C3 Sulfenylation of N-Heteroarenes in Water under Catalyst-Free Conditions

Article abstract of DOI:10.1002/ejoc.201700487

A method for the catalyst-free C–H sulfenylation of imidazo[1,2-a]pyridines by using sulfonothioates as an odorless thioarylated reagent in aqueous medium was developed. This protocol was used for a variety of substituted imidazo[1,2-a]pyridines with broad functional-group tolerance and was extended to the sulfenylation of indoles and imidazothiazoles. The sulfonothioates were activated exclusively in aqueous medium rather than in an organic solvent, and the feasibility of the process for scale-up studies was demonstrated.

Full text of DOI:10.1002/ejoc.201700487

A Journal of
Accepted Article
Title: C-3 Sulfenylation of N-heteroarenes in water under catalyst-free
conditions
Authors: Adimurthy Subbarayappa, Chitrakar Ravi, and Abhisek Joshi
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To be cited as: Eur. J. Org. Chem. 10.1002/ejoc.201700487
Link to VoR: http://dx.doi.org/10.1002/ejoc.201700487
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10.1002/ejoc.201700487
European Journal of Organic Chemistry
COMMUNICATION
C-3 Sulfenylation of N-heteroarenes in water under catalyst–free
conditions
Chitrakar Ravi, Abhisek Joshi, and Subbarayappa Adimurthy*
Abstract: We describe herein
sulfenylation of imidazo[1,2–a]pyridines
a
catalyst-free C−H group tolerance. The methodology has been extends to
using sulfernylation of indoles and imidazothiazoles. The
sulfonothioates as odorless source of thioarylated reagent in sulfonothioates are activated exclusively in aqueous medium
an aqueous medium. The method works for a variety of rather than organic solvent media and the feasibility of the
substituted imidazo[1,2–a]pyridines with broad functional
process for scale-up studies have been demonstrated.
these reactions are effective only in organic solvent media.
Introduction
In
the
past
decade,
transition-metal-catalyzed
C−H
Recently, water has been employed as a solvent-cum-catalyst
for the construction of heterocycles.[12] Aqueous reactions have
attracted much attention due to their unique reactivity and
functionalization has received extensive attention in organic
synthesis, due to its powerful and versatile tool for directly
introducing the desired new functionalities via C−H bond
transformation.[1] The cost associated with precious metal
catalysts and their contamination in the products of human
consumption are also the matter of concern in the synthesis of
bio-organic molecules.[2] Sulfenylation of heteroarenes through
C−H activation[3] for the construction of carbon-sulfur (C─S)
bond has attracted special interest in various fields such as
pharmaceutical, agrochemical industries, natural products and
organic materials.[4] Synthesis of C-3 substituted imidazo[1,2–
a]pyridines are very challenging to achieve, as these structures
are widely distributed in variety of natural products and synthetic
molecules with diverse pharmaceutical applications.[5] The C-3
substituted imidazo[1,2–a]pyridine derivatives such as
necopidem, saripidem, and zolpidem are clinically used as
neuroactive drugs including alpidem, (as an anxiolytic agent),
minodronic acid[6] and optically active GSK812397 (HIV
infection).[7] In addition, functionalized imidazo[1,2–a]pyridines
were also recognized in medicinal chemistry for antiproliferative
activity against melanoma cells, tubulin polymerization and
protein kinase inhibitors.[8] In recognition of the importance of
these molecules, recently many research groups have
demonstrated the sulfenylation of imidazo[1,2–a]pyridines using
disulfides, thiols, sulfenyl chlorides, sodium sulfinates and
sulfonyl hydrazines as a source for sulfenylation.[9-11] Though
these strategies have been successfully employed for the
sulfenylation of imidazo[1,2–a]pyridines, they require the
activators such as copper catalyst, iodine, peroxides and also
Scheme 1. sulfenylation of imidazo[1, 2–a]pyridine.
selectivity observed which were difficult to achieve in
conventional organic solvents.[13] Organic substrates are
generally insoluble in water, however reactions reported to
proceed “on water” to obtain the desired products.[14]
Results and Discussion
Despite the significance of these approaches, the synthetic
simplicity as well as environmentally benign process, to provide
the direct strategy for the synthesis of 3-sulfenyl imidazo[1,2–
a]pyridines via a transition metal-free and organic solvent-free
protocol is still an attractive.[15] To the best of our knowledge, the
sulfenylation of imidazo[1,2–a]pyridines in water under metal-
free conditions were not reported. In continuation of our interest
on the synthesis of functionalized imidazo[1,2–a]pyridines,[15a]
herein, we report an efficient metal−free C−H sulfenylation of
imidazo[1,2-a]pyridines with S-phenyl sulfonothioates in an
aqueous medium (Scheme 1). Compared to the sulfenylation in
organic solvents,[15a] water has been identified as a selective
solvent to activate the organic substrates to convert into desired
products.
At the outset of our investigation, we selected 2-phenyl
imidazo[1,2–a]pyridine (1a) and S-phenyl sulfonothioate (2a) as
model substrates to optimize the reaction conditions (Table 1).
Initially our hypothesis was examined by the reaction of 1a with
S-phenyl benzenesulfonothioate 2a in the presence of 5 mol %
of NaI in isopropyl alcohol (IPA) at room temperature, after 24 h,
the desired sulfenylated product 3a was isolated in 26% yield
Academy of Scientific & Innovative Research, CSIR–Central Salt &
Marine Chemicals Research Institute, G.B. Marg, Bhavnagar-364 002.
Gujarat (INDIA)
e-mail: adimurthy@csmcri.res.in
http://adimurthy.wixsite.com/drsadimurthygroup
Supporting information for this article is given via a link at the end of the
document.((Please delete this text if not appropriate))
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(Table 1, entry 1).[15b,c] When the same reaction was performed
at 45 °C, 3a was isolated in 33% yield (Table 1, entry 2). Upon
further raising of the reaction temperature to 60 °C and 80 °C,
the desired product 3a was isolated in 40% yield in both cases
(Table 1, entries 3 and 4). In the latter entry, there was no
advantage when the temperature raised to 80 °C. However, with
increasing the catalyst loading to 10 mol%, at 60 °C, marginal
improvement in yield was observed (Table 1, entry 5). Attempts
Upon extensive screening of various sulfenylation sources (2b–
2e); various iodine sources (KI, TBAI, I2) and different solvents
(acetonitrile and methanol), the yield of the product was not
improved under these conditions (Table 1, entries 8–16).
Fortunately, when the reaction of 1a and 2a was conducted in
water as solvent, with NaI (10 mol %) at 60 °C, the desired
product 3a was isolated in 58% yield (Table 1, entry 17). When
the reaction temperature in water was increased to 80 °C and
100 °C, gratifyingly, the yield was increased to 84% and 85%
respectively (Table 1, entries 18 and 19). Surprisingly, 82% of
3a was isolated without NaI in water at 100 °C (Table 1, entry
20). We anticipated to raise product yield further, accordingly
increased the temperature to 110°C and 120°C, the desired
product 3a was obtained in 88% and 92% yield respectively
(Table 1, entries 21 and 22). Also, the reaction was performed
under oxygen (O2 balloon) and closed tube instead of argon
atmosphere to check the efficiency the transformation and the
yield of 3a was reduced to 84% and 81% respectively. (Table 1,
entries 23 and 24). Finally, we performed the reactions in protic
solvents such as acitic acid, methanol and ethanol and observed
only 18%, 14% and 22% yield respectively (Table 1, entries 25-
27), it indicates that, water promotes the present sulfenylation
reaction.
Table 1. Optimization of reaction conditions.a
Table 2. Substrate scope of imidazo heterocyclic compoundsa
aReaction conditions: 0.25 mmol of 1a, 0.3 mmol of 2, catalyst (10 mol %),
solvent (1mL), 24 h, argon balloon, isolated yields. b36 h. cO2 balloon. d
Closed tube.
were made to further increase the catalyst loading and reaction
time, no improvement in the yield of the product was observed
(Table 1, entries 6 and 7).
aReaction conditions: 0.25 mmol of 1, 0.3 mmol of 2a, Water (1mL), argon
balloon, 24 h, isolated yields.
Under the set of optimized conditions (Table 1, entry 22), the
sulfenylation of various imidazo[1,2-a]pyridines 1 with S-phenyl
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10.1002/ejoc.201700487
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benzenesulfonothioate 2a was examined (Table 2). The results
in Table 2 demonstrate that, the reaction has a high degree of
functional group tolerance with broad substrate scope. Initially,
2-arylimidazo[1,2-a]pyridines bearing electron-donating groups
(Me, Et, and OMe,) at the para position of the phenyl ring could
react with 2a smoothly and afford the C-3 sulfenylated products
3b−3d in good to excellent yields (77−93%). Similarly, the
presence of electron-withdrawing groups (Cl, Br, and CN) either
at para or meta -position of the phenyl ring of 2-
phenylimidazo[1,2-a]pyridines provided the corresponding
sulfenylated products 3e−3i in good yields. The reaction of S-
phenyl benzenesulfonothioate 2a with various substituted
imidazo[1,2-a]pyridines, [substituents on pyridine ring of 1 such
as methyl, methoxy, bromide, chloride, and cyanide] under the
optimized conditions gave the C-3 sulfenylated products 3j–3o
in good to excellent yields (41 –94%). When the zolimidine drug
was subjected to the present reaction conditions, it gave 44%
yield of desired sulfenylated product 3p. The electronic effects
associated with (either electron-donating or electron-withdrawing
groups) on the benzene ring of 2-phenyl imidazo[1,2–a]pyridines
has little influence on the yield of sulfenylation reaction.
obtained in 65% and 59% yields respectively. The 2-
methylimidazo[1,2-a]pyridine also gave the desired product 3s in
57% yield. To our delight, the reaction of alkylsulfonothioates
were also reacted well with various imidazo[1,2-a]pyridines and
gave moderate to good yields of corresponding 3-
(methylthio)imidazo[1,2-a]pyridine derivatives 3t-3y including 3-
(methylthio)-2-(thiophen-2-yl)imidazo[1,2-a]pyridine
3z
in
moderate to good yields (42-87%). Furthermore, the present
conditions were extended to the sulfenylation of other
imidazoheterocycles like benzo[d]imidazo[2,1–b]thiazole to
ascertain the scope of the methodology and obtained
corresponding sulfenylated products 3aa and 3ab in 64% and
56% yield respectively. Unfortunately the present conditions not
suitable for benzimidazole 3ac.
We then verified the reactivity of other sulfenylation sources S-
phenyl alkyl/ arylsulfonothioates
thioarylations with 1a under the optimized reaction conditions
(Table 3). The reaction of 1a with S-phenyl
2 and subjected to C-H
methanesulfonothioate 2aa gave the desired product 3a in 80%
yield. Variety of S-phenyl alkyl/arylsulfonothioates 2 bearing
various substituents such as methyl (2ab & 2ac), fluoro (2ad &
2ae), chloro (2af & 2ag), nitro (2ah & 2ai) and thiophene 2aj
groups were well tolerated and delivered the desired thioarylated
imidazo[1,2-a]pyridine derivatives 4a-4e in moderate to excellent
yields (46- 93%).
The present methodology is also applicable to alkenyl and
heterocyclic substituted derivatives (E)-2-styrylimidazo[1,2-
a]pyridine 3q and 2-(thiophen-2-yl)imidazo[1,2-a]pyridine 3r and
Table 3. Scope for different sulfonothioatesa
Scheme 2. Gram scale sulfenylation of 1a.
In addition, to confirm the feasibility of the process for scale-up
studies, we synthesized two products (from two different
sulfenylating agents) at gram scale under the same optimized
conditions. The reaction of 1a (1.164 g, 6 mmol), with two
different sulfenylating agents 2a (1.800 g, 7.2 mmol) and 2aa
(1.346 g, 7.2 mmol) were subjected and the corresponding
sulfenylated product 3a was obtained in 81% (1.460 g) and 76%
(1.369 g) yield respectively (Scheme 2). This study indicates the
feasibility of the method for industrial/commercial production as
the reactions performed only in water.
Further, the present strategy has been extended to another
important heterocyclic compounds like indoles under the set of
optimized conditions. Various indoles
5
with different
benzenesulfonothioates 2 were examined (Table 4). Initially the
reaction of S-phenyl benzenesulfonothioate 2 was conducted
with N-methyl indole 5a, N-benzyl indole 5b and indole 5c these
indoles reacted smoothly and gave good yields (67 –81%) of
corresponding sulfenylated products (6a-6c). The presence of
electron-donating/withdrawing substituted (Me, Br, F, CN, NO2,
CO2Me) indoles were also reacted well and afford the desired
products (6d-6j) in good to excellent yields. Further, the different
aReaction conditions: 0.25 mmol of 1a, 0.3 mmol of 2, Water (1mL), argon
balloon, 24 h, isolated yields
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10.1002/ejoc.201700487
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COMMUNICATION
sulfernylation
sources
including
S-(thiophen-2-yl)
These reactions indicates that other sulfur sources 2b-e were
not activated under the present conditions only 2a selectively
active to dissociation in water and also selectively 2a was
activated without need of any catalyst. Further, the reaction of
unsubstituted imidazo[1,2-a]pyridine 7 was reacted with 2a, the
C-3 sulfenylated product 8 was isolated in 63% yield (Scheme 3,
eq. (2)). When C-3 substituted substrate 3-methylimidazo[1,2-
a]pyridine 9 was subjected to the same reaction conditions, the
formation of desired product 10 was not observed (Scheme 3,
eq. (3)).These experiments (Scheme 3, eqs. 2 and 3) designates
that, the present transformation is highly desirable for selective
synthesis of structural isomers and supports the formation of
imidazolium intermediate to propose the probable reaction
mechanism as shown in scheme 4. The reaction with D2O gave
89% yield of 3a (Scheme 3, eq. (4)) Also when the reaction was
performed with TEMPO as a radical scavenger, 78% yield of 3a
was obtained under optimized conditions.15c It indicates that, the
reaction does not proceed through radical pathway (Scheme 3,
eq. (5)).
benzenesulfonothioate 2 were also compatible for a range of
indole derivatives and provided good yields of products (6k-6o).
The reactions of alkylsulfonothioate with indole also reactive and
afford the corresponding 3-(methylthio)-1H-indole 6p in good
yield (89%).
Table 4. Substrate scope of indole compounds.a
aReaction conditions: 0.25 mmol of 5, 0.3 mmol of 2a, Water (1mL), argon
balloon, 24 h, isolated yields.
To gain insight into the reaction mechanism, some selective and
control experiments were performed (Scheme 3). Principally, we
subjected the reactivity of 1a with other sulfur sources such as
1,2-diphenyldisulfane 2b, thiophenol 2c, and benzenesulfonyl
chloride 2d and sodium benzenesulfonate 2e under the standard
reaction conditions, along with S-phenyl phenylsulfonothioates
2a (Scheme 3, eq 1). No reactions or traces of product was
observed with 2b-2e, however, with 2a 92% of desired product
was obtained (Scheme 3, eq. 1).
Scheme 4. Plausible Reaction Mechanism
Based on the literature reports[15,16] and our above observations
in the present work, a plausible reaction mechanism has been
proposed (Scheme 4). In the presence of water, S-phenyl
phenylsulfonothioate 2a disassociate into A. Subsequently,
attack of A to imidazo[1,2-a]pyridine 2a through the electrophilic
attack of PhS+ on the C-3 position of 1a and generates the
imidazolium intermediates B and C. Finally, elimination of
sulfinic acid from the intermediate B provides the desired
product 2-phenyl-3-(phenylthio)imidazo- [1,2-a]pyridine 3a.
Similar plausible mechanism also applicable for the indole
derivatives.
Conclusions
In conclusion, we have developed an efficient strategy for the
sulfenylation
of
imidazo[1,2-a]pyridine,
phenylbenzo[d]imidazo[2,1-b]thiazoles and indole derivatives
under catalyst-free aqueous conditions with high degree of
functional group tolerance. The scope of the methodology has
been extended to variety of aryl and alkyl sulfonothioates and
Scheme 3. Selectivity and control experiments
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preparation of sulfenylated imidazo[1,2-a]pyridines.
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A clean washed boiling tube equipped with a magnetic stir bar was
charged with 2-phenylimidazo[1,2-a]pyridine 1a (0.0485 g, 0.25 mmol),
S-phenyl benzenesulfonothioate 2a (0.075 g, 0.30 mmol) and H2O (1mL),
the above mixture was stirred for 24h at 120°C temperature in argon
balloon. After completion of the reaction, the mixture was poured into
10mL of NaHCO3 solution. The product was extracted with ethyl acetate
(10 mL × 3) and dried with anhydrous Na2SO4. Removal of the solvent
under reduced pressure, the left out residue was purified through column
chromatography using silica gel (20% EtOAc/hexane) to obtain 2-phenyl-
3-(phenylthio)imidazo[1,2-a]pyridine 3a in 92 % yield (0.0696g).
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Keywords: water • imidazopyridines • sulfenylation • catalyst
free • C-H Functionalisation.
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COMMUNICATION
C-H Sulfenylation
Chitrakar Ravi, Abhisek Joshi, and
Subbarayappa Adimurthy*
Page No. – Page No.
Text for Table of Contents
C-3 Sulfenylation of N-heteroarenes
in water under catalyst–free
conditions
The synthesis of sulfinylated imidazo[1,2-a]pyridine, phenylbenzo[d]imidazo[2,1-b]thiazoles and indole derivatives have
been reporting under catalyst-free conditions with high degree of functional group tolerance in aqueous medium. The
sulfenylation scope of the this methodology has been extended to variety of aryl and alkyl sulfonothioates.
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