Visible-Light-Induced Photocatalytic Synthesis of β-Keto Dithiocarbamates via Difunctionalization of Styrenes

Article abstract of DOI:10.1021/acs.orglett.1c01059

A facile photocatalyzed strategy for difunctionalization of styrenes in the presence of CS2 and amines providing β-keto dithiocarbamates has been developed. In the case of 4-nitrostyrene and 2-vinylpyridine, however, only 2-arylethylthiocarbamates are interestingly formed without the aid of photoredox catalysis/TBHP.

Full text of DOI:10.1021/acs.orglett.1c01059

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Letter
Visible-Light-Induced Photocatalytic Synthesis of β‑Keto
Dithiocarbamates via Difunctionalization of Styrenes
Ramesh Kumar Vishwakarma, Saurabh Kumar, and Krishna Nand Singh*
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ABSTRACT: A facile photocatalyzed strategy for difunctionalization of styrenes in the presence of CS2 and amines providing β-keto
dithiocarbamates has been developed. In the case of 4-nitrostyrene and 2-vinylpyridine, however, only 2-arylethylthiocarbamates are
interestingly formed without the aid of photoredox catalysis/TBHP.
he difunctionalization approach has attracted a great deal
Tof attention in organic synthesis owing to its excellent
regioselectivity and good functional group tolerance.1−3
Simultaneous introduction of two functional groups onto a
carbon−carbon double bond paves an exigent way for creating
complex molecules and a new window for the formation of
unconventional bonds by simple procedures. Lately, styrenes
have been extensively used as valuable and readily available
building blocks of many natural products, pharmaceuticals, and
materials.4 Two exemplary difunctionalization reactions of
styrenes to form C−O and C−S bonds are shown in Scheme
1,4d,f yet to the best of our knowledge, there exists no report on
the synthesis of β-keto dithiocarbamates using styrenes.
Organic dithiocarbamates constitute an important class of
sulfur-containing compounds having potential biological
activities (Figure 1).5 Besides their phenomenal use in cancer
treatment,6 they play a key role in agriculture chemistry,7 the
rubber industry,8 and solid-phase organic synthesis.9 They also
Figure 1. Some dithiocarbamates with biological activities.
Scheme 1. Difunctionalization of Styrenes Creating C−O
and C−S Bonds
act as a versatile intermediate in organic synthesis.10,11
Different methods are known for the synthesis of dithiocarba-
mates using alkyl halide,12 aryl halide,13 and other substrates,14
including the synthesis of 2-aryl-2-hydroxyethyl dithiocarba-
mates via regioselective addition of tetraalkylthiuram disulfides
to styrenes.4f
Received: March 28, 2021
Published: May 14, 2021
https://doi.org/10.1021/acs.orglett.1c01059
© 2021 American Chemical Society
Org. Lett. 2021, 23, 4147−4151
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a
Mechanistically, the difunctionalization of styrenes involves
either transition-metal catalysis or radical-mediated pro-
cesses,15,16 of which the later one concerning visible light
photoredox catalysis via single electron transfer is more
captivating. Photoredox catalysis has recently stimulated
organic chemists to ensure sustainable, facile, and rapid entry
to the valued products under mild conditions.17,18 Visible-light
activation of traditional substrates to reactive intermediates at
room temperature has influenced various functionalization
reactions that otherwise require high energy influx. Another
striking change manifests the use of organic photoredox
catalysts as an alternative to transition metal (Ru and Ir) based
photocatalysts due to its low cost, high solubility, and eco-
safety.19
In view of the above and considering the importance of
multicomponent reactions (MCRs),20 we report herein an
efficient visible-light-promoted synthesis of β-keto dithiocarba-
mates by the MCR of styrenes, carbon disulfide, and secondary
amines in the presence of rhodamine B base as organic
photoredox catalyst under open air atmosphere at room
temperature (cf. Scheme 1, Present work).
To comprehend our idea to achieve the synthesis of β-keto
dithiocarbamates, an archetypal reaction employing styrene
(1a), carbon disulfide (2), and pyrrolidine (3a) was studied in
detail. To begin, the reactants 1a, 2, and 3a were irradiated
with blue LED (470 nm) as the visible light source in the
presence of eosin Y (5 mol %) as photocatalyst and K2S2O8 as
oxidant in dimethylformamide (DMF), which gave rise to the
desired product 2-oxo-2-phenylethylpyrrolidine-1-carbodi-
thioate 4a in 30% yield (Table 1, entry 1). Prompted by this
initial observation, the model reaction was thoroughly
optimized by varying different parameters such as visible-
light source, photocatalyst, oxidant, and solvent (Table 1). The
use of other photocatalysts like rose bengal and rhodamine B
base improved the product yield to 58 and 70% respectively
(Table 1, entries 2 and 3). Markedly, the reaction offered 15%
product yield in the absence of the photocatalyst, no product
without oxidant, and just 5% yield without LED source and
photocatalyst (Table 1, entries 4−6). The use of white LED
and green LED (530 nm) also remained inferior (entries 7 and
8). The proficiency of other oxidants such as aq TBHP, DTBP,
and Na2S2O8 was also examined (entries 9−11), of which aq
TBHP enhanced the product yield significantly (78%, entry 9).
The effect of aq TBHP concentration was thereafter
scrutinized, which revealed no further improvement in product
yield with 3 equiv of aq TBHP, while use of 1 equiv of aq
TBHP reduced the yield to 51%. As regards the effect of other
solvents viz. CH3CN, DMSO, DMA, toluene, ethanol, DCE,
and DMF−water mixture (3:1) on the reaction (entries 12−
18), none of them could surpass the value of DMF (entry 9).
Keeping all other parameters of entry 9 as such, the
photocatalyst concentration was then examined, which
disclosed 3 mol % as optimum and entry 19 as the best fit.
The scope and versatility of the reaction were subsequently
examined under the established conditions (Scheme 2). A
diverse array of styrenes containing electron-donating groups
such as Me, tBu, Ph, and MeO, as well as electron-withdrawing
groups such as F, Cl, Br, and CH3COO, participated nicely
with CS2 and amines under the optimized conditions to afford
the predictable products 4a−4ae in moderate to good yields.
Notably 2-vinylnaphthalene also worked well to afford the
dithiocarbamates 4r and 4s. A sterically hindered 2,4,6-
Table 1. Optimization of the Reaction Conditions
b
yield
(%)
entry photocatalyst
oxidant
light
solvent
1
2
3
4
5
6
7
8
eosin Y
rose bengal
rhodamine B
K2S2O8
K2S2O8
K2S2O8
K2S2O8
blue
blue
blue
blue
blue
DMF
DMF
DMF
DMF
DMF
DMF
30
58
70
15
0
rhodamine B
K2S2O8
K2S2O8
K2S2O8
TBHP
DTBP
Na2S2O8
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
5
rhodamine B
rhodamine B
rhodamine B
rhodamine B
rhodamine B
rhodamine B
rhodamine B
rhodamine B
rhodamine B
rhodamine B
rhodamine B
rhodamine B
white DMF
green DMF
25
52
78
72
0
45
68
71
42
10
5
9
blue
blue
blue
blue
blue
blue
blue
blue
blue
blue
DMF
DMF
DMF
CH3CN
DMSO
DMA
toluene
ethanol
DCE
DMF/water
(3:1)
10
11
12
13
14
15
16
17
18
trace
c
19
20
rhodamine B TBHP
rhodamine B TBHP
blue
blue
DMF
DMF
78
70
d
a
Reaction conditions: 1a (1 mmol), 2 (2.4 mmol), 3a (1.2 mmol),
photocatalyst (5 mol %), oxidant (2 equiv), solvent (2 mL), rt, 36 h.
Isolated yield after column chromatography. Photocatalyst (3 mol
%). Photocatalyst (2 mol %). Blue (470 nm) and green LEDs (530
nm) were used for irradiation.
b
c
d
trimethylstyrene also delivered the desired product 4t in
good yield.
As regards amines, a variety of secondary amines viz.
pyrrolidine, morpholine, piperidine, dimethylamine, diethyl-
amine, dipropylamine, dibutylamine, dicyclohexylamine, tert-
butyl piperazine-1-carboxylate (1-Boc-piperazine), N-methyl-
aniline, and N-methyl-1-phenylmethanamine reacted effec-
tively with different styrenes to give the corresponding
dithiocarbamates 4. Markedly, the diminished yield of the
product 4ae (36%) is due to the lower solubility of the
resulting dithiocarbamic acid formed by N-Boc piperazine with
carbon disulfide during the course of reaction. However, the
reaction of other alkenes like stilbene, 4-bromobutene,
cyclohexene, allyl alcohol, and N-allylphthalimide, as well as
amines like n-butylamine, 2-phenylethylamine, aniline, 2-
aminopyridine, and diphenylamine, remained futile under the
stipulated conditions.
4-Nitrostyrene and 2-vinylpyridine brought about the
formation of alkylated dithiocarbamates rather than the
expected β-keto dithiocarbamates, even without using photo-
redox catalysis/TBHP. To ascertain the likeliness of this mode
of reaction, some more alkylated dithiocarbamates 5a−5f have
also been prepared (Scheme 3).
All of the products have been fully characterized based on
their NMR (1H, 13C, and 19F) and HRMS (SI). The structure
of a representative product 4j is conclusively confirmed by the
single-crystal studies (Figure 2, CCDC 2061891).
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a,b
Scheme 2. Scope and Versatility of the Reaction
Scheme 4. Control Experiments
eqs 1−3). When the standard reactions of 1a, 2, and 3a were
performed under nitrogen atmosphere, no desirable product
was formed, implying a key role of atmospheric air in the
reaction (Scheme 4, eq 1). Addition of (2,2,6,6-tetramethylpi-
peridin-1-yl)oxyl (TEMPO) or butylated hydroxyl toluene
(BHT) to the standard reaction also impeded the desired
conversion, thereby suggesting a radical pathway (eq 2). In the
case of TEMPO, the formation of the adduct 4b was also
confirmed by HRMS. To ascertain the intermediacy of the
compound pyrrolidine-1-carbodithioic acid (I), a discrete
reaction of pyrrolidine and CS2 was performed under the
stipulated conditions which readily led to the formation of I
(cf. SI for NMR). When the intermediate I was afterward made
to react with styrene under the same conditions, it gave rise to
the product 4a in 82% yield (eq 3).
The cyclic voltammogram of the intermediate I (0.01 mM)
was recorded at a glassy carbon electrode in 0.1 M CH3CN/
TBAP at a scan rate of 550 mVs−1 (cf. Figure S2), which
revealed its oxidation potential to be +0.73 V vs SCE. The
redox potential of the excited state of rhodamine B base (EE*/
Ered = +1.26 and EE*/Eox = −1.31 V vs SCE)19a may thus
allow the oxidation of the intermediate I.
On the basis of product isolation, control experiments, and
literature precedent,4b,c,f,21−23 a plausible mechanism for the
formation of 4 is outlined in Scheme 5.
a
Reaction conditions: 1 (1.0 mmol), 2 (2.4 mmol), 3 (1.2 mmol), aq
b
TBHP (2 equiv), DMF (2 mL), rt. Isolated yield after column
chromatography.
Scheme 3. Products Using p-Nitrostyrene and 2-
Vinylpyridine
a,b
To probe the formation of alkylated dithiocarbamates 5,
more control experiments were carried out (Scheme 4, eq 3−
6). Remarkably, the presence of TEMPO in the reaction of 2-
vinylpyridine, CS2, and pyrrolidine under the standard
conditions did not inhibit the formation of product 5b,
thereby discarding the possibility of a radical pathway. Further,
the same reaction without using blue LED and rhodamine B
also gave rise to 5b readily in the presence as well in the
absence of TBHP, suggesting an ionic Michael-type addition.
Accordingly, a mechanism for the formation of product 5 is
depicted in Scheme 5.
a
Reaction conditions: 1 (1.0 mmol), 2 (2.4 mmol), 3 (1.2 mmol),
b
DMF (2 mL), 10 min, rt. Isolated yield after column chromatog-
raphy.
The amine 3 easily reacts with carbon disulfide 2 to form the
dithiocarbamic acid I.13b The electrochemical studies (cf.
Figure S2) corroborate an oxidative photoinduced electron
transfer (PIET) to I by the photoexcited rhodamine B
(PC*)21 to form the radical cation II. The rhodamine B
̇
radical anion (PC
̅
) thus generated, in turn, is quenched by
Figure 2. ORTEP diagram of 4j.
TBHP via single-electron transfer to make the catalyst (PC)
free for the next catalytic cycle, with concurrent decomposition
of TBHP to tBuO• and hydroxide ion, followed by the reaction
To gain insight into the β-keto dithiocarbamate 4 formation,
a number of control experiments were carried out (Scheme 4,
22
of BuO• with TBHP to give BuOO•. Deprotonation of II
t
t
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Letter
Scheme 5. Plausible Mechanism for Products 4 and 5
AUTHOR INFORMATION
■
Corresponding Author
Krishna Nand Singh − Department of Chemistry, Institute of
Science, Banaras Hindu University, Varanasi 221005, India;
orcid.org/0000-0001-9147-5463; Email: knsingh@
bhu.ac.in, knsinghbhu@yahoo.co.in
Authors
Ramesh Kumar Vishwakarma − Department of Chemistry,
Institute of Science, Banaras Hindu University, Varanasi
221005, India
Saurabh Kumar − Department of Chemistry, Institute of
Science, Banaras Hindu University, Varanasi 221005, India
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.orglett.1c01059
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We are thankful to SERB, New Delhi (File No.CRG/2019/
003649) for financial assistance. R.K.V. and S.K. thank CSIR,
New Delhi, for the awards of SRF and RA, respectively.
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4j, characterization data, and copies of H, 13C, and 19F
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Accession Codes
CCDC 2061891 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge
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