Acid-Catalyzed Oxidative Addition of Thiols to Olefins and Alkynes for a One-Pot Entry to Sulfoxides

Article abstract of DOI:10.1055/s-0035-1562480

An oxidative variant of the thiol-ene reaction has been developed, achieving the direct addition of thiols to olefins to form sulfoxides. The reaction uses tert-butyl hydroperoxide as oxidant and methanesulfonic acid as catalyst. The latter is believed to catalyze the oxidation of the intermediate sulfide to the sulfoxide. No special precautions are necessary to exclude oxygen, yet the products are formed without oxidation at the β-position. Styrenes, acrylic acid derivatives, alkynes, and thiophenols gave the highest yields, while aliphatic olefins and thiols were less effective.

Full text of DOI:10.1055/s-0035-1562480

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© Georg Thieme Verlag Stuttgart · New York
2016, 27, A–E
letter
A
H.-L. Yue, M. Klussmann
Letter
Synlett
Acid-Catalyzed Oxidative Addition of Thiols to Olefins and
Alkynes for a One-Pot Entry to Sulfoxides
Hui-Lan Yue1
R
R
O
S
10 mol% MsOH
3 equiv t-BuOOH
Martin Klussmann*
RS
+
RS SH
R
R
MeCN, 40 °C
2–48 h
R
R
Max-Planck-Institut für Kohlenforschung,
Kaiser Wilhelm-Platz 1, 45479 Mülheim an
der Ruhr, Germany
46 examples,
up to 99% yield
R = aryl, alkyl, EWG, H
RS = aryl, (alkyl)
klusi@mpi-muelheim.mpg.de
Received: 01.06.2016
Accepted after revision: 25.06.2016
Published online: 25.07.2016
a) previous methods
H(O)O
O
0,1
O2/Ox
O2/Ox
S
R
+
RS SH
RS SH
R
RS
R
DOI: 10.1055/s-0035-1562480; Art ID: st-2016-d0374-l
R
O
O
S
1,2
RS
Abstract An oxidative variant of the thiol-ene reaction has been devel-
oped, achieving the direct addition of thiols to olefins to form sulfox-
ides. The reaction uses tert-butyl hydroperoxide as oxidant and meth-
anesulfonic acid as catalyst. The latter is believed to catalyze the
oxidation of the intermediate sulfide to the sulfoxide. No special pre-
cautions are necessary to exclude oxygen, yet the products are formed
without oxidation at the β-position. Styrenes, acrylic acid derivatives,
alkynes, and thiophenols gave the highest yields, while aliphatic olefins
and thiols were less effective.
R
+
R
R
R
R
b) this work
O
S
[MsOH]
t-BuOOH
R
+
RS SH
R
RS
R
Scheme 1 Oxidative variants of thiyl radical addition reactions to olefins
Key words addition, homogeneous catalysis, oxygenations, perox-
ides, radical reaction, sulfoxides
Sulfoxides are valuable intermediates and building
blocks in organic chemistry, for example, for the construc-
tion of complex carbocycles and heterocycles via the Pum-
merer reaction.7 Furthermore, they can serve as ligands in
asymmetric synthesis8 or as modifiable directing groups in
metal-catalyzed ortho-directed C–H bond functionalization
reactions of arenes.9 In addition, sulfoxides are prevalent in
medicinal chemistry and natural products, and they serve
as precursors to other sulfur-containing compounds of
pharmaceutical interest such as sulfoximines.10 Sulfoxides
can be prepared by oxidation of sulfides or by nucleophilic
substitution at the sulfur atom of sulfinate esters or amide
derivatives.10c,11 An alternative procedure consists of the in
situ generation of nucleophilic sulfenate anions that can be
employed in substitution or cross-coupling reactions.12
Herein, we report the catalytic and direct oxidative ad-
dition of thiols to alkenes, leading to sulfoxides without
functionalization at the β-position (Scheme 1, b). The
method employs methanesulfonic acid (MsOH) as catalyst
and tert-butyl hydroperoxide (t-BuOOH) as oxidant. Brønst-
ed acids are known catalysts for the oxidation of sulfides
The addition of thiols to carbon–carbon double bonds,
also referred to as the thiol-ene reaction and as a ‘click’ re-
action, has been investigated extensively in the fields of or-
ganic synthesis and materials chemistry due to its high
atom economy, regioselectivity, minimum number of side
products, and insensitivity to ambient oxygen or water.2
Under oxidative conditions, variants of this reaction enable
the direct synthesis of oxygenated products of considerable
synthetic interest (Scheme 1, a). Kharasch studied and dis-
cussed the general principle of this type of reaction back in
1951, observing the formation of β-hydroxysulfoxides in
the presence of oxygen.3 More recent variants include the
synthesis of β-hydroxy-,4 β-peroxy-,5 and β-oxy-6 sulfides,
sulfoxides, and sulfones, respectively, in a single step from
thiols and olefins. Yet, the combination of thiyl radical addi-
tion with exclusive oxygenation of sulfur is apparently still
lacking in the repertoire of direct oxidative thiol-ene reac-
tion variants.
© Georg Thieme Verlag Stuttgart · New York — Synlett 2016, 27, A–E

B
H.-L. Yue, M. Klussmann
Letter
Synlett
with hydroperoxides, such as t-BuOOH,13 and peroxides can
also act as initiators of the thiol-ene radical chain reaction.
The absence of oxygen as oxidant should avoid rapid trap-
ping of the intermediate radical and thus prevent oxidation
at the β-position.
use of hydrogen peroxide as oxidant. Acetonitrile was found
to be the best solvent, although ethanol, ethyl acetate, 1,4-
dioxane, and THF were also similarly effective (see Support-
ing Information for further details of optimization studies).
Using the optimized conditions (Table 1, entry 4), the
scope and limitations of the reaction of thiophenol with
various alkenes was investigated (Scheme 2).
In an initial experiment, 10 mol% MsOH and t-BuOOH
(0.5 equiv, as a solution in decane) were added to a mixture
of thiophenol (2.0 equiv) and styrene (limiting reagent) in
MeCN. After stirring for 20 hours in a closed vial at 40 °C
without precautions to exclude oxygen, sulfoxide 1a was
generated in 48% yield along with sulfide 2a in 21% yield
(Table 1, entry 1). Increasing the amount of oxidant im-
proved the yield of sulfoxide 1a and reduced the amount of
sulfide as well as the reaction time, achieving 95% yield of
1a after 8 hours with 3.0 equivalents of t-BuOOH (Table 1,
entries 2–4).14 Reducing the amount of thiophenol to 1.2
equivalents diminished the yield only slightly (cf. Table 1,
entries 3 and 5), indicating that oxidative formation of di-
sulfide from thiophenol only occurs to a minor extent. Re-
ducing the amount of acid decreased the rate of formation
of 1a a little and increased the amount of sulfide 2a (Table
1, entries 6 and 7). Without acid, similar amounts of sulfox-
ide and sulfide were formed, again supporting the role of
acid in sulfur oxidation (Table 1, entry 8). In the absence of
oxidant or oxidant and acid, clean formation of the thiol-
ene product 2a was observed, but only after an extended
reaction time of 20 hours (Table 1, entries 9 and 10). Other
Brønsted acids were also found to be suitable, but weaker
acids gave less sulfoxide and more sulfide. Using simple
metal salts instead of MsOH gave inferior results, as did the
R2
R2
O
S
10 mol% MsOH
3 equiv t-BuOOH
R1
R1
Ph
+
Ph SH
O
MeCN, 40 °C
2–48 h
R3
R3
Br
O
S
O
S
S
Ph
Ph
Ph
Ph
3, 83%
4, 90%
R
1a, R = H, 89%
1b, R = F, 86%
1c, R = Cl, 90%
1d, R = Br, 98%
O
S
O
S
Ph
1e, R = CN, 86%
1f, R = Me, 81%
1g, R = OMe, 95%
5, 95%
6, 66%
O
S
O
S
Ph
O
S
Ph
Ph
Ph
Ph
7, 83%
d.r. = 1.8:1
8, 24%
d.r.=1.2:1
9, 37%
O
O
S
O
S
O
S
S
Ph
Ph
Ph
10, 47%
11, 56%
12, 5%
13, 40%
15a, R = Me, 14%
15b, R = Et, 52%
15c, R = n-Bu, 69%a
15d, R = t-Bu, 54%a
15e, R = Bn, 72%a
O
O
S
S
O
O
C6H13
Ph
N
R
Ph
14, 13%
O
S
O
O
S
Table 1 Optimization of the Reaction Conditionsa
S
N
NC
Ph
Ph
Ph
Ph
18, 8%
16, 18%
17, 77%
d.r. = 1.2:1
O
O
O
catalyst
oxidant
O
S
O
S
O
S
S
S
H
N
H
Ph
Ph SH
Ph
Ph
Ph
Ph
N
N
MeCN
40 °C
Ph
Ph
Ph
Ph
Ph
1a
2a
19, 72%
20, 71%a
21, 80%b
d.r. = 1.1:1
O
O
O
O
Entry
Time
(h)
Catalyst
(mol%)
t-BuOOH
(equiv)
Yield of 1a Yield of 2a
(%)b
(%)b
H2N
O
S
Ph 22, 71%
1
2
20
18
20
8
MsOH (10)
MsOH (10)
MsOH (10)
MsOH (10)
MsOH (10)
MsOH (5)
MsOH (2.5)
–
0.5
1.0
2.0
3.0
2.0
2.0
2.0
2.0
–
48
79
92
95
91
85
88
34
–
21
14
Scheme 2 Products and isolated yields for the reaction of thiophenol
with various olefins; reaction conditions as given in Table 1, entry 4. a
Thiophenol (3 equiv). b Thiophenol (4 equiv).
3
trace
4
trace
trace
trace
7
5c
6
18
18
18
18
20
20
In general, styrene and its derivatives bearing electron-
donating or electron-withdrawing groups on the aryl ring
were suitable for this protocol, and the corresponding prod-
ucts 1a–g and 3–5 were obtained in good to excellent yields
(81–98%). Vinylnaphthalene gave a lower yield than sty-
rene, providing 6 in 66% yield. The sterically more hindered
α-methylstyrene led to sulfoxide 7 in 83% yield. In contrast,
phenylpropene and α-phenylstyrene resulted in marked re-
duction in yields, and the corresponding products 8 and 9
were only isolated in 24% and 37% yields, respectively. Both
alkenes were incompletely converted under the standard
7
8
30
9
MsOH (10)
–
98
10
–
–
99
a Reaction conditions: styrene (0.5 mmol), thiophenol (1 mmol), solvent (0.5
mL), 40 °C, 8–24 h. t-BuOOH was used as a 5.5 M solution in decane.
b Yields were determined by 1H NMR analysis of the crude reaction mixture
using 1,3,5-trimethylbenzene as internal standard.
c 0.6 mmol of thiophenol was used.
© Georg Thieme Verlag Stuttgart · New York — Synlett 2016, 27, A–E

C
H.-L. Yue, M. Klussmann
Letter
Synlett
conditions, and prolonged reaction times or increased reac-
tion temperatures had no significant effect. Better yields
were achieved with cyclic styrene derivatives, generating
sulfoxides 10 and 11 in around 50% yield. Vinyl pyridines
gave no sulfoxide at all, most likely because these basic sub-
strates deactivated the acid catalyst.
action of alkyl thiols with aliphatic olefins gave similar
yields of products 33 and 34 as was the case with thiophe-
nol (cf. 12 and 14).
This catalytic system could also be used for the oxida-
tive addition of thiophenol to alkynes to afford the vinyl
sulfoxides 35–37 (Scheme 4). The product yield with a ter-
minal alkyne was higher than with internal alkynes (cf.
product 35 with 36 and 37). The E/Z diastereoselectivity
was not very high in all cases, attaining 4.9:1 in the case of
product 36. However, the diastereomers of 35 could be
readily separated by column chromatography. For 37, the
diastereomers could not be distinguished by NMR spectros-
copy.
Cyclic and open-chain aliphatic olefins gave the corre-
sponding products 12–14 in low amounts only, with the ex-
ception of cyclopentadiene, which furnished the allylic
sulfoxide 13 in a reasonable yield of 40%. Better yields were
achieved with electron-deficient olefins. While methyl ac-
rylate afforded the desired product 15a in low yield of 15%
only, other acrylate esters gave products 15b–e in yields
ranging from 52–72%. In most of these reactions, an addi-
tional equivalent of thiophenol was added to the reaction
mixture because an increased formation of disulfide was
observed. Acrylonitrile and acrylamides could also be suc-
cessfully employed. The nitrile sulfoxide 16 was only
formed in low amounts, but the amides 17–22 were formed
in good yields of up to 80%. Radical cascade cyclization to
oxindoles was not observed with the N-phenyl acrylamide
substrates.15
O
10 mol% MsOH
3 equiv t-BuOOH
S
R1
Ph
+
Ph SH
Ph
Ph
MeCN, 40 °C
2–48 h
R1
O
O
O
S
S
S
Ph
Ph
Ph
Ph
37, 41%
35, 82%
E/Z = 1.3:1
36, 40%
E/Z = 4.9:1
Scheme 4 Substrate scope for the reaction of thiophenol with various
alkynes. Reagents and conditions: alkyne (0.5 mmol), thiol (1 mmol),
solvent (0.5 mL), 40 °C, 2–48 h, under air; yields of isolated pure prod-
ucts.
The scope of the reaction was further investigated using
a variety of thiols (Scheme 3).
O
10 mol% MsOH
3 equiv t-BuOOH
S
+
R2 SH
R1
R1
R2
MeCN, 40 °C
2–48 h
R1 = Ph, PhCH2CH2
For further mechanistic insight, the reaction between
styrene and thiophenol was conducted in the presence of
2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO). The adduct
38 could be obtained in 5% yield, while the formation of 1a
was completely suppressed. The reaction of diphenyl disul-
fide with styrene did not provide 1a under standard reac-
tion conditions, ruling out the intermediacy of disulfides.
Trace amounts of β-hydroxysulfide 39, β-hydroxysulfoxide
40, and β-keto sulfoxide 41 could be isolated as byproducts
(Figure 1), in addition to the above-mentioned disulfide.
O
S
O
S
O
S
Ph
Ph
F
Ph
23, 99%
24, 90%
25, 91%
Cl
Br
O
S
O
S
O
S
Ph
Ph
Ph
Ph
Ph
26, 93%
27, 92%
28, 92%
O
S
O
S
O
Ph
S
Ph
Ph
29, 93%
30, 17%
31, 15%
N
O
S
O
S
O
S
O
O
S
OH
OH
O
S
O
O
S
Ph
S
Ph
Ph
Ph
Ph Ph
Ph Ph
Ph Ph
Ph
38, 5%
39
40
41
32, 8%
33, 9%
34, 12%
Figure 1 Isolated byproducts of the reaction and in the presence of
TEMPO, respectively
Scheme 3 Products and isolated yields for the reaction of styrene with
various thiols; reaction conditions as given in Table 1, entry 4.
Substituted thiophenols containing either electron-do-
nating or electron-withdrawing groups were all suitable
substrates and generated the corresponding products 23–
28 in excellent yields. 2-Thionaphthol could also be used in
the reaction to give the expected product 29 in 93% yield.
The aliphatic thiols benzylthiol, tert-butylthiol, and cyclo-
hexylthiol afforded the desired products 30–34 in low
yields only, indicating that stable arylthiyl radicals are re-
quired for high yields in the present transformation. The re-
On the basis of the above results and the generally ac-
cepted radical mechanism of the thiol-ene reaction,2,3
a
likely reaction mechanism is depicted in Scheme 5. Thiyl
radicals are formed by hydrogen atom transfer (HAT) from
the thiol, a process that can be accelerated by the addition
of t-BuOOH. Addition to the olefin gives alkyl radical 42,
which affords the sulfide through HAT from a second thiol
molecule, regenerating a thiyl radical and thus leading to a
radical-chain reaction. In the case of electron-deficient
© Georg Thieme Verlag Stuttgart · New York — Synlett 2016, 27, A–E

D
H.-L. Yue, M. Klussmann
Letter
Synlett
alkenes, a Michael addition reaction might take place in-
stead of a radical chain.16 Finally, oxidation of the sulfide by
t-BuOOH is accelerated by the acid catalyst and leads to the
observed products.
(2) (a) Pan, X.-Q.; Zou, J.-P.; Yi, W.-B.; Zhang, W. Tetrahedron 2015,
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+
R'
[t-BuOOH]
S
(4) For selected examples, see: (a) Surendra, K.; Krishnaveni, N. S.;
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Org. Lett. 2015, 17, 1180.
R
SH
R'
R
S
R'
R
42
R
SH
R
+
O
[HX]
t-BuOOH
S
R
S
R'
X
H
(5) For selected examples, see: (a) Oswald, A. J. Org. Chem. 1959, 24,
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H
O
t-Bu
O
S
R'
R
43
Scheme 5 Suggested mechanism of product formation
A certain accelerating effect of t-BuOOH addition on the
radical chain reaction (‘peroxide effect’)17 is supported by
the increased conversion of styrene in the presence of high-
er loadings (see Supporting Information, Table S1). The cat-
alytic effect of added acid on the sulfur oxidation is evident
from the fact that, without acid, nearly equal amounts of
sulfide and sulfoxide are formed under standard conditions
(Table 1, entries 3 and 7). Possibly the acid is accelerating
this step by hydrogen bonding, acting as a proton shuttle, as
indicated in the transition state model 43 (Scheme 5). Such
an activation mode has been suggested before in asymmet-
ric sulfoxidations with chiral acids.13b,c
In summary, we have developed a method for sulfoxide
synthesis through direct oxidative addition of thiols to
alkenes or alkynes by employing t-BuOOH as oxidant and
methanesulfonic acid as catalyst. Aliphatic olefins and thi-
ols were less effective, but good yields were achieved with
styrenes, acrylic acid derivatives, alkynes, and thiophenols.
Acknowledgment
H.-L.Y. thanks the Chinese Scholarship Council for support and M.K.
thanks the DFG for support by a Heisenberg scholarship (KL 2221/4-
1).
Supporting Information
Supporting information for this article is available online at
http://dx.doi.org/10.1055/s-0035-1562480.
S
u
p
p
ortiInfogrmoaitn
S
u
p
p
ortioInfgrmoaitn
References and Notes
(1) New address: Key Laboratory of Tibetan Medicine Research,
Northwest Institute of Plateau Biology, Chinese Academy of Sci-
ences, Xining 810001, P. R. of China.
© Georg Thieme Verlag Stuttgart · New York — Synlett 2016, 27, A–E

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H.-L. Yue, M. Klussmann
Letter
Synlett
(14) Synthesis of (Phenethylsulfinyl)benzene (1a) – Typical Proce-
dure
tography on silica gel (hexane–acetone = 8:1) to afford 1a as a
clear oil (102 mg, 89%). 1H NMR (500 MHz, CDCl3): δ = 7.64–
7.62 (m, 2 H), 7.54–7.48 (m, 3 H), 7.29–7.26 (m, 2 H), 7.22–7.16
(m, 3 H), 3.12–2.99 (m, 3 H), 2.91–2.86 (m, 1 H). 13C NMR (125
MHz, CDCl3): δ = 143.63 (Ar q), 138.77 (Ar q), 131.05 (Ar CH),
129.30 (Ar CH), 128.76 (Ar CH), 128.57 (Ar CH), 126.72 (Ar CH),
124.02 (Ar CH), 58.33 (CH2), 28.19 (CH2). ESI-HRMS: m/z calcd
for [C14H14OSNa]+: 253.065756 [M + Na+]; found: 253.065900.
(15) Chen, J.-R.; Yu, X.-Y.; Xiao, W.-J. Synthesis 2015, 47, 604.
(16) Nair, D. P.; Podgórski, M.; Chatani, S.; Gong, T.; Xi, W.; Fenoli, C.
R.; Bowman, C. N. Chem. Mater. 2014, 26, 724.
To a 1 mL screw-cap vial charged with a magnetic stirring bar,
anhydrous MeCN (0.5 mL), styrene (0.5 mmol, 1 equiv), thio-
phenol (1.0 mmol, 2 equiv), t-BuOOH (5.5 M solution in decane,
1.5 mmol, 3 equiv), and methanesulfonic acid (3.55 μL, 10
mol%) were added in that order. The reaction mixture was
stirred and heated to 40 °C. The vial was closed containing a
small headspace of air. After full conversion was reached, as
indicated by TLC, the reaction mixture was diluted, a small
amount of silica was added, and the solvent was removed under
vacuum. The resulting residue was purified by column chroma-
(17) Kharasch, M. S.; Mayo, F. R. J. Am. Chem. Soc. 1933, 55, 2468.
© Georg Thieme Verlag Stuttgart · New York — Synlett 2016, 27, A–E