Copper-Catalyzed Intramolecular Chalcogenoamination of Enynyl Azides: Synthesis of 5-Selenyl/Sulfenyl Nicotinates

Article abstract of DOI:10.1021/acs.orglett.8b03695

A novel methodology for the synthesis of 5-selenyl/sulfenyl nicotinates involving copper-catalyzed organochalcogenyl aza-annulation of enynyl azide with diorganyl-dichalcogenides has been described. This method offers difunctionalization of alkynes via regioselective intramolecular chalcogenoamination in one pot to provide substituted 5-chalcogenyl nicotinates in good to excellent yields. The resulting nicotinates provide access to their oxides, sulfones, and acid derivatives.

Full text of DOI:10.1021/acs.orglett.8b03695

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Copper-Catalyzed Intramolecular Chalcogenoamination of Enynyl
Azides: Synthesis of 5‑Selenyl/Sulfenyl Nicotinates
Chada Raji Reddy,*,†,‡ Ravi Ranjan,†,‡,§ and Santosh Kumar Prajapti†,§
†Department of Organic Synthesis & Process Chemistry, CSIR−Indian Institute of Chemical Technology, Hyderabad 500007, India
‡Academy of Scientific and Innovative Research (AcSIR), Ghaziabad 201 002, India
S
* Supporting Information
ABSTRACT: A novel methodology for the synthesis of 5-selenyl/
sulfenyl nicotinates involving copper-catalyzed organochalcogenyl
aza-annulation of enynyl azide with diorganyl-dichalcogenides has
been described. This method offers difunctionalization of alkynes via
regioselective intramolecular chalcogenoamination in one pot to
provide substituted 5-chalcogenyl nicotinates in good to excellent
yields. The resulting nicotinates provide access to their oxides,
sulfones, and acid derivatives.
Scheme 1. Synthesis of Chalcogenyl Heterocycles from
Alkynyl Substrates
halcogenyl (selenyl and sulfenyl) compounds are gaining
Cprominence, because of their prospective biological
activities and utilization as important frameworks in material
chemistry. Hence, the synthesis of such skeletons based on
pharmaceutically significant compounds is an essential task to
find the novel therapeutic agents.1 Accessing chalcogenyl
heterocycles, including medicinally proven privileged scaffolds
such as pyridines, is of immense interest.2,3 Selenyl/sulfenylpyr-
idine derivatives have attracted considerable attention from the
organic chemist, because of their therapeutic value in several
diseases, such as HIV, cancer, inflammation, and microbial
infection.3 To the best of our knowledge, synthetic methods to
access selenyl/sulfenylpyridines are sparse and require a
multistep synthetic approach.4 Hence, the development of a
new method for the direct access of selenyl/sulfenylpyridine is
highly desirable.
On the other hand, intense effort has been directed toward the
development of approaches for the synthesis of chalcogenyl
(selenyl and sulfenyl) derivatives. Conventionally, carbon−
chalcogen (Se, S) bond is accomplished via the coupling of aryl
or heteroaryl lithium/Grignard reagent with chalcogenide
precursors.5 Alternatively, transition-metal-catalyzed coupling
reactions of chalcogenide with aryl/heteroaryl halides,6
triflates,6e,7 or boronic acids8 are also reported in the literature.
However, construction of chalcogenyl heterocycles through the
annulation of corresponding acyclic precursors remains rare.9
Literature reports revealed that the vicinal difunctionalization of
alkynes is a powerful technique to build functionalized
heterocycles from readily accessible starting materials.9 For
example, in 2009, Larock and co-workers elegantly reported an
electrophilic cyclization of N,N-dialkyl-2-(1-alkynyl)anilines
with arylsulfenyl/arylselenyl chlorides to provide the corre-
sponding 3-selenylindoles and 3-sulfenylindoles (Scheme 1a).9a
Afterward, the Li group described a novel PdI2/I2-catalyzed thio-
annulation of azides with disulfides for the synthesis of 4-
sulfenylisoquinolines (Scheme 1b).9b Zeni and co-workers
demonstrated an iron(III) chloride promoted cyclization of
alkynylaryl esters with diorganyl dichalcogenides to give
substituted organochalcogenyl-isochromenones in good yield
(Scheme 1c).9c In 2016, N-sulfanylsuccinimides were utilized as
an electrophilic source in the presence of BF3·Et2O to induce
Received: November 19, 2018
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.8b03695
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Letter
cyclization of aryl alkynoates to achieve 3-sulfenylated
coumarins (Scheme 1d).9d Recently, a Pd-catalyzed propargyl
substitution and cycloisomerization reactions of propargyl
acetates bearing acyl and imidoyl groups with indium organo-
thiolates have been accomplished for the synthesis of substituted
furan and pyrrole derivatives (Scheme 1e).9e Inspired by these
elegant procedures, we speculate that the (E)-2-en-4-ynyl azides,
derived from MBH acetates of acetylenic aldehydes,10 could be
interesting substrates to offer the corresponding selenyl/sulfenyl
nicotinates. In continuation of our recent interest in exploring
the ene-yne-assisted annulation reactions,11 we disclose our
preliminary results on the synthesis of 5-selenyl/sulfenyl
nicotinates via intramolecular chalcogenoamination of enynyl
azides using Cu(II) as a catalyst (Scheme 1f). To the best of our
knowledge, this is the first one-pot synthetic protocol that
installs the selenyl/sulfenyl group on the pyridine during the ring
construction.
amount of catalyst led to the longer reaction time and declined
yield (Table 1, entry 9). The influence of other solvents such as
1,2-dichloroethane (DCE), toluene, dimethylformamide
(DMF), and dimethylsulfoxide (DMSO) has also been
evaluated, and the results revealed that acetonitrile was superior
to give excellent yield of the product (Table 1, entries 10−13).
The reaction was unfruitful in the absence of catalyst, signifying
the crucial role of the copper catalyst in this intramolecular
selenoamination process (Table 1, entry 14). Hence, 5 mol % of
CuCl2 in CH3CN was selected as the optimal reaction condition
for the present annulation reaction (entry 8, Table 1).
To investigate the general scope and limitations of the
selenoamination, various (E)-2-en-4-ynyl azides (1), derived
from corresponding MBH-acetate of acetylenic aldehydes, were
subjected to the optimal reaction conditions and the results are
summarized in Scheme 2. In all cases, the intramolecular
The required (E)-2-en-4-ynyl azides (1) for the synthesis of
selenyl nicotinates are readily prepared from the allylic
substitution of MBH-acetate of acetylenic aldehydes with
sodium azide.10 In order to determine the suitable reaction
conditions for electrophilic selenyl aza-annulation, we com-
menced the intramolecular selenoamination using enynyl azide
1a as a model substrate and commercially available diphenyl
diselenide as the selenium source. Initially, desired product 2a
was not detected under 5 mol % of CuCl in DCE at 0 °C (room
temperature) (Table 1, entry 1). However, 42% yield of selenyl
Scheme 2. Synthesis of 5-Selenyl Nicotinates
a
Table 1. Optimization of Reaction Conditions
catalyst
(mol %)
temperature
time
(h)
yield
b
entry
solvent
(°C)
(%)
1
2
3
4
5
6
7
8
9
CuCl (5)
CuCl (5)
CuCl (5)
CuBr (5)
CuBr2 (5)
Cu(OAc)2 (5) CH3CN
Cu(OTf)2 (5)
CuCl2 (5)
DCE
DCE
CH3CN
CH3CN
CH3CN
0 (rt)
90
90
90
90
90
90
0
0
0
0
0
10
10
10
10
10
10
10
1
2
1
1
1
00
42
58
25
40
23
28
98
87
57
68
48
52
00
selenoamination progressed well to afford the corresponding
selenyl-nicotinates (2) in good to excellent yields (78%−98%)
in a short reaction time. The results indicated that the (E)-2-en-
4-yn-1-azides bearing aryl groups either with electron-donating
[4-MeO (1b), 4-Me (1c), 3,5-dimethyl (1d)] or electron-
withdrawing groups [4-Cl (1e), 4-F (1f)] were well-tolerated in
the annulation process with diphenyl diselenide to give the
selenyl nicotinates 2b−2f in excellent yield (89%−98%).
Similarly, 3-(trifluoromethyl)phenyl enynyl azide 1g reacted
smoothly under the present reaction conditions to obtain 2g in
86% yield. Interestingly, 1-naphthyl substitution on alkyne of
enynyl azide 1h did not affect the selenoamination, which
delivered methyl 6-(naphthalen-1-yl)-5-(phenylselanyl)-nicoti-
nate (2h) in 93% yields. As we expected, 2-thiophenyl
enynylazide 1i was also a compatible substrate for this reaction
to provide methyl 5-(phenylselanyl)-6-(thiophen-2-yl)-
nicotinate (2i), albeit in 78% yield. In addition, the scope of
this protocol was further investigated using different selenides
with 1a under the optimized reaction conditions. Interestingly,
the diaryldiselenides with electron-donating and electron-
withdrawing substituents reacted efficiently to produce the
desired products 2j and 2k in excellent yields. The reaction of 1a
CH3CN
CH3CN
CH3CN
DCE
toluene
DMF
CuCl2 (5)
10 CuCl2 (5)
11 CuCl2 (5)
12 CuCl2 (5)
13 CuCl2 (5)
DMSO
CH3CN
0
90
1
12
c
14

a
Reaction conditions: enynyl azide (0.41 mmol) and Ph2Se2 (0.29
mmol), air. Isolated yield. In the absence of catalyst, no reaction was
observed.
b
c
nicotinate 2a was isolated, when the reaction temperature was
increased to 90 °C (Table 1, entry 2). The yield of 2a was
enhanced to 58% when the reaction was performed in
acetonitrile (Table 1, entry 3). Furthermore, the examination
of different copper catalysts such as CuBr, CuBr2, Cu(OAc)2,
and Cu(OTf)2 resulted in inferior yield of 2a (Table 1, entries
4−7). Attractively, the reaction of 1a with diphenyl selenide with
5 mol % of CuCl in acetonitrile provided the selenyl-nicotinate
2a in 98% yield (Table 1, entry 8). Further reduction in the
B
DOI: 10.1021/acs.orglett.8b03695
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Letter
with heteroaromatic diselenide, 1,2-di(thiophen-2-yl)diselane,
gave the selenyl nicotinate 2l in 44% yield. Excitingly, dialkyl
diselenide also underwent the selenoamination with 1a to give
2m in 95% yield.
aforementioned results, the possible mechanism of reaction is
outlined in Scheme 4. First, CuCl2 will activate Ph2X2 (X = Se or
Scheme 4. Proposed Reaction Mechanism
Motivated by the above achievement, we next set out to
explore the scope of present annulation for the synthesis of
biologically important 5-sulfenyl nicotinates. Initially, we
attempted the sulfenylation reaction of 1a under the above
optimized reaction conditions using diphenyl disulfide in place
of diphenyl selenide. Disappointingly, the sulfenyl-amination of
enynyl azide 1a failed to give 5-sulfenyl-nicotinate 3a. However,
the prolonged reaction time to 10 h, 25% yield of the desired
product was obtained, along with recovery of the starting
material. Interestingly, complete conversion of the starting
material was noticed at a reaction temperature of 90 °C, and the
corresponding sulfenyl nicotinate 3a was isolated in 86% yield.
Having the success in hand, we explored the generality of the
method using diverse (E)-2-en-4-ynyl azides and disulfides
(Scheme 3). Delightfully, a wide range of enynyl azides having
S) through polarization of the X−X bond to Cu-complex (A),
which promotes an electrophilic addition onto alkyne for the
generation of intermediate B. Consequently, the intramolecular
nucleophilic attack by azide to C and subsequent deprotonation
leads to the formation of the product 2a/3a and seleno/
thiophenol, which oxidizes to their corresponding diselenide/
sulfide to re-enter the catalytic cycle.
Scheme 3. Synthesis of 5-Sulfenyl Nicotinates
To demonstrate the potential application of this protocol, we
extended our study by utilizing selenium, sulfur, and ester as
versatile functionalities present in the products for further
transformations. As shown in Scheme 5, selenonicotinate 2c was
Scheme 5. Diversification of Selenyl/Sulfenyl Nicotinates
different electronic and steric substitution patterns were found
to be suitable in affording the corresponding 5-sulfenyl
nicotinates 3b−3g in good yield (70%−95%).
The reaction of enynyl azide with electron-donating groups
[4-OMe (1b), 4-Me (1c), 3,5-dimethyl (1d)] on phenyl ring,
underwent the Cu-catalyzed intramolecular sulfeno-amination
to give sulfenyl nicotinates 3b−3d in 90%−95% yield. The
enynyl azide substrates having electron-withdrawing groups [4-
Cl (1e), 4-F (1f)] delivered the corresponding products 2e and
2f in 82% and 79%, respectively. In addition, the selenoamina-
tion of 1-naphthylenynyl azide 1h with diphenyldisulfide was
also successful to afford 3g in 74% yield. Similarly, 2-thiophenyl-
enynyl azide 1i could also be used under the standard reaction
conditions to obtain 3h in 70% yield. The possibility of this
methodology was further extended to other disulfides, p-tolyl
disulfide and bis(3-fluorophenyl)disulfide, by demonstrating the
successful sulfenyl annulation of 1a to give the sulfenyl
nicotinates 3i and 3j in 90% and 77% yields, respectively.
To gain insights into the reaction mechanism, various control
experiments were performed (see Scheme S2 in the Supporting
Information).12 Based on the literature precedence13 and the
successfully oxidized with mCPBA (1 equiv) in dichloro-
methane to afford the corresponding selenoxide derivative 4 in
83% yield.14 Besides, 2c was converted to 5-(phenylselanyl)-6-
(p-tolyl)nicotinic (5) acid in 94% yield by employing KOH in
MeOH.10a Interestingly, oxidation of sulfenyl nicotinate 3c,
using 1 equiv of mCPBA as an oxidant, furnished the sulfoxide 6
exclusively in 78% yield, while the formation of sulfone 7 in 83%
yield was observed when 2.5 equiv of mCPBA was used in the
reaction.14
In summary, we have developed an efficient copper-catalyzed
intramolecular chalcogenoamination of enynyl azides, derived
from the Morita−Baylis−Hillman acetates of acetylenic
aldehydes. The developed protocol provides a convenient
approach to biologically important diversely substituted 5-
selenyl/sulfenyl nicotinates in good to excellent yields. The
method features a broad substrate scope and mild reaction
conditions. The synthesized 5-selenyl/sulfenyl nicotinate were
further diversified to their oxides, sulfone, and acid analogues.
C
DOI: 10.1021/acs.orglett.8b03695
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Chem. 2018, 143, 1185. (c) Zhang, J.; Duan, D.; Xu, J.; Fang, J. ACS
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.orglett.8b03695.
■
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Experimental procedures, characterization details and 1H
and 13C NMR spectra of new compounds (PDF)
Accession Codes
CCDC 1887366 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge via
www.ccdc.cam.ac.uk/data_request/cif, or by emailing data_
request@ccdc.cam.ac.uk, or by contacting The Cambridge
Crystallographic Data Centre, 12 Union Road, Cambridge
CB2 1EZ, UK; fax: +44 1223 336033.
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AUTHOR INFORMATION
■
Corresponding Author
*E-mail: rajireddy@iict.res.in.
ORCID
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Volante, R. P. J. Org. Chem. 1998, 63, 9606. (b) Mispelaere-Canivet, C.;
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(c) Taniguchi, N. J. Org. Chem. 2007, 72, 1241.
Chada Raji Reddy: 0000-0003-1491-7381
Ravi Ranjan: 0000-0003-3958-3875
Author Contributions
§These authors contributed equally.
Notes
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H. Org. Lett. 2017, 19, 452. (f) Glenadel, Q.; Ismalaj, E.; Billard, T. Org.
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(12) The control experiment (Scheme S2-d in the Supporting
Information) using the enynyl azide without ester substitution provided
the inseparable mixture of selenyl nicotinate and acyl pyrrole, which
suggests that the electron-withdrawing group play an important role in
the formation of the pyridine ring.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
C.R.R. and S.K.P. thanks the Science and Engineering Research
Board (SERB) and the Department of Science and Technology,
New Delhi for funding the project (Nos. EMR/2016/006253
and PDF/2017/000509), respectively. R.R. thanks the Council
of Scientific and Industrial Research (CSIR), New Delhi for
research fellowship. (CSIR-IICT Communication No. IICT/
Pubs./2018/357).
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