b]
Table 1. Optimization on reaction conditions[a]
Entry
Base
Solvent
H2O
T (oC)
reflux
120
Yield (%)[b]
1
TMEDA
TMEDA
TMEDA
TMEDA
TMEDA
NaOH
38
42
44
39
20
41
63
74
55
77
59
85
66
2
DMF
3
DMSO
MeCN
toluene
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
120
4
reflux
reflux
120
5
6
7
t-BuONa
NaHCO3
DABCO
NaHCO3
NaHCO3
NaHCO3
NaHCO3
120
8
120
9
120
10[c]
11[d]
12[c,e]
13[c,f]
120
[a] General conditions: 1 (0.2 mmol), 2 (0.3 mmol), NaHCO3 (0.3 mmol) in
DMSO (2.5 mL), stirred at 120 oC for 12h. [b] Yield of isolated prouducts based
on 1.
120
120
120
2
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10.1002/ejoc.202000938
European Journal of Organic Chemistry
COMMUNICATION
Encouraged by the successful synthesis of the heterocycle
fused 1,2,3-triazoles, we then turned to extend the synthetic
scope of the reaction by employing acyclic diamino functionalized
enamines 4 as alternative substrates of 1. Delightfully, the
reactions of 4 with tosyl azide proceeded efficiently under the
standard conditions to provide 5-amino functionalized 1,2,3-
triazoles 5. As shown in Scheme 2, the synthesis of 1,2,3-triazoles
5 containing a series of different functional groups such as methyl,
methoxyl, halogen and nitro group in the N-aryl or the ketone aryl
segment were provided with moderate to good yield, further
illustrating the advantageous application of this transition metal-
free protocol in the synthesis of nitrogen side chain functionalized
1,2,3-triazoles with exceptional product diversity. According to the
present data, the substituent in both the ketone aryl and the aryl
ring connected to the amino group didn’t show regular influence
to the reaction results.
Scheme 3 The proposed reaction mechanism
has exhibited broad application scope in the synthesis of both N-
heterocycle fused 1,2,3-triazoles and 5-amino-1,2,3-triazoles.
Besides offering a reliable transition metal-free route the useful
1,2,3-triazole scaffolds, the present work discloses further the
high potential of enamines in the designation of valuable and
sustainable methods of synthesis.
Scheme 2 Scope on the metal-free synthesis of 5-amino-1,2,3-triazoles
Following the work in scope examination, the gram scale
synthesis on the model reaction was carried out. Notably, the 5
mmol scale reaction under the standard reaction condition led to
the synthesis of 3a with high yield of 70% (Eq 1), proving the
applicability of the present method in the synthesis of the
functional 1,2,3-triazoles with large amount.
Acknowledgements
The work is financially supported by National Natural Science
Foundation of China (21861019).
Keywords: enamine • tosyl azide • transition metal-free • N-side
chain • 1,2,3-triazole
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intramolecular annulation on 9 then leads to the production of
1,2,3-triazoles 3 and 5.
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European Journal of Organic Chemistry
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4
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European Journal of Organic Chemistry
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1,2,3-Triazole synthesis
Entry for the Table of Contents
The synthesis of versatile 5-amino functionalized 1,2,3-triazoles, both in the forms of N-heterocycle fused and acyclic amino group
functionalized forms, have been realized via the annulation reactions of gem-diamino enaminones and tosyl azide under transition
metal-free conditions.
5
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