Zn-catalyzed dihydrosulfenylation of alkynes using thiols
a
b
Nobukazu Taniguchi and Kenji Kitayama
a
b
Department of Chemistry, Fukushima Medical University, Fukushima, Japan; Innovation Park, Daicel Corporation, Himeji, Hyogo, Japan
ABSTRACT
ARTICLE HISTORY
Zinc-catalyzed hydrosulfenylation of alkenes afforded regioselectively the corresponding sulfides in
good yields. Furthermore, the method can promote dihydrosulfenylation of alkynes. The procedure
could produce anti-Markovnikov type dithioacetals in excellent yields.
Received 28 March 2019
Accepted 1 April 2019
KEYWORDS
Hydrosulenylation; alkene;
alkyne; thiol; zinc catalyst
GRAPHICAL ABSTRACT
Introduction
were obtained when the reaction was performed with
LaCl or ZnCl (Entries 4–5). Moreover, the zinc-catalyzed
reaction gave good regioselective results in various sol-
vents, and the formation of disulfides was suppressed
3
2
Dithioacetals are important compounds in organic synthe-
[
1]
sis. These are usually prepared from carbonyl compounds
with thiols, [2]and are widely used as convenient
intermediates.
(Entries 6–8). On the other hand, a reaction using PdCl2
and CuF2 afforded unsatisfactory results (Entries 9–10).
Although the method has been well established, a synthe-
sis by alkyne dihydrosulfenylation has been hardly
researched. However, several reports have been published to
Thus, the ZnCl catalyst efficiently promoted the hydrosul-
2
fenylation of styrene.
On the basis of the obtained results, we then focused our
attention on the addition of thiols to alkynes. Initially, to
find suitable conditions, a reaction of 1-phenylacetylene
with toluenethiol was investigated. When the reaction was
treated by zinc catalyst in toluene, the corresponding
dithioacetal 6a was obtained in 76–79% yield (Entries 1–2 in
Table 2). However, as shown in entry 3, the reaction with 4-
bromobenzenethiol afforded vinyl sulfide 7a in 90% yield,
while dihydrosulfenylation hardly proceeded. To promote
this procedure, several zinc salts were examined. When the
[
3,4]
date (Scheme 1).
These procedures employ alkynes hav-
ing electron-withdrawing groups or a specialized calcium
catalyst under microwave irradiation.
As a general rule, it is known that the reaction of alkynes
with thiols affords the corresponding vinyl sufides via rad-
[
5]
ical process (Figure 1). The reaction proceeds via anti-
Markovnikov addition. However, the dihydrosulfenylation
hardly proceeds.
Therefore, we challenged to synthesize regioselectively
anti-Markovnikov type dithioacetals.
To solve this problem, the choice of metal catalysts is
very important. Initially, the activity of numerous metal cat-
alysts for alkene hydrosulfenylations was investigated.
ZiBr catalyst was employed, the expected dithioacetal 6a
2
was obtained in 60% yield, and 7a was produced at 33%
yield (Entry 4). Fortunately, the ZnI catalyzed reaction was
2
increased the yield to 71%, and the formation of vinyl sul-
fide was suppressed (Entry 5). Other zinc catalysts were
inferior to ZnI (Entries 6–7).
2
Results and discussion
Sequentially, various dihydrosulfenylations of alkynes
To establish suitable conditions, the hydosulfenylation of were evaluated (Table 3). When a mixture of terminal aryl
alkenes was surveyed. As shown in Table 1, when a mix- alkynes with thiols was treated by ZnI , the corresponding
2
ture of 4-toluenethiol with styrene was conducted in dithioacetals 6 were obtained in excellent yields. The proced-
CH Cl , the disulfide was obtained in 71% yield (Entry 1). ure with arenethiols afforded excellent results whereas using
2
2
The addition of a CeCl catalyst in DMF gave no product alkyl thiols slightly decreased the reactivity.
3
(
Entry 2). However, the reaction in CH Cl produced the
Furthermore, internal alkynes as well as terminal alkynes
2
2
expected sulfide 2a in 75% yield (Entry 3). Similar results gave good results (Entries 15–16).
CONTACT Nobukazu Taniguchi
ß 2019 Taylor & Francis Group, LLC
Department of Chemistry, Fukushima Medical University, Fukushima, 960-1295, Japan.
Taniguchi, Nobukazu
