Switching Selectivity of α-Enolic Dithioesters: One Pot Access to Functionalized 1,2- and 1,3-Dithioles

Article abstract of DOI:10.1021/acs.joc.6b01802

An operationally simple cascade protocol has been developed for the construction of 1,2- and 1,3-dithiole derivatives from α-enolic dithioesters. 1,2-Dithioles are achieved by the reaction of dithioesters with elemental sulfur in the presence of InCl₃ under solvent-free conditions. 1,3-Dithioles have been constructed via DABCO mediated self-coupling of dithioesters in open air enabling the formation of two new C-S bonds and one ring in a single operation in contiguous fashion. The reactions proceeded smoothly affording the desired sulfur-rich heterocycles in good to excellent yields, exhibiting gram-scale ability and broad functional group tolerance utilizing easy to handle cheap and easily available reagents. The probable mechanisms for the formation of 1,2- and 1,3-dithioles from α-enolic dithioesters have been suggested.

Full text of DOI:10.1021/acs.joc.6b01802

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Article
Switching Selectivity of alpha-Enolic dithioesters: One
Pot Access to Functionalized 1,2- and 1,3-Dithioles
Suvajit Koley, Tanmoy Chanda, Subhasis Samai, and Maya Shankar Singh
J. Org. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.joc.6b01802 • Publication Date (Web): 04 Nov 2016
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Switching Selectivity of α-Enolic dithioesters: One Pot Access to
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Functionalized 1,2- and 1,3-Dithioles
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Suvajit Koley†, Tanmoy Chanda†, Subhasis Samai‡ and Maya Shankar Singh*,†
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†Department of Chemistry, Institute of Science, Banaras Hindu University, Varanasi 221005,
India
‡Department of Chemistry, University Colleges of Science and Technology, University of
Calcutta, 92, A.P.C. Road, Kolkata 700009, India
Abstract Graphic
ABSTRACT: An operationally simple cascade protocol has been developed for the
construction of 1,2- and 1,3-dithiole derivatives from α-enolic dithioesters. 1,2-Dithioles are
achieved by the reaction of dithioesters with elemental sulfur in the presence of InCl3 under
solvent-free conditions. 1,3-Dithioles have been constructed via DABCO mediated self-
coupling of dithioesters in open air enabling the formation of two new CS bonds and one
ring in a single operation in contiguous fashion. The reactions proceeded smoothly affording
the desired sulfur-rich heterocycles in good to excellent yields, exhibiting gram-scale ability
and broad functional groups tolerance utilizing easy to handle cheap and easily available
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reagents. The probable mechanisms for the formation of 1,2- and 1,3-dithioles from α-enolic
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dithioesters have been suggested.
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Keywords: α-Enolic dithioesters; 1,2- and 1,3-Dithioles; Cascade reaction; DABCO; InCl3;
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Elemental sulfur.
INTRODUCTION
Among the various heterocyclic frameworks,1 heterocycles containing sulfur atom(s) are
versatile privileged scaffolds present in many biologically active molecules and
pharmaceuticals.2 1,2-Dithiole derivatives show many significant pharmacological activities.3
Compounds having 3H-1,2-dithiole-3-thione as a core nucleus exhibit chemotherapeutic,
antioxidant and radio-protective properties.4 Further, dithiolethiones are used as
chemopreventive, choleretic, and sialagogue agents in various bio models.5 On the other
hand, 1,3-dithioles are versatile building blocks that can be employed in many chemical
transformations for the synthesis of natural products.6 Furthermore, 1,3-dithioles have been
widely utilized for the synthesis of organic charge-transfer materials, optical tools, and
electronic conductors.7 In addition to above applications, the 1,2- and 1,3-dithiole derivatives
exhibit anti-HIV activities8,9 and cytoprotective effects in a variety of cell/tissue types and
disease models.10 Besides these properties, many 1,2-dithiole-3-thiones are used as precursor
for the synthesis of vinylogues of tetrathiafulvalene, which amplify non-linear optical (NLO)
property.11 Furthermore, they are used as π-donor moiety for fabricating photoconductive
materials that could be used as electron transport materials for hologram recording.12
Among available reports, the most common methods for the synthesis of 3H-1,2-
dithiole-3-thiones include the reaction of oxoesters with P4S10/Lawesson’s reagent/molecular
sulfur13/β-oxothioic acid or their salts with polysulfanes14/hexamethyldisilathiane (HMDT).15
Timoshenko and co-workers16 synthesized β-bromo-β-trifluoromethyl dithiocrotonic ester by
the reaction of perfluoroketene dithioacetals with sulfur promoted by magnesium bromide
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under solvent-free conditions. However, the reaction required very high temperature (210 ºC)
and excess of MgBr2. In this context, utilization of elemental sulfur, which is cheap,
nontoxic, stable under ambient conditions, easy to handle and readily available in pure form
could be an excellent solution. On the other hand, different strategies have been developed
for the construction of 1,3-dithioles.17 Hartung et al.,18a Gao et al.18b and Voronkov and co-
workers18c synthesized 1,3-dithiole derivatives utilizing external sulfur source such as CS2
and Na2S. Most of the above reactions were performed not only in the presence of hazardous
reagents/solvents at high temperature, but also suffer from one or more limitations such as
lack of generality and substituent compatibility, tedious isolation procedures, expensive and
detrimental metal precursors and unsatisfactory product yield. Therefore, owing to their
immense applications, exploring and improving of new, efficient and general synthetic routes
to access sulfur-rich 1,2-/1,3-dithioles from easily accessible starting materials is still of great
significance.
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Cascade reactions play an important role in organic synthesis and are a powerful tool for
generating molecular complexity and diversity with greater efficiency in a one-pot process.19
In this context, a simple polyfunctional molecule -oxo/α-enolic dithioesters (DTEs) have
drawn significant attention as practical key intermediate in various organic transformations.20
Our own interest in dithiole synthesis derives from our continuous endeavours aimed at
devising new synthetic methods for five and six membered heterocycles employing
organosulfur building blocks like -oxo/α-enolic dithioesters and their newly developed
synthetic variants.21-23 The reactivity of DTEs was demonstrated to access diverse structurally
challenging heterocycles in the authors' laboratory over the past ten years.21,22 Recently, we
reported the synthesis of dithiole motifs employing -enolic dithioesters in good yields.22
This time, herein we disclose a simple and safe new method to access 1,2- and 1,3-dithioles
from dithioesters under solvent-free and metal-free conditions (Scheme 1).
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Scheme 1. Strategies toward 1,2- and 1,3-Dithiole Derivatives
RESULTS AND DISCUSSION
In continuation of our efforts towards the advancement of synthetic strategies to access
heterocyclic scaffolds21-23 using α-enolic dithioesters (DTEs) as a key substrate, herein we
disclose a highly selective construction of 3H-1,2-dithiole-3-thiones 3 and densely
functionalized 2-alkylidene-1,3-dithioles 4 from a common acyclic dithioester precursor 1
simply by varying the easily available reagents (Scheme 2). Intermolecular cross-coupling
reactions for the construction of carbon–sulfur bonds are interesting area in synthetic
chemistry. In recent years, sulfur-mediated/catalyzed/participated reactions have attracted a
great deal of attention in organic synthesis. The simplest and most straightforward synthetic
equivalent of the sulfur synthon should unquestionably be elemental sulfur.24
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R1
Ph
Ph
R2
R1
R2
R1
R2
Me
Et
n-Pr
Me
Me
Me
n-Pr
Me
Et
4-OMeC6H4 i-Bu
2-furyl
2-furyl
2-furyl
2-thienyl
3-pyridyl
1a
1b
1c
1d
1e
1f
1i
1j
1q
1r
1s
1t
1u
1v
4-MeC6H4
4-MeC6H4
4-CF3C6H4
4-ClC6H4
4-BrC6H4
2-naphthyl
1-naphthyl
Me
n-Pr
Me
n-Bu
n-Pent
Me
2-ClC6H4
2-ClC6H4
2-BrC6H4
3-OMeC6H4 Me
3-MeC6H4
1k
1l
1m
1n
1o
1p
Me
ferrocenyl Me
n-Pent
1g
1h
3-OMeC6H4 Me
Et
Scheme 2. Synthesis of 1,2-Dithiole-3-thiones (3) and 1,3-Dithioles (4)
On the basis of literature survey, we started our study by the reaction of 1.0 mmol of
methyl 3-hydroxy-3-(4-methoxyphenyl) prop-2-enedithioate (1h) with 4.0 mmol of elemental
sulfur (2) in 5 mL of acetic acid (AcOH) at reflux temperature. Workup of the reaction
mixture afforded 15% of the desired product 5-(4-methoxyphenyl)-3H-1,2-dithiole-3-thione
(3f), and most of the dithioester remained unconsumed even after 24 h of reflux (Table 1,
entry 1). The above observation was encouraging enough to broaden the optimization studies.
Keeping in mind the benefits of the solvent-free protocol, we performed the above model
reaction under solvent-free conditions at 100 ºC in the presence of 20 mol % of InCl3. To our
great pleasure, the desired product 3f was obtained in 85% yield within 1 h (Table 1, entry 2).
Screening of some other metal promoters such as FeCl3, Sc(OTf)3 and Y(OTf)3 did trigger
the reaction albeit in lower yields (Table 1, entries 3-5). PdCl2 and Cu(OTf)2 provided
reported
one
2-(4-(4-methoxybenzoyl)-5-(methylthio)-3H-1,2-dithiol-3-ylidene)-1-(4-
methoxyphenyl)ethanone22a,b (Table 1, entries 6 and 7).
Table 1. Optimization of Conditions for the Synthesis of 3fa
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entry
promoter
(mmol)
solvent
(5 mL)
AcOH
temp
(ºC)
time yieldb
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(h)
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(%)
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c
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reflux
InCl3 (0.2)c
FeCl3 (0.2)c
Sc(OTf)3 (0.2)c
Y(OTf)3 (0.2)c
PdCl2 (0.2)c
Cu(OTf)2 (0.2)c
InCl3 (0.2)c
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none
HCl
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rt
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0 (66)d
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0 (79)d
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InCl3 (0.2)c
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InCl3 (0.2)c
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InCl3 (0.2)c
90
InCl3 (0.33)c
InCl3 (0.33)
90
1
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c
-
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-
c
-
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a1h (1.0 mmol) was taken as model substrate for optimization. Isolated yield. Reactions
b
c
were carried out with 4 mmol of elemental sulfur. d3H-1,2-dithiol-3-ylidene was obtained.22a,b
Thus, after establishing InCl3 as a choice of promoter, next we optimized its loading,
and temperature of the reaction (Table 1, entries 8-12). InCl3 (20 mol %) at room
temperature and at 50 °C was found to be completely ineffective (Table 1, entries 8 and 9). It
was found that 33 mol % of InCl3 under solvent-free conditions at 90 °C is enough for the
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completion of the reaction and provided the desired product 3f in 95% yield within 1 h (Table
1, entry 12). Since the desired product 3f was obtained in almost quantitative yield under
solvent-free conditions, we further did not screen the solvent for the reaction. Blank reactions
either in the absence of elemental sulfur or without InCl3 did not provide the desired product
even in trace after 24 h, and the starting material remained completely unconsumed (Table 1,
entries 13 and 14). HCl also triggered the reaction affording the desired 1,2-dithiole 3f in
32% yield along with some inseparable side products (Table 1, entry 15). Henceforth, the
optimum condition for the formation of 3f was established as 1.0 mmol of dithioester 1h, 4.0
mmol of elemental sulfur, 33 mol % of InCl3 at 90 °C under solvent-free conditions in open
air (Table 1, entry 12).
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With the optimized reaction conditions in hand, we then set out to explore the scope of
the reaction by subjecting a range of structurally diverse α-enolic dithioesters 1; the results
are summarized in Table 2. The variants of substituents in the phenyl ring (R1 moiety) of
DTEs 1 did not hamper the reaction process, and the approach tolerated DTEs bearing both
electron-donating and electron-withdrawing groups. Various substituents such as OMe, Me,
Cl, Br, and CF3 at ortho, meta, and para-positions of phenyl ring were found to be
compatible (Table 2, 3a-j). Notably, DTEs bearing extended aromatic moiety such as 2-
naphthyl and 1-naphthyl at R1 were also tolerated well and furnished the corresponding 1,2-
dithioles in high yields (Table 2, 3k and 3l). It is noteworthy that DTEs bearing not only
aromatic and extended aromatic R1 moieties but hetero-aromatics such as 2-furyl, 2-thienyl
and 3-pyridyl also worked well under the optimized conditions affording the desired
compounds in high yield (Table 2, 3m, 3n, 3o). Importantly, dithioester 1v bearing ferrocenyl
moiety at R1 is also shown to be suitable substrate and furnished the corresponding 1,2-
dithiole in 91% yield within 1 h (Table 2, 3p).
Table 2. Reaction of DTEs 1 with Elemental Sulfur 2 to Give 1,2-Dithioles 3a
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aAll the reactions were carried out using 1.0 mmol of 1, and 4.0 mmol of 2. bIsolated yield.
After the successful synthesis of 1,2-dithioles and motivated by a report from Li and co-
workers,25 next we envisioned to see the effect of base on the above reaction. Consequently,
InCl3 in above standard reaction was replaced with 33 mol % of K2CO3. After 24 h of heating
under solvent-free conditions, an entirely new spot was observed on the TLC plate, which
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was isolated and characterised as 2-(4-(4-methoxybenzoyl)-5-(methylthio)-1,3-dithiol-2-
ylidene)-1-(4-methoxyphenyl)ethanone (4d) in 20% yield (Table 3, entry 1). This interesting
observation encouraged us to optimize the reaction conditions for the formation of 4d.
Consequently, increasing the amount of K2CO3 increased the yield of 4d to 51% (Table 3,
entries 2 and 3). Next, we performed the model reaction employing various bases in the
absence of sulfur. K2CO3 and KOtBu did mediate the reaction providing the desired product
4d in 51% and 42% yields, respectively (Table 3, entries 4 and 5). KOH and NaOH did not
enable the desired transformation even after 24 h (Table 3, entries 6 and 7). Use of secondary
amine bases like piperidine, pyrrolidine and N-phenylpyrazine furnished the previously
reported corresponding thioamides20d (Table 3, entries 8-10). Screening of tertiary amine
bases such as triethyl amine (TEA), dimethylamino pyridine (DMAP), 1,8-
diazabicyclo[5.4.0]undec-7-ene (DBU) and 1,4-diazabicyclo[2.2.2]octane (DABCO) under
solvent-free conditions promoted the reaction well affording the desired product 4d in 61-
78% yields, respectively (Table 2, entries 11-14). DABCO provided the maximum yield of
4d within minimum time (Table 3, entry 14).
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Next, the model reaction was investigated under different solvents in the presence of
DABCO (1.0 equiv). Polar solvents like EtOH, CH3CN and THF at their reflux temperature
could not improve the result (Table 3, entries 15-17). The reaction in DMF at 90 °C resulted
an inseparable mixture of several products (Table 3, entry 18). Then, we performed the
reaction in a non-polar solvent like toluene at 90 ºC, which gave the desired product 4d in
81% yield (Table 3, entry 19). Pleasingly, increasing the temperature to its reflux, the product
4d was obtained almost in near quantitative yield (90%) within 6 h (Table 3, entry 20).
Further increase of the loading of DABCO could not improve the result (Table 3, entry 21).
Thus, the optimum condition for the formation of 4d was found to be equimolar amount of
DTE and DABCO under refluxing toluene in open atmosphere (Table 3, entry 20).
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Table 3. Optimization of Conditions for the Synthesis of 4da
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entry
promoter
(mmol)
K2CO3 (0.33)c
solvent temp time yieldb
(5 mL) (ºC) (h) (%)
24 20
1
2
3
4
5
6
7
8
9
none
none
none
none
none
none
none
none
none
90
90
90
90
90
90
90
90
90
90
90
90
90
90
K2CO3 (0.5)c
K2CO3 (1.0)c
K2CO3 (1.0)
KOtBu (1.0)
KOH (1.0)
24
15
15
24
24
24
2
30
51
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42
-
NaOH (1.0)
-
d
piperidine (1.0)
pyrrolidine (1.0)
-
d
2
-
d
10 N-phenylpyrazine (1.0) none
2.5
8
-
11 TEA (1.0)
none
none
none
none
EtOH
61
64
69
78
54
48
34
12 DMAP (1.0)
13 DBU (1.0)
10
8
14 DABCO (1.0)
15 DABCO (1.0)
16 DABCO (1.0)
17 DABCO (1.0)
18 DABCO (1.0)
19 DABCO (1.0)
8
reflux 24
CH3CN reflux 24
THF
reflux 24
e
DMF
90
7
-
toluene 90
12
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20 DABCO (1.0)
21 DABCO (1.5)
toluene reflux 6
toluene reflux 6
90
88
7
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b
aDithioester 1h (1.0 mmol) was taken as model substrate for optimization. Isolated yield.
cReactions were carried out with 4.0 mmol of sulfur. dCorresponding thioamide was formed
exclusively.20d eMixtures of several inseparable spots.
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Once the optimal conditions had been identified, the scope of the reaction is
demonstrated by 12 examples presented in Table 4. Our studies revealed that DTEs 1 bearing
different substituents at R1 such as aromatic (bearing both electron-donating and electron-
withdrawing substituents), extended aromatic (2-naphthyl and 1-naphthyl) as well as hetero-
aromatic (2-furyl) were tolerated well and furnished the corresponding 1,3-dithioles in good
to high yields (Table 4, 4a-l). Gratefully, steric bulk posed no problem in this reaction, as
exemplified by products 4j and 4k. Depending on the substitution pattern, the E:Z isomer
ratio (diastereoisomeric ratio) of the products 4a-l ranges from 59:41 to 90:10 (calculated
from HPLC). The highest dr of 90:10 was observed in the reaction with DTE 1f, which gave
4c in 85% yield. However, electron-withdrawing groups at the 4-position of phenyl moiety of
R1 substituent of DTEs 1 accelerate the intermolecular homocoupling reaction (Table 4, 4h),
may be due the fact that the 4-substituted electron-withdrawing groups could stabilize the
intermediate F (Scheme 6b). Further, varying the groups at R2 of dithioesters 1 enabled
structural diversification of product 4. Pleasingly, the groups such as Me, Et, n-Pr, i-Bu, and
n-Pentyl at R2 enabled the reaction to occur smoothly, resulting the corresponding 1,3-
dithiole derivatives 4 in good yields. Various types of -enolic dithioesters are amenable to
this coupling.
Table 4. Substrates Scope for the Synthesis of 4a
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aAll the reactions were carried out using 1.0 mmol of 1 with 1.0 mmol of DABCO in
refluxing toluene in open air. bIsolated yield. n-Pent = Pentyl.
The structures of all the synthesized 1,2-dithioles (3a–p) and 1,3-dithioles (4a–l) were
confirmed by their satisfactory spectral (1H & 13C NMR and HRMS) studies, and
unequivocally established by the X-ray single crystal diffraction analysis of two
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representative molecules 3f and 4a (See SI).26 Thus, by using this one-pot operationally
simple procedure, we have synthesized a small library (28 compounds) of 1,2- and 1,3-
dithioles in good to high yields. The thus obtained sulfur-rich dithioles, bearing additional
sulfur atom as exocyclic/ring substituent with various functional groups could be building
blocks for the construction of biologically active compounds including natural products. In
order to demonstrate the practical application of this method, the reaction of dithioester 1h
(10 mmol) was performed under standard conditions, which afforded the desired products 3f
(2.11 g, 88%) and 4d (1.73 g, 81%) in good yields.
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After the effective application of diverse DTEs 1 for the construction of 1,3-dithioles
4 via homocoupling, next we tried to investigate the possibility of heterocoupling. To this
end, an intermolecular crossover experiment between two different dithioesters 1a and 1h
was performed under the standard conditions. Workup of the reaction afforded exclusive
formation of the corresponding homocoupled products 4a and 4d along with a trace amount
of heterocoupled products 4ad and 4da, which could not be separated (detected from the
mass study) (Scheme 3).
Scheme 3. Exclusive Formation of Homocoupled 1,3-Dithioles 4a and 4d
To further explore the reaction pathway for the synthesis of 1,3-dithioles 4, some
competition experiments were performed. Observing the high preferential homocoupling of
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DTEs 1, we became attracted to establish the order of relative coupling abilities through
competition experiments. Thus, intermolecular competition experiment between two different
dithioesters bearing electron-donating group (EDG) 1h and electron-withdrawing group
(EWG) 1l at R1 moiety was performed (Scheme 4a). Notably, electron-withdrawing
dithioester 1l preferentially converted to the corresponding 1,3-dithiole 4h more efficiently
than the conversion rate of electron-donating dithioester 1h to 4d. From this selectivity
pattern, it could be suggested that EWG in the phenyl ring facilitates the reaction more
prominently than the R1 moiety bearing EDG. Next, we performed the reaction between two
dithioesters having similar R1 moiety but different R2 groups (1a and 1b) under standard
condition. The results showed insignificant selectivity in this case (Scheme 4b). In both the
cases formation of a trace amount of heterocoupled products were observed on TLC but
could not be isolated.
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Scheme 4. Intermolecular Competition Experiments with Different DTEs 1
Following the successful finding on the tuneable synthesis of diverse 1,2- and 1,3-
dithiole derivatives, we designed some control experiments to explore the reaction
mechanism. To shed some light on this cascade strategy, and to check the possibility of
oxidative cyclization that could occur at higher temperature, we treated dithioester 1h with
elemental sulfur under standard reaction condition in the presence of TEMPO (1 equiv). The
reaction proceeded smoothly providing the desired 1,2-dithiole (3f) in 92% isolated yield,
which is comparable to the yield obtained in the absence of TEMPO (95%), suggesting that
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the reaction does not involve the radical cyclization pathway (Scheme 5, eqn. 1). Similarly,
refluxing of DTE 1h in toluene with DABCO in the presence of TEMPO (1 equiv) provided
the desired 1,3-dithiole (4d) in 88% isolated yield (Scheme 5, eqn. 2). Similar results were
obtained in the presence of 4,4′-di-tert-butylbiphenyl also.
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Scheme 5. Mechanistic Studies
Although the mechanism of the transformation is not clear at this moment, on the basis
of present observations and literature reports,22a,27 a plausible reaction scenario is outlined in
Scheme 6. For the formation of 1,2-dithiole, we speculated that at first the enethiol tautomer
of dithioester 1′ reacts with InCl3 generating the intermediate A.27a Intermediate A undergoes
nucleophilic attack by chloride ion to alkyl group attached to sulfur to form intermediate B
with elimination of alkyl chloride, which in turn converted to thermodynamically more
favourable intermediate C. Then intermediate C could react with elemental sulphur inserting
sulfur atom to generate intermediate D, which undergoes intramolecular cyclization to form a
negatively charged tetrahedral intermediate E. Finally, intermediate E produced the desired
1,2-dithiole 3 by the elimination of OInCl2 (Scheme 6a).27b In case of formation of 1,3-
‾
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dithiole (Scheme 6b), the first step could involve the selective nucleophilic attack of the
enethiol sulfur of DTE 1ʹ at the  (sp2) carbon of its second molecule to give -oxoketene
dithioacetal intermediate F. The open-chain intermediate F undergoes intramolecular
cyclization by the elimination of R2SH to form cyclic intermediate G. Subsequent oxidation
of intermediate G gave the desired 1,3-dithiole 4.
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Scheme 6. Probable Mechanistic Routes for 3 and 4
CONCLUSION
In summary, we have developed an operationally simple and efficient one-pot practical
methodology for the synthesis of densely functionalized sulfur-rich 1,2- and 1,3-dithiole
derivatives employing a common acyclic α-enolic dithioester precursor. The outcome of this
cascade reaction was effectively controlled by tailoring the choice of cheap and easily
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available reagents. This one-pot approach involves the formation of new SS and CS bonds
in contiguous fashion involving in situ generated open-chain intermediates followed by
intramolecular heterocyclization. The relevance of this method is demonstrated by
straightforward access to sulfur-rich heterocycles, which are omnipresent structural motifs in
a number of biologically active compounds and functional materials. Switchable selectivity,
non-toxic conditions, methodical simplicity, flexible structural modification, broad substrate
scope and good functional group tolerance make this strategy practical and highly viable for
future applications. The described one pot open-flask cascade chemistry is general and low
cost, making this protocol a good alternative to existing ones. The presence of several
functional groups at various positions of the 1,2- and 1,3-dithioles may be of special interest,
as it could act as an effective chemical handle for further functionalization.
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EXPERIMENTAL SECTION
General Considerations: All the chemicals except α-enolic dithioesters are commercially
available and were used as received. α-Enolic dithioesters were prepared following the
literature procedure.28 Thin-layer chromatography (TLC) was performed using silica gel 60
F254 precoated plates. Column chromatography was performed on silica gel (100-200 mesh).
Infrared (IR) spectra were measured in KBr, and wavelengths (ν) are reported in cm-1. 1H and
13C NMR spectra were recorded on NMR spectrophotometer operating at 300 and 75 MHz,
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respectively. Chemical shifts for H and C NMR spectra are reported as δ in units of parts
per million (ppm) downfield from SiMe4 (δ 0.0). Coupling constant (J) values are given in
Hertz (Hz). HPLC has been done with the help of C18 column using MeOH-CH3CN (1:1)
solvents. HRMS were measured in EI or ESI mode, and the mass analyzer of the HRMS was
TOF. Melting points are uncorrected.
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General Procedure for the Synthesis of 3H-1,2-Dithiole-3-thiones (3a-p). To a mixture
of α-enolic dithioester 1 (1.0 mmol) and sulfur powder 2 (4.0 mmol), InCl3 (0.33 mmol) was
added followed by heating at 90 °C till the completion of the reaction. After completion of
the reaction (monitored by TLC), water (20 mL) was added to the reaction mixture followed
by extraction with ethyl acetate (2 × 10 mL). The combined organic layer was dried over
anhydrous Na2SO4 and evaporated in vacuum. The crude residue was purified by column
chromatography over silica gel using ethyl acetate/hexane as eluent to afford pure 3H-1,2-
dithiole-3-thiones 3.
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Characterization Data of the Isolated Compounds:
5-Phenyl-3H-1,2-dithiole-3-thione (3a). Eluent composition: 2% ethyl acetate : n-hexane;
Yield: 92% (193 mg), Red solid, mp 125-126 C (lit.29a mp 126 C); FT IR (KBr, cm-1):
3421, 2922, 1663, 1574, 1203, 1062; 1H NMR (300 MHz, CDCl3): δ 7.64 (d, J = 8.1 Hz, 2H),
7.58-7.45 (m, 3H), 7.42 (s, 1H); 13C NMR (75 MHz, CDCl3): δ 215.4, 172.8, 135.8, 132.0,
131.5, 129.5, 126.8.
5-(2-Chlorophenyl)-3H-1,2-dithiole-3-thione (3b). Eluent composition: 2% ethyl acetate : n-
hexane; Yield: 89% (217 mg), Red solid, mp 128-129 °C (lit.29b mp 129 °C); FT IR (KBr,
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cm-1): 3359, 2917, 1665, 1416, 1273, 1072; H NMR (300 MHz, CDCl3): δ 7.47-7.43 (m,
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2H), 7.39-7.23 (m, 3H); C NMR (75 MHz, CDCl3): δ 215.1, 168.9, 140.0, 132.3, 132.0,
130.8, 130.5, 129.9, 127.3.
5-(2-Bromophenyl)-3H-1,2-dithiole-3-thione (3c). Eluent composition: 3% ethyl acetate : n-
hexane; Yield: 85% (244 mg), Red solid, mp 92-94 °C; FT IR (KBr, cm-1): 3421, 2969, 1672,
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1416, 1269, 1096; H NMR (300 MHz, CDCl3): δ 7.62 (d, J = 6.9 Hz, 1H), 7.43-7.17 (m,
4H); 13C NMR (75 MHz, CDCl3): δ 215.1, 170.6, 140.1, 134.0, 132.0, 131.8, 130.7, 127.8,
121.8. HRMS [M+H]+ calcd. For C9H6BrS3 288.8815, found 288.8817.
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5-(3-Methoxyphenyl)-3H-1,2-dithiole-3-thione (3d). Eluent composition: 5% ethyl acetate :
n-hexane; Yield: 88% (211 mg), Red solid, mp 113-114 °C (lit.29a mp 114 °C); FT IR (KBr,
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cm-1): 3087, 2947, 1640, 1445, 1271, 1086; H NMR (300 MHz, CDCl3): δ 7.39-7.35 (m,
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2H), 7.22 (s, 1H), 7.11-7.06 (m, 2H), 3.85 (s, 3H); C NMR (75 MHz, CDCl3): δ 215.2,
172.6, 160.0, 135.8, 132.6, 130.5, 119.1, 117.6, 112.1, 55.4.
5-(3-Tolyl)-3H-1,2-dithiole-3-thione (3e). Eluent composition: 2% ethyl acetate : n-hexane;
Yield: 83% (186 mg), Red liquid; FT IR (KBr, cm-1): 3113, 2921, 1618, 1461, 1277, 1092;
1H NMR (300 MHz, CDCl3): δ 7.45-7. 43 (m, 2H), 7.41 (s, 1H), 7.36-7.34 (m, 2H), 2.41 (s,
3H); 13C NMR (75 MHz, CDCl3): δ 215.4, 173.1, 139.5, 135.7, 132.9, 131.5, 129.4, 127.4,
124.0, 21.3; HRMS [M+H]+ calcd. For C10H9S3 224.9866, found 224.9866.
5-(4-Methoxyphenyl)-3H-1,2-dithiole-3-thione (3f). Eluent composition: 5% ethyl acetate :
n-hexane; Yield: 95% (228 mg), Red solid, mp 110-111 °C (lit.15 mp 111 °C); FT IR (KBr,
cm-1): 3446, 2924, 1619, 1459, 1259, 1083; H NMR (300 MHz, CDCl3): δ 7.61 (d, J = 8.7
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Hz, 2H), 7.38 (s, 1H), 6.97 (d, J = 8.4 Hz, 2H), 3.80 (s, 3H); 13C NMR (75 MHz, CDCl3): δ
215.0, 173.0, 162.8, 134.5, 128.5, 124.0, 114.9, 55.5.
5-(4-Tolyl)-3H-1,2-dithiole-3-thione (3g). Eluent composition: 2% ethyl acetate : n-hexane;
Yield: 93% (208 mg), Red solid, mp 118-119 °C (lit.29a mp 119 °C); FT IR (KBr, cm-1):
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3447, 2922, 1601, 1415, 1275, 1072; H NMR (300 MHz, CDCl3): δ 7.43 (br, 2H), 7.31 (s,
1H), 7.17 (br, 2H), 2.32 (s, 3H); 13C NMR (75 MHz, CDCl3): δ 215.2, 173.1, 143.0, 135.2,
130.1, 128.7, 126.6, 21.5.
5-(4-(Trifluoromethyl)phenyl)-3H-1,2-dithiole-3-thione (3h). Eluent composition: 3% ethyl
acetate : n-hexane; Yield: 85% (236 mg), Red liquid; FT IR (KBr, cm-1): 3437, 2969, 1672,
1416, 1269, 1096; 1H NMR (300 MHz, CDCl3): δ 7.79-7.73 (m, 4H), 7.43 (s, 1H); 13C NMR
(75 MHz, CDCl3): δ 215.4, 170.1, 136.8 (q, J = 1.4 Hz), 133.5 (q, J = 32.9 Hz), 130.1, 127.2,
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126.5 (q, J = 3.6 Hz), 123.3 (q, J = 270.7 Hz); HRMS [M+H]+ calcd. For C10H6F3S3
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278.9584, found 278.9574.
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5-(4-Chlorophenyl)-3H-1,2-dithiole-3-thione (3i). Eluent composition: 2% ethyl acetate : n-
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hexane; Yield: 83% (203 mg), Red solid, mp 135-136 °C (lit.29a mp 136 °C); FT IR (KBr,
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cm-1): 3359, 2923, 1607, 1426, 1246, 1097; H NMR (300 MHz, CDCl3): δ 7.51 (d, J = 8.4
Hz, 2H), 7.37 (d, J = 8.4 Hz, 2H), 7.18 (s, 1H); 13C NMR (75 MHz, CDCl3): δ 215.2, 171.0,
138.3, 135.9, 129.9, 129.8, 127.9.
5-(4-Bromophenyl)-3H-1,2-dithiole-3-thione (3j). Eluent composition: 3% ethyl acetate : n-
hexane; Yield: 81% (234 mg), Red solid, mp 128-129 °C (lit.29a mp 129 °C); FT IR (KBr,
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cm-1): 3341, 2916, 1679, 1464, 1263, 1083; H NMR (300 MHz, CDCl3): δ 7.54 (d, J = 8.4
Hz, 2H), 7.48-7.42 (m, 2H), 7.31 (s, 1H); 13C NMR (75 MHz, CDCl3): δ 215.3, 171.0, 136.0,
132.8, 130.4, 128.1, 126.7.
5-(Naphthalen-2-yl)-3H-1,2-dithiole-3-thione (3k). Eluent composition: 2% ethyl acetate :
n-hexane; Yield: 91% (236 mg), Red solid, mp 68-71 °C; FT IR (KBr, cm-1): 3440, 2957,
1628, 1470, 1243, 1077; 1H NMR (300 MHz, CDCl3): δ 8.18 (d, J = 12.0 Hz, 1H), 7.92-7.85
(m, 3H), 7.65-7.54 (m, 4H); 13C NMR (75 MHz, CDCl3): δ 215.3, 172.7, 136.0, 134.7, 132.9,
129.5, 128.9, 128.8, 128.3, 127.9, 127.5, 127.1, 123.3; HRMS [M+H]+ calcd. For C13H9S3
260.9866, found 260.9868.
5-(Naphthalen-1-yl)-3H-1,2-dithiole-3-thione (3l). Eluent composition: 2% ethyl acetate : n-
hexane; Yield: 80% (208 mg), Red solid, mp 140-141 °C (lit.29c 140-142 °C); FT IR (KBr,
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cm-1): 3394, 2967, 1624, 1475, 1275, 1032; H NMR (300 MHz, CDCl3): δ 8.04-7.91 (m,
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3H), 7.64-7.47 (m, 2H), 7.32 (s, 1H); C NMR (75 MHz, CDCl3): δ 215.4, 171.6, 139.8,
135.5, 131.6, 130.0, 128.5, 128.4, 127.7, 127.6, 126.8, 124.9, 124.3.
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5-(Furan-2-yl)-3H-1,2-dithiole-3-thione (3m). Eluent composition: 3% ethyl acetate : n-
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hexane; Yield: 88% (176 mg), Red solid, mp 109-110 °C (lit.29d mp 110 °C); FT IR (KBr,
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cm-1): 3433, 2924, 1601, 1486, 1231, 1053; H NMR (300 MHz, CDCl3): δ 7.63 (d, J = 1.5
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Hz, 1H), 7.34 (s, 1H), 6.98 (d, J = 3.6 Hz, 1H), 6.59 (d, J1 = 1.5 Hz, J2 = 3.3 Hz, 1H); 13C
NMR (75 MHz, CDCl3): δ 214.0, 159.9, 146.2, 132.8, 113.3, 113.2, 113.1.
5-(Thiophen-2-yl)-3H-1,2-dithiole-3-thione (3n). Eluent composition: 2% ethyl acetate : n-
hexane; Yield: 82% (177 mg), Red solid, mp 129-130 °C (lit.29b mp 129 °C); FT IR (KBr,
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cm-1): 3430, 2923, 1636, 1471, 1278, 1071; H NMR (300 MHz, CDCl3): δ 7.58 (d, J = 5.1
Hz, 1H), 7.53 (d, J = 3.6 Hz, 1H), 7.32 (s, 1H), 7.14 (t, J = 2.2 Hz, 1H); 13C NMR (75 MHz,
CDCl3): δ 214.4, 165.0, 134.6, 133.8, 130.9, 129.1, 129.0.
5-(Pyridin-3-yl)-3H-1,2-dithiole-3-thione (3o). Eluent composition: 25% ethyl acetate : n-
hexane; Yield: 86% (181 mg), Red solid, mp 162-164 °C; FT IR (KBr, cm-1): 3444, 2927,
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1659, 1425, 1275, 1095; H NMR (300 MHz, CDCl3): δ 8.93 (s, 1H), 8.78 (d, J = 4.2 Hz,
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1H), 7.95 (dd, J1 = 1.2 Hz, J2 = 9.3 Hz, 1H), 7.47-7.43 (m, 2H); C NMR (125 MHz,
CDCl3): δ 215.5, 168.6, 152.8, 147.3, 136.7, 134.1, 128.0, 124.2; HRMS [M+H]+ calcd. For
C8H6NS3 211.9662, found 211.9663.
5-Ferrocenyl-3H-1,2-dithiole-3-thione (3p). Eluent composition: 4% ethyl acetate : n-
hexane; Yield: 91% (289 mg), Red solid, mp 158-159 °C (lit.13b mp 157-159 °C); FT IR
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(KBr, cm-1): 3436, 2956, 1648, 1448, 1247, 1070; H NMR (300 MHz, CDCl3): δ 7.20 (s,
1H), 4.71 (br, 1H), 4.60 (br, 2H), 4.23 (br, 4H); 13C NMR (75 MHz, CDCl3): δ 214.4, 176.6,
133.6, 133.5, 74.9, 72.2, 71.19, 71.13, 68.7, 68.6.
General Procedure for the Synthesis of 1,3-Dithiol-2-ylidene (4a-l). To a mixture of
α-enolic dithioester 1 (1.0 mmol) and DABCO (1.0 mmol), 5 mL of toluene was added and
the reaction mixture was heated at 110 °C till the completion of the reaction. After
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completion of the reaction (monitored by TLC), the solvent was evaporated under vacuum
and then water (20 mL) was added to the reaction mixture followed by extraction with ethyl
acetate (2 × 10 mL). The combined organic layer was dried over anhydrous Na2SO4 and then
evaporated in vacuum. The crude residue was purified by column chromatography over silica
gel using ethyl acetate/hexane as eluent to afford a mixture of E- and Z-isomers of
trisubstituted 1,3-dithioles 4.
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2-(4-Benzoyl-5-(methylthio)-1,3-dithiol-2-ylidene)-1-phenylethanone (4a). (Mixture of E-
and Z-isomers) dr 82:18, Eluent composition: 5% ethyl acetate : n-hexane; Yield: 90% (166
1
mg), Yellow solid, FT IR (KBr, cm-1): 3424, 2946, 1610, 1432, 1235, 1088; H NMR (300
MHz, CDCl3): δ 7.96-7.92 (m, 2H), 7.84-7.77 (m, 2H), 7.59-7.37 (m, 7H), 2.72, 2.62 (2s,
3H); 13C NMR (125 MHz, CDCl3): δ 185.7, 184.7, 160.9, 151.9, 143.6, 138.7, 138.6, 137.5,
133.6, 132.8, 132.3, 132.2, 131.1, 129.9, 129.6, 129.5, 129.1, 129.0, 128.7, 128.6, 128.3,
127.79, 127.72, 127.5, 126.9, 126.1, 121.1, 115.8, 104.4, 103.6, 18.8, 18.3; HRMS [M+H]+
calcd. For C19H15O2S3 371.0234, found 371.0256.
2-(4-Benzoyl-5-(propylthio)-1,3-dithiol-2-ylidene)-1-phenylethanone (4b). (Mixture of E-
and Z-isomers) dr 87:13, Eluent composition: 5% ethyl acetate : n-hexane; Yield: 82% (163
1
mg), Sticky solid; FT IR (KBr, cm-1): 3435, 2929, 1654, 1447, 1254, 1079; H NMR (300
MHz, CDCl3): δ 7.96-7.91 (m, 1H), 7.84-7.74 (m, 2H), 7.57-7.36 (m, 8H), 3.15-2.94 (m, 2H),
1.78-1.71 (m, 2H), 1.05-0.82 (m, 3H); 13C NMR (75 MHz, CDCl3): δ 185.8, 184.5, 161.1,
138.2, 137.4, 137.3, 132.88, 132.83, 132.1, 131.9, 129.7, 128.8, 128.6, 128.56, 128.50, 128.4,
127.6, 127.5, 127.3, 104.2, 103.3, 37.7, 37.1, 22.7, 22.6, 13.2; HRMS [M+H]+ calcd. For
C21H19O2S3 399.0547, found 399.0550.
2-(4-(3-Methoxybenzoyl)-5-(methylthio)-1,3-dithiol-2-ylidene)-1-(3-methoxyphenyl)ethano-
ne (4c). (Mixture of E- and Z-isomers) dr 90:10, Eluent composition: 5% ethyl acetate : n-
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hexane; Yield: 85% (183 mg), Sticky solid; FT IR (KBr, cm-1): 3081, 2923, 1603, 1410,
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1262, 1057; 1H NMR (300 MHz, CDCl3): δ 7.51-7.43 (m, 2H), 7.41-7.33 (m, 4H), 7.16-7.05
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(m, 3H), 3.86, 3.85, 3.83, 3.80 (four s, 6H), 2.71, 2.59 (two s, 3H); C NMR (75 MHz,
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CDCl3): δ 185.1, 184.3, 160.8, 159.8, 159.6, 139.7, 138.7, 129.5, 129.4, 120.8, 120.5, 119.9,
119.8, 119.1, 119.0, 118.8, 118.6, 118.2, 112.8, 112.7, 111.9, 111.8, 104.2, 103.6, 55.3, 55.2,
18.6, 18.1; HRMS [M+H]+ calcd. For C21H19O4S3 431.0445, found 431.0437.
2-(4-(4-Methoxybenzoyl)-5-(methylthio)-1,3-dithiol-2-ylidene)-1-(4-methoxyphenyl)ethano-
ne (4d). (Mixture of E- and Z-isomers) dr 59:41, Eluent composition: 8% ethyl acetate : n-
hexane; Yield: 90% (193 mg), Yellow solid, FT IR (KBr, cm-1): 3422, 2922, 1630, 1439,
1
1232, 1116; H NMR (300 MHz, CDCl3): δ 7.95-7.80 (m, 4H), 7.32, 7.26 (two s, 1H), 6.97-
6.91 (m, 4H), 3.88, 3.85, 3.85, 3.81 (four s, 6H), 2.66, 2.56 (two s, 3H); 13C NMR (75 MHz,
CDCl3): δ 184.5, 183.5, 163.56, 163.51, 162.8, 162.7, 160.17, 160.10, 132.4, 132.0, 131.2,
131.0, 130.7, 130.4, 130.2, 130.0, 129.7, 129.6, 129.4, 128.3, 128.1, 114.7, 114.2, 113.85,
113.81, 104.0, 103.3, 55.4, 55.3, 18.7, 18.2; HRMS [M+H]+ calcd. For C21H19O4S3 431.0445,
found 431.0471.
2-(4-(Isobutylthio)-5-(4-methoxybenzoyl)-1,3-dithiol-2-ylidene)-1-(4-methoxyphenyl)ethan-
one (4e). (Mixture of E- and Z-isomers) dr 83:17, Eluent composition: 7% ethyl acetate : n-
hexane; Yield: 78% (184 mg), Sticky solid; FT IR (KBr, cm-1): 3436, 2924, 1678, 1496,
1
1252, 1066; H NMR (300 MHz, CDCl3): δ 7.95-7.81 (m, 4H), 7.32, 7.31 (two s, 1H), 6.96-
6.92 (m, 4H), 3.87, 3.84, 3.83, 3.79 (four s, 6H), 2.97, 2.82 (two d, J = 6.9 Hz, 2H), 1.96-1.84
(m, 1H), 1.10, 1.01 (two d, J = 6.6 Hz, 6H); 13C NMR (75 MHz, CDCl3): δ 184.8, 183.3,
163.66, 163.62, 162.7, 162.6, 160.4, 145.0, 132.3, 131.4, 131.3, 131.2, 130.5, 130.4, 130.3,
130.2, 129.69, 129.60, 129.3, 124.4, 113.8, 113.7, 104.0, 103.1, 55.4, 55.3, 44.6, 43.9, 28.7,
21.76, 21.71; HRMS [M+H]+ calcd. For C24H25O4S3 473.0915, found 473.0918.
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2-(4-(4-Methylbenzoyl)-5-(methylthio)-1,3-dithiol-2-ylidene)-1-(p-tolyl)ethanone
(4f).
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(Mixture of E- and Z-isomers) dr 84:16, Eluent composition: 6% ethyl acetate : n-hexane;
Yield: 74% (147 mg), Sticky solid; FT IR (KBr, cm-1): 3337, 2919, 1677, 1475, 1263, 1057;
1H NMR (300 MHz, CDCl3): δ 7.79-7.61 (m, 3H), 7.27-7.16 (m, 6H), 2.62, 2.51 (two s, 3H),
2.35, 2.34, 2.33, 2.27 (four s, 6H); 13C NMR (75 MHz, CDCl3): δ 184.4, 184.0, 160.4, 143.7,
142.9, 135.7, 134.8, 130.0, 129.7, 129.35.129.30, 129.2, 128.8, 128.7, 128.5, 127.7, 127.6,
127.5, 121.4, 104.2, 103.4, 21.6, 21.5, 18.7, 18.2; HRMS [M+H]+ calcd. For C21H19O2S3
399.0547, found 399.0576.
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2-(4-(4-Methylbenzoyl)-5-(propylthio)-1,3-dithiol-2-ylidene)-1-(p-tolyl)ethanone
(4g).
(Mixture of E- and Z-isomers) dr 79:21, Eluent composition: 5% ethyl acetate : n-hexane;
Yield: 70% (150 mg), Sticky solid; FT IR (KBr, cm-1): 3426, 2922, 1618, 1471, 1236, 1067;
1H NMR (300 MHz, CDCl3): δ 7.87-7.67 (m, 4H), 7.36-7.15 (m, 5H), 3.10-2.95 (m, 2H),
2.42, 2.41, 2.40, 2.34 (4s, 6H), 1.79-1.68 (m, 2H), 1.11-0.88 (m, 3H); 13C NMR (125 MHz,
CDCl3): δ 192.1, 190.6, 185.9, 184.5, 170.3, 163.5, 161.1, 147.2, 145.7, 144.1, 143.8, 142.9,
142.8, 142.4, 141.2, 137.0, 135.9, 135.6, 135.4, 135.1, 135.0, 134.7, 133.6, 133.4, 130.1,
129.8, 129.4, 129.2, 129.1, 128.9, 128.7 127.8, 127.6, 123.9, 106.5, 104.3, 37.9, 37.3, 22.9,
22.6, 21.8, 21.7, 13.4, 13.2; HRMS [M+H]+ calcd. For C23H23O2S3 427.0860, found
427.0890.
2-(4-(Methylthio)-5-(4-(trifluoromethyl)benzoyl)-1,3-dithiol-2-ylidene)-1-(4-
(trifluoromethyl)phenyl)ethanone (4h). (Mixture of E- and Z-isomers) dr 82:18, Eluent
composition: 7% ethyl acetate : n-hexane; Yield: 92% (232 mg), Yellow solid, FT IR (KBr,
1
cm-1): 3351, 2926, 1684, 1478, 1236, 1071; H NMR (300 MHz, CDCl3): δ 8.05-8.01 (m,
2H), 7.92-7.85 (m, 2H), 7.77-7.70 (m, 4H), 7.38, 7.36 (two s, 1H), 2.76, 2.66 (two s, 3H); 13C
NMR (125 MHz, CDCl3): δ 185.1, 184.3, 183.4, 182.9, 161.9, 161.7, 141.7, 140.3, 134.2,
134.0, 133.8, 133.7, 133.6, 133.4, 130.6, 128.8, 128.7, 128.5, 128.0, 127.9, 125.9, 125.89,
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125.85, 125.83, 125.6, 124.8, 124.6, 122.6, 122.4, 104.3, 103.6, 18.8, 18.3; HRMS [M+H]+
calcd. For C21H13F6O2S3 506.9982, found 507.0005.
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2-(4-(4-Bromobenzoyl)-5-(pentylthio)-1,3-dithiol-2-ylidene)-1-(4-bromophenyl)ethanone
(4i). (Mixture of E- and Z-isomers) dr 78:22, Eluent composition: 6% ethyl acetate : n-
hexane; Yield: 81% (236 mg), Yellow solid, FT IR (KBr, cm-1): 2923, 2853, 1630, 1480,
1230, 1069; 1H NMR (300 MHz, CDCl3): δ 7.82-7.77 (m, 2H), 7.73-7.49 (m, 6H), 7.30, 7.26
(two s, 1H), 3.18-2.98 (m, 2H), 1.75-1.66 (m, 2H), 1.41-1.29 (m, 4H), 0.90 (t, J = 7.0 Hz,
3H); 13C NMR (75 MHz, CDCl3): δ 185.4, 183.4, 161.6, 136.2, 132.4, 131.97, 131.92,
131.91, 132.1, 131.2, 130.4, 130.1, 130.0, 129.18, 129.11, 128.9, 127.2, 103.9, 103.1, 36.0,
35.4, 30.7, 28.8, 22.1, 13.8; HRMS [M+H]+ calcd. For C23H21Br2O2S3 584.9050, found
584.9060.
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2-(4-(2-Naphthoyl)-5-(methylthio)-1,3-dithiol-2-ylidene)-1-(naphthalen-2-yl)ethanone (4j).
(Mixture of E- and Z-isomers) dr 83:17, Eluent composition: 6% ethyl acetate : n-hexane;
Yield: 79% (186 mg), Sticky solid; FT IR (KBr, cm-1): 3446, 2923, 1633, 1485, 1223,1087;
1H NMR (300 MHz, CDCl3): δ 8.43-8.32 (m, 2H), 7.98-7.83 (m, 7H), 7.57-7.50 (m, 5H),
7.29, 7.23 (two s, 1H), 2.68, 2.57 (two s, 3H); 13C NMR (75 MHz, CDCl3): δ 185.4, 184.4,
160.7, 143.4, 135.6, 135.2, 135.1, 134.7, 133.9, 132.9, 132.6, 132.2, 131.7, 129.7, 129.6,
129.3, 129.1, 128.79, 128.73, 128.6, 128.5, 128.4, 128.3, 128.2, 128.0, 127.8, 127.7, 127.2,
127.0, 126.8, 126.6, 126.4, 124.6, 124.2, 123.8, 123.2, 115.9, 104.5, 18.7, 18.2; HRMS
[M+H]+ calcd. For C27H19O2S3 471.0547, found 471.0570.
2-(4-(1-Naphthoyl)-5-(ethylthio)-1,3-dithiol-2-ylidene)-1-(naphthalen-1-yl)ethanone (4k).
(Mixture of E- and Z-isomers) dr 87:13, Eluent composition: 5% ethyl acetate : n-hexane;
Yield: 74% (180 mg), Sticky solid; FT IR (KBr, cm-1): 3424, 2924, 1627, 1478, 1231, 1116;
1H NMR (300 MHz, CDCl3): δ 8.79, 8.36 (two d, J = 8.1 Hz, 2H), 8.08-7.89 (m, 7H), 7.66-
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7.22 (m, 8H), 7.15-6.99 (m, 1H), 3.22-3.06 (m, 2H); 13C NMR (75 MHz, CDCl3): δ 192.8,
186.8, 170.6, 136.9, 134.8, 134.2, 134.1, 134.0, 133.7, 131.7, 131.2, 130.7, 130.3, 128.7,
128.6, 128.4, 128.2, 127.5, 127.2, 127.0, 126.8, 126.3, 126.1, 125.6, 125.5, 124.9, 124.7,
124.6, 124.3, 110.9, 108.6, 30.1, 14.4, 14.1; HRMS [M+H]+ calcd. For C28H21O2S3 485.0704,
found 485.0705.
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2-(4-(Furan-2-carbonyl)-5-(propylthio)-1,3-dithiol-2-ylidene)-1-(furan-2-yl)ethanone (4l).
(Mixture of E- and Z-isomers) dr 88:12, Eluent composition: 6% ethyl acetate : n-hexane;
Yield: 81% (153 mg), Sticky solid; FT IR (KBr, cm-1): 3426, 2924, 1661, 1465, 1258, 1076;
1H NMR (300 MHz, CDCl3): δ 7.71-7.66 (m, 1H), 7.56-7.38 (m, 2H), 7.28-6.97 (m, 2H),
6.61-6.49 (m, 2H), 3.22-3.00 (m, 2H), 1.90-1.71 (m, 2H), 1.15-0.97 (m, 3H); 13C NMR (125
MHz, CDCl3): δ 178.6, 177.0, 174.2, 174.1, 170.7, 169.3, 164.2, 161.4, 154.9, 153.3, 153.2,
152.5, 151.9, 151.8, 148.5, 146.7, 146.5, 145.5, 145.3, 143.3, 139.4, 135.9, 132.8, 122.0,
119.7, 119.2, 118.0, 116.0, 115.4, 114.8, 113.1, 113.0, 112.6, 112.5, 112.4, 106.0, 103.9,
103.5, 39.8, 38.1, 22.8, 22.6, 13.5, 13.2; HRMS [M+H]+ calcd. For C17H15O4S3 379.0132,
found 379.0158.
Experimental Details for the Crossover Experiment:
A mixture of α-enolic dithioester 1a (0.5 mmol) and 1h (0.5 mmol) was dissolved in 5 mL of
toluene followed by addition of DABCO (1.0 mmol). The reaction mixture was heated at
110 °C till the completion of the reaction. After completion of the reaction (monitored by
TLC), the solvent was evaporated under vacuum and then water (20 mL) was added to the
reaction mixture followed by extraction with ethyl acetate (2 × 10 mL). The combined
organic layer was dried over anhydrous Na2SO4 and then evaporated under vacuum. From the
crude residue major and separable spots were purified by column chromatography over silica
gel using ethyl acetate/hexane as eluent.
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ASSOCIATED CONTENT
Supporting Information. Full experimental details, analytical and spectroscopic data (copies
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of H and C NMR). X-ray structures and crystallographic information files (PDF & CIF).
This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*M.S.S.: fax, (+91)-542-236-8127; tel, (+91)-542-670-2502;
E-mail: mayashankarbhu@gmail.com; mssingh@bhu.ac.in
Notes
The authors declare no competing financial interest.
ACKNOWLEDGEMENTS
M.S.S. is grateful to Prof. H. Ila, JNCASR, Bangalore for her discussion and suggestions.
The authors gratefully acknowledge the generous financial support from the Science and
Engineering Research Board, New Delhi (SB/S1/OC-30/2013) and the Council of Scientific
and Industrial Research (02(0072)/12/EMR-II), New Delhi.
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