Metal-free oxidative coupling of alkyl chlorides with thiols: An efficient access to sulfoxides

Article abstract of DOI:10.1016/j.tetlet.2019.151492

An efficient and step-economical access to sulfoxides from thiols and alkyl halides in the presence of I₂O₅ and DBU via direct oxidative couplings is described here. It is the first case that combined Williamson sulfide synthesis and subsequent sulfide oxidation into one step manipulation for sulfoxides preparation. This protocol features wide substrate scope, mild and metal-free conditons, the use of naturally abundant starting materials and avoidance of over-oxidation.

Full text of DOI:10.1016/j.tetlet.2019.151492

Journal Pre-proofs
Metal-free oxidative coupling of alkyl chlorides with thiols: an efficient access
to sulfoxides
Qian Liu, Xiaoqian Zhao, Feng Xu, Gaoqiang Li
PII:
S0040-4039(19)31291-2
DOI:
https://doi.org/10.1016/j.tetlet.2019.151492
TETL 151492
Reference:
Tetrahedron Letters
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Revised Date:
Accepted Date:
15 November 2019
2 December 2019
4 December 2019
Please cite this article as: Liu, Q., Zhao, X., Xu, F., Li, G., Metal-free oxidative coupling of alkyl chlorides with
thiols: an efficient access to sulfoxides, Tetrahedron Letters (2019), doi: https://doi.org/10.1016/j.tetlet.
2019.151492
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Tetrahedron Letters
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Metal-free oxidative coupling of alkyl chlorides with thiols: an efficient access to
sulfoxides
Qian Liu, Xiaoqian Zhao, Feng Xu and Gaoqiang Li
Key Laboratory of Macromolecular Science of Shaanxi Province, School of Chemistry and Chemical Engineering, Shaanxi Normal University, Xi'an, Shaanxi
710062, P. R. China
———
ARTICLE INFO
ABSTRACT
 Corresponding author. Tel.: +0-000-000-0000; fax: +0-000-000-0000; e-mail: author@university.edu
Article history:
Received
Received in revised form
Accepted
An efficient and step-economical access to sulfoxides from thiols and alkyl halides in the
presence of I2O5 and DBU via direct oxidative couplings is described here. It is the first case
that combined Williamson sulfide synthesis and subsequent sulfide oxidation into one step
manipulation for sulfoxides preparation. This protocol features wide substrate scope, mild and
metal-free conditons, the use of naturally abundant starting materials and avoidance of over-
oxidation.
Available online
Keywords:
Oxidation
Coupling
2009 Elsevier Ltd. All rights reserved.
Sulfoxides
Metal-free
Introduction
Because of this, a number of reagents and protocols have been
established to produce sulfoxide analogues,8 mainly including i)
Sulfoxides are the most privileged scaffolds in natural
products,1 medicinal chemistry,2 and functionalized materials.3
Their derivatives were generally employed as important
intermediates in the preparation of biologically active molecules,
especially marketed pharmaceuticals such as Omeprazole,4a
Provigil,4b Mesoridazine,4c and Sulfinpyrazone (Figure 1).4d
Furthermore, they have been extensively applied to reactions of
carbon-carbon bond formation, molecular rearrangements, and
chemical transformations.5 In recent years, sulfoxides have also
been utilized to serve as chiral ligands in enantioselective
catalysis6 or as tunable directing groups in metal-catalyzed C-H
oxidation of sulfides;8a,9 ii) nucleophilic substitution of sulfinyl
derivatives with organometallic reagents;10 and iii) recently
developed trapping of in situ generated sulfenate anions with
i) Oxidation of sulfides
O
R1X
Williamson ether synthesis
step 1
[O]
S
S
+
R1
R2
R1
R2
step 2
R2SM
M = Na, K, etc.
ii) Substitution of sulfinyl derivatives with organometallic reagents
O
S
O
O
S
H
N
O
S
O
S
R4M
O
S
or
NH2
N
R1
R4
M = Mg, Li
R1
OR2
R1
NR2R3
N
CH3
OCH3
H3C
Omeprazole
iii) alkylation or arylation of in situ generated sulfenate anions
Provigil
(heartburn and esophagitis)
O
S
(gastric acid inhibitor)
O
S
R1
O
S
[Pd] or [Cu]
R2X
O
S
N
O
R1
LG
R1
R2
N
O
S
O
N
N
S
S
O
LG = CO2R, SiR3
CH3
R1
Scheme 1. Representative strategies for the synthesis of sulfoxides
Mesoridazine
(neuroleptic drugs)
Sulfinpyrazone
(platelet inhibitor)
electrophiles;11 Among them, the selective oxidation of thioethers
is undoubtly the most straightforward and widely applied strategy
for the synthesis of corresponding sulfoxides. Although a wide
range of oxidizing systems have been well established and shown
excellent performance in sulfoxidation, there are still various
limitations such as the utilization of expensive or heavy metal
Figure 1. Sulfoxides in marketed pharmaceuticals
bond functionalization reactions.7

2
Tetrahedron
catalysts, super acidic or basic conditions, unfavorable toxic
I2O5. To our delight, the desired sulfoxide 3aa was isolated in
wastes, over-oxidation to sulfones and so on. Hence, the
development of metal-free, mild, efficient and highly selective
methods to prepare sulfoxides is still highly desirable.
60% yield when potassium hydroxide was used as a base in
acetonitrile at 90 C (Table 1, entry 1). We next investigated the
o
effects of other bases, including K2CO3, KOtBu, Et3N, DBU and
DMAP. It can be found that KOtBu and K2CO3 both led to
sharply decreased yields and DBU proved to be the most
effective to enhance the yield into 84% (Table 1, entry 5).
Attempt to increase and reduce the amounts of DBU both failed
to improve the reaction performance (Table 1, entries 7 and 8).
Further screening of the solvents suggested that toluene was the
best reaction medium, affording the desired sulfoxide 3aa in 95%
yield (Table 1, entry 15). The reaction was ineffective in water.
Nevertheless, it is important to emphasize that alcohols can also
serve as suitable mediums, albiet with a slightly dropped yield
without consuming any oxidant. Further increase of the loading
of I2O5 failed to improve the reaction performance (Table 1, entry
16). In contrast, decreased yield was found when the loading of
the oxidant was reduced to 1 equivalent (Table 1, entry 17). The
reaction temperature was next examined. When the reaction was
performed at 80oC, only 78% yield of sulfoxide 3aa was isolated.
When the reaction was carried out at 100oC, the sulfoxide 3aa
was delivered in 87% yield. Thus, the optimal conditions were
determined as I2O5 (1.2 equiv.) and DBU (2.0 equiv.) in toluene
at 90oC.
Recently, iodine pentoxide (I2O5) as a cheap, green, and
reliable inorganic oxidant has received increasingly attention and
been successfully applied for metal-free oxidative couplings of
alkenes with thiols to obtain sulfones (Scheme 2).12 On the other
hand, the thiols are generally employed as starting materials to
prepare sulfides by Williamson’s method, which could be easily
further oxidized to sulfoxides. Nevertheless, so far, the direct
synthesis of sulfoxides from thiols and halides via a direct
oxidative coupling process has remained unreported. Herein, we
wish to report a novel, efficient and step-economical access to
sulfoxides from thiols and alkyl halides via direct oxidative
couplings (Scheme 2).
Previous work
(Preparation of sulfones)
O
S
R3
I2O5
R2
+
R3SH
R1
R1
O
DBU, THF
R2
This work
(Preparation of sulfoxides)
I2O5
With the optimal reaction conditions in hand, we next
investigated the scope and generality of this transformation using
various benzyl halides and 4-methylbenzenethiol. In general,
benzyl chlorides bearing either electron-withdrawing or electron-
donating groups on the phenyl rings were suitable for this
transformation, affording the desired sulfoxides in moderate to
excellent yields. It should be noted that electron-rich benzyl
chlorides are more reactive than electron-poor ones, generating
the corresponding sulfoxides in much higher yields (up to 99%
O
S
R1X
+
R2SH
DBU, toluene
R1
R2





Step-economical synthesis
Metal-free conditions
Without super acid or base
Simple and widely available starting materials
Avoidance of over-oxidation
Scheme 2. Preparation of sulfones and sulfoxides via oxidative couplings
Table 2. Results for oxidative coupling between 4-methylbenzenethiol and
with I2O5
various alkyl halidesa,b
Results and discussion
I2O5 (1.2 equiv.)
DBU (2.0 equiv.)
toluene, 90oC
O
S
SH
R
Cl
+
R
Table 1. Optimization of the reaction conditionsa
O
SH
I2O5, base
Cl
S
3aa-3am
2a-m
1a
+
solvent
O
S
O
temp. 8 h
O
S
S
1a
3aa
2a
Entry
1
2
3
4
Base
Solvent
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
1,4-dioxane
ethyl acetate
THF
H2O
MeOH
EtOH
toluene
toluene
toluene
toluene
toluene
Temp./oC
90
90
90
90
90
90
90
Yieldb/%
60
10
trace
nd
84
45
78
82
60
84
89
nd
87
89
95
90
85
3ac
, 7h, 97%
3ab
, 8h, 79%
3aa
, 8h, 95%
KOH
K2CO3
KOtBu
NEt3
O
S
O
S
O
S
OCH3
3ae
3af
, 4h, 92%
, 6h, 98%
3ad
, 6h, 99%
5
DBU
DMAP
DBU
DBU
DBU
DBU
DBU
DBU
DBU
DBU
DBU
DBU
DBU
DBU
DBU
Cl
O
Cl
6
O
S
O
S
7c
S
Cl
Br
8d
9
90
90
3ag
3ah
3ai
, 5h, 47%
, 7h, 97%
, 5h, 88%
10
11
12
13
14
15
16e
17f
18
19
reflux
reflux
90
reflux
reflux
90
90
90
80
100
F
O
S
O
S
O
S
F
3aj
, 9h, 73%
3al
, 9h, 64%
3h, 90%c
3ak
, 9h, 51%
NO2
O
S
78
87
3am
, 9h, 56%
a Reaction conditions: 1a (0.5 mmol), 2a (0.5 mmol), I2O5 (1.2 equiv.), base
(2 equiv.), solvent (2 mL). b Isolated yields. c DBU (1.5 equiv.). d DBU (2.5
equiv.). e I2O5 (1.5 equiv.). f I2O5 (1 equiv.).
a Reaction conditions: 1a (0.5 mmol), 2a-m (0.5 mmol), I2O5 (1.2 equiv.), DBU
(2 equiv.), toluene (2 mL), 90 oC. b Isolated yields. c 4-bromobenzyl bromide was
used.
Initially, 4-methylbenzenethiol (1a) and benzyl chloride (2a)
were selected as the model substrates to determine the optimized
reaction conditions for sulfoxide synthesis in the presence of

yield). Significantly, the halo substituents such as fluoro, chloro,
and bromo on the benzene ring were well tolerated in this
oxidative coupling process. The sulfoxides with halo substituents
are very useful for they can be easily further derivatized by
means of simple coupling strategies and the likes. It can be found
that these transformations were seriously affected by the steric
effect. Especially, steric-demanding ortho-substituted benzyl
chlorides showed exceptionally high reactivity than para-
substituted ones in those transformations (Table 2, products 3ac,
3ag and 3aj). When 4-bromobenzyl chloride was replaced by 4-
bromobenzyl bromide to perform the transformation, the yield of
product 3al was greatly enhanced from 64% to 90% within 3
hours.
Several control experiments were further conducted to
understand the reaction mechanism as shown in eqs1-3 in
Scheme 3. When the template reaction was carried out in the
absence of DBU as a base, sulfoxide 3aa was completely not
detected in the reaction system. Furthermore, when disulfide was
employed to replace thiol to reacted with benzyl chloride under
standard reaction conditions, it still failed to give any desired
sulfoxide 3aa. This result indicated that disulfide, which was
easily formed by the dimerization of thiyl radical,12 was not a
reaction intermediate. Nevertheless, benzyl tolyl sulfide did
successfully afforded the desired sulfoxide 3aa in 87% yield in
the presence of I2O5. The above results indicated that benzyl tolyl
sulfide might be a key intermediate in current reaction system.
Inspired by the success of the benzyl chlorides, we next
explored the generality of a variety of thiols. The representative
results were summarized in Table 3. All the examined aromatic
and heteroaromatic thiols reacted well and provided the
corresponding benzyl sulfoxides in yields ranging from 21% to
98% (Table 3). A majority of aryl thiols with electron-donating
groups or electron-withdrawing groups exhibited excellent
reactivities, affording the expected sulfoxides in good to
excellent yields. Notably, halogens were also tolerable, which
benefits further functionalization. When heteroaromatic thiols
such as 2-thiophenthiol, 2-benzothiazolethiol and 3-pyridinethiol
were investigated, the corresponding sulfoxides still smoothly
formed, albeit with decreased yields (Table 3, products 3la, 3ma
and 3na). Besides these aryl thiols, several typical alkyl thiols
such as cyclohexanethiol, 1-butanethiol and -toluenethiol were
also selected to perform the transformations under standard
conditions. Unfortunately, no desired product was observed for
these thiols even if stronger base such as KOH was utilized.
O
SH
I2O5 (1.2 equiv.)
S
Cl
(1)
(2)
+
toluene, 90oC, 8 h
3aa
(0%)
2a
1a
O
S
I2O5 (1.2 equiv.)
S
DBU (2 equiv.)
toluene, 90oC, 8 h
Cl
+
S
3aa
(0%)
4a
2a
O
S
I2O5 (1.2 equiv.)
S
(3)
toluene, 90oC, 8 h
3aa
(87%)
5a
Scheme 3. Control experiments
On the basis of the above results, a tentative reaction pathway
was proposed in Scheme 4. Initially, the thiols reacted with
benzyl chlorides in the presence of DBU as a base, producing the
corresponding sulfides. Then the generated sulfides were
oxidized to sulfoxides. This suggested pathway was in
accordance with our preliminary thoughts.
Table 3. Results for oxidative coupling between benzyl chloride with
various thiolsa,b
I2O5 (1.2 equiv.)
DBU (2.0 equiv.)
toluene, 90oC
O
S
Cl
R
SH
+
O
R
I2O5
DBU
S
Ar
S
Ar
+
RSH
ArCH2Cl
1b-o
3ba-3oa
2a
R
R
O
O
S
O
S
O
S
Scheme 4. Tentative reaction pathway
O
Conclusions
3ca
3da
, 8h, 90%
, 8h, 67%
3ba
,8h, 98%
In conclusion, a novel and convenient method has been
successfully developed for the preparation of sulfoxides through
direct oxidative couplings between thiols and alkyl chlorides in
the presence of I2O5 and DBU. This is the first case that
combined Williamson sulfide synthesis and subsequent sulfide
oxidation into one step manipulation for sulfoxides preparation.
A wide range of functionalized benzyl sulfoxides were generated
in good to excellent yields. The salient features of this protocol
including metal-free conditions, without super acid or base,
avoidance of over-oxidation to sulfones, well functional group
tolerance, and readily available starting materials make this
transformation extremely attractive.
O
S
O
S
O
S
Br
Cl
3ea
3ga
, 10h, 74%
, 8h, 56%
3fa
, 8h, 43%
O
S
O
S
O
S
F
Cl
F
3ia
, 8h, 87%
3ha
, 8h, 80%
3ja
, 8h, 64%
O
S
O
S
O
S
S
S
N
Acknowledgments
3ka
, 10h, 64%
3la
3ma
, 10h, 21%
, 10h, 45%
We are grateful for financial support from the National
Natural Science Foudation of China (No.21302117), the Natural
Science Foundation of Shaanxi Province (2018JM2009) and
Fundamental Research Funds for the Central Universities
(GK201903034).
O
S
N
3na
, 8h, 50%
a Reaction conditions: 1b-o (0.5 mmol), 2a (0.5 mmol), I2O5 (1.2 equiv.), DBU
(2 equiv.), toluene (2 mL), 90 oC. b Isolated yields.

4
Tetrahedron
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Metal-free oxidative coupling of alkyl
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sulfoxides
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Qian Liu, Xiaoqian Zhao, Feng Xu and Gaoqiang Li*
O
S
I2O5 (1.2 equiv.)
X
R
R
+
(Het)ArSH
(Het)Ar
DBU (2.0 equiv.),
toluene, 90 oC
X= Cl, Br


Without super acid or base
Simple and widely available starting
Highlights
materials
Avoidance of over-oxidation
 Step-economical synthesis
Metal-free conditions



5
Declaration of interests
☒ The authors declare that they have no known
competing financial interests or personal
relationships that could have appeared to influence
the work reported in this paper.
☐The authors declare the following financial
interests/personal relationships which may be
considered as potential competing interests: