Ionic Liquid Promoted Diazenylation of N-Heterocyclic Compounds with Aryltriazenes under Mild Conditions

Article abstract of DOI:10.1021/acs.orglett.6b00605

An efficient, mild, and metal-free approach to direct diazenylation of N-heterocyclic compounds with aryltriazenes using Br?nsted ionic liquid as a promoter has been developed for the first time. Many N-heterocyclic azo compounds were synthesized in good to excellent yields at room temperature under an open atmosphere. Notably, the promoter 1,3-bis(4-sulfobutyl)-1H-imidazol-3-ium hydrogen sulfate could be conveniently recycled and reused with the same efficacies for at least four cycles.

Full text of DOI:10.1021/acs.orglett.6b00605

Letter
pubs.acs.org/OrgLett
Ionic Liquid Promoted Diazenylation of N‑Heterocyclic Compounds
with Aryltriazenes under Mild Conditions
Dawei Cao, Yonghong Zhang,* Chenjiang Liu,* Bin Wang, Yadong Sun, Ablimit Abdukadera, Haiyan Hu,
and Qiang Liu
The Key Laboratory of Oil and Gas Fine Chemicals, Ministry of Education & Xinjiang Uygur Autonomous Region, Urumqi Key
Laboratory of Green Catalysis and Synthesis Technology, School of Chemistry and Chemical Engineering, Physics and Chemistry
Detecting Center, Xinjiang University, Urumqi 830046, P. R. China
S
* Supporting Information
ABSTRACT: An efficient, mild, and metal-free approach to
direct diazenylation of N-heterocyclic compounds with
aryltriazenes using Brønsted ionic liquid as a promoter has
been developed for the first time. Many N-heterocyclic azo
compounds were synthesized in good to excellent yields at
room temperature under an open atmosphere. Notably, the
promoter 1,3-bis(4-sulfobutyl)-1H-imidazol-3-ium hydrogen sulfate could be conveniently recycled and reused with the same
efficacies for at least four cycles.
Scheme 1. Traditional and IL-Promoted Methods for the
Synthesis of Diazenylation Derivatives
zo compounds have widespread applications in medicinal
Achemistry and materials science, such as therapeutic
agents, food additives, and textile fibers.1,2 In particular, those
with aromatic or heterocyclic substituents as directing groups,
initiators, dyes, nonlinear optics, and photochemical switches
have attracted considerable attention in the field of chemistry.3
Therefore, intensive research has been directed toward the
development of simple and efficient methods for the synthesis
of azo derivatives.
The synthesis of azo derivatives is often performed by the
coupling reactions of the diazonium salts and Mills coupling of
aromatic nitroso compounds with aromatic amines.4,5 Sym-
metrical azo compounds can also be obtained by oxidation
reaction of amines/hydrazines or reduction reaction of
nitroaromatic compounds (Scheme 1, A).6 In addition, there
are other methods for preparing azo compounds, such as the
Wallach rearrangement of azoxy derivatives, dimerization
reaction of the diazonium salts, and pyrolysis of azides.7
However, these methods often suffer from harsh reaction
conditions, such as anaerobic anhydrous conditions, high
temperature, and pressure. In particular, some reactions involve
reducing agents and oxidizing agents containing heavy metal
ions and use toxic solvents, which do not meet the concept of
green chemistry. Furthermore, the limited substrate scope of
some traditional strategies involving only arenes impedes their
applications, and heterocyclic compounds have rarely been
studied. Therefore, the development of mild and convenient
methods for the diazenylation of heterocyclic compounds is still
in high demand.
heteroatom bonds by transition-metal-catalyzed cross-coupling
reactions.9,10 However, the applications of triazenes in
diazenylation reactions have rarely been reported. Recently,
Luo11 reported a Sc(OTf)3-catalyzed method for the synthesis
of aliphatic azo compounds with triazenes in the presence of
TMSCl (Scheme 1, B), which successfully solved the problem
of instability of the diazonium salts in the reaction process. The
diazenylation of heterocyclic compounds with triazenes has
never been reported so far.
Ionic liquids (ILs) have been widely used as an environ-
mentally acceptable reaction medium or catalyst in green
synthesis over the past decade owing to their advantages of
Aryltriazenes have emerged as highly powerful building
blocks in a variety of synthetic transformations,8 which
attracted extensive attention due to the advantages of good
reactivity and stability. Aryltriazenes as masked diazonium ions
have been widely utilized in the construction of C−C and C−
Received: March 3, 2016
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.6b00605
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
nonvolatility, thermal stability, recyclability, and tunable
chemistry.12 In particular, the application of task-specific ILs
including Brønsted or Lewis acid ILs in the preparation of
heterocyclic compounds has become an attractive field in
organic synthesis (Figure 1).13 With this background in mind,
Scheme 2. Scope of Diazenylation of Aromatic Triazenes
and 2-Phenylindole
a
Figure 1. ILs examined in this work.
we report that a Brønsted acid IL promoted diazenylation
reaction of N-heterocyclic compounds with aryl triazenes at
room temperature under air (Scheme 1, C).
We commenced our study using 1-phenyltriazenes (1) and 2-
phenylindole (2a) as model substrates (Table S1). Initially, we
examined the influence of the ILs for the reaction (Table S1,
entries 1−5). 1,3-Bis(4-sulfobutyl)-1H-imidazol-3-ium hydro-
gen sulfate (IL5) was found to be superior to the others (Table
S1, entry 5), which produced the desired product 3a in 96%
isolated yield. However, reducing the amount of IL5 led to a
relatively lower yield (Table S1, entry 6). Gratifyingly, among
the organic solvents tested (Table S1, entries 7−15), green
solvent ethanol was found to be the best one. However, the
yield of product 3a was greatly decreased with water as solvent
or under solvent-free conditions, which might be due to the
insolubility of substrates (Table S1, entries 16−17). We next
examined the effect of leaving group of the 1-aryltriazene
compounds. For cyclic substitution, the ring size exhibited
almost no influence on yield (Table S1, entries 18 and 19), and
the substrates with various linear chains, such as ethyl, allyl, and
ethoxyl, could produce the desired compound in more than
87% yields (Table S1, entries 20−22). Consequently, the
reaction was carried out with 1 equiv of IL5 as the promoter in
EtOH (1 mL) at 25 °C for 1.5 h.
a
General conditions: 1 (0.2 mmol), 2a (0.2 mmol), and IL5 (0.2
b
mmol) at 25 °C for 1.5 h in the EtOH (1 mL) under air. At 50 °C for
5 h. Direct filtration. At 80 °C for 8 h.
c
d
Scheme 3. Scope of Diazenylation of Triazophenyl Reagent
and N-Heterocyclic Compounds
a
With the optimized conditions established, the scope of
aromatic triazo reagents was examined (Scheme 2). A variety of
electron-rich and electron-poor groups at the para-position of
the aromatic ring readily reacted to give the corresponding
products in good to excellent yields. For example, methyl-,
methoxy-, and halogen-substituted products were isolated in
79%−96% yields (compounds 3a−d). In particular, the strong
electron-withdrawing group of nitro-substituted triazene can be
involved well in the reaction (compound 3e). It is important to
note that the above groups whether in ortho- or meta- also
reacted smoothly (compounds 3f−l). To our gratification, the
multisubstituted substrate could be transformed well with good
yield (compound 3m). Surprisingly, the carboxyl-substituted
substrate furnished the corresponding product 3n in good yield.
In addition, some special groups were also tolerated in this
transformation, such as sensitive groups azo, alkaline group
N,N-dimethyl, condensed ring, and heteroaromatic groups
(entries 3o−r).
a
General conditions: 1a (0.2 mmol), 2 (0.2 mmol), and IL5 (0.2
mmol) at 25 °C for 1.5 h in the EtOH (1 mL) under air.
formed into the corresponding 2-position diazenylation
products in excellent yields (compounds 4a,b). Next, a series
of 2-methyl-substituted indoles, whether with electron-rich or
electron-poor groups, proceeded smoothly to produce the
desired compounds in more than 93% yields (compounds 4c−
f). Furthermore, N-substituted indoles could furnish the
corresponding products in excellent yields (compounds 4g,h).
To our delight, the aromatic ring moiety or 2-aryl of indoles
with electron-withdrawing groups (e.g., 4-Cl, 4-F) led to
compounds 4i−k smoothly. Interestingly, 1-methylpyrrole
could also be transformed in this reaction (compound 4l).
The substrate scope of a series of N-heterocyclic derivatives
was next expanded to explore the generality of this reaction
under the optimized reaction conditions, and the results are
summarized in Scheme 3. This protocol was found to be
general, affording the desired products in excellent yields. First,
3-methylindole and 5-bromo-3-methylindole can be trans-
B
DOI: 10.1021/acs.orglett.6b00605
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Organic Letters
Letter
However, other heterocycle substrates, such as thiophene, 2,6-
dimethylthiophene, benzothiophene, and benzofuran, did not
deliver the desired products.
Scheme 5. Plausible Reaction Mechanism
In order to further demonstrate the potential industrial
application of current protocol, a gram-scale experiment of 1a
and 2a was carried out in 15 mL of EtOH at 25 °C for 1.5 h in
the presence of 45 mol % of IL5. The result is shown in
Scheme 4. As the IL5 is highly water-soluble, it could be
Scheme 4. Gram-Scale Reaction
removed by filtration with water, and the product 3a was
obtained after drying in vacuo. It is noted that the purity of 3a
was achieved without requiring chromatographic purification or
recrystallization.
To further develop a recyclable catalytic system, the recycling
of the IL5 was investigated under the optimized conditions
(Figure 2). Following each cycle, water and ethyl acetate were
reaction in 54% yield. Meanwhile, the same product was
obtained in the presence of tert-butyl nitrite using 1,4-dioxane
as solvent under air at room temperature for 0.5 h in 84% yield.
The results are shown in Scheme 6.
Scheme 6. Derivatization Reactions
Figure 2. Recycling of IL5 in the synthesis (E)-2-phenyl-3-
(phenyldiazenyl)-1H-indole.
added, and the IL5 was recovered from the water layer and
recycled for the next reaction after drying in vacuo. For the first
run, the activity of IL5 remained the same. In the subsequent
two cycles, the desired products were still reached with 100%
conversions and high yields by extension of the reaction time
from 1.5 to 3 and 5 h. In the fourth cycle, even if the reaction
time was prolonged to 24 h, the conversion could not reach
100%. However, the corresponding product was afforded in
94% yield after addition of another 5 mol % of IL5 and then
reacted for 8 h. The decrease of promoter activity may be
because the acidity of IL5 is affected by tetrahydropyrrole
generated during the reaction.
In summary, we have found a Brønsted acidic IL-promoted
diazenylation reaction of N-heterocyclic derivatives with
triazenes. This system can provide some advantages, such as
being metal-free, providing excellent yields, avoiding the use of
highly toxic organic solvents, and using mild conditions. Most
importantly, the promoter is very easy to handle and can be
recycled and reused for four runs with no significant loss of
activity. Furthermore, the protocol can be used as an
economical synthetic method for the diazenylation reaction of
N-heterocyclic derivatives.
ASSOCIATED CONTENT
* Supporting Information
■
According to the above result and previous reports,14
a
S
plausible mechanism is depicted in Scheme 5. First, the
aryltriazene 1a was activated by promoter IL5 to give the
tetrahydropyrrolyl cation A, which was then transformed into
azo cation B through the release of tetrahydropyrrole.
Subsequently, the intermediate C was afforded via electrophilic
addition of diazonium species B to 2-phenylindole 2a. Finally,
the product 3a was generated by deprotonation of C.
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.6b00605.
Experimental procedure, NMR spectra, and analytical
data for all new compounds (PDF)
We found that this reaction is useful in the synthesis of 3-
nitroindole. 3-Nitroindoles are useful precursors in the
preparation of novel antidiabetic agents.15 They are also the
key building block in the formation of amino acid derivatives.16
Therefore, the compound 2-phenyl-3-nitroindole can be
synthesized by IL5 as promoter, AgNO3 as the catalyst, and
Na2S2O8 as the oxidant in CH2Cl2 at 25 °C after 36 h of
AUTHOR INFORMATION
Corresponding Authors
■
*E-mail: pxylcj@126.com.
*E-mail: zhzhzyh@126.com.
Notes
The authors declare no competing financial interest.
C
DOI: 10.1021/acs.orglett.6b00605
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
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ACKNOWLEDGMENTS
■
We are grateful to Key Laboratory of Xinjiang Uyghur
Autonomous Region (2015KL014), NSFC (21572195,
21502162, 21262035, and 21162025), the Program for
Outstanding Youth Science and Technology Innovation
Talents Training in Xinjiang Uygur Autonomous Region
(2014721004), and the Natural Science Foundation of Xinjiang
University (BS110133 and BS150230) for financial support of
this research.
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