Transition metal free, green and facile halogenation of ketene dithioacetals using a KX-oxidant system

Article abstract of DOI:10.1039/d0nj03737k

A facile oxidative halogenation of α-oxo ketene dithioacetals is achieved by using a potassium halide and oxidant combination under transition metal free conditions at ambient temperature. Using this methodology, halogenated ketene dithioacetals are synth

Full text of DOI:10.1039/d0nj03737k

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Transition metal free, green and facile
halogenation of ketene dithioacetals
using a KX–oxidant system†‡
Cite this: New J. Chem., 2021,
45, 1109
Vishakha Rai,
Ganesh Shivayogappa Sorabad
and
Mahagundappa Rachappa Maddani
*
Received 24th July 2020,
Accepted 16th November 2020
A facile oxidative halogenation of a-oxo ketene dithioacetals is achieved by using a potassium halide and
oxidant combination under transition metal free conditions at ambient temperature. Using this methodology,
halogenated ketene dithioacetals are synthesized in good to excellent yields with a short reaction time. The
halogenated products are successfully transformed into nitrogen containing heterocyclic molecules.
DOI: 10.1039/d0nj03737k
rsc.li/njc
DMSO,10 cerium(IV) ammonium nitrate (CAN)/KBr or LiBr,11 Select-
fluors/KBr12 and NaBrO3–NaBr13 and metal–oxo catalysts.14
Introduction
Bromination of organic molecules is an important and well- Among several oxidizing agents, oxone is increasingly being
known process in synthetic organic chemistry that results in either applied as an effective oxidant in synthetic protocols due to its
addition or substitution products depending upon the substrates non-toxicity, inexpensiveness, high stability, and versatility.15
and conditions employed. Formerly, these bromination reactions
On the other hand, a-oxo ketene dithioacetals containing a
involved the use of toxic and hazardous molecular bromine as donor–acceptor moiety have been found to be valuable three
the reagent, mainly in chlorinated solvents. To avoid the poten- carbon synthons in organic syntheses16 for the construction of
tially hazardous neat bromine reagent, NBS, tert-butyl hypohalite1 various C–C and C–hetero bonds. Carbonyl groups, alkenes and
and several carrying agent supported bromine reagents such as S atoms are the reactive sites, which make ketene dithioacetals
crystalline tribromides2 have been developed. However, these an admirable class of compounds. In spite of their huge popu-
reagents also suffer from several drawbacks such as limited larity as powerful building blocks in the synthesis of a variety of
application to reactive ring systems,2b,c low yields of products,2d heterocyclic molecules,17 their utility in C(sp2)–H functionaliza-
expensive ammonium bromides,2e excess loading of reagents,2f tion is not well explored. Although, metal18 mediated and a few
environmental issues and waste production.2g Moreover, toxic metal-free19 C–H functionalisation reactions are available, the
bromine was used in the preparation of these reagents in most direct C–H halogenation reactions under transition metal free
cases. In spite of that, there is a continuous demand for bromi- conditions are less studied. Only very few reports are available on
nated organic substrates due to their increasing importance as the halogenation of ketene dithioacetals, as shown in Scheme 1.
medicinally potent compounds,3 synthetic intermediates for nat-
Synthesis of a-bromo ketene dithioacetals was first performed by
ural products, agrochemicals and other industrially valuable Ila and Junjappa20 in 1985. They synthesized various brominated
molecules.4 Therefore, environmentally safe procedures have been ketene dithioacetals by using N-bromosuccinimide as the bromine
developed for halogenation reactions that involve an in situ source in carbon tetrachloride as the solvent at room temperature.
preparation of electrophilic halonium species by oxidation of a Later on, Liu21 and coworkers employed excess CuBr2 for the direct
halide ion with various oxidants.5 In the present scenario, oxida- bromination of ketene dithioacetals at room temperature. Similarly,
tive halogenation continues to be of great interest as it eliminates another transition metal mediated chlorination22 was reported by
the use of toxic molecular halogens. A number of protocols are Qiu and He. They used FeCl3 as the chloride source and PhI(OAc)2
available to achieve oxidative halogenation such as t-BuOOH/ as the oxidant for the direct chlorination of ketene dithioacetals.
H2O2–HBr,6 H2O2–V2O5–Et4NBr,7 oxone/NaBr,8 oxone/HBr,9 HBr/ Additionally, Wang and co-workers synthesized a-iodinated23 and
brominated24 ketene dithioacetals through halo-decarboxylation
reaction of a-carboxy-a-cinnamoyl ketene dithioacetals using mole-
Department of Chemistry, Mangalore University, Mangalagangothri, 574199
cular halogens under basic reaction conditions. In the same way,
Mangalore, Karnataka, India. E-mail: mahagundappa@gmail.com,
De Kimpe and co-workers reported the synthesis of a-fluorinated25
mahagundappa@mangaloreuniversity.ac.in
ketene N,S-acetals through direct fluorination using selectfluor as
† Dedicated to Prof. S. Chandrasekaran on his 75th birthday.
‡ Electronic supplementary information (ESI) available. See DOI: 10.1039/d0nj03737k
the fluorinating reagent.
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Table 1 Optimization of reaction conditionsa
MBr
Oxidant
T
Yieldb
(1C) (%)
Entry (1.1 equiv.) (1.1 equiv.) Solvent
1
2
3
4
5
6
7
8
KBr
KBr
KBr
KBr
KBr
KBr
KBr
KBr
KBr
KBr
KBr
KBr
LiBr
NaBr
KBr
KBr
Oxone
Oxone
Oxone
Oxone
Oxone
Oxone
Oxone
Oxone
Oxone
H2O2
DCE
THF
rt
rt
rt
rt
rt
rt
rt
rt
69
72
74
12
14
58
84
80
90
10
12
88
52
63
74
65
Toluene
DMF
DMSO
Ethanol
MeCN
H2O
MeCN : H2O (1 : 1) rt
MeCN : H2O (1 : 1) rt
MeCN : H2O (1 : 1) rt
MeCN : H2O (1 : 1) rt
MeCN : H2O (1 : 1) rt
MeCN : H2O (1 : 1) rt
MeCN : H2O (1 : 1) 50
MeCN : H2O (1 : 1) 80
9
10
11
12
13
14
15
16
TBHP
Scheme 1 Direct halogenation of a-oxo ketene dithioacetals.
PhI(OAc)2
Oxone
Oxone
Oxone
Oxone
Although a few methods for the synthesis of halogenated
ketene dithioacetals are available, there is a need for the devel-
opment of mild and selective halogenation protocols using
inexpensive and non-toxic reagents under transition metal free
conditions. Moreover, the resulting a-halo ketene dithioacetals
would be potential precursors for the preparation of functiona-
lized ketene dithioacetals via substitution of the vinylic halogen
by various nucleophilic species.20 Therefore, our strategy is to
employ oxidative halogenation for the facile synthesis of haloge-
nated ketene dithioacetals using non-organic halogen reagents
in an environmentally acceptable way. Oxidation of halide salts
to generate in situ electrophilic halonium species is an effective
approach for halogenation26 with high atom economy and step
economy. To the best of our knowledge, the oxidative halogena-
tion strategy has not been applied to ketene dithioacetals so far. In
continuation of our success in the oxidative C–H functionalization27
strategies described herein, we wish to report a transition metal
free, direct halogenation of ketene dithioacetals using an environ-
mentally benign KX–oxidant system.
a
All reactions were carried out by using ketene dithioacetal 1a (1 equiv.
0.44 mmol), MBr (1.1 equiv.) and oxidant (1.1 equiv.) at room temperature
for 2 h. Isolated yield.
b
entry 7). Similarly, we also carried out the reaction in water as a
green solvent and found that the product 2a was formed in a
good yield of 80% (Table 1, entry 8). The above two reactions
encouraged us to perform the reaction in aqueous acetonitrile.
Accordingly, the present bromination reaction in aqueous
acetonitrile (1 : 1) as the solvent mixture at room temperature
furnished 2a in an excellent yield (90%, Table 1, entry 9).
Based on the above results, further screening studies were
conducted in aq. acetonitrile as the solvent medium. We then
examined the reactivity of other oxidants. In the present
oxidative bromination reaction, peroxide-based oxidants did
not give successful results and product 2a was obtained in very
low yields (Table 1, entries 10 and 11). Furthermore, reaction
using diacetoxyiodobenzene as an oxidant also gave a very good
yield of 2a (Table 1, entry 12). In addition, our efforts in
improving the yield by using other halide salts such as LiBr
Results and discussion
We begin with the reaction of ketene dithioacetal 1a with and NaBr were unsuccessful (Table 1, entries 13 and 14). The
different bromide salts (MBr, 1.1 equiv.) in the presence of impact of the reaction temperature was also examined by
oxone (1.1 equiv.) as the oxidant at room temperature to carrying out the reactions at higher temperatures. However, it
optimize the oxidative bromination reaction and the results was observed that increasing the reaction temperature resulted
are tabulated in Table 1. In the reactions of 1a with KBr and in decreasing the yield of product 2a (Table 1, entries 15 and
oxone in dichloroethane, tetrahydrofuran and toluene, we 16). Thus, based on all the above optimization studies, we
observed the formation of the desired brominated ketene found the optimized reaction conditions for the oxidative
dithioacetal 2a in good yields (Table 1, entries 1–3). Similar bromination of ketene dithioacetals to be the use of KBr
reactions in polar aprotic solvents such as DMF and DMSO (1.1 equiv.) and oxone (1.1 equiv.) in aq. acetonitrile (1 : 1) at
were not successful and produced the product 2a in very low room temperature for 2 h (Table 1, entry 9).
yields (Table 1, entries 4 and 5). Ethanol was also not effective
We then continued to examine the substrate scope using the
for the present oxidative bromination reaction and gave the above optimised reaction conditions. Ketene dithioacetals
product 2a in moderate yield (Table 1, entry 6). We were pleased functionalised with various substituents exhibited excellent
to observe the formation of product 2a in 84% yield when the tolerance to the present oxidative halogenations and the results
reaction was carried out in acetonitrile as the solvent (Table 1, are tabulated in Table 2. The substrates containing electron
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Table 2 Substrate scope for the bromination of ketene dithioacetalsa
dithioacetals also underwent smooth halogenation to afford the
corresponding products 2m and 2n in excellent yields. Further-
more, ketene cyclic dithioacetals also participated in the present
halogenation to provide the expected products in excellent yields.
For example, the parent ketene cyclic dithioacetal, electron donat-
ing group substituted, halogen substituted, and naphthyl and
furanyl derived cyclic dithioacetals took part in the reaction and
furnished the corresponding products 2o–w in good to excellent
yields. It is worth noting that our protocol was also successfully
applied to aliphatic ketone and ester functionalized ketene
dithioacetals. Both these substrates furnished their brominated
products 2x and 2y in excellent yields under optimised reaction
conditions.
Encouraged by the production of the above brominated pro-
ducts, we continued to explore chlorinations of ketene dithioace-
tals. Unexpectedly, we did not observe the formation of the
chlorinated product when KCl was used under optimised condi-
tions. We thought that the KCl–oxone combination may not be
effective for chlorination reactions. Therefore, based on the results
of optimization studies, chlorination of the ketene dithioacetal
derivatives was studied by employing KCl as the halogen source
and diacetoxyiodobenzene as the oxidant. Accordingly, ketene
dithioacetals with different substituents were subjected to
chlorination and the corresponding chlorinated products 3a–i
were obtained in 70–90% yields, as shown in the Table 3.
Furthermore, a scale-up experiment for bromination was also
conducted as outlined in Scheme 2. The reactions on the 2.0 g
scale also went well in furnishing the brominated products 2a
and 2o in very good yields. Hence, the methodology presented
here was found to be advantageous for the quantitative produc-
tion of synthetically valuable bromo ketene dithioacetals.
Table 3 Substrate scope for chlorination of ketene dithioacetalsa
a
All reactions were carried out by using ketene dithioacetals 1a (1 equiv.
0.44 mmol), KBr (1.1 equiv.) and oxone (1.1 equiv.) in aq. acetonitrile
(1:1) at room temperature for 1–2 h.
donating groups furnished the desired brominated products
2a–e in excellent yields.
Similarly, reactions of halogen substituted ketene dithioacetals
underwent smooth bromination and afforded the brominated
products 2f–i in high yields. In addition, naphthyl and anthracenyl
functionalized ketene dithioacetals also successfully produced
their corresponding brominated products 2j and 2k in very good
yields. Interestingly, a substrate containing –NO2, a strong electron
withdrawing group, also led to the formation of the desired
product 2l in good yield. Heterocycle derived ketene dithioacetals
were also tested for their reactivity towards the present halogena-
a
All reactions were carried out by using ketene dithioacetals 1a
(1 equiv. 0.44 mmol), KCl (1.1 equiv.) and PhI(OAc)2 (1.1 equiv.) in
tion reaction. We found that furanyl and thiophenyl ketene aq. acetonitrile (1 : 1) at room temperature for 1–2 h.
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Scheme 2 Gram scale experiment.
Scheme 5 Tentative reaction mechanism.
To demonstrate the synthetic utility of the synthesized products,
a few transformations were planned. Accordingly, product 2a was
subjected to reaction with guanidine nitrite in acetonitrile under
reflux conditions. This reaction transformed the starting bromoke-
tene dithioacetal into a pyrimidine derivative 4a in 78% yield, as
shown in Scheme 3. Similarly, the reaction of 2a with hydrazine
hydrate in methanol under reflux conditions gives phenyl pyrazole in
71% yield. Furthermore, reduction of 2a with sodium borohydride
resulted in the formation of 6a in 98% yield. Later, the addition of
boron trifluoride etherate to the reduced product 6a under reflux
conditions in THF led to the formation of 7a in an excellent yield
(95%). Similarly, cyclic ketene dithioacetal 2x was reacted smoothly
with hydrazine hydrate in DMF at 100 1C to produce the annulated
and 1,2-sulfur migrated product 8a in 72% yield.
the present halogenation did not proceed in the absence of an
oxidant, and the starting material was isolated unaffected. This
reaction indicates that an oxidizing agent is necessary for the
present halogenation reaction.
Based on the control experiments and literature precedence,28
we herein propose a tentative mechanism, as shown in Scheme 5.
This mechanism is proposed to involve the oxidation of KBr to the
bromonium ion in the presence of oxone. The nucleophilic attack of
1a leads to the thionium cation species A. The subsequent loss of H+
results in the formation of the desired a-haloketene dithioacetal 2a.
Conclusions
We have developed an efficient and operationally simple oxidative
halogenation of ketene dithioacetals. This reaction is superior to
the previously reported methods as it utilises the stable, inexpen-
sive and non-toxic KX/oxidant system under transition metal free
conditions. The present halogenation in an aqueous acetoni-
trile medium provides an environmentally benign method for
the synthesis of various halogenated ketene dithioacetal deriva-
tives under ambient conditions with good to excellent yields.
Applications of the synthesized products were also shown for
the synthesis of nitrogen containing heterocyclic molecules.
The mechanism of the present reaction was studied by
performing some control experiments (Scheme 4). The present
halogenation reaction in the presence of the radical inhibitor
2,2,6,6-tetramethylpiperidin-1-oxyl (TEMPO) proceeded smoothly
without a significant decrease of the yield of 2a, thus excluding
the involvement of a radical pathway. In addition, as expected,
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
The authors thank DST-INSPIRE (DST/INSPIRE Fellowship/2017/
IF70456), New Delhi, India for financial support.
Notes and references
Scheme 3 Transformation of bromo ketene dithioacetals.
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