Regio- and stereoselective synthesis of 1-aryl-1-thio-2-thiophosphinylethene derivatives via a radical process

Article abstract of DOI:10.1016/j.tet.2008.12.071

Radical additions of S-thiophosphinyl dithiocarbonate to terminal aromatic alkynes afford (E)-1-aryl-1-thio-2-thiophosphinylethene derivatives regio- and stereoselectively in high yields. The transformations of the products are also described.

Full text of DOI:10.1016/j.tet.2008.12.071

Tetrahedron 65 (2009) 1553–1558
Contents lists available at ScienceDirect
Tetrahedron
journal homepage: www.elsevier.com/locate/tet
Regio- and stereoselective synthesis of 1-aryl-1-thio-2-thiophosphinylethene
derivatives via a radical process
*
*
Akinori Sato, Hideki Yorimitsu , Koichiro Oshima
Department of Material Chemistry, Graduate School of Engineering, Kyoto University, Kyoto-daigaku Katsura, Nishikyo-ku, Kyoto 615-8510, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Radical additions of S-thiophosphinyl dithiocarbonate to terminal aromatic alkynes afford (E)-1-aryl-1-
thio-2-thiophosphinylethene derivatives regio- and stereoselectively in high yields. The transformations
of the products are also described.
Received 4 December 2008
Received in revised form 20 December 2008
Accepted 23 December 2008
Available online 27 December 2008
Ó 2009 Elsevier Ltd. All rights reserved.
Keywords:
Radical reaction
C–P bond formation
C–S bond formation
Dithiocarbonate
1. Introduction
After reporting the reaction shown in Scheme 1,4b we explored
another intermolecular radical addition reaction to introduce both
Radical additions of heteroatom–heteroatom bonds to alkenes
and alkynes are fundamental methods to introduce two hetero-
atoms to organic molecules in one operation. Among them, radical
additions of dicharcogenides to carbon–carbon multiple bonds
have been widely investigated.1 However, in spite of the increasing
utilities of organophosphorus compounds in organic chemistry,2
examples of radical additions of phosphorus–heteroatom bonds are
limited.3,4 Our group recently reported a radical thiophosphination
of alkynes with thiophosphines (Scheme 1).4b The reaction pro-
ceeds through the addition of sulfur-centered radical to the ter-
minal carbon of alkyne followed by the efficient reaction of the
resulting vinyl radical with the phosphino group of thiophosphine.
phosphorus and sulfur atoms to alkynes with the regioselectivity
opposite to the previous reaction. Here we report the synthesis of
(E)-1-aryl-1-thio-2-thiophosphinylethene derivatives via a radical
process using
a
thiophosphinylated dithiocarbonate.5 Stereo-
selective syntheses of (E)-1-aryl-1-thio-2-thiophosphinylethenes
are scarcely reported.6
2. Result and discussion
A mixture of phenylacetylene (1a), S-diphenylthiophosphinyl
O-ethyl dithiocarbonate (2), and a catalytic amount of dilauroyl
peroxide (DLP, (nC11H23CO2)2)7 was heated in benzene at reflux
temperature for 4 h (Table 1, entry 1). NMR analysis of the crude
mixture indicated the formation of the corresponding adduct, S-2-
diphenylthiophosphinyl-1-phenylethenyl O-ethyl dithiocarbonate
(91%, a single isomer). Silica gel column chromatography followed
by gel permeation chromatography (GPC)8 afforded 3a in 76% yield.
We confirmed by X-ray analysis that the E isomer was exclusively
formed.
R
SR''
radical initiator
heat
+
R
C
C
H
R'2P SR''
SR''
R'2P
SR''
R
R
C C H
R''S PR'2
via
This reaction would proceed as follows (Scheme 2). Initially, an
undecyl radical, thermally generated from DLP, would attack the
sulfur atom of the thiocarbonyl moiety of 2 to generate diphe-
nylthiophosphinoyl radical (A) with the formation of O-ethyl S-
Scheme 1. Our previous work.4b
undecyl dithiocarbonate.5 The phosphorus-centered radical
A
would react with the triple bond of 1a to furnish vinyl radical B.9 An
equilibrium would exist between radicals B and B0. However, only
radical B would react with 2 to furnish radical C (vide infra). Finally,
* Corresponding authors. Tel.: þ81 75383 2441; fax: þ81 75383 2438.
E-mail addresses: yori@orgrxn.mbox.media.kyoto-u.ac.jp (H. Yorimitsu),
oshima@orgrxn.mbox.media.kyoto-u.ac.jp (K. Oshima).
0040-4020/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2008.12.071

1554
A. Sato et al. / Tetrahedron 65 (2009) 1553–1558
the 1H NMR spectra of 3b–3k with the spectrum of 3a. Both elec-
Table 1
Addition of 2 to various terminal alkynes
tron-rich and electron-deficient aryl-substituted acetylenes reac-
ted with 2 to afford 3 in high yields. The addition of 2 to sterically
hindered alkynes like 1c and 1d proceeded smoothly (entries 3 and
4). Functional groups such as alkoxy (entry 5), keto (entry 6), cyano
(entry 7), ester (entry 8), formyl (entry 9), and bromo (entry 10)
moieties remained unchanged under the reaction conditions. It is
worth noting that a hydroxy group did not affect the reactivity
(entry 11). However, attempts to perform addition reactions of 2
across 1-dodecyne, trimethylsilylacetylene, methyl propiolate, and
1-phenylpropyne resulted in failure, suffering from low conversion.
The reaction of thiophosphinyl dithiocarbonate 2 with p-
diethynylbenzene (1l) efficiently afforded p-bis(1-thio-2-thio-
phosphinylethenyl)benzene 3l. The double addition proceeded
with perfect regio- and stereoselectivity in a very high yield
(Scheme 4).
S
Ar
S
P
S
S
P
5 mol% DLP
benzene
reflux, 4 h
+
C C H
1
Ar
Ph
Ph
Ph
Ph
EtO
S
EtO
S
2 (1.2 eq)
3 (E/Z > 99/1)
Entry
Ar
Ph (1a)
4-Me–C6H4 (1b)
2,4,6-Me3–C6H2 (1c)
1-Naphthyl (1d)
4-MeO–C6H4 (1e)
4-Ac–C6H4 (1f)
Products
Isolated yield (%)
1
2
3
4
5
6
7
8
3a
3b
3c
3d
3e
3f
3g
3h
3i
76a (91)c
71b (89)c
83a
73
88
92
85
79
4-NC–C6H4 (1g)
4-MeO2C–C6H4 (1h)
4-OHC–C6H4 (1i)
4-Br–C6H4 (1j)
9
10
11
88
3j
3k
87b
4-HOCH2–C6H4 (1k)
88
a
Yield after purification by GPC.
Yield after recrystallization.
b
c
S
Yield in parentheses was determined by 31P NMR.
H
C C
S
C C H
1l
EtO
S
Ph2P
S
S
5 mol% DLP
+
S
S
benzene
reflux, 4 h
S
fragmentation of radical C would produce 3a and regenerate the
phosphorus-centered radical A to complete the radical chain.
PPh2
OEt
P
Ph
EtO
S
2 (3.0 eq)
Ph
3l 92%
Scheme 4.
S
P
We next attempted to carry out stereospecific transformations
of product 3. It was found that hydrolysis of the dithiocarbonate
moieties of 3 followed by S-alkylation proceeded10 in the presence
of excess potassium hydroxide and reactive alkyl halides 4 to pro-
vide thiophosphinylated vinyl sulfides 5 in excellent yields with
retention of configuration (Table 2).11
Ph
Ph
Ph
B'
Ph
S
P
Ph
C
1a
C
H
DLP
Ph
Ph
B
n
C11H23
S
S
P
S
P
Ph
Ph
Ph
EtO
S
2
2
Ph
A
Table 2
n
S
P
C11H23
Hydrolysis of 3 followed by alkylation with 4
S
Ph
Ph
S
Ph
S
P
S
Ar
S
P
Ar
S
P
5 eq KOH
MeOH, r.t.
EtO
S
3a
+
Ph
Ph
R
X
Ph
Ph
R
Ph
Ph
EtO
S
EtO
S
S
4 (5 eq)
C
3
5
Scheme 2.
Entry
3
R–X (4)
Me–I (4a)
Allyl–Br (4b)
Bn–Br (4c)
Me–I (4a)a
Products
Isolated yield (%)
E/Z
The E selectivity of the reaction can be explained as outlined in
Scheme 3. Vinyl radical B0 would reside in larger population than
radical B0 in the equilibrium because radical B has a favorable E
configuration. However, the diphenylthiophosphinyl moiety and
reagent 2 are so large that the steric repulsion between B0 and 2
would prevent the reaction. Hence, only B can react with 2 to form
product 3a exclusively.
1
2
3
4
3a
3a
3a
3b
5aa
5ab
5ac
5ba
98
98
88
93
98:2
98:2
99:1
98:2
a
KOH and 4a (3.2 equiv) were used.
Finally, cross-coupling reactions of vinyl sulfide 5aa with or-
ganometallic reagents were examined.12 Coupling reactions with
Grignard reagents resulted in failure. However, treatment of 5aa
Ph
Ph
S
OEt
with diethylzinc in the presence of
a catalytic amount of
P
S
NiCl2(dppp) in cyclopentyl methyl ether (CPME) at 0 ꢀC afforded
ethylated compound 6 effectively (Scheme 5). This cross-cou-
pling reaction proceeded without loss of stereochemistry, albeit
S
S
2
Ph
S
P
S
P
Ph
Ph
S
Ph
Ph
Ph
Ph
P
S
Ph
S
S
B
B'
2
fast
too slow
OEt
S
Ph
S
P
Ph
Ph
Ph
S
P
MeS
EtO
S
P
Ph
S
P
Ph
5aa
Ph
Ph
+
15 mol% NiCl2(dppp)
CPME, 0 °C, 4.5 h
Ph
Ph
Ph
Ph
P
Ph
Ph
EtO
S
(E/Z = 98/2)
Ph
Et
Et
+
6 75%
(Z/E = 98/2)
7 14%
(Z/E = 96/4)
3a
3a'
Et2Zn
(3 eq)
Scheme 3.
2.0 eq (TMS)3SiH
20 mol% AIBN
benzene, reflux, 4 h
88% (Z/E = 98/2)
A variety of aryl-substituted terminal alkynes underwent the
radical addition reaction with complete regio- and stereoselectivity
(Table 1). The E configurations of 3 were deduced from comparing
Scheme 5.

A. Sato et al. / Tetrahedron 65 (2009) 1553–1558
1555
partial desulfidation at the thiophosphinyl moiety was observed.
4.2. Experimental procedures
Phosphine sulfide 6 could be converted to 7 by the action of
tris(trimethylsilyl)silane via a radical pathway.13 The overall re-
action can be regarded as a trans-carbophosphination of the
4.2.1. Preparation of S-diphenylthiophosphinyl O-ethyl
dithiocarbonate (2)
starting alkyne to prepare a
b
-disubstituted alkenylphosphine
Under argon, diphenylthiophosphinyl chloride (3.08 mL,
15.0 mmol) was added to a suspension of potassium O-ethyl
dithiocarbonate (2.64 g, 16.5 mmol) in THF (20.0 mL) at ambient
temperature. After 4 h, a diluted NaCl solution was poured into the
suspension and the mixture was extracted twice with hexane/
EtOAc¼2:1. The organic extracts were dried over Na2SO4, filtered,
and concentrated in vacuo. Silica gel column chromatography
(hexane/EtOAc¼20:1 to 10:1) afforded S-diphenylthiophosphinyl
O-ethyl dithiocarbonate (2) as a yellow solid in 84% yield (4.25 g,
12.6 mmol).
stereoselectively.14
3. Conclusion
We have developed an intermolecular addition of S-thio-
phosphinyl dithiocarbonate to arylacetylenes to produce (E)-1-
aryl-1-thio-2-thiophosphinylethene derivatives, which were
difficult to be prepared. Taking advantage of mild radical condi-
tions, the reactions proceed without loss of various functional
groups. Further transformation of the products enabled us to syn-
thesize stereocontrolled, multisubstituted alkenyl phosphines.
In light of the importance of organophosphorus and organo-
sulfur compounds, the products or their derivatives can be useful
for diverse purposes such as a new type of ligands, building blocks
for organic synthesis, and components of supramolecular
structures.
4.2.2. General procedure for additions of thiophosphinyl
dithiocarbonate 2 to alkynes 1
Under argon,
a mixture of alkyne 1 (0.50 mmol), S-
diphenylthiophosphinyl O-ethyl dithiocarbonate (2, 0.20 g,
0.60 mmol), and DLP (0.010 g, 0.025 mmol) was heated in boiling
benzene (2.0 mL) for 4 h. After cooled to room temperature, the
solvent was removed in vacuo. Silica gel column chromatography
(hexane/EtOAc) followed by GPC or recrystallization (if needed)
afforded a 1-aryl-1-thio-2-thiophosphinylethene derivative 3 as
a white or yellow solid (E/Z>99:1). The E configuration was
assigned by X-ray analysis (See 4.2.7.2).
4. Experimental
4.1. General
4.2.3. Double addition of thiophosphinyl dithiocarbonate 2 to 1l
Under argon, a mixture of p-diethynylbenzene (1l, 0.063 g,
0.50 mmol), S-diphenylthiophosphinyl O-ethyl dithiocarbonate (2,
0.51 g, 1.5 mmol), and DLP (0.010 g, 0.025 mmol) was heated in
boiling benzene (3.0 mL) for 4 h. After cooled to room temperature,
the solvent was removed in vacuo. Filtration with EtOAc followed
by short column chromatography on silica gel (CHCl3) afforded 1,4-
bis{(E)-2-diphenylthiophosphinyl-1-(ethoxythiocarbonylsulfanyl)-
ethenyl}benzene (3l) as a pale yellow solid in 92% yield (0.37 g,
0.46 mmol).
4.1.1. Instrumentation
1H NMR (500 MHz) and 13C NMR (125.7 MHz) spectra were
taken on a Varian UNITY INOVA 500 spectrometer and were
obtained in CDCl3 with tetramethylsilane as an internal standard.
31P NMR (121.5 MHz) spectra were taken on a Varian MERCURY 300
spectrometer and were obtained in CDCl3 with 85% H3PO4 solution
as an external standard. Chemical shifts (d) are in parts per million
relative to tetramethylsilane at 0.0 ppm for 1H, relative to CDCl3 at
77.0 ppm for 13C, and relative to H3PO4 at 0.0 ppm for 31P. DEPT, H–
H COSY, H–C COSY, and NOE analyses allowed the assignments of
the signals of each compounds. IR spectra were determined on
a SHIMADZU FTIR-8200PC spectrometer. Mass spectra were de-
termined on a JEOL Mstation 700 spectrometer. Melting points
were determined on Yanaco micro melting point apparatus. TLC
analyses were performed on commercial glass plates bearing
a 0.25-mm layer of Merck Silica gel 60F254. Silica gel (Wakogel 200
mesh) was used for column chromatography. Gel permeation
chromatography (GPC) was performed by LC-908 (Japan Analytical
Industry Ltd., two in-line JAIGEL-2H, toluene, 10 mL/min, UV and RI
detectors). Elemental analyses were carried out at the Elemental
Analysis Center of Kyoto University.
4.2.4. General procedure for transformation of dithiocarbonates 3
to sulfides 5
Transformation of 3a to 5aa is representative. Under argon,
ground potassium hydroxide (0.056 g, 1.0 mmol) was added to
a suspension of dithiocarbonate 3a (0.088 g, 0.20 mmol) and
iodomethane (4a, 0.062 mL, 1.0 mmol) in methanol (3.0 mL) at
ambient temperature. After stirring for 30 min, the mixture was
quenched with a diluted NaCl/NH4Cl solution and extracted three
times with hexane/EtOAc¼1:1. The organic extracts were dried
over Na2SO4, filtered, and concentrated in vacuo. Silica gel column
chromatography (hexane/EtOAc¼10:1) afforded {2-diphenylthio-
phosphinyl-1-(methylthio)ethenyl}benzene (5aa) as a white solid
in 98% yield (0.072 g, 0.20 mmol, E/Z¼98:2).
4.1.2. Materials
Unless otherwise noted, materials obtained from commercial
suppliers were used without further purification. Benzene,
methanol, CPME, diphenylthiophosphinyl chloride, and potas-
sium hydroxide were purchased from Wako Pure Chemicals.
Potassium O-ethyl dithiocarbonate and NiCl2(dppp) were avail-
able from TCI. DLP and iodomethane were obtained from Nacalai
Tesque. Neat diethylzinc was purchased from Aldrich and was
diluted with degassed dry hexane to prepare a 1.0 M solution,
which was stored under argon. Benzene was dried over slices of
sodium. Alkynes were commercially available or readily prepared
by the conventional Sonogashira reactions. S-Diphenylthio-
phosphinyl O-ethyl dithiocarbonate (2) was synthesized from
potassium O-ethyl dithiocarbonate and diphenylthiophosphinyl
chloride (vide infra).
4.2.5. Cross-coupling reaction of 5aa with diethylzinc
Under argon, diethylzinc (1.0 M hexane solution, 0.60 mL,
0.60 mmol) was added to a suspension of (E)-{2-diphenylthio-
phosphinyl-1-(methylthio)ethenyl}benzene
(5aa,
0.073 g,
0.20 mmol) and NiCl2(dppp) (0.016 g, 0.030 mmol) in CPME
(3.0 mL) at 0 ꢀC. The reaction was monitored by TLC. After 4.5 h,
the mixture was quenched with diluted NaCl solution and
extracted three times with EtOAc, dried over Na2SO4, filtered
through a pad of FlorisilÒ, and concentrated in vacuo. Silica gel
column chromatography (hexane/EtOAc¼20:1 to 10:1) afforded 1-
diphenylthiophosphinyl-2-phenyl-1-butene (6) as a white solid in
75% yield (0.052 g, 0.15 mmol, Z/E¼98:2) and 1-diphenylphos-
phino-2-phenyl-1-butene (7) in 14% yield (8.7 mg, 0.027 mmol, Z/
E¼96:4).

1556
A. Sato et al. / Tetrahedron 65 (2009) 1553–1558
4.2.6. Desulfidation of vinylphosphine sulfide 6
(m, 2H), 7.50–7.54 (m, 1H), 7.54–7.58 (m, 1H), 7.67 (dd, J¼7.0, 1.0 Hz,
1H), 7.73–7.80 (m, 3H); 13C NMR (CDCl3)
13.36, 70.56, 124.26,
Under argon, a mixture of vinylphosphine sulfide 6 (0.035 g,
0.10 mmol), tris(trimethylsilyl)silane (0.062 mL, 0.20 mmol), and
AIBN (3.2 mg, 0.020 mmol) was heated in boiling benzene (2.0 mL)
for 4 h. After cooled to room temperature, the solvent was removed
in vacuo. Silica gel column chromatography (hexane/EtOAc¼50:1 to
20:1) afforded 1-diphenylphosphino-2-phenyl-1-butene (7) as
a colorless oil in 88% yield (0.028 g, 0.088 mmol, Z/E¼98:2).
d
125.40, 125.84, 126.11, 127.15 (d, J¼12.9 Hz), 128.08, 128.24 (d,
J¼12.5 Hz), 129.30 (d, J¼1.4 Hz), 129.38 (d, J¼85.4 Hz), 129.89,
129.93 (d, J¼1.0 Hz), 130.21 (d, J¼2.9 Hz), 130.99 (d, J¼11.0 Hz),
131.10 (d, J¼10.0 Hz), 131.27 (d, J¼2.9 Hz), 132.53 (d, J¼5.8 Hz),
132.96, 133.07 (d, J¼86.9 Hz), 133.48 (d, J¼75.0 Hz), 147.43 (d,
J¼1.5 Hz), 208.82 (d, J¼1.4 Hz); 31P NMR (CDCl3)
d 29.31. Found: C,
66.25; H, 4.82%. Calcd for C27H23OPS3: C, 66.10; H, 4.73%. Mp: 115–
116 ꢀC.
4.2.7. Characterization data
4.2.7.1. S-Diphenylthiophosphinyl O-ethyl dithiocarbonate (2). IR
4.2.7.6. (E)-S-2-Diphenylthiophosphinyl-1-(4-methoxyphenyl)ethen-
(neat) 1436, 1366, 1254, 1096, 1033, 722, 689, 653 cmꢁ1
;
1H NMR
yl O-ethyl dithiocarbonate (3e). IR (Nujol) 1609, 1504, 1437, 1253,
(CDCl3)
d
1.00 (t, J¼7.0 Hz, 3H), 4.40 (q, J¼7.0 Hz, 2H), 7.47–7.52 (m,
1241, 1041, 834, 747, 712, 629 cmꢁ1 1H NMR (CDCl3)
; d 1.12 (t,
4H), 7.53–7.58 (m, 2H), 7.95–8.02 (m, 4H); 13C NMR (CDCl3)
d
12.65,
J¼7.0 Hz, 3H), 3.68 (s, 3H), 4.44 (q, J¼7.0 Hz, 2H), 6.48–6.52 (m, 2H),
71.12, 128.67 (d, J¼13.4 Hz), 131.62 (d, J¼11.4 Hz), 132.23 (d,
6.91 (d, J¼16.0 Hz, 1H), 7.26–7.32 (m, 4H), 7.32–7.37 (m, 2H), 7.45–
J¼2.9 Hz), 132.67 (d, J¼84.5 Hz), 204.27 (d, J¼4.4 Hz); 31P NMR
7.49 (m, 2H), 7.77–7.84 (m, 4H); 13C NMR (CDCl3)
d 13.28, 55.23,
(CDCl3)
d
59.46. Found: C, 53.36; H, 4.43%. Calcd for C15H15OPS3: C,
70.49, 112.81, 128.26 (d, J¼12.9 Hz), 128.72 (d, J¼74.0 Hz), 129.01 (d,
53.23; H, 4.47%. Mp: 58–59 ꢀC.
J¼4.8 Hz), 131.21 (d, J¼3.3 Hz), 131.35 (d, J¼10.5 Hz), 131.39, 132.22
(d, J¼85.5 Hz), 148.88, 160.41, 209.71; 31P NMR (CDCl3)
d 29.09.
4.2.7.2. (E)-S-2-Diphenylthiophosphinyl-1-phenylethenyl O-ethyl di-
Found: C, 61.25; H, 4.98%. Calcd for C24H23O2PS3: C, 61.25; H, 4.93%.
Mp: 105–106 ꢀC.
thiocarbonate (3a). IR (Nujol) 1438, 1240, 1103, 1041, 715, 693,
635 cmꢁ1; 1H NMR (CDCl3)
d
1.07 (t, J¼7.0 Hz, 3H), 4.42 (q, J¼7.0 Hz,
2H), 6.96–7.01 (m, 2H), 7.02 (d, J¼15.5 Hz, 1H), 7.01–7.05 (m, 1H),
4.2.7.7. (E)-S-1-(4-Acetylphenyl)-2-(diphenylthiophosphinyl)ethenyl
7.25–7.30 (m, 4H), 7.31–7.36 (m, 2H), 7.48–7.52 (m, 2H), 7.75–7.82
O-ethyl dithiocarbonate (3f). IR (Nujol) 1681, 1436, 1247, 1099, 1037,
(m, 4H); 13C NMR (CDCl3)
d
13.14, 70.53, 127.42, 128.27 (d,
819, 708, 627 cmꢁ1; 1H NMR (CDCl3)
d
1.12 (t, J¼7.0 Hz, 3H), 2.49 (s,
J¼12.9 Hz), 129.28, 129.65, 130.13 (d, J¼73.1 Hz), 131.27
3H), 4.43 (q, J¼7.0 Hz, 2H), 7.13 (d, J¼15.5 Hz,1H), 7.26–7.31 (m, 4H),
(d, J¼3.4 Hz), 131.30 (d, J¼10.5 Hz), 132.03 (d, J¼86.0 Hz), 136.44
7.32–7.37 (m, 2H), 7.54–7.60 (m, 4H), 7.75–7.81 (m, 4H); 13C NMR
(d, J¼4.8 Hz), 149.15, 209.24 (d, J¼1.5 Hz); 31P NMR (CDCl3)
d
28.97.
(CDCl3)
d
13.25, 26.62, 70.70, 127.28, 128.38 (d, J¼12.9 Hz), 129.94,
Found: C, 62.74; H, 5.04%. Calcd for C23H21OPS3: C, 62.70; H, 4.80%.
Mp: 106–107 ꢀC. The E configuration was determined by X-ray
analysis. Crystallographic data for the structure has been deposited
with the Cambridge Crystallographic Data Centre (CCDC 710609).
Copies of the data can be obtained, free of charge, on application to
the Director, CCDC, 12 Union Road, Cambridge, CB2 1EZ, UK. Fax: 44
1223 336033 or e-mail: deposit@ccdc.cam.ac.uk.
131.33 (d, J¼11.0 Hz), 131.45 (d, J¼2.9 Hz), 131.81 (d, J¼86.9 Hz),
132.67 (d, J¼71.6 Hz), 137.01, 141.05 (d, J¼4.8 Hz), 147.53, 197.26,
208.53; 31P NMR (CDCl3)
d 28.73. Found: C, 62.35; H, 4.77%. Calcd
for C25H23O2PS3: C, 62.22; H, 4.80%. Mp: 102–103 ꢀC.
4.2.7.8. (E)-S-1-(4-Cyanophenyl)-2-(diphenylthiophosphinyl)ethenyl
O-ethyl dithiocarbonate (3g). IR (Nujol) 2233, 1436, 1248, 1105,
1028, 901, 841, 716, 694, 636 cmꢁ1 1H NMR (CDCl3)
; d 1.14 (t,
4.2.7.3. (E)-S-2-Diphenylthiophosphinyl-1-(4-methylphenyl)ethenyl
J¼7.0 Hz, 3H), 4.44 (q, J¼7.0 Hz, 2H), 7.18 (d, J¼15.0 Hz, 1H), 7.26–
O-ethyl dithiocarbonate (3b). IR (Nujol) 1559, 1506, 1438, 1244,
7.30 (m, 2H), 7.30–7.35 (m, 4H), 7.37–7.42 (m, 2H), 7.58–7.62 (m,
1102, 1043, 827, 715, 631 cmꢁ1; 1H NMR (CDCl3)
d
1.10 (t, J¼7.0 Hz,
2H), 7.74–7.80 (m, 4H); 13C NMR (CDCl3)
d 13.26, 70.82, 112.43,
3H), 2.16 (s, 3H), 4.43 (q, J¼7.0 Hz, 2H), 6.75–6.79 (m, 2H), 6.97 (d,
118.16, 128.48 (d, J¼12.9 Hz), 130.30, 131.03, 131.28 (d, J¼10.9 Hz),
131.48 (d, J¼86.0 Hz), 131.67 (d, J¼2.8 Hz), 134.08 (d, J¼70.6 Hz),
141.09 (d, J¼4.8 Hz), 146.31, 208.10 (d, J¼1.5 Hz); 31P NMR (CDCl3)
J¼16.0 Hz, 1H), 7.25–7.30 (m, 4H), 7.32–7.38 (m, 4H), 7.75–7.82 (m,
4H); 13C NMR (CDCl3)
d 13.20, 21.19, 70.50, 128.03, 128.22 (d,
J¼12.9 Hz), 129.55 (d, J¼74.0 Hz), 129.63, 131.11 (d, J¼2.9 Hz), 131.35
d 28.41. Found: C, 61.97; H, 4.14%. Calcd for C24H20NOPS3: C, 61.91;
(d, J¼11.0 Hz), 132.22 (d, J¼85.9 Hz), 133.61 (d, J¼4.9 Hz), 139.49,
H, 4.33%. Mp: 132–133 ꢀC.
149.27, 209.53; 31P NMR (CDCl3)
d 29.11. Found: C, 63.27; H, 5.07%.
Calcd for C24H23OPS3: C, 63.41; H, 5.10%. Mp: 123–124 ꢀC.
4.2.7.9. (E)-S-2-Diphenylthiophosphinyl-1-(4-methoxycarbonylphe-
nyl)ethenyl O-ethyl dithiocarbonate (3h). IR (Nujol) 1722, 1717, 1436,
4.2.7.4. (E)-S-2-Diphenylthiophosphinyl-1-mesitylethenyl
O-ethyl
1283, 1244, 1111, 1036, 707, 629 cmꢁ1 1H NMR (CDCl3)
; d 1.09 (t,
dithiocarbonate (3c). IR (Nujol) 1436, 1245, 1104, 1031, 852, 712,
J¼7.0 Hz, 3H), 3.88 (s, 3H), 4.41 (q, J¼7.0 Hz, 2H), 7.12 (d, J¼15.5 Hz,
691, 633 cmꢁ1; 1H NMR (CDCl3)
d
1.35 (t, J¼7.0 Hz, 3H), 2.13 (s, 3H),
1H), 7.26–7.32 (m, 4H), 7.33–7.38 (m, 2H), 7.52–7.56 (m, 2H), 7.63–
2.16 (s, 6H), 4.68 (q, J¼7.0 Hz, 2H), 6.46 (d, J¼0.5 Hz, 2H), 7.24 (d,
J¼15.5 Hz, 1H), 7.26–7.31 (m, 4H), 7.36–7.41 (m, 2H), 7.65–7.71 (m,
7.67 (m, 2H), 7.74–7.80 (m, 4H); 13C NMR (CDCl3)
d 13.22, 52.17,
70.67, 128.38 (d, J¼12.9 Hz), 128.57, 129.69, 130.30, 131.31 (d,
J¼10.5 Hz), 131.47 (d, J¼2.9 Hz), 131.80 (d, J¼84.5 Hz), 132.31 (d,
J¼71.6 Hz), 140.94 (d, J¼5.3 Hz), 147.77, 166.27, 208.49; 31P NMR
4H); 13C NMR (CDCl3)
d 13.54, 20.27, 20.92, 70.48, 125.14 (d,
J¼78.3 Hz), 128.05, 128.06 (d, J¼12.9 Hz), 129.97 (d, J¼6.3 Hz),
131.04 (d, J¼2.9 Hz), 131.40 (d, J¼11.0 Hz), 132.41 (d, J¼86.4 Hz),
136.33, 138.72, 148.99 (d, J¼2.9 Hz), 208.48; 31P NMR (CDCl3)
(CDCl3) d 28.65. Found: C, 60.16; H, 4.75%. Calcd for C25H23O3PS3: C,
60.22; H, 4.65%. Mp: 132–133 ꢀC.
d
29.83. Found: C, 64.65; H, 5.64%. Calcd for C26H27OPS3: C, 64.70; H,
5.64%. Mp: 93–94 ꢀC.
4.2.7.10. (E)-S-2-Diphenylthiophosphinyl-1-(4-formylphenyl)ethenyl
O-ethyl dithiocarbonate (3i). IR (Nujol) 1699,1684,1558,1439,1243,
4.2.7.5. (E)-S-2-Diphenylthiophosphinyl-1-(1-naphthyl)ethenyl O-
1100, 1041, 825, 715 cmꢁ1; 1H NMR (CDCl3)
d
1.11 (t, J¼7.0 Hz, 3H),
ethyl dithiocarbonate (3d). IR (Nujol) 1559, 1507, 1436, 1231, 1049,
4.43 (q, J¼7.0 Hz, 2H), 7.17 (d, J¼15.5 Hz, 1H), 7.26–7.32 (m, 4H),
1036, 798, 714, 633 cmꢁ1; 1H NMR (CDCl3)
d
1.20 (dd, J¼7.0, 7.0 Hz,
7.33–7.38 (m, 2H), 7.49–7.52 (m, 2H), 7.64–7.67 (m, 2H), 7.75–7.81
3H), 4.48 (dq, J¼11.0, 7.0 Hz, 1H), 4.53 (dq, J¼11.0, 7.0 Hz, 1H), 6.75–
6.80 (m, 2H), 6.85–6.90 (m, 1H), 7.20 (dd, J¼8.0, 7.0 Hz, 1H), 7.26–
7.32 (m, 2H), 7.32–7.40 (m, 3H), 7.37 (d, J¼16.5 Hz, 1H), 7.41–7.47
(m, 4H), 9.85 (s, 1H); 13C NMR (CDCl3)
d 13.23, 70.74, 128.42 (d,
J¼12.9 Hz), 128.60, 130.35 (d, J¼1.0 Hz), 131.31 (d, J¼10.5 Hz), 131.54
(d, J¼3.3 Hz), 131.69 (d, J¼86.0 Hz), 133.20 (d, J¼71.1 Hz), 136.11,

A. Sato et al. / Tetrahedron 65 (2009) 1553–1558
1557
142.44 (d, J¼4.8 Hz), 147.23, 191.39 (d, J¼1.4 Hz), 208.34 (d,
6.97–7.02 (m, 2H), 7.03–7.08 (m, 1H), 7.18–7.23 (m, 4H), 7.26–7.31
(m, 4H), 7.31–7.40 (m, 5H), 7.54–7.60 (m, 4H); 13C NMR (CDCl3)
J¼1.4 Hz); 31P NMR (CDCl3)
d 28.61. Found: C, 61.35; H, 4.62%. Calcd
for C24H21O2PS3: C, 61.52; H, 4.52%. Mp: 110–111 ꢀC.
d
35.57, 114.31 (d, J¼86.0 Hz), 127.60 (two signals are overlapped),
128.03 (d, J¼12.4 Hz), 128.71, 128.82, 128.98, 129.23 (d, J¼0.9 Hz),
130.71 (d, J¼2.8 Hz), 131.21 (d, J¼11.0 Hz), 133.63 (d, J¼85.9 Hz),
135.18, 135.64 (d, J¼6.3 Hz), 157.86 (d, J¼2.4 Hz); 31P NMR (CDCl3)
4.2.7.11. (E)-S-1-(4-Bromophenyl)-2-(diphenylthiophosphinyl)ethen-
yl O-ethyl dithiocarbonate (3j). IR (Nujol) 1480, 1436, 1264, 1248,
1107, 1030, 819, 707 cmꢁ1; 1H NMR (CDCl3)
d
1.13 (t, J¼7.0 Hz, 3H),
d 28.55. Found: C, 72.97; H, 5.54%. Calcd for C27H23PS2: C, 73.27; H,
4.44 (q, J¼7.0 Hz, 2H), 7.06 (d, J¼15.5 Hz, 1H), 7.07–7.12 (m, 2H),
5.24%. Mp: 153–154 ꢀC.
7.28–7.36 (m, 6H), 7.36–7.41 (m, 2H), 7.74–7.81 (m, 4H); 13C NMR
(CDCl3)
d
13.24, 70.67, 123.61, 128.36 (d, J¼12.4 Hz), 130.50, 131.18,
4.2.7.17. 4-{(E)-2-Diphenylthiophosphinyl-1-(methylthio)ethenyl}to-
131.34 (d, J¼10.5 Hz), 131.41 (d, J¼2.9 Hz), 131.59 (d, J¼74.0 Hz),
luene (5ba). IR (Nujol) 1558, 1503, 1437, 1098, 914, 825, 752, 709,
131.75 (d, J¼87.4 Hz), 135.44 (d, J¼4.8 Hz), 147.60, 208.72; 31P NMR
629 cmꢁ1
;
1H NMR (CDCl3)
d
2.17 (s, 3H), 2.36 (s, 3H), 6.01 (d,
J¼15.5 Hz, 1H), 6.75–6.79 (m, 2H), 7.12–7.16 (m, 2H), 7.23–7.28 (m,
4H), 7.29–7.34 (m, 2H), 7.73–7.79 (m, 4H); 13C NMR (CDCl3)
16.40
(CDCl3)
d 28.76. Found: C, 53.04; H, 3.74%. Calcd for C23H20BrOPS3:
C, 53.18; H, 3.88%. Mp: 118–119 ꢀC.
d
(d, J¼1.5 Hz), 21.14, 111.77 (d, J¼88.3 Hz), 127.99 (d, J¼12.4 Hz),
128.22, 128.89 (d, J¼1.0 Hz), 130.63 (d, J¼2.9 Hz), 131.34 (d,
J¼10.5 Hz), 133.11 (d, J¼6.6 Hz), 133.81 (d, J¼86.0 Hz), 138.88,
4.2.7.12. (E)-S-2-Diphenylthiophosphinyl-1-(4-hydroxymethylphen-
yl)ethenyl O-ethyl dithiocarbonate (3k). IR (Nujol) 3600–3300 (br),
1654, 1506, 1436, 1242, 1041, 826, 710, 692, 626 cmꢁ1
;
1H NMR
160.03 (d, J¼2.9 Hz); 31P NMR (CDCl3)
d 29.03. Found: C, 69.37; H,
(CDCl3)
d
1.12 (t, J¼7.0 Hz, 3H), 1.50 (br s, 1H), 4.44 (q, J¼7.0 Hz, 2H),
5.56%. Calcd for C22H21PS2: C, 69.44; H, 5.56%. Mp: 106–107 ꢀC.
4.51 (s, 2H), 6.96–7.00 (m, 2H), 7.03 (d, J¼16.0 Hz, 1H), 7.26–7.31 (m,
4H), 7.32–7.37 (m, 2H), 7.46–7.51 (m, 2H), 7.75–7.82 (m, 4H); 13C
4.2.7.18. (Z)-1-Diphenylthiophosphinyl-2-phenylbutene
(6). IR
1.10
NMR (CDCl3)
d
13.24, 64.70, 70.56, 125.73, 128.28 (d, J¼12.4 Hz),
(Nujol) 1559, 1436, 1098, 725, 712, 697 cmꢁ1; 1H NMR (CDCl3)
d
129.94 (d, J¼1.0 Hz), 130.62 (d, J¼73.0 Hz), 131.21 (d, J¼3.4 Hz),
(t, J¼7.5 Hz, 3H), 2.57 (qdd, J¼7.5, 1.5, 0.5 Hz, 2H), 6.35 (d, J¼19.0,
131.33 (d, J¼10.5 Hz), 132.14 (d, J¼85.9 Hz), 135.78 (d, J¼4.8 Hz),
1.5 Hz, 1H), 6.94–7.02 (m, 3H), 7.12–7.16 (m, 2H), 7.22–7.27 (m, 4H),
142.05, 148.73, 209.20 (d, J¼1.4 Hz); 31P NMR (CDCl3)
d
28.97.
7.28–7.33 (m, 2H), 7.71–7.77 (m, 4H); 13C NMR (CDCl3)
d 12.35, 35.14
Found: C, 61.05; H, 4.90%. Calcd for C24H23O2PS3: C, 61.25; H, 4.93%.
Mp: 111–112 ꢀC.
(d, J¼15.8 Hz), 118.82 (d, J¼86.8 Hz), 127.47, 127.67, 128.01 (d,
J¼12.4 Hz), 128.02 (d, J¼1.5 Hz), 130.66 (d, J¼2.8 Hz), 131.26 (d,
J¼10.5 Hz), 133.48 (d, J¼85.4 Hz), 138.57 (d, J¼6.8 Hz), 162.87; 31P
4.2.7.13. p-Bis{(E)-2-diphenylthiophosphinyl-1-(ethoxythiocarbonyl-
NMR (CDCl3) d 28.44. Found: C, 75.96; H, 6.09%. Calcd for C22H21PS:
sulfanyl)ethenyl}benzene (3l). IR (Nujol) 1577, 1559, 1436, 1246,
C, 75.83; H, 6.08%. Mp: 79–80 ꢀC. The Z configuration of the major
1098, 1041, 846, 752, 715, 632 cmꢁ1
;
1H NMR (CDCl3)
d
1.39 (t,
isomer was determined by NOE experiments.
J¼7.0 Hz, 6H), 4.62 (q, J¼7.0 Hz, 4H), 7.06 (s, 4H), 7.10 (d, J¼15.5 Hz,
2H), 7.24–7.29 (m, 8H), 7.31–7.36 (m, 4H), 7.71–7.78 (m, 8H); 13C
4.2.7.19. (Z)-1-Diphenylphosphino-2-phenylbutene (7). IR (neat)
NMR (CDCl3)
d
13.70, 70.96, 128.34 (d, J¼12.9 Hz), 128.77, 131.29 (d,
2966, 1585, 1478, 1433, 1095, 1026, 837, 742, 696 cmꢁ1 1H NMR
;
J¼10.5 Hz), 131.54 (d, J¼3.4 Hz), 131.86 (d, J¼85.9 Hz), 134.41 (d,
(CDCl3)
(d, J¼3.5, 1.5 Hz, 1H), 7.17–7.20 (m, 2H), 7.26–7.33 (m, 9H), 7.36–7.41
(m, 4H); 13C NMR (CDCl3)
d
1.09 (t, J¼7.5 Hz, 3H), 2.60 (qdd, J¼7.5,1.5, 0.5 Hz, 2H), 6.25
J¼71.6 Hz), 136.99 (d, J¼4.8 Hz), 146.75, 209.85 (d, J¼1.9 Hz); 31P
NMR (CDCl3)
d
28.41. Found: C, 59.56; H, 4.47%. Calcd for
d
13.00, 33.98 (d, J¼6.3 Hz), 123.41 (d,
C40H36O2P2S6: C, 59.83; H, 4.52%. Mp: 158–159 ꢀC.
J¼7.6 Hz), 127.56, 127.80, 128.11, 128.21 (d, J¼3.8 Hz), 128.32
(d, J¼6.3 Hz), 132.56 (d, J¼18.1 Hz), 140.38 (d, J¼10.1 Hz), 141.11
4.2.7.14. (E)-{2-Diphenylthiophosphinyl-1-(methylthio)ethenyl}ben-
zene (5aa). IR (Nujol) 1684, 1653, 1558, 1540, 1507, 1437, 1095, 715,
(d, J¼7.6 Hz), 160.86 (d, J¼24.9 Hz); 31P NMR (CDCl3)
d
ꢁ28.20.
Calcd for C22H21PS: C, 83.52; H, 6.69%. HRMS found: 316.1382
697 cmꢁ1
;
1H NMR (CDCl3)
d
2.38 (s, 3H), 6.05 (d, J¼15.0 Hz, 1H),
6.97–7.01 (m, 2H), 7.03–7.08 (m, 1H), 7.24–7.29 (m, 6H), 7.29–7.34
(m, 2H), 7.73–7.80 (m, 4H); 13C NMR (CDCl3)
(D
¼þ0.2 ppm); calcd for C22H21P: 316.1381 [M]þ. The Z configura-
tion of the major isomer was determined by NOE experiments.
d
16.40 (d, J¼1.5 Hz),
112.09 (d, J¼88.3 Hz), 127.59, 128.08 (d, J¼12.4 Hz), 128.91, 128.99
(d, J¼1.0 Hz), 130.78 (d, J¼2.9 Hz), 131.31 (d, J¼10.5 Hz), 133.72 (d,
J¼85.9 Hz), 135.95 (d, J¼6.3 Hz), 159.89 (d, J¼2.9 Hz); 31P NMR
Acknowledgements
This work is supported by Grants-in-Aid for Scientific Research
and GCOE Research from the Ministry of Education, Culture, Sports,
Science, and Technology, Japan. A.S. acknowledges JSPS for financial
support.
(CDCl3)
d 28.83. Found: C, 68.55; H, 5.25%. Calcd for C21H19PS2: C,
68.82; H, 5.23%. Mp: 106–107 ꢀC. The E configuration of the major
isomer was determined by NOE experiments.
4.2.7.15. (E)-{1-Allylthio-2-(diphenylthiophosphinyl)ethenyl}benze-
Supplementary data
ne (5ab). IR (Nujol) 1558, 1539, 1507, 1437, 1096, 906, 719,
632 cmꢁ1; 1H NMR (CDCl3)
d
3.43 (ddd, J¼6.5, 1.0, 1.0 Hz, 2H), 5.19–
NOE spectra of 5aa, 6, and 7. Supplementary data associated
with this article can be found in the online version, at doi:10.1016/
j.tet.2008.12.071.
5.26 (m, 2H), 5.87 (ddt, J¼17.0, 10.0, 6.5 Hz, 1H), 6.25 (d, J¼15.5 Hz,
1H), 6.97–7.03 (m, 2H), 7.03–7.08 (m, 1H), 7.23–7.35 (m, 8H), 7.71–
7.79 (m, 4H); 13C NMR (CDCl3)
d
35.97, 114.35 (d, J¼86.0 Hz), 119.04,
127.59, 128.06 (d, J¼12.5 Hz), 128.97, 129.23 (d, J¼0.9 Hz), 130.80 (d,
References and notes
J¼2.9 Hz),131.31 (d, J¼10.5 Hz),131.76,133.65 (d, J¼85.9 Hz),135.70
(d, J¼6.3 Hz), 157.91 (d, J¼2.4 Hz); 31P NMR (CDCl3)
d 28.77.
1. Reviews: (a) Ogawa, A. J. Synth. Org. Chem. Jpn. 1995, 53, 869–880; (b) Ogawa,
A.; Hirao, T. Rev. Heteroat. Chem. 1998, 18, 1–10; (c) Renaud, P. Top. Curr. Chem.
2000, 208, 81–112.
2. (a) Murphy, P. J. In Organophosphorus Reagents; Murphy, P. J., Ed.; Oxford Uni-
versity Press: New York, NY, 2004; Chapter 1; (b) Quin, L. D. Guide to Organo-
phosphorus Chemistry; John Wiley & Sons: New York, NY, 2000.
3. (a) Tzschach, V. A.; Baensch, S. J. Prakt. Chem. 1971, 313, 254–258; (b) Morse,
J. G.; Mielcarek, J. J. J. Fluorine Chem. 1988, 40, 41–49; (c) Kawaguchi, S.; Nagata,
S.; Shirai, T.; Tsuchii, K.; Nomoto, A.; Ogawa, A. Tetrahedron Lett. 2006, 47,
Found: C, 70.37; H, 5.37%. Calcd for C23H21PS2: C, 70.38; H, 5.39%.
Mp: 81–82 ꢀC.
4.2.7.16. (E)-{1-Benzylthio-2-(diphenylthiophosphinyl)ethenyl}benz-
ene (5ac). IR (Nujol) 1653, 1558, 1540, 1507, 1437, 1097, 906, 772,
719 cmꢁ1
;
1H NMR (CDCl3)
d
4.05 (s, 2H), 6.17 (d, J¼15.0 Hz, 1H),

1558
A. Sato et al. / Tetrahedron 65 (2009) 1553–1558
3919–3922; (d) Carta, P.; Puljic, N.; Robert, C.; Dhimane, A.-L.; Fensterbank, L.;
de Andrade, L. H. Tetrahedron Lett. 2000, 41, 161–163; (c) Kondoh, A.; Yorimitsu,
H.; Oshima, K. Org. Lett. 2007, 9, 1383–1385.
ˆ
Lacote, E.; Malacria, M. Org. Lett. 2007, 9, 1061–1063; (e) Shirai, T.; Kawaguchi,
S.; Nomoto, A.; Ogawa, A. Tetrahedron Lett. 2008, 49, 4043–4046.
4. (a) Sato, A.; Yorimitsu, H.; Oshima, K. Angew. Chem., Int. Ed. 2005, 44, 1694–
1696; (b) Wada, T.; Kondoh, A.; Yorimitsu, H.; Oshima, K. Org. Lett. 2008, 10,
1155–1157.
5. Zard and co-workers recently reported many radical reactions using dithio-
carbonates. See: (a) Zard, S. Z. Angew. Chem., Int. Ed. 1997, 36, 672–685; (b)
Quiclet-Sire, B.; Zard, S. Z. Phosphorus, Sulfur Silicon Relat. Elem. 1999, 153, 137–
145; (c) Zard, S. Z. In Radicals in Organic Synthesis; Renaud, P., Sibi, M. P., Eds.;
Wiley-VCH: Weinheim, 2001; Vol. 1, pp 90–108; (d) Quiclet-Sire, B.; Zard, S. Z.
Top. Curr. Chem. 2006, 264, 201–236; (e) Quiclet-Sire, B.; Zard, S. Z. Chem.dEur.
J. 2006, 12, 6002–6016; (f) Zard, S. Z. Org. Biomol. Chem. 2007, 5, 205–213; (g)
Corbet, M.; Ferjancic, Z.; Quiclet-Sire, B.; Zard, S. Z. Org. Lett. 2008, 10, 3579–
3582.
7. The use of AIBN as a radical initiator instead of DLP worked as well.
8. GPC, which can separate compounds by the sizes of molecules, was used when
the conventional silica gel column chromatography could not separate 3 from 2.
9. Parsons, A. F.; Sharpe, D. J.; Taylor, P. Synlett 2005, 2981–2983.
10. Sato, A.; Yorimitsu, H.; Oshima, K. Chem. Asian J. 2007, 2, 1568–1573.
11. Thiophosphinylated vinyl sulfides 5 are gradually isomerized overnight to a 1:1
mixture of E/Z isomers in CDCl3.
12. (a) Dubbaka, S. R.; Vogel, P. Angew. Chem., Int. Ed. 2005, 44, 7674–7684 and
references cited therein; (b) Kanemura, S.; Kondoh, A.; Yorimitsu, H.; Oshima,
K. Synthesis 2008, 2659–2664.
13. Romeo, R.; Wozniak, L. A.; Chatgilialoglu, C. Tetrahedron Lett. 2000, 41, 9899–
9902.
14. (a) Meijer, J.; Westmijze, H.; Vermeer, P. Recl. Trav. Chim. Pays-Bas 1976, 95, 102–
104; (b) Kanemura, S.; Kondoh, A.; Yorimitsu, H.; Oshima, K. Org. Lett. 2007, 9,
2031–2033.
6. Some examples to synthesize Z isomers were reported. See: (a) Han, L.-B.;
Tanaka, M. Chem. Lett. 1999, 863–864; (b) Braga, A. L.; Alves, E. F.; Silveira, C. C.;