Stereoselective hydrozirconation of alkynylsulfide and regioselective synthesis of haloalkenyl sulfide via electrophile-switched halogenation of thioalkenyl zirconocene

Article abstract of DOI:10.1016/j.tetlet.2013.04.122

Stereoselective preparation of alkenyl sulfide was carried out via syn-hydrozirconation of the alkynyl sulfide. Regiochemistry of halogenation of the thioalkenyl zirconocene could be switched by different halides. α-Chloroalkenyl sulfide or β-haloalkenyl

Full text of DOI:10.1016/j.tetlet.2013.04.122

Tetrahedron Letters 54 (2013) 3643–3646
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Tetrahedron Letters
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Stereoselective hydrozirconation of alkynylsulfide and regioselective
synthesis of haloalkenyl sulfide via electrophile-switched halogenation of
thioalkenyl zirconocene
⇑
Weixin Zheng , Ya Hong, Ping Wang, Fenfen Zheng, Yanjing Zhang, Wei Wang
College of Material, Chemistry and Chemical Engineering, Hangzhou Normal University, Xiasha, Hangzhou 310036, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Stereoselective preparation of alkenyl sulfide was carried out via syn-hydrozirconation of the alkynyl sul-
Received 20 February 2013
Revised 10 April 2013
Accepted 29 April 2013
Available online 9 May 2013
fide. Regiochemistry of halogenation of the thioalkenyl zirconocene could be switched by different
halides.
a-Chloroalkenyl sulfide or b-haloalkenyl sulfide (Br, I) could be obtained by the treatment of
NCS or NBS (NIS), respectively. Possible mechanism of halogenation of the thioalkenyl zirconocene was
set up herein.
Ó 2013 Elsevier Ltd. All rights reserved.
Keywords:
Alkynyl sulfide
Hydrozirconation
Syn-addition
Electrophile-switched halogenation
Stereo- and regioselectivity
Alkenyl sulfides are ubiquitous in the nature, which are wide-
spread with therapeutic properties1 and regarded as the significant
synthetic intermediates for their attractive reactivity.2 Generally,
the synthesis of the alkenyl sulfide could be performed by the
cross-coupling of alkenyl halide with thiols in the presence of var-
ious catalysts.3 Hydrothiolation4 of alkyne using catalysts has been
exploited to access alkenyl sulfide, which configured the syn-5 or
anti-addition6 depending on the type of the involved transition-
metal-catalyst or stoichiometric amount of the Lewis acid. Re-
cently, Ying and co-workers have proved that the stereoselectivity
of copper-catalyzed hydrothiolation could be determined by the
presence/absence of a CO2 atmosphere.7 Palladium-catalyzed syn-
thioboration of terminal alkynes also leads to alkenyl sulfide.8
In addition, the reduction of alkynyl sulfide to alkenyl sulfide is
quite limited.9 As we know, hydrozirconation of functionalized al-
kyne10 followed by halogenation10c–e plays an important role in the
synthesis of functionalized alkenyl halide, which could lead to the
formation of polysubstituted alkene. To the best of our knowledge,
electrophilic substitution of alkenyl zirconocene usually positions
on the sp2 carbon attached to the zirconium atom so far.10
abnormal owing to a trans-hydrozirconation and the generation of
b-heteroalkenyl zirconocene,17 which actuated us to study on the
chemistry of the alkynyl sulfide which also carries C–S bond in
the molecular. Therefore, we now wish to report the chemistry of
hydrozirconation of alkynylsulfide. Furthermore, as useful inter-
mediates to prepare structure-defined alkenes,18 haloalkenyl sul-
fides will be produced via regioselectively electrophile-switched
halogenation of thioalkenyl zirconocene.
To the suspension of Schwartz’s reagent (Cp2Zr(H)Cl, 1.2 mmol)
in CH2Cl2 was added 1.0 equiv of alkynyl sulfide 1 at room temper-
ature. The reaction mixture was then stirred till a clearly khaki
solution of (E)-thioalkenyl zirconocene 2 was obtained. Hydrolysis
of 2 with saturated ammonium chloride aqueous afforded (Z)-alke-
nyl sulfide 3a–i in moderate to good yields (Scheme 1).19 The re-
sults are listed in Table 1.
R1
H
SR2
R1
SR2
H
Cp2Zr(H)Cl
CH2Cl2, r.t.
R1
SR2
ZrCp2Cp2Zr
Cl Cl
1
In most cases, the syn-hydrozirconation of internal alkynes with
(E)-2'
(E)-2
heteroatomic groups, such as chalcogenides,11 ZnX,12 BR2,13 SnR3,14
NH4Cl aqueous
SR2
SiR3,15 I+R216etc., provide
a-heteroalkenyl zirconocene as the major
product. However, the behaviors of alkynyl sulfone or sulfoxide are
R1
H
H
3
⇑
Corresponding author. Tel./fax: +86 571 2886 7899.
E-mail address: wxzheng@hznu.edu.cn (W. Zheng).
Scheme 1. Preparation and hydrolysis of (E)-thioalkenyl zirconocene 2.
0040-4039/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2013.04.122

3644
W. Zheng et al. / Tetrahedron Letters 54 (2013) 3643–3646
According to the 3JH–H between the two vinyl protons in product
R1
SR2
H
R1
H
SR2
R1
E
SR2
H
R1
H
SR2
El
3 (Table 1), all of the coupling constants are less than 12 Hz, which
indicated the Z configuration of the product 3. The investigation of
1H NMR spectra of the crude products 3 showed the purity of (Z)-
isomer was above 99%. That is, the hydrozirconation of alkenyl sul-
fide underwent selectively a syn-addition.
and/or
E
ZrCp2Cp2Zr
Cl Cl
4
α−
β −4
(E)-2
(E)-2'
Scheme 2. Reaction of (E)-thioalenyl zirconocene 2 with electrophiles.
To determine the regiochemistry of the hydrozirconation, the
in situ prepared (E)-thioalkenyl zirconocene 2 in CH2Cl2 was
quenched with 1.0 equiv of other electrophiles at 0 °C and gave
the corresponding trisubstituted alkenyl sulfide 4 (Scheme 2).20
The results are listed in Table 2.
Table 2
Reaction of (E)-2 with electrophiles
Entry
R1
R2
Reagents
a
-4:b-4a
Yield of 4b (%)
With the treatment of DCl or NCS, the
a-substituted products
1
2
3
4
5
6
nBu
nBu
nHex
nHex
nBu
nPr
nBu
nPr
nBu
nPr
Bn
DCI/D20
DCI/D20
DCI/D20
DCI/D20
NCS
NCS
NCS
NCS
NBS
NBS
NBS
NBS
NBS
NIS
NIS
NIS
NIS
>99:1
>99:1
>99:1
>99:1
>99:1
94:6
>99:1
>99:1
<1:99
<1:99
<1:99
12:88
<1:99
<1:99
10:90
22:78
26:74
56 (4a)
48 (4b)
52 (4c)
47 (4d)
48 (4e)
55 (4f)
52 (4g)
57 (4h)
85 (4i)
90 (4j)
78 (4k)
88 (4l)
68 (4m)
89 (4n)
85 (4o)
82 (4p)
79 (4q)
4a–h were generated in moderate to good yield. Owing to the cou-
pling and splitting data in the 1H NMR spectra of the products, the
regiochemistry of 4a–h are similar to the results of the literatures
(entries 1–8 in Table 2).11–16 Surprisingly, bromination or iodin-
ation of 2 chiefly gave b-substituted alkenyl sulfide (entries 9–17
in Table 2). Based on the same substrates (entry 5 and entry 10),
the regiochemistry of chlorination of 2 was contrary to that of
the bromination. Therefore, regiochemistry of halogenation of the
thioalkenyl zirconocene could be switched by different halides.
(E)-1-chloroalkenyl sulfide or (E)-2-haloalkenyl sulfide (Br, I) could
be obtained by treatment of NCS or NBS (NIS), respectively.
nBu
7
8
9
nBu
nHex
nBu
nBu
nBu
Et
10
11
12
13
14
15
16
17
nBu
nBu
nHex
nHex
nBu
nBu
nHex
nHex
nPr
nBu
nPr
nBu
Et
Bn
Et
Bn
As to the formation of the
a- or b-haloalkenyl sulfides, a
possible mechanism was hypothesized as follows (Scheme 3).11a
(E)-Thioalkenyl zirconocene 2 was generated from the hydrozirco-
nation of the alkynyl sulfide 1 followed by another addition of
a
Ratio of a
-4:b-4 is based on the 1H NMR spectra of the crude products of the
reactions.
Cp2Zr(H)Cl to 2 to result in
a,b-dizirconium species 5. There
b
Isolated yields.
achieved an equilibrium between the two intermediates of (E)-2
and 5 in this reactive system. Compound 5 was readily converted
into (E)-2 by syn-elimination with release of Cp2Zr(H)Cl because
the empty orbital in sulfur atom could stabilize the carbon anion
attached to zirconium. While Cl+ or D+ was addressed in the reac-
tion, the stabilized carbon anion could undergo electrophilic sub-
R1
H1
[Zr]1
H2
nBu
Cl
NCS
R1= nBu
R1
SR2
1
H1
H2
stitution as well as syn-elimination of [Zr]2–H2 to afford
a-4
6
7
Cp Zr(H)Cl
2
1.2 equiv.
(path a in Scheme 3). b-SR2 elimination with [Zr]2 would occur to
lead to (Z)-alkenyl zirconium 6, which was the probable reason
why the yields of 4a–h (entries 1–8 in Table 2) were lower than
those of b-bromides (4i–m) or iodides (4n–q). The chlorinated
product 7 was detected by GCMS.21 In the case of Br+ or I+, it was
a rigorous challenge for the larger size of the electrophiles to attack
β-Elimination
SR2
2[Zr]1
R1
H1
R1
H1
SR2
Cp Zr(H)Cl
2
[Zr]1
2H
[Zr]
+
(E)-2
+
E
L
E
2
S
5
b
a
2
1
1
-[Zr] H -[Zr] H
the sterically hindered a-C with two bulky groups, thioalkyl and
Syn-Elimination
R1
H
SR2
ES
zirconium. Thus b-anion of 5 was engaged in and underwent a
R1
EL
SR2
syn-elimination of [Zr]1–H1 to give b-4.
The in situ NMR of the mixture of the hydrozirconation of alkynyl
sulfide (R1 = nBu, R2 = nPr) was carried out (Fig. 1). Compound 5 and
(E)-2 were detected in the ratio of 63:37 as well as a trace amount of
(E)-20. As to compound 5, the C–H correlation spectroscopy showed
two dt peaks at 5.50 (J = 7.5, 5.8 Hz, H1) and 5.82 (J = 7.5, 1 Hz, H2)
ppm assignable to the two protons, respectively. In the species
H
α-4
-4
β
+
+
+
E
E
= Large electrophiles: Br , I
= Small electrophiles: D , Cl
[Zr] =
ZrCp2
Cl
L
+
+
+
S
Scheme 3. Mechanism of formation of thioalkenyl sulfide 2 and its regioselective
halogenation.
129.2
Table 1
126.0
nBu
SnPr
Synthesis of (Z) alkenyl sulfide 3
180.3
C
C
nBu
SnPr
148.9
H1
6.17(t)
C
[Zr]1
Entry
R1
R2
1H NMR of
a
-H to thio group
Yield of 3a (%)
H1
C
2[Zr]1
5.50(dt)
2H
3
[Zr]
1
2
3
4
5
6
7
8
9
nBu
nBu
nBu
nBu
nBu
nHex
nHex
Ph
Et
nPr
iPr
nBu
Ph
nPr
nBu
nPr
Bn
5.91 (d, JH–H = 9.2 Hz, 1H)
72 (3a)
68 (3b)
54 (3c)
62 (3d)
59 (3e)
65 (3f)
61 (3g)
60 (3h)
66 (3i)
5.82(dt)
(E)-2
3
5
5.89 (d, JH–H = 9.6 Hz, 1H)
3
5.89 (d, JH–H = 9.6 Hz, 1H)
3
Figure 1. In situ NMR data of the mixture of hydrozirconation.
5.90 (d, JH–H = 9.2 Hz, 1H)
3
6.19 (d, JH–H = 9.2 Hz, 1H)
3
5.89 (d, JH–H = 9.6 Hz, 1H)
3
(E)-2, the sp2 carbon attached zirconium atom appeared character-
istically at 180.3 ppm and the other sp2 carbon at 148.9 ppm.22
When alkynyl sulfide (R1 = nBu, R2 = nPr), Schwart’s reagent, and
NBS were mixed together according to the comments of the re-
5.91 (d, JH–H = 9.2 Hz, 1H)
3
6.43 (d, JH–H = 10.8 Hz, 1H)
3
Ph
6.42 (d, JH–H = 11.2 Hz, 1H)
a
Isolated yields.

W. Zheng et al. / Tetrahedron Letters 54 (2013) 3643–3646
3645
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Scheme 4. Selective conversion of b-haloalkenyl sulfide.
viewer, a mixture of b-bromoalkenyl sulfides 4j and a-product in
the ratio of 90:10 was obtained based on the NMR of crude product.
Initially, before the excess amount of Cp2Zr(H)Cl was consumed, the
ratio of
that after accomplishment of hydrozirconation. It was a favorable
opportunity for bromination of -position. Along with the process-
a-thioalkenyl zirconocene 2 in Scheme 3 was higher than
a
ing of the hydrozirconation, the amount of dizirconium species 5
would increase to result in the formation of b-bromoalkenyl sulfides
4j. Thus b-bromoalkenyl sulfides dominated and the results of this
reaction were quite similar with our former study. Therefore, the
formation of 5 via second hydrozirconation of the species 2 followed
by syn-elimination of [Zr]1H1 could be concluded.
Owing to two functional groups in the molecule, the iodoalke-
nyl sulfide 4o was involved in the NiCl2(dppp)2 catalyzed cross-
coupling reaction to give C–I bond cleavage product 8 in the
isolated yield of 59% and the C–S bond was reserved selectively
(Scheme 4).23 Therefore, the two functional groups could be con-
verted stepwisely, which could be utilized to prepare a polysubsti-
tuted alkene.
It was reported that hydrozirconation of internal alkynyl chalc-
ogenides (Se, Te) in THF mainly gave the a-zirconated alkenyl chal-
cogenide intermediates, in which 2.0 equiv of Cp2Zr(H)Cl was
crucial to perform the total hydrozirconation of alkynyl selenides
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romethane rather than THF was favorable for the reaction of alke-
nyl sulfide with acceptable yield and regio- or stereoselectivity.
The chemistry of 2 differs from that of sele- or tellualkenyl
zirconocene.
In conclusion, we have demonstrated the efficient formation of
thioalkenyl zirconocene species via hydrozirconation of alkynyl
sulfide, which can readily give (Z)-disubstituted alkenyl sulfides
in good yields. Also, the regioselective synthesis of
a- or b-halo-
alkenyl sulfide via electrophile-switched halogenation of thioalke-
nyl zirconocene has been developed, which can be regarded as the
precursor for the synthesis of polysubstituted alkene derivatives.
Further investigation into the reactivity of thioalkenyl zirconecene
is in progress.
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Acknowledgments
This work was supported by the National Natural Science Foun-
dation of China (20972037), the Special Funds for key innovation
team of Zhejiang Province (2010R50017), the Natural Science
Foundation of Zhejiang Province (Y406341), HNUEYT (2011-01-
013), and PCSIRT (IRT 1231).
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19. General experimental procedure for the hydrozirconation of the alkynyl
Supplementary data
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.tetlet.2013.04.
122.
sulfide: to
temperature was added alkynyl sulfide (1.0 mmol). The above reaction
mixture was then stirred till to be clearly khaki solution, which was
a suspension of Cp2Zr(H)Cl (1.2 mmol) in CH2Cl2 at room
References and notes
a
quenched with saturated ammonium chloride aqueous followed extracted
with ether for three times. The combined organic layer was washed with brine
and dried over anhydrous MgSO4. After rotary evaporation, the residue was
purified by column chromatograph (silica gel, hexane as the eluent) to afford
(Z)-alkenyl sulfide 3a–i.
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20. General procedures for the synthesis of trisubstituted alkenyl sulfide 4: at 0 °C,
the solution of in situ prepared (E)-thioalkenyl zirconocene 2 in CH2Cl2 was
quenched with 1.0 equivalent of DCl/D2O (for 4a–d), NCS (1.2 mmol, 0.16 g for
4e–4h), NBS (1.2 mmol, 0.21 g for 4i–4m) and NIS (1.2 mmol, 0.27 g for 4n–
4q), respectively and extracted with diethyl ether. The combined organic layer
was washed with brine and dried over anhydrous MgSO4. After rotary
evaporation, the residue was purified by column chromatograph (silica gel,
hexane as the eluent) to afford 4a–q.
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