Hydrothiolation of Alkenes and Alkynes Catalyzed by 3,4-Dimethyl-5-vinylthiazolium iodide and Poly(3,4-dimethyl-5-vinylthiazolium) iodide

Article abstract of DOI:10.1002/cctc.201600363

The highly selective anti-Markovnikov addition of thiols to unactivated alkenes and alkynes was demonstrated by using 3,4-dimethyl-5-vinylthiazolium iodide or its polymer, poly(3,4-dimethyl-5-vinylthiazolium) iodide, as a complementary catalyst. The reaction proceeded cleanly under base-free conditions in air with both aromatic and aliphatic thiols. The polymer catalyst showed a high turnover number (≈5800) and could be reused up to four times without any loss of catalytic activity. DFT calculations supported stabilization of the thiyl radical intermediate by the thiazolium cation, which resulted in reaction of the radical with unsaturated C?C bonds.

Full text of DOI:10.1002/cctc.201600363

DOI: 10.1002/cctc.201600363
Communications
Hydrothiolation of Alkenes and Alkynes Catalyzed by 3,4-
Dimethyl-5-vinylthiazolium iodide and Poly(3,4-dimethyl-
5-vinylthiazolium) iodide
Supill Chun, Junyong Chung, Ji Eun Park, and Young Keun Chung*[a]
The highly selective anti-Markovnikov addition of thiols to un-
activated alkenes and alkynes was demonstrated by using 3,4-
dimethyl-5-vinylthiazolium iodide or its polymer, poly(3,4-di-
methyl-5-vinylthiazolium) iodide, as a complementary catalyst.
The reaction proceeded cleanly under base-free conditions in
air with both aromatic and aliphatic thiols. The polymer cata-
lyst showed a high turnover number (ꢀ5800) and could be
reused up to four times without any loss of catalytic activity.
DFT calculations supported stabilization of the thiyl radical in-
termediate by the thiazolium cation, which resulted in reaction
of the radical with unsaturated CÀC bonds.
dition of CO2 to epoxides, the benzoin condensation reaction,
and the tandem formation of g-butyrolactone from benzalde-
hyde and methyl acrylate. These polymer-based systems can
be reused several times without any significant loss of catalytic
activity. We also reported the synthesis of poly(3,4-dimethyl-5-
vinylthiazolium) iodide (B) from 3,4-dimethyl-5-vinylthiazolium
iodide (A) (Scheme 1) and their use in thioesterification reac-
tions.[11] Continuing our work on expanding the scope of poly-
mer-based organocatalytic systems, we screened a variety of
reactions in the presence of B.
Organosulfur compounds have widespread applications in ma-
terials chemistry and chemical biology.[1] Thus, the develop-
ment of efficient synthetic methods for the incorporation of
sulfur into organic frameworks is of significant interest.[2] In
this context, the hydrothiolation reaction, that is, the direct ad-
dition of the SÀH bond of thiols to unsaturated carbon–carbon
bonds, is a simple and atom-economical approach to the syn-
thesis of organosulfur compounds.[3] The development of effi-
cient catalytic systems that can promote hydrothiolation is an
important challenge for synthetic organic chemists. Hydrothio-
lation can be catalyzed by strong bases,[4a–d] strong acids, or
free radicals.[4e,f] Recently, several metal compounds were re-
vealed to catalyze hydrothiolation reactions and were re-
viewed.[5] However, some of the reported procedures have
many disadvantages, such as unsatisfactory yields, long reac-
tion times,[6] formation of unwanted byproducts, the use of
highly carcinogenic and hazardous organic solvents,[7] and the
use of expensive and/or difficult to obtain catalysts.[8] Thus, the
development of a more efficient and convenient method for
the hydrothiolation of alkenes and alkynes is necessary.
Scheme 1. Synthesis of poly(3,4-dimethyl-5-vinylthiazolium) iodide (B) from
3,4-dimethyl-5-vinylthiazolium iodide (A). AIBN=2,2’-azobisisobutyronitrile.
Interestingly, we found that A and B were highly effective
catalysts for the hydrothiolation of alkenes and alkynes under
mild conditions. Herein, we report a simple and efficient proto-
col for the synthesis of linear and vinyl thioethers through the
anti-Markovnikov addition of thiols to alkenes and alkynes by
using N-heterocyclic carbene catalysts, A and B. Our catalysts
were highly effective, even in the presence of air, and did not
requires the use of metal complexes or free-radical initiators.
High turnover numbers were observed in the presence of poly-
meric catalyst B. To the best of our knowledge, catalyst B is
the first successfully recyclable organic polymer catalyst for the
hydrothiolation reaction.
As a model reaction for hydrothiolation, we initially exam-
ined the reaction of styrene with thiophenol in the presence of
polymeric B (1 mol%) for 60 min (Table 1). The reaction was
highly sensitive to the medium and temperature. In CH2Cl2 and
toluene, the expected product was not formed (Table 1, en-
tries 1 and 2). However, upon changing the solvent to DMF at
408C, the expected product was formed, albeit in low yield
(14%; Table 1, entry 3). Increasing the amount of catalyst B to
2 mol% failed to produce any noticeable increase in yield
(15%; Table 1, entry 4). To optimize the reaction conditions fur-
ther, we screened a range of temperatures. No reaction was
observed at 258C. Upon increasing the reaction temperature
to 608C, the yield increased dramatically to 82%. However,
a further increase in the temperature to 708C was not helpful
in increasing the yield (81%; Table 1, entry 7). An increase in
Organocatalysts are usually inexpensive, readily available,
robust, and nontoxic.[9] However, they are not usually reusable
because of the difficulties involved in their recovery. To over-
come the problems related to the isolation of these catalysts
from a reaction mixture, we recently developed a polymer-
based organocatalytic system, poly(4-vinyl N-heterocyclic car-
bene).[10] Theses catalysts showed high activity in the cycload-
[a] S. Chun, J. Chung, J. E. Park, Prof. Y. K. Chung
Department of Chemistry, College of Natural Sciences
Seoul National University
Seoul 151-747 (Korea)
E-mail: ykchung@snu.ac.kr
Supporting Information for this article can be found under http://
dx.doi.org/10.1002/cctc.201600363.
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sponding hydrothiolated products in good to high yields. If
a methyl group was introduced to the a position (see products
3ja and 3ka), the yield decreased and catalyst A became more
active than catalyst B. The yields observed in the presence of
A were slightly higher than those in the presence of B (product
3ja, 76 and 65%; product 3ka, 77 and 70%). This observation
might be explained by considering the feasibility of the sub-
strate encountering the catalytically active sites. In polymeric
catalyst B, the catalytically active sites are densely situated on
the polymer backbone and are quite effective for the reaction
with ordinary substrates. However, B is less effective than A in
reactions involving sterically hindered substrates. The active
sites in monomeric catalyst A might be more accessible to
sterically demanding substrates than the active sites in B. It is
interesting that catalysts A and B are complementary in terms
of the hydrothiolation reaction of thiophenol with various styr-
enes. Among substrates having a halogen substituent, the sub-
strate bearing a bromine atom showed the highest yield (prod-
ucts 3ga–ia: 78, 80, and 92%; products 3na and 3ma: 71 and
76%). 3-Chlorostyrene (91%) showed the highest yield among
the 2-, 3-, and 4-chlorostyrenes. Interestingly, the formation of
a b-nitrosulfide was observed in the reaction of (E)-(2-nitrovi-
nyl)benzene (3la).[13]
Table 1. Optimization of the reaction conditions.[a]
Entry
B [mol%]
Solvent
T [8C]
t [min]
Yield[b] [%]
1
2
3
4
5
6
7
8
1
1
1
2
1
1
1
1
CH2Cl2
toluene
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
40
40
40
40
25
60
70
60
60
60
60
60
60
60
60
60
60
60
90
90
90
60
N.R.
N.R.
14
15
N.R.
82
81
96
9
10
11
1
0.01
1
90[c]
58
9[d]
[a] Conditions: 1a (1 mmol), 2a (1.1 mmol), B, in air. [b] Yield of isolated
product; N.R.=no reaction. [c] Catalyst A was used. [d] Under a N2 atmos-
phere.
the reaction time to 90 min at 608C increased the yield of the
product (96%; Table 1, entry 8). The optimum reaction condi-
tions were found to involve the use of B (1 mol%), 1a
(1 mmol), and 2a (1.1 mmol) in DMF (1 mL) at 608C for 90 min.
Moreover, the catalytic activity of B was higher than that of
monomeric A (96:90; Table 1, entry 8 vs. 9). The
The scope of the reaction was then investigated further with
a variety of thiols (Table 3). In general, high yields were ob-
amount of catalyst B could be lowered to 0.01 mol%
Table 2. Hydrothiolation of different styrenes with thiophenol.[a]
(Table 1, entry 10). A turnover number of 5800 was
observed for the catalyst under our optimized condi-
tions.[12] However, owing to experimental difficulties
in measuring the turnover number with the small
amount of catalyst B in our reaction system, the
maximum turnover number was not investigated.
We also examined the reusability of B in successive
reactions (see the Supporting Information). For ex-
perimental feasibility, 5 mol% of B was used. Our cat-
alytic system turned out to be quite robust and
could be reused up to four times without any loss of
catalytic activity (96, 97, 97, and 95% in runs 1, 2, 3,
and 4, respectively). Interestingly, under a nitrogen
atmosphere a poor yield (9%) was observed (Table 1,
entry 11).
The hydrothiolation reaction under our standard
reaction conditions tended to be quite clean and
produced no significant side products. Next, the sub-
strate scope of the hydrothiolation reaction with re-
spect to the styrenes was examined under our opti-
mized reaction conditions (Table 2). Using a standard
thiol (thiophenol), reactions were performed with
a variety of styrenes in the presence of both A and B
as catalysts to compare their catalytic activities. For
the substrates delivering products 3aa–ia, except for
3ba and 3da, higher yields were observed in the
presence of polymeric catalyst B than in the pres-
ence of monomeric A. Substrates with an electron-
withdrawing substituent (e.g., CF3, F, Cl, and Br) re-
acted smoothly with the thiol to afford the corre-
[a] Conditions: 1a (1 mmol), 2 (1.1 mmol), A or B (1 mol%), DMF (1 mL), 608C, 90 min.
[b] Yield of isolated product (by A/B). [c] 3 h. [d] 2 (2 equiv.). [e] a-Hydrothiolation.
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ratio in a rather poor yield (31%). It seemed
that as the chain length of the aliphatic thiol in-
creased (products 5ao, 5aj, and 5ak), both the
yield and the selectivity for the E isomer in-
creased. However, if dodecane thiol was used,
poor regioselectivity was observed (E/Z=
42:58). The best E/Z ratio was observed upon
using decane thiol. A branched alkane thiol
such as 2-ethylhexane-1-thiol gave the E isomer
as a major product. If a substrate with a bulky
substituent, such as (triisopropylsilyl)acetylene,
was treated with thiophenol, a high E/Z ratio
(product 5pa; 90:10) was observed. In the reac-
tion of 1,2-ethanedithiol, bis-hydrothiolation
was observed, which resulted in the formation
of 2-benzyl-1,3-dithiolane in 61% yield.[14]
Table 3. Hydrothiolation of styrene with different thiols.[a]
To gain some insight into the reaction mech-
anism, we performed the reaction in the pres-
ence of a common radical trap [i.e., 2,2,6,6-tet-
ramethylpiperidin-1-yl)oxyl
(TEMPO),
1.21 equiv.]. No hydrothiolation product was
formed in this case. Instead, 2,2,6,6-tetramethyl-
1-[1-phenyl-2-(phenylthio)ethoxy]piperidine[15]
[a] Conditions: 1a (1 mmol), 2 (2 mmol), A or B (1 mol%), DMF (1 mL), 608C, 90 min.
[b] Yield of isolated product. [c] The yield was calculated by 1H NMR spectroscopy with
mesitylene as an internal standard. [d] 2 h. [e] 6 h.
was obtained in
(Scheme 2).
a very low 9% yield
Our observation that the hydrothiolation re-
action could be performed in air suggested di-
served for products 3ab–ag. However, as the aliphatic charac-
ter of the thiols increased, the yields decreased (products 3ae–
ak). Interestingly, higher yields were observed with monomeric
catalyst A for most aliphatic thiols. For example, a poor yield
(38%) was observed for pentanethiol in the presence
oxygen as a radical initiator in our reaction.[16] Moreover, the
formation of a product was not observed in the absence of
catalyst A. This result suggests that catalyst A plays an impor-
tant role in the reaction.
of B (product 3aj), but the yield increased to 62% in
Table 4. Hydrothiolation of different alkynes with thiols.[a]
the presence of catalyst A. Upon using decane-1-
thiol as the substrate in the presence of A, the yield
was still moderate at 50% (product 3ak). In the case
1
of product 3ad, the yield was calculated by H NMR
spectroscopy, because of difficulties in separating
the product from the reactant. Unfortunately, 2-ami-
nobenzenethiol and 2-chlorobenzenethiol were un-
reactive under our reaction conditions. Overall, the
substrate scope of our hydrothiolation process sug-
gests that both steric and electronic factors may play
a major role in the reaction.
To expand the scope of this reaction to alkynes,
we investigated the hydrothiolation of ethynylben-
zene with various thiols under our optimized reac-
tion conditions (Table 4). It was encouraging to ob-
serve that the reactions went to completion within
30 min. In most cases, the (E)-vinyl sulfide was the
predominant product. The regioselectivity of the hy-
drothiolation varied depending upon the substrate.
Introduction of a substituent on the thiophenol led
to a decrease in the E/Z ratio from 89:11 to 80:20
(products 5aa, 5al, and 5am). If an electron-donat-
ing group ÀNH2 was present on the thiophenol, the
Z isomer was favored over the E isomer with a 91:9
[a] Conditions: 4a (1 mmol), 2 (1.1 mmol), B (1 mol%), DMF (1 mL), RT, 30 min. [b] Yield
of isolated product. [c] The E/Z ratio was determined by analysis of the isolated product
by 1H NMR spectroscopy.
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To gain further information about the reaction, we
prepared C and tested it as a catalyst in the hydro-
thiolation of styrene with thiophenol. Reaction in the
presence of C led to the formation of diphenyl disul-
fide (42%) and phenethyl phenyl sulfide (43%)
(Scheme 3). This result shows that the substituent at
the C2 position of the thiazolium highly affects the re-
action; furthermore, the yield of the hydrothiolation
Scheme 3. Experiment with 2,3,4-trimethylthiazol-3-ium iodide (C) as the catalyst.
product obtained in the presence of catalyst A with a hydrogen
atom at the C2 position was much higher than that obtained
in the presence of catalyst C with a methyl group at the C2 po-
sition.[17]
On the basis of our observations and those of others,[18] we
suggest that the reaction follows
a radical mechanism
(Figure 1). The key point of the hydrothiolation is the role of
the thiazolium, which interacts with the thiyl radical; this re-
Figure 1. Plausible reaction mechanism for the hydrothiolation reaction.
Scheme 2. Reaction of styrene with thiophenol in the presence of TEMPO.
Figure 2. Energy profiles for the catalyzed and uncatalyzed thiol–ene reactions. For further details, please see the Supporting Information.
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sults in stabilization of the radical and enhances its addition to
unsaturated cÀc bonds.
Keywords: alkenes
polymers · radical reactions
· hydrothiolation · organocatalysis ·
The role of the thiazolium catalyst was confirmed by DFT cal-
culations (Figure 2). The thiyl radical produced by the reaction
with dioxygen is stabilized by catalyst A or C relative (stabilized
by À10.53 and À7.86 kcalmolÀ1 in the presence of A and C, re-
spectively, relative to the free thiyl radical), and the addition of
the thiyl radical to styrene in the presence of A shows the
lowest activation energy barrier 12.01 kcalmolÀ1 (TS_Cat.A).
This is reflected in the high yield of the hydrothiolation prod-
uct (90%). In the absence of catalyst A or C, no reaction is ob-
served within 90 min. However, in the presence of C or without
any catalyst, the energy of the transition state does not match
the trend in the yield. Thus, stabilization of the thiyl radical
seems to be critical to the hydrothiolation reaction.
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In conclusion, we developed a robust method for the vinyl-
thiazolium- or poly(vinylthiazolium)-catalyzed hydrothiolation
of alkenes and alkynes. The reaction can be employed for both
aliphatic and aromatic thiols. Our optimized reaction condi-
tions afforded linear thioethers from both activated and unacti-
vated alkenes with aliphatic and aromatic thiol partners in
good to excellent yields under metal-free, additive-free, and
open-air conditions. For most aromatic thiols, higher yields
were observed in the presence of polymer catalyst B, but with
aliphatic thiols, higher yields were generally observed with
monomeric catalyst A. The polymeric catalyst showed a high
turnover number (ꢀ5800) and was recyclable.
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Experimental Section
Procedure for the hydrothiolation of alkenes
A tube-type Schlenk flask equipped with a stirring bar was charged
sequentially with catalyst A or B (2.6 mg, 0.01 mmol, 1 mol%), the
styrene derivative (1 mmol), the thiol (ꢀ1.1–2 mmol), and DMF
(1 mL). The mixture was heated at 608C for 90 min. After the solu-
tion was cooled to room temperature, the solvent was evaporated
under reduced pressure. Purification by flash chromatography
(silica gel, n-hexane/ethyl acetate) afforded the thioether.
Procedure for the hydrothiolation of alkynes
A Schlenk flask equipped with a stirring bar was charged sequen-
tially with catalyst B (2.6 mg, 0.01 mmol, 1 mol%), the acetylene
derivative (1 mmol), the thiol (1.1 mmol), and DMF (1 mL). The mix-
ture was stirred at room temperature for 30 min. The solvent was
evaporated under reduced pressure, and purification by flash chro-
matography (silica gel, n-hexane/ethyl acetate) afforded the thio-
ether.
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This work was supported by a National Research Foundation of
Korea (NRF) grant funded by the Korean government
(2014R1A5A1011165 and 2007-0093864). S.C., J.C., and J.P. are re-
cipients of BK21 plus Fellowships.
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Received: March 29, 2016
Revised: May 9, 2016
Published online on && &&, 0000
&
ChemCatChem 2016, 8, 1 – 7
www.chemcatchem.org
6
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!

COMMUNICATIONS
S. Chun, J. Chung, J. E. Park, Y. K. Chung*
&& – &&
Hydrothiolation of Alkenes and
Alkynes Catalyzed by 3,4-Dimethyl-5-
vinylthiazolium iodide and Poly(3,4-
dimethyl-5-vinylthiazolium) iodide
Thio, thio, it’s off to work we go!
zolium) iodide, as a complementary cat-
alyst. DFT calculations support a mecha-
nism in which the thiazolium cation sta-
bilizes a thiyl radical and enhances its
reaction with unsaturated CÀC bonds.
Highly selective anti-Markovnikov addi-
tion of thiols to unactivated alkenes and
alkynes is demonstrated by using 3,4-di-
methyl-5-vinylthiazolium iodide or its
polymer, poly(3,4-dimethyl-5-vinylthia-
ChemCatChem 2016, 8, 1 – 7
www.chemcatchem.org
7
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
&
These are not the final page numbers! ÞÞ