Visible-Light-Mediated Synthesis of 1,2,4-Dithiazolidines from β-Ketothioamides through a Hydrogen-Atom-Transfer Photocatalytic Approach of Eosin y

Article abstract of DOI:10.1021/acs.joc.9b00406

An efficient protocol for visible-light-mediated synthesis of a specific class of 1,2,4-dithiazolidines from β-ketothioamides is devised employing eosin Y as a photoinitiator at ambient temperature in an open pot. The reaction proceeds via an in situ-gene

Full text of DOI:10.1021/acs.joc.9b00406

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Article
Visible-Light Mediated Synthesis of 1,2,4-Dithiazolidines
from #-Ketothioamides through Hydrogen Atom
Transfer Photocatalytic Approach of Eosin Y
Monish A Ansari, Dhananjay Yadav, Sonam Soni, Abhijeet Srivastava, and Maya Shankar Singh
J. Org. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.joc.9b00406 • Publication Date (Web): 05 Apr 2019
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Visible-Light Mediated Synthesis of 1,2,4-Dithiazolidines from -
Ketothioamides through Hydrogen Atom Transfer Photocatalytic
Approach of Eosin Y
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Monish A. Ansari, Dhananjay Yadav, Sonam Soni, Abhijeet Srivastava, and Maya Shankar Singh*
Department of Chemistry, Institute of Science, Banaras Hindu University, Varanasi 221005, India
ABSTRACT: An efficient visible-light-mediated construction of a specific class of 1,2,4-dithiazolidines
from -ketothioamides is devised employing Eosin Y as a photoinitiator at ambient temperature in open-
pot. The reaction proceeds via in situ generated thiyl radical followed by dimerization/deaminative
cyclization cascade enabling the creation of dithiazolidine ring through successive formation of SS and
NC bonds under metal- and additive-free conditions. Remarkably, the benign conditions, sustainability,
and quantifying forbearance of wide horizons of functional groups are added characteristics to the
strategy. Developed HAT methodology will be helpful in post-synthetic modification via embedded
synthetic handles.
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INTRODUCTION
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Photo-induced organic synthesis occupies an impeccable place in ecological equilibria. Visible-light
photoredox catalytic (VLPC) reactions devised by some specific groups and their co-workers are
fundamental and path breaking in organic synthesis, and have seen remarkable growths over the recent
years.1 Visible-light promoted reactions revealed several noteworthy applications owing to its intrinsic
characteristics of simplicity, safety, cost-effectiveness, and sustainability. In this 21st century, visible-
light alone or in combination with an appropriate catalyst has set the remarkable innovations and
enabled numerous expedient synthetic manipulations, which were earlier unapproachable by traditional
methods.2 Upon irradiation, photocatalyst after getting excited could activate the substrate either by
direct energy-transfer3a-d (ET) or via single-electron-transfer3e-h (SET) mode resulting numerous efficient
synthetic transformations.3
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Besides above approach, another possibility could be a hydrogen-atom transfer (HAT) pathway,
recognized as an important activation mode in photocatalyzed reactions.4 The key benefit of this mode is
that the HAT catalyst activates the substrate even if they are electrochemically unmatchable. Usually,
there are three modus operandi of HAT process in photocatalysis.4a In the first strategy, a hydrogen atom
is abstracted by the excited photocatalyst from the substrate followed by a reverse hydrogen-atom
transfer (RHAT) to newly produced intermediate completing the catalytic cycle.5 Second, the co-catalyst
was activated by previously excited photocatalyst. Thus, activated co-catalyst next triggers the reaction
via HAT pathway.6 Third mode is the synchronized transfer of an electron (e-) and a proton (H+) from
the substrate known as proton-coupled electron-transfer (PCET) process,7 enabling the generation of a
radical, which could be employed in several transformations. Since, the indirect HAT and PCET modes
could progress only if some additional reagents are available, the most competent and cost-effective
procedure is the direct HAT catalysis amidst these three modes. Nevertheless, the key constraint for the
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extensive use of HAT process is restricted by insufficient known photocatalyst,4 such as aromatic
ketones, polyoxometalates (POMs)8 and the recently reported species uranyl cation.5e Furthermore, the
above-mentioned catalysts need some additional additives, and suffer from undesired side reactions.
Thus, exploring viable and metal-free catalysts, which can promote direct HAT processes, would be of
great significance.
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Organic dyes are employed as ideal photocatalytic candidates,9 exhibiting the benefits of non-
toxicity, eco-compatibility, and cheapness over the metal-based photocatalysts. Based upon redox
potentials of chemical quenchers, Eosin Y dye (Figure 1), one amongst extensively used
organophotocatalysts, could follow either reductive or oxidative quenching cycles.10,11
Figure 1. Photo- and Electrochemical properties of Eosin Y.
For the success of any protocol, particularly, the selection of substrate is highly crucial and
governing factor. One such simple substrate is β-ketothioamide that has been well-documented as an
intriguing synthon because of its exceptional reactivity.12 β-Ketothioamides are comprehensively
exploited to access valuable heterocyclic scaffolds.12 Furthermore; they not only behave as ambiphilic
species (having both nucleophilic and electrophilic character), but also could be transformed into radical
intermediates for further manipulations. Sulfur-centered radicals (SCRs) are appealing intermediates in
organic synthesis. Based on our experience on thermal reactivity/transformation of β-ketothioamides,13
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we envisaged that the transformation of β-ketothioamides via visible-light driven photocatalysis could
be a viable alternative for valuable scaffolds. Rare reports are available on the formation of X−X bonds
through organo-photocatalysis.9 Yadav and co-workers have formulated the synthesis of 1,2,4-
thiadiazoles via cyclization of thioamides catalyzed by organo-photocatalyst Eosin Y.14a Lei and co-
workers14b developed the synthesis of benzothiazoles from thioamides promoted by Ru(bpy)3(PF6)2.
Nicewicz et al.14c observed that sulfur radical cation (S˙+) could experience Newman-Kwart
rearrangement. Recently, the role of Eosin Y dye as a hydrogen-atom transfer photocatalyst toward CH
functionalization has also been disclosed.15 Based upon literature reports, we envisioned that the Eosin
Y could play the role of an ideal HAT photocatalyst and may react with enethiol tautomer of thioamide
to generate thiyl radical, which could undergo dimerization enabling the formation of intramolecular
disulfide (S-S) bond.
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The utility of sulfur heterocycles has become prevalent in medicinal and agricultural industries.16a
Amongst the diverse heterocyclic frameworks, 1,2,4-dithiazoles and their derivatives are privileged
scaffolds having various applications in pharmaceutical sciences.16b,c Kühle and Zumach successfully
synthesized 1,2,4-dithiazolidine-3,5-diones by the reaction of chlorocarbonylsulfenylchloride with O-
esters of N,N-dialkylthiocarbamic acids (Scheme 1a).17a Barany et al. reported the synthesis of 1,2,4-
dithiazolidines via tandem reaction of thioamides with (chlorocarbonyl)sulfenyl chloride (Scheme
1b).17b A. P. Guzaev17c reported the construction of 1,2,4-dithiazolidine-3-thiones, which were further
utilized as sulfurizing agents for oligonucleotide synthesis (Scheme 1c). Recently, Pan and co-
workers17d devised the synthesis of 1,2,4-dithiazoles via photocatalytic reaction employing thioamides,
para-quinonemethides, and (NH4)2S (Scheme 1d). Li and co-workers18a developed the synthesis of
thiazolylidenes and 1,4-dithiines from -ketothioamides (KTAs) catalyzed by molecular iodine (Scheme
1e). For the synthesis of 1,2,4-dithiazolidine derivatives, various other reagents such as CF3SCl,18b
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18d
SO2Cl2,18c and (NH4)2Ce(NO3)6
have been executed. Despite the above examples, the synthetic
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potential of the visible-light photoredox generation of sulfur radicals from thioamides remains
surprisingly underexplored. Hence, the development of efficient and sustainable visible-light induced
cyclizations employing thioamides for the synthesis of heterocyclic structures is still highly desirable.
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Scheme 1. Previous Attempts and Current Approach for the Synthesis of 1,2,4-Dithiazolidines
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To the extent of our knowledge, synthesis of 1,2,4-dithiazolidines from β-ketothioamides via
photocatalysis has not been disclosed till yet. Driven by the need for an efficient and unified synthetic
route to 1,2,4-dithiazolidine derivatives, we were fascinated by the probability of visible-light-promoted
site-selective dimerization of -ketothioamides. Herein, we describe the visible-light-enabled
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transformation of -ketothioamides into 1,2,4-dithiazolidines catalyzed by Eosin Y dye as a HAT
photocatalyst at ambient temperature (Scheme 1f).
RESULTS AND DISCUSSION
To check the credibility of our hypothesis, we commenced our study by using 3-oxo-N,3-
diphenylpropanethioamide (1a) as a model substrate for optimizing the reaction conditions. The
outcomes of the reactions are enlisted in Table 1. The solution of 1a in 2 mL of dichloromethane (DCM)
or in 2 mL of dichloroethane (DCE) separately, was irradiated under green LED with 2 mol % of Eosin
Y dye for 12 h in open-pot at room temperature. The anticipated product could not be detected, and the
substrate 1a was recovered completely unreacted (Table 1, entries 1 and 2). Next, we carried out the
model reaction in various solvents (Table 1, entries 3-8). To our pleasure, dimethyl formamide (DMF)
was found as a solvent of choice providing the expected product 2a in 76% yield (Table 1, entry 5).
Encouraged by this favourable result, to identify the most suitable catalyst, a series of other organic
photocatalysts such as Rhodamine B, Rose-Bengal, Acridine Red, Fluorescein and Methylene Blue
(Table 1, entries 9-13) were screened. Although the above tested photocatalysts triggered the reaction,
but none of them furnished superior result than Eosin Y. Further decrease in the amount of Eosin Y to 1
mol % had a detrimental effect, whereas increasing the amount of Eosin Y (4 mol %) could not improve
the efficacy of the reaction (Table 1, entries 14 and 15). In control experiments, we found that visible-
light and dye are inevitable components for this novel transformation (Table 1, entries 16 and 17).
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Table 1. Optimization of Reaction Conditions for the Synthesis of 2a
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entry
photocatalyst (mol %)
Eosin Y (2 mol %)
Eosin Y (2 mol %)
Eosin Y (2)
solvent time (h) yieldb (%)
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DCM
DCE
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NRc
NRc
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CH3CN
THF
Eosin Y (2)
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Eosin Y (2)
DMF
DMSO
Toluene
Dioxane
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
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Eosin Y (2)
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NRc
Eosin Y (2)
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Eosin Y (2)
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Rhodamine B (2)
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10 Rose Bengal (2)
11 Acridine Red (2)
12 Fluorescein (2)
13 Methylene blue (2)
14 Eosin Y (1)
15 Eosin Y (4)
16 Eosin Y (2)d
17 none
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NRc
NRc
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18 Eosin Y (2)e
19 Eosin Y (2)f
20 Eosin Y (2)g
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aReaction conditions unless otherwise stated: all the reactions were performed with 1a (0.25 mmol),
Eosin Y (2 mol %) in 2 mL of solvent at rt in open air. bYields. cNR = No reaction. dWithout irradiation.
eWith blue LED. fWith white LED. gWith cyan LED.
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The optimized condition for this reaction was determined as 1a (0.25 mmol), Eosin Y (2 mol %) in
DMF under green LED irradiation for 4 h at room temperature in open air. In place of green LED, when
the reaction was performed under the irradiation of different light sources such as blue LED, white LED
and cyan LED, the yield of observable product 2a was diminished by 14-28% (Table 1, entries 18-20),
indicating that the green LED is the suitable source for excitation of photocatalyst.
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We explored the substrate scope of the protocol employing a range of structurally diverse -
ketothioamides (1a-1x) with above mentioned optimized set of reaction conditions. The results of our
investigations are enlisted in Scheme 2. Explicitly, R1 moiety substituted at o-, m- and p-positions with
electron-donating and electron-withdrawing groups (such as -Me, -OMe, -F, -Cl, -Br, and -CF3) were
viable participants, and furnished the desired 1,2,4-dithiazolidines (2b-2m) in 66-82% yields. Notably,
thioamides appended with substrates bearing aromatic, heteroaromatic (furyl and thienyl), and extended
aromatic moieties as R1 worked well under the standard conditions. xtended aromatic appendants such
as 1-naphthyl and biphenyl groups were also tolerated well furnishing the corresponding dithiazolidines
(2n, 2o) in 67% and 65% yields, respectively.
Next, when R1 moiety was swapped to -electron-deficient 3-pyridyl substituent and-electron-rich
motifs such as 2-thienyl and 2-furyl, the corresponding desired products 2p, 2q, and 2r were obtained in
66%, 70%, and 69% yield, respectively. R1 appended with cyclopropyl group (thioamide prepared from
cyclopropyl methyl ketone), produced the product 2s in relatively low (46%) yield. After the effective
employment of R2 as neutral phenyl group, we intended to employ R2 as phenyl substituted with
electron-withdrawing and electron-donating groups to realize the effect of the electronic property of R2
on the reaction. Consequently, substrate 1 bearing R2 moiety as 3-chlorophenyl and 4-methoxyphenyl
groups gave the corresponding desired products 2t and 2u in 58% and 65% yield, respectively.
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Scheme 2. Scope and Limitation of -Ketothioamides
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Concurrently, to examine vulnerability of this protocol, moieties as aliphatic groups were employed
instead of aromatic groups at R2. Thus, when R2 phenyl group was switched to alkyl groups such as
methyl, n-butyl and cyclohexyl, unfortunately, no expected product was obtained (Scheme 2, 2v-2x).
Further, to ascertain the importance of the -keto group of thioamide, we carried out the reaction with
nonketone thioamides such as 2-(3-methoxyphenyl) ethanethioamide and N-(p-tolyl) benzothioamide
under the previously optimized conditions. The observable product cannot be obtained suggesting that
-keto group is necessary for the progress of the reaction. This could be due the stabilization of enethiol
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tautomer 1 of -ketothioamide via intramolecular hydrogen bonding. The structures of all the
compounds 2a-2u were characterized by satisfactory spectral (1H, 13C NMR and HRMS) studies.
Additionally, the structure was unambiguously established by the single crystal X-ray diffraction
analysis of (2Z,2'Z)-2,2'-(4-phenyl-1,2,4-dithiazolidine-3,5-diylidene)bis(1-phenylethanone) 2a as a
representative compound (see Supporting Information for details).19
For deeper understanding of mechanistic investigation, some controlled experiments were
performed (Scheme 3). In the presence of radical scavengers such as TEMPO or BHT, the yield of the
desired product 2a was drastically reduced to 6-10% indicating the radical pathway. To check whether
addition of oxygen improves reaction efficiency, the test reaction was carried out under oxygen purging
conditions. No improvement in the outcome of the reaction was observed. The model reaction of 1a
under argon atmosphere dramatically dropped the yield of product 2a to 8%, indicating that air (O2) is
necessary for the reaction.
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Scheme 3. Key Control Experiments
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To validate the synthetic utility of the strategy, we performed a large scale experiment with 1k (3
mmol) in the presence of green LED (530 nm) under the optimal conditions (Scheme 4). The anticipated
product 2k was achieved in 78% yield (566 mg), and found to be comparable with small-scale reaction
(Scheme 2, entry 2k, 82%). The above reaction indicated that this method could be employed for higher-
scale synthesis of 1,2,4-dithiazolidine derivatives 2.
Scheme 4. A 3 mmol Scale Synthesis of 2k
Ensued by control experiments and literature reports,4,20 the following plausible mechanism has
been suggested (Scheme 5). The redox potential of Eosin Y indicated that it was not capable of oxidizing
or reducing the substrate, since we obtained the potential of β-ketothioamide in the range of (E1/2 = +1.4
V to E1/2 = +1.6 V vs Ag/AgCl) in 0.1M TBAP/CH3CN solution. The above values rule out SET mode
for this reaction. Eosin Y (EY) is transformed into its excited state (EY*) by irradiation with visible-
light, which abstract enethiol hydrogen from substrate 1 producing the radical intermediates A and C.
The thiyl radical A undergoes dimerization to give disulfide tethered enamine intermediate B, which
upon intramolecular deaminative cyclization enabled 1,2,4-dithiazolidine derivative 2. The intermediate
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C undergoes aerial oxidation to regenerate ground state (EY) and produced HO2˙ radical, which suffers
disproportionation to give oxygen and hydrogen peroxide.21 Thus, H2O2 formed was detected by
potassium iodide (KI) and starch as indicator.4c
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Scheme 5. Proposed Reaction Pathway
CONCLUSION
In conclusion, we have devised an efficient protocol to access highly functionalized 1,2,4-
dithiazolidines bearing two exocyclic enone moieties at 3- and 5-positions via visible light-mediated
oxidative symmetrical dimerizative of -ketothioamides at room temperature. Remarkably, this strategy
employing inexpensive Eosin Y dye as a hydrogen-atom transfer (HAT) photocatalyst and air as an
oxidant is not only operationally simple, economic, eco-compatible, metal-/additive-free, but also
exhibits excellent functional group compatibility with electron-rich/electron-deficient arenes and
heteroarenes. No external redox mediators or prefunctionalization of the substrate is required. This
strategy enables a decent alternative to access symmetrical 1,2,4-dithiazolidines to existing ones,
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consequently, broadening the chemistry of ketothioamides. Overall, this Eosin Y dye based
photocatalytic HAT strategy should be promising towards functionalization of a wide range of CH,
NH and SH bonds in a sustainable manner, and can allow to overcome traditional metal-catalyzed
reactions, which are environmentally notorious.
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EXPERIMENTAL SECTION
General Information. Unless otherwise mentioned, all commercially available solvents and reagents
(procured from Merck, Aldrich, and Avra synthesis) were utilized as such without any purification. The
β-ketothioamides were synthesized by reported procedure.13,22 All the reactions were monitored by
analytical thin layer chromatography (TLC) using Merck precoated aluminium sheets and visualized by
1
13
UV lamp. Flash column chromatography was performed on silica gel (230-400 mesh). The H and C
NMR spectra were recorded on JEOL 500 FT-NMR spectrometer operating at 500 and 125 MHz,
respectively. Chemical shifts (δ) for H and C{1H} NMR are given in parts per million (ppm) using
the residual solvent peaks as reference relative to tetramethylsilane (TMS). Coupling constant (J) values
are reported in Hz. Mass spectra were recorded on Agilent Q-TOF and Waters-Q-TOF Premier-HAB213
instrument. The green light irradiation was done using high-power LUXEON Rebel LEDs (λ = 530 nm,
161 lm @ 700mA). The power of each light is 1W. All the reactions were carried out using a round
bottle (25 mL) borosilicate glass without special photochemical equipment. There is 8.0 cm distance
between the reactor and LEDs. The melting points are uncorrected. Compound stereochemistry is
supported by an X-ray crystallographic structure determination of one of the representative molecule 2a.
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General experimental procedure for compound 2: β-Ketothioamide 1 (0.25 mmol) and Eosin Y (2
mol %) were added to 2 mL of dimethyl formamide (DMF). The resulting mixture was stirred at room
temperature in open air for 4 h under the irradiation of green LED (530 nm) placed 8 cm from the flask.
After the completion of the reaction (monitored by TLC), the reaction mixture was quenched with water.
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The aqueous layer was extracted with ethyl acetate followed by drying over anhydrous MgSO4, and the
solvent was evaporated under reduced pressure. The crude residue thus obtained was purified by flash
column chromatography over silica gel (230-400 mesh) using ethyl acetate/hexane (20:80) as eluent to
afford the desired pure dithiazolidine products 2 in good to excellent yields. Chemical yields refer to
pure isolated compounds.
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The spectral and analytical data of all the compounds are given as follows:
(2Z,2'Z)-2,2'-(4-phenyl-1,2,4-dithiazolidine-3,5-diylidene)bis(1-phenylethanone) (2a): The product
o
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was obtained as yellow solid (76%, 39 mg); mp 200-201 C; H NMR (500 MHz, CDCl3) δ 7.78-7.72
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(m, 3H), 7.69 (d, J = 10.0 Hz, 4H), 7.47-7.42 (m, 4H), 7.37 (t, J = 7.5 Hz, 4H), 6.16 (s, 2H); C {1H}
NMR (125 MHz, CDCl3) δ 185.8, 164.9, 138.2, 137.1, 131.6, 131.2, 130.6, 128.2, 127.67, 127.1, 96.7;
HRMS (ESI-TOF) m/z: [M + H]+ Calcd for C24H18NO2S2 416.0773; Found 416.0743.
(2Z,2'Z)-2,2'-(4-phenyl-1,2,4-dithiazolidine-3,5-diylidene)bis(1-(p-tolyl)ethanone) (2b): The product
o
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was obtained as yellow solid (68%, 38 mg); mp 215-216 C; H NMR (500 MHz, CDCl3) δ 7.77-7.70
(m, 3H), 7.58 (d, J = 5.0 Hz, 4H), 7.41 (d, J = 5.0 Hz, 2H), 7.16 (d, J = 10.0 Hz, 4H), 6.13 (s, 2H), 2.35
(s, 6H); 13C {1H} NMR (125 MHz, CDCl3) δ 185.6, 164.5, 142.4 134.5, 131.2, 130.5, 128.9, 127.7,
127.2, 96.6, 21.3; HRMS (ESI-TOF) m/z: [M + Na]+ Calcd for C26H21NO2S2Na 466.0906; Found
466.0906.
(2Z,2'Z)-2,2'-(4-phenyl-1,2,4-dithiazolidine-3,5-diylidene)bis(1-(4-methoxyphenyl) ethanone) (2c):
The product was obtained as yellow solid (66%, 39 mg); mp 224-225 oC; 1H NMR (500 MHz, CDCl3) δ
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7.71-7.61 (m, 7H), 7.37 (d, J = 5.0 Hz, 2H), 6.81 (d, J = 10.0 Hz, 4H), 6.06 (s, 2H), 3.77 (s, 6H); C
{1H} NMR (125 MHz, CDCl3) δ 185.3, 164.9, 163.0, 139.1, 131.8, 131.1, 130.5, 129.8, 128.4, 114.0,
97.0, 55.7; HRMS (ESI-TOF) m/z: [M + H]+ Calcd for C26H22NO4S2 476.0985; Found 476.0991.
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(2Z,2'Z)-2,2'-(4-phenyl-1,2,4-dithiazolidine-3,5-diylidene)bis(1-(4-fluorophenyl)ethanone) (2d): The
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product was obtained as yellow solid (78%, 44 mg); mp 220-221 C; H NMR (500 MHz, CDCl3) δ
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7.80-7.68 (m, 7H), 7.42 (d, J = 5.0 Hz, 2H), 7.04 (t, J = 10 Hz, 4H), 6.10 (s, 2H); C {1H} NMR (125
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MHz, CDCl3) δ 185.3, ,166.1, 165.8 (d, JCF = 251.25 Hz), 139.2, 134.4, 132.3, 131.7, 130.6 (d, JCF =
2
8.75 Hz), 128.6, 116.3 (d, JCF = 22.5 Hz), 97.4; HRMS (ESI-TOF) m/z: [M + Na]+ Calcd for
C24H15F2NO2S2Na 474.0404; Found 474.0404.
(2Z,2'Z)-2,2'-(4-phenyl-1,2,4-dithiazolidine-3,5-diylidene)bis(1-(4-chlorophenyl)ethanone)
(2e):
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The product was obtained as pale yellow solid (78%, 47 mg); mp 275-276 C; H NMR (500 MHz,
CDCl3) δ 7.76 (t, J = 10.0 Hz, 3H), 7.62 (d, J = 10.0 Hz, 4H), 7.41 (d, J = 5.0 Hz, 2H), 7.33 (d, J = 5.0
Hz, 4H), 6.10 (s, 2H); C {1H} NMR (125 MHz, CDCl3) δ 184.6, 165.5, 138.1, 135.5, 131.5, 130.9,
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128.6, 127.7, 96.6; HRMS (ESI-TOF) m/z: [M + H]+ Calcd for C24H16Cl2NO2S2 483.9994 (35Cl); Found
483.9995; Calcd for C24H16Cl2NO2S2 485.9970 (37Cl); Found 485.9970.
(2Z,2'Z)-2,2'-(4-phenyl-1,2,4-dithiazolidine-3,5-diylidene)bis(1-(4-bromophenyl)ethanone) (2f): The
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product was obtained as yellow solid (76%, 54 mg); mp 238-239 C; H NMR (500 MHz, CDCl3) δ
7.81-7.77 (m, 3H), 7.67 (s, 2H), 7.53 (d, J = 5.0 Hz, 2H), 7.42 (d, J = 5.0 Hz, 4H), 7.30 (t, J = 7.5 Hz,
2H), 6.10 (s, 2H); 13C {1H} NMR (125 MHz, CDCl3) δ 184.5, 165.9, 139.0, 138.2, 134.7, 131.8, 131.7,
131.2, 129.8, 127.7, 127.5, 125.4, 96.7; HRMS (ESI-TOF) m/z: [M + H]+ Calcd for C24H16Br2NO2S2
571.8984 (79Br); Found 571.8992; Calcd for C24H16Br2NO2S2 573.8969 (81Br); Found 573.8973.
(2Z,2'Z)-2,2'-(4-phenyl-1,2,4-dithiazolidine-3,5-diylidene)bis(1-(4-(trifluoromethyl)
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phenyl)ethanone)(2g): The product was obtained as yellow solid (78%, 54 mg); mp 219-220 C; H
NMR (500 MHz, CDCl3) δ 7.80-7.76 (m, 7H), 7.63 (d, J = 5.0 Hz, 4H), 7.43 (d, J = 5.0 Hz, 2H), 6.16 (s,
2H); C {1H} NMR (125 MHz, CDCl3) δ 184.8, 166.3, 140.4, 138.3, 133.5 (q 2JCF3 = 32.5 Hz), 131.8,
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131.3, 127.9, 127.8, 125.7, 125.6, 123.8 (q JCF3 = 273.3 Hz), 97.1; HRMS (ESI-TOF) m/z: [M + H]+
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Calcd for C26H16F6NO2S2 552.0521; Found 552.0544.
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(2Z,2'Z)-2,2'-(4-phenyl-1,2,4-dithiazolidine-3,5-diylidene)bis(1-(3-bromophenyl)ethanone)
(2h):
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The product was obtained as yellow solid (72%, 52 mg); mp 205-206 oC; 1H NMR (500 MHz, CDCl3) δ
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7.79-7.71 (m, 5H), 7.53-7.51 (m, 4H), 7.37 (d, J = 10.0 Hz, 2H), 7.20-7.17 (m, 2H), 6.04 (s, 2H); C
{1H} NMR (125 MHz, CDCl3) δ 184.3, 165.8, 139.0, 138.0, 134.6, 131.6, 131.0, 130.4, 129.9, 127.6,
125.7, 122.7, 96.6; HRMS (ESI-TOF) m/z: [M + H]+ Calcd for C24H16Br2NO2S2 571.8984 (79Br); Found
571.8990; Calcd for C24H16Br2NO2S2 573.8969 (81Br); Found 573.8976.
(2Z,2'Z)-2,2'-(4-phenyl-1,2,4-dithiazolidine-3,5-diylidene)bis(1-(m-tolyl)ethanone) (2i): The product
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was obtained as yellow solid (67%, 37 mg); mp 205-206 C; H NMR (500 MHz, CDCl3) δ 7.78-7.71
(m, 3H), 7.56 (s, 2H), 7.42 (t, J = 7.5 Hz, 4H), 7.27-7.22 (m, 4H), 6.14 (s, 2H), 2.35 (s, 6H); C{1H}
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NMR (125 MHz, CDCl3) δ 186.1, 165.0, 138.2, 137.3, 132.6, 131.4, 130.7, 128.1, 127.9, 127.8, 124.3,
97.0, 21.2; HRMS (ESI-TOF) m/z: [M + H]+ Calcd for C26H22NO2S2 444.1086; Found 444.1092.
(2Z,2'Z)-2,2'-(4-phenyl-1,2,4-dithiazolidine-3,5-diylidene)bis(1-(3-methoxyphenyl)ethanone) (2j):
The product was obtained as yellow solid (67%, 40 mg); mp 223-224 oC; 1H NMR (500 MHz, CDCl3) δ
7.77-7.71 (m, 3H), 7.41 (d, J = 5.0 Hz, 2H), 7.35 (s, 2H), 7.25 (t, J = 7.5 Hz, 2H), 7.15 (d, J = 5.0 Hz,
2H), 6.99 (d, J = 5.0 Hz, 2H), 6.14 (s, 2H), 3.81 (s, 6H); 13C {1H} NMR (125 MHz, CDCl3) δ 185.8,
165.2, 159.8, 138.8, 138.4, 131.5, 130.9, 129.3, 127.9, 119.7, 118.3, 112.1, 97.1, 55.4; HRMS (ESI-
TOF) m/z: [M + H]+ Calcd for C26H22NO4S2 476.0985; Found 476.0985.
(2Z,2'Z)-2,2'-(4-phenyl-1,2,4-dithiazolidine-3,5-diylidene)bis(1-(2-chlorophenyl)ethanone)
(2k):
The product was obtained as yellow solid (82%, 50 mg); mp 225-226 oC; 1H NMR (500 MHz, CDCl3) δ
7.67-7.59 (m, 3H), 7.50 (d, J = 10.0 Hz, 2H), 7.37 (d, J = 10.0 Hz, 2H), 7.31-7.28 (m, 6H), 6.05 (s, 2H);
13C {1H} NMR (125 MHz, CDCl3) δ 186.8, 164.7, 138.3, 138.2, 131.5, 131.4, 131.4, 130.9, 130.5,
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130.2, 127.9, 127.0, 101.4; HRMS (ESI-TOF) m/z: [M + H]+ Calcd for C24H16Cl2NO2S2 483.9994
(35Cl); Found 483.9993; Calcd for C24H16Cl2NO2S2 485.9970 (37Cl); Found 485.9968.
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(2Z,2'Z)-2,2'-(4-phenyl-1,2,4-dithiazolidine-3,5-diylidene)bis(1-(o-tolyl)ethanone) (2l): The product
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was obtained as yellow solid (68%, 38 mg); mp 224-225 oC; H NMR (500 MHz, CDCl3) δ 7.67-7.58
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(m, 3H), 7.35 (d, J = 5.0 Hz, 2H), 7.28-7.25 (m, 4H), 7.17-7.12 (m, 4H), 5.86 (s, 2H), 2.41 (s, 6H); C
{1H} NMR (125 MHz, CDCl3) δ 190.2, 164.3, 138.9, 138.5, 137.1, 131.5, 131.4, 130.8, 130.3, 127.9,
127.8, 125.7, 100.7, 20.6; HRMS (ESI-TOF) m/z: [M + H]+ Calcd for C26H22NO2S2 444.1086; Found
444.1080.
(2Z,2'Z)-2,2'-(4-phenyl-1,2,4-dithiazolidine-3,5-diylidene)bis(1-(2-methoxyphenyl)ethanone) (2m):
The product was obtained as yellow solid (67%, 40 mg); mp 224-225 oC; 1H NMR (500 MHz, CDCl3) δ
7.83 (d, J = 10.0 Hz, 2H), 7.71-7.62 (m, 3H), 7.42-7.34 (m, 4H), 6.98 (t, J = 7.5 Hz, 2H), 6.82 (d, J =
10.0 Hz, 2H), 6.46 (s, 2H), 3.58 (s, 6H); 13C {1H} NMR (125 MHz, CDCl3) δ 185.2, 163.8, 158.0,
139.3, 132.8, 131.0, 130.9, 130.1, 128.3, 127.2, 120.8, 111.6, 102.6, 55.1; HRMS (ESI-TOF) m/z: [M +
H]+ Calcd for C26H22NO4S2 476.0985; Found 476.0981.
(2Z,2'Z)-2,2'-(4-phenyl-1,2,4-dithiazolidine-3,5-diylidene)bis(1-([1,1'-biphenyl]-4-yl)ethanone)
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(2n): The product was obtained as white solid (67%, 47 mg); mp 248-249 C; H NMR (500 MHz,
CDCl3) δ 7.75-7.70 (m, 7H), 7.53 (t, J = 7.5 Hz, 8H), 7.41-7.37 (m, 6H), 7.33-7.30 (m, 2H), 6.15 (s,
2H); 13C {1H} NMR (125 MHz, CDCl3) δ 185.4, 165.0, 144.5, 139.9, 138.4, 136.0, 131.4, 130.8, 128.8,
127.8, 127.0, 96.9; HRMS (ESI-TOF) m/z: [M + H]+ Calcd for C36H26NO2S2 568.1399; Found
568.1398.
(2Z,2'Z)-2,2'-(4-phenyl-1,2,4-dithiazolidine-3,5-diylidene)bis(1-(naphthalen-1-yl)ethanone)
(2o):
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The product was obtained as pale yellow solid (65%, 42 mg); mp 254-255 C; H NMR (500 MHz,
CDCl3) δ 8.44 (d, J = 5.0 Hz, 2H), 7.88 (d, J = 10.0 Hz, 2H), 7.83 (d, J = 5.0 Hz, 2H), 7.64 (t, J = 7.5
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Hz, 2H), 7.58-7.48 (m, 8H), 7.41-7.39 (m, 3H), 6.03 (s, 2H); 13C {1H} NMR (125 MHz, CDCl3) δ
189.5, 164.6, 136.8, 133.6, 131.3, 131.1, 130.7, 130.1, 128.1, 127.6, 127.0, 126.2, 126.2, 125.6, 124.4,
101.2; HRMS (ESI-TOF) m/z: [M + H]+ Calcd for C32H22NO2S2 516.1086; Found 516.1115.
(2Z,2'Z)-2,2'-(4-phenyl-1,2,4-dithiazolidine-3,5-diylidene)bis(1-(pyridin-3-yl)ethanone) (2p): The
product was obtained as yellow solid (66%, 34 mg); mp 198-199 oC; 1H NMR (500 MHz, CDCl3) δ 8.89
(s, 2H), 8.72 (d, J = 5.0 Hz, 2H), 8.11 (d, J = 5.0 Hz, 2H), 7.83 (t, J = 10.0 Hz, 4H), 7.49 (d, J = 5.0 Hz,
3H), 6.22 (s, 2H); 13C {1H} NMR (125 MHz, CDCl3) δ 183.0, 151.2, 147.4, 139.1, 134.3, 130.9, 126.7,
122.8, 119.5, 119.1, 96.9; HRMS (ESI-TOF) m/z: [M + H]+ Calcd for C22H16N3O2S2 418.0678; Found
418.0677.
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(2Z,2'Z)-2,2'-(4-phenyl-1,2,4-dithiazolidine-3,5-diylidene)bis(1-(thiophen-2-yl)ethanone) (2q): The
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product was obtained as yellow solid (70%, 37 mg); mp 201-202 C; H NMR (500 MHz, CDCl3) δ
7.77-7.70 (m, 3H), 7.49 (d, J = 5.0 Hz, 2H), 7.41 (d, J = 5.0 Hz, 2H), 7.33 (d, J = 3.5 Hz, 2H), 7.01 (t, J
= 5.0 Hz, 2H), 5.98 (s, 2H); 13C{1H} NMR (125 MHz, CDCl3) δ 179.0, 164.1, 144.0, 138.1, 131.8,
131.3, 130.8, 129.2, 127.9, 127.8, 96.8; HRMS (ESI-TOF) m/z: [M + H]+ Calcd for C20H14NO2S4
427.9902; Found 427.9869.
(2Z,2'Z)-2,2'-(4-phenyl-1,2,4-dithiazolidine-3,5-diylidene)bis(1-(furan-2-yl)ethanone) (2r): The
product was obtained as brown (69%, 34 mg); mp 204-205 oC; 1H NMR (500 MHz, CDCl3) δ 7.76-7.70
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(m, 3H), 7.40 (d, J = 10.0 Hz, 4H), 7.02 (d, J = 5.0 Hz, 2H), 6.44 (d, J = 5.0 Hz, 2H), 6.05 (s, 2H); C
{1H} NMR (125 MHz, CDCl3) δ 175.6, 164.3, 152.7, 145.1, 138.3, 131.5, 130.9, 129.3, 128.0, 118.6,
115.1, 114.9, 112.4, 96.8; HRMS (ESI-TOF) m/z: [M + H]+ Calcd for C20H14NO4S2 396.0359; Found
396.0357.
(2Z,2'Z)-2,2'-(4-phenyl-1,2,4-dithiazolidine-3,5-diylidene)bis(1-cyclopropylethanone)(2s):
The
product was obtained as greenish yellow (46%, 20 mg); mp 236-237 oC; 1H NMR (500 MHz, CDCl3) δ
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4H); C {1H} NMR (125 MHz, CDCl3) δ 195.5, 162.0, 138.5, 131.3, 130.6, 128.0, 100.0, 20.2, 10.5;
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HRMS (ESI-TOF) m/z: [M + H]+ Calcd for C18H18NO2S2 344.0773; Found 344.0772.
(2Z,2'Z)-2,2'-(4-(3-chlorophenyl)-1,2,4-dithiazolidine-3,5-diylidene)bis(1-(p-tolyl)ethanone)(2t):
The product was obtained as yellow (58%, 35 mg); mp 220-221 oC; 1H NMR (500 MHz, CDCl3) δ 7.71
(d, J = 5.0 Hz, 2H), 7.60 (d, J = 5.0 Hz, 4H), 7.46 (s, 1H), 7.36 (s, 1H), 7.18 (d, J = 5.0 Hz, 4H), 6.12 (s,
2H), 2.36 (s, 6H); 13C {1H} NMR (125 MHz, CDCl3) δ 185.9, 164.3, 142.7, 139.5, 136.9, 134.6, 132.4,
131.2, 129.2, 128.5, 127.5, 126.4, 96.8, 21.5; HRMS (ESI-TOF) m/z: [M + H]+ Calcd for
C26H21ClNO2S2 478.0697 (35Cl); Found 478.0703; Calcd for C26H21ClNO2S2 480.0673 (37Cl); Found
480.0666.
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(2Z,2'Z)-2,2'-(4-(4-methoxyphenyl)-1,2,4-dithiazolidine-3,5-diylidene)bis(1-(p-tolyl)ethanone)(2u):
The product was obtained as yellow (65%, 38 mg); mp 224-225 oC; 1H NMR (500 MHz, CDCl3) δ 7.60
(d, J = 10.0 Hz, 4H), 7.30 (d, J = 10.0 Hz, 2H), 7.20 (d, J = 10.0 Hz, 3H), 7.16 (d, J = 10.0 Hz, 3H), 6.17
(s, 2H), 3.98 (s, 3H), 2.35 (s, 6H); 13C {1H} NMR (125 MHz, CDCl3) δ 185.9,165.2, 160.8, 142.5,
134.8, 130.9, 129.1, 129.1, 127.5, 116.4, 96.9, 55.7, 21.5; HRMS (ESI-TOF) m/z: [M + H]+ Calcd for
C27H24NO3S2 474.1192; Found 474.1199.
ASSOCIATED CONTENT
Supporting Information
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Copies of H, C{1H} NMR spectra (PDF), X-ray crystallographic data (CIF) and cyclic voltamogram
of thioamides. The Supporting Information is available free of charge on the ACS Publications website.
AUTHOR INFORMATION
Corresponding Author
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*E-mail: mayashankarbhu@gmail.com; Fax: +91-542-2368127
ORCID ID:
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Maya Shankar Singh: 0000-0002-3199-0823
Notes
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The authors declare no competing financial interest.
ACKNOWLEDGMENTS
We gratefully acknowledge the financial support from the Science and Engineering Research Board
(SERB/EMR/2015/002482) and the Council of Scientific and Industrial Research 02(0263)/16/EMR-II),
New Delhi. The authors (M.A.A., D.Y., S.S. & A.S.) are thankful to UGC and CSIR, New Delhi for
research fellowship.
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Srivastava, A.; Singh, M. S. Chemoselective one-pot access to benzo[e]indole-4,5-diones and
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