A facile [4+1] type synthetic route to thiophenes from dienol silyl ethers and elemental sulfur

Article abstract of DOI:10.1016/j.tet.2005.10.024

A facile method for the synthesis of thiophene derivatives via the reaction of readily accessible dienol silyl ethers with elemental sulfur is described. Dienol silyl ethers and elemental sulfur, when heated at 180°C in the presence of MS4A, provided 3-si

Full text of DOI:10.1016/j.tet.2005.10.024

Tetrahedron 62 (2006) 537–542
A facile [4C1] type synthetic route to thiophenes
from dienol silyl ethers and elemental sulfur
Yukihiro Kasano,a Aoi Okada,a Daisuke Hiratsuka,a Yoji Oderaotoshi,a
Satoshi Minakataa and Mitsuo Komatsua,b,c,
*
aDepartment of Applied Chemistry, Graduate School of Engineering, Osaka University, Yamadaoka 2-1, Suita, Osaka 565-0871, Japan
bCenter for Atomic and Molecular Technologies, Graduate School of Engineering, Osaka University,
Yamadaoka 2-1, Suita, Osaka 565-0871, Japan
cResearch Center for Environmental Preservation, Osaka University, Yamadaoka 2-4, Suita, Osaka 565-0871, Japan
Received 13 September 2005; revised 13 October 2005; accepted 13 October 2005
Available online 3 November 2005
Abstract—A facile method for the synthesis of thiophene derivatives via the reaction of readily accessible dienol silyl ethers with elemental
sulfur is described. Dienol silyl ethers and elemental sulfur, when heated at 180 8C in the presence of MS4A, provided 3-siloxythiophene
derivatives in yields up to 98%. In this reaction, thiophene derivatives might be formed through 1,2-dithiines and eight-membered cyclic
tetrasulfides.
q 2005 Elsevier Ltd. All rights reserved.
1. Introduction
and azomethine imines, via an intramolecular 1,2- or 1,4-
silatropic shift and their cycloaddition reactions.4
By applying the 1,4-silatropic protocol to the generation
of thiocarbonyl ylides, we developed a convenient route for
the preparation of enol and dienol silyl ethers from readily
accessible starting materials, as shown in Scheme 1 for the
case of dienol silyl ethers.5 These reactions proceed via the
generation of thiocarbonyl ylides by the thermal 1,4-
silatropy of S-a-silyl thioesters and electrocyclization of
the thiocarbonyl ylides to give thiiranes followed by sulfur
extrusion under completely neutral conditions without the
need for any catalyst or additive. In these reactions,
thiophene derivatives were also obtained as minor products.
The thiophene ring structure is widespread in nature and
many of these compounds are biologically active.1
Thiophene derivatives are also widely utilized as
functional materials in dyes, liquid crystals and as
components of organic conducting polymers.1 Because
of their significance, heterocycles are important target
molecules from a synthetic point of view, and a number
of methods for their preparation have been reported to
date.1 Although the Gewald, Hinsberg and Paal
syntheses are well-known procedures for the construction
of the ring, new or improved methodologies have
been developed more recently.1 Among these pro-
cedures, [4C1] type ring formation from 4-carbon
units and elemental sulfur should be practical because
of the low cost sources and, in fact, one of these
reactions is used in the commercial production of
thiophenes.2 While the thermal reaction of dienes with
elemental sulfur is a simple approach to thiophene
synthesis,3 a high reaction temperature is required (300–
420 8C), and the efficiency is also low (up to 51%).
We previously reported on efficient methods of generation
of nitrogen-centered 1,3-dipoles, azomethine ylides
Keywords: Thiophene; Dienol silyl ether; Elemental sulfur.
*
Corresponding author. Tel.: C81 6 6879 7400; fax: C81 6 6879 7402;
e-mail: komatsu@chem.eng.osaka-u.ac.jp
Scheme 1. A convenient route to dienol silyl ethers via thiocarbonyl ylides.
0040–4020/$ - see front matter q 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2005.10.024

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Y. Kasano et al. / Tetrahedron 62 (2006) 537–542
In preliminary experiments, the precursors of the thiophenes
were found to be the dienol silyl ethers and elemental sulfur
generated in situ. This fact stimulated us to develop a new
method for the synthesis of siloxythiophenes. The desired
siloxythiophenes should be equivalent to hydroxythio-
phenes, which have been prepared only with difficulty by
low yielding and/or multi-step procedures especially for
simple 3-hydroxythiophenes.6
(1)
Based on the above results, a solution of dienol silyl ether 2a
and 5 equiv of elemental sulfur in benzene was heated at
180 8C for 36 h in a sealed glass tube (Eq. 2) to give
thiophene 3a in 66% yield. These results verify that a dienol
silyl ether is a precursor of a thiophene.
From these points of view, we present the full details of an
efficient and convenient approach to siloxythiophenes from
dienol silyl ethers and elemental sulfur as the S1 unit via a
[4C1] type reaction.
2. Results and discussion
As mentioned in the introduction, to clarify the mechanism
of generation of thiophene derivatives, the time course for
the reaction of silylmethylthioester 1a was monitored by 1H
NMR (Fig. 1). The thermal reaction of 1a for 1.5 h provided
a mixture of 2a (82%) and 3a (9%), which led to a mixture
of 2a (6%) and 3a (55%) upon continued heating under the
same conditions for 80 h. Thus, the formation of the
thiophene 3a must have been related to the consumption of
the dienol silyl ether 2a. Since sulfur might be generated
in situ in this reaction, as shown in Scheme 1, it appears
that the dienol silyl ether was consumed by a reaction
with sulfur.
(2)
To improve the efficiency of the reaction, the effect of
additives was examined (Table 1). Although triethylamine
and tri-n-butylphosphine are well known activators of
elemental sulfur,7 the use of these reagents had no effect
on the results (entries 2 and 3). Lewis acids were also
ineffective because of their tendency to cause the
polymerization of dienol silyl ether 2a (entries 4, 5, 6 and
˚
7). Interestingly, when molecular sieves 4A (MS4A) were
employed as an additive, the desired thiophene was obtained
in almost quantitative yield (entry 8). To clarify the effect of
MS4A, reactions using other porous inorganic materials and
dehydrating agents were examined. Porous inorganic
materials, such as zeolite, silica gel and alumina (entries
9, 10 and 11) and a dehydrating agent, sodium sulfate (entry
12) were ineffective except for potassium carbonate, which
gave a quantitative yield (entry 13). Even when the reaction
was carried out without these additives, the hydrolysis of
starting dienol silyl ether did not occur, suggesting that
MS4A does not function as a dehydrating reagent. Since the
presence of potassium carbonate requires a longer reaction
time than that with MS4A, the following experiments were
carried out using MS4A as an additive.
Reactions of dienol silyl ethers 2b–e with elemental sulfur
in the presence of MS4A were carried out (Table 2). The
starting dienol silyl ether 2b, in which the substituent R1 is a
methyl group, was efficiently converted to the correspond-
ing thiophene (entry 1). Thiophene derivative 3c possessing
no substituent at the 2-position was also obtained in good
yield from dienol silyl ether 2c (entry 2). Dialkyl substituted
dienol silyl ether 2d having no aromatic substituent reacted
to give 2,5-dialkyl substituted thiophene 3d (entry 3).
Although improvements in the efficiency are needed,
2,5-unsubstituted thiophene 3e was synthesized from the
unsubstituted dienol silyl ether 2e (entry 4). The low yield
might be because of the condensation polymerization of 2e.
Figure 1. A time course for the reaction of a,b-unsaturated thioester 1a.
To verify the reaction pathway proposed above, an
additional experiment was carried out (Eq. 1). As shown
in Figure 1, the thermal treatment of thioester 1a at 180 8C
for 80 h led to the formation of thiophene 3a in 55% yield.
In the presence of elemental sulfur (5 equiv), the reaction
proceeded at an increased rate and gave a higher yield of
product (61%).
The present method was applicable to other types of dienol
silyl ethers.
Thermal reaction of 2,3-bis(trimethylsilyloxy)-1,3-
butadiene 2f with elemental sulfur gave 3,4-

Y. Kasano et al. / Tetrahedron 62 (2006) 537–542
539
Table 1. Effect of additives on the formation of thiophene 3a from dienol silyl ether 2a and elemental sulfur
Entry
Additive
None
Time (h)
Yield (%)a
1
2
3
4
5
6
7
8
36
40
40
6
66
Et3N
n-Bu3P
BF3$OEt2
AgOTf
Mg(OTf)2
Zn(OTf)2
MS4A
(1 equiv)
(1 equiv)
(1 equiv)
(1 equiv)
(1 equiv)
(1 equiv)
(10 mg)
(10 mg)
(10 mg)
(10 mg)
(1 equiv)
(1 equiv)
27b
54
0
0
28
1
24
18
48
84
40
63
39
90
20
98 (87)c
57
66
9
Zeolited
Silica gele
Aluminaf
Na2SO4
K2CO3
10
11
12
13
37
40
98
1
a Determined by H NMR.
b Dienol silyl ether 2a (37%) was recovered.
c Isolated yield. 2,5-Diphenyl-3-hydroxythiophene was also obtained in 8% yield as a result of hydrolysis of 3a on work up.
d Zeolite A-4 from Wako Pure Chem. Ind.
e Silica gel 60 from Merck Co.
f
Aluminium oxide 90 active basic from Merck Co.
Table 2. Reactions of dienol silyl ethers with elemental sulfur
Entry
Dienol silyl ether
R1
R2
Time (h)
Thiophene
Yield (%)a
1
2
3
4
2b
2c
2d
2e
Me
H
Me
H
Ph
Ph
Me
H
10
10
20
10
3b
3c
3d
3e
80 (82)
77 (76)
44 (51)
14 (15)
a Isolated yield. 1H NMR yields before purification are presented in the parenthesis.
bis(trimethylsilyloxy)thiophene 3f in 29% yield (Eq. 3).
reaction. 1-Trimethylsilyloxy-1,3-butadiene 2h was found
to function as a starting dienol silyl ether to afford
2-siloxythiophene 3h (Eq. 5).
(3)
Since Danishefsky’s diene 2g should be a good candidate
for synthesis of a functionalized thiophene, the reaction of
2g with elemental sulfur was carried out, unexpectedly
affording 5-unsubstituted thiophene 3e in 51% yield (Eq. 4).
Although the reaction mechanism is unclear at present,
Danishefsky’s diene is a better starting material rather than
2e for the formation of 2,5-unsubstituted thiophene 3e.
(5)
These results present the potential for applying this reaction
to synthesis of a variety of thiophene derivatives.
While no by-products were observed in the synthesis of
thiophenefromdienolsilylethersexceptfor2e, 1,2-dithiine 4e
(5%) and the eight-membered cyclic tetrasulfide 5e (2%) were
obtained after 10 h in the case of the reaction of dienol silyl
ether 2e. To clarify the route of formation of the thiophene
derivative, the reaction of 2e with sulfur was followed by 1H
NMR and the time course is illustrated in Figure 2. Even after
the complete consumption of dienol silyl ether 2e, the amount
of thiophene 3e increased with the decrease in the amounts of
4e and 5e. Thus, it is conceivable that the cyclic polysulfides
(4)
To expand the utility of the system, a 1,3-butadiene having
trimethylsiloxy group at 1-position was evaluated in the

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Y. Kasano et al. / Tetrahedron 62 (2006) 537–542
couldbeprecursorsofthethiophenederivativeinthisreaction.
Inpractice,examplesoftheformationofthiophenederivatives
from 1,2-dithiine have been reported.8
the dienol silyl ether occurs, and the S8 ring is opened. The
elimination of S6 followed by an attack of sulfur at the
b-position of the generated carboxonium group gives the
1,2-dithiine skeleton. The elimination of hydrogen sulfide
from the 1,2-dithiine then gives the thiophene derivative.
Indeed, the generation of hydrogen sulfide was confirmed by
using CuSO4 test paper. Based on this fact, it is presumed
that the effect of MS4A or potassium carbonate is to act as a
scavenger of hydrogen sulfide.
Scheme 3. Plausible reaction mechanism for the generation of a thiophene
derivative from a dienol silyl ether and elemental sulfur.
3. Conclusions
Figure 2. Time course for the reaction of dienol silyl ether 2e with
elemental sulfur.
A convenient route to 3-siloxythiophene derivatives from
readily accessible dienol silyl ethers and elemental sulfur is
described. The presence of MS4A improved the efficiency
of the reaction, providing thiophene derivatives in up to
98% yield. Furthermore, the isolation of a 1,2-dithiine
derivative and an eight-membered cyclic tetrasulfide and a
study of the reaction profile suggest that the reaction
proceeds via these intermediates to form the thiophene
derivatives. Further studies in this area are currently
underway in our laboratory.
To confirm that thiophene derivative 3e was formed via 4e
and 5e, these compounds were treated under the same
conditions as were used for the formation of thiophene from
2e, respectively (Scheme 2). As a result, the reaction of 1,2-
dithiine 4e gave thiophene derivative 3e in 19%. In the case
of 5e, 4e was also formed in situ and was gradually
converted to the thiophene derivative. These results suggest
that dienol silyl ether 2e was initially converted to a mixture
of cyclic polysulfides 4e and 5e, which were then altered to
thiophene derivative 3e. Examples of an equilibrium
between 1,2-dithiines and cyclic tetrasulfides have been
reported,9 a fact that also supports the proposed reaction
pathway.
4. Experimental
4.1. General experimental methods
IR spectra were obtained on a Jasco FT/IR-410 infrared
spectrophotometer. 1H and 13C NMR spectra were recorded
on a JEOL FT NMR JNM EX 270 spectrometer (1H NMR,
270 MHz; 13C NMR, 68 MHz). Chemical shifts are reported
in parts per million (d), relative to internal TMS at 0.00 for 1H
NMR and chloroformat 77.0for 13C NMR. Mass spectra were
obtained with a Shimadzu Model GC–MS-QP5000 spec-
trometer. High-resolution mass spectral data were obtained on
a JEOL DX-303 mass spectrometer. All reactions were
performed under an atmosphere of argon. Organic solvents
were dried and distilled prior to use. Column chromatography
was performed using silica gel 60, spherical, neutrality
(Nacalai Tesque), which was dried at 140 8C under reduced
pressure for 3 h prior to use. Because of high moisture
sensitivity, high-resolution mass spectral (HRMS) data are
given for the products instead of elemental analyses.
Scheme 2. Thermal treatment of 4e and 5e leading to thiophene 3e.
4.2. Synthesis of starting dienol silyl ethers
A plausible path for the formation of a thiophene derivative
from a dienol silyl ether and elemental sulfur is proposed on
the basis of the above results (Scheme 3). A nucleophilic
attack on elemental sulfur by the carbon of the 1-position of
Starting dienol silyl ethers 2b,10 2c11 and 2d10 were
prepared according to the published procedures except for
dienol silyl ether 2a.5

Y. Kasano et al. / Tetrahedron 62 (2006) 537–542
541
4.2.1. 1,4-Diphenyl-2-(trimethylsilyloxy)-1,3-butadiene
(2a). trans-1,4-Dipehnyl-but-3-en-2-one12 (267 mg,
1.20 mmol), DBU (237 mg, 1.56 mmol) and trimethylsilyl
chloride (156 mg, 1.40 mmol) were placed in a 20 mL
round bottom flask and was refluxed in CH2Cl2 (4 mL) for
12 h. After being cooled to room temperature, hexane
(20 mL) was added to the reaction mixture and washed
twice with saturated aqueous Na2CO3 (10 mL). The organic
layer was dried over MgSO4, and filtered. Removal of the
solvent and purification by rapid column chromatography
on dried neutral silicagel afforded dienol silyl ether 2a as a
pale yellow oil. Yield: 248 mg (84%). (1Z,3E)-2a/(1E,3E)-
2aZ96/ 4. (1Z,3E)-2a: 1H NMR (CDCl3) d 7.57 (br d, 2H,
JZ7.3 Hz, o-aromatic H), 7.44 (br d, 2H, JZ7.3 Hz,
o-aromatic H), 7.37–7.14 (m, 6H, aromatic H), 6.82 (d,
1H, JZ15.8 Hz, vinyl H-4), 6.72 (d, 1H, JZ15.8 Hz, vinyl
H-3), 5.90 (s, 1H, vinyl H-1), 0.15 [s, 9H, Si(CH3)3]; 13C
NMR (CDCl3) d 149.8, 136.8, 136.3, 128.9, 128.8, 128.6,
128.0, 127.8, 127.5, 126.5, 126.2, 115.4, 0.9; MS (EI) m/z
(relative intensity): 294 (MC, 15), 203 (21), 131 (96), 73
(Me3SiC, 100); HRMS (EI) calcd for C19H20OSi (MC)
294.1440, found 294.1424. Key spectra of the minor
compound (1E,3E)-2a obtained from the mixture of 2a:
1H NMR (C6D6) d 6.23 (s, 1H, vinyl H-1), 0.28 [s, 9H,
Si(CH3)3]. Only distinct signals are listed.
7.56 (d, 2H, JZ7.3 Hz, o-aromatic H at thiphene-C2), 7.37
(dd, 2H, JZ7.3 Hz, m-aromatic H), 7.29 (d, 1H, JZ7.3 Hz,
p-aromatic H), 6.93 (d, 1H, JZ1.6 Hz, 3-position in
thiophene), 6.32 (d, 1H, JZ1.6 Hz, 5-position in thiophene),
0.30 [s, 9H, Si(CH3)3]; 13C NMR (CDCl3) d 152.4, 141.8,
134.3, 128.7, 127.4, 125.1, 118.3, 104.1, 0.13; MS (EI) m/z
(relative intensity): 248 (MC, 100), 233 (MCKMe, 46), 205
(30), 173 (7), 115 (14), 73 (Me3SiC, 17); HRMS (EI) calcd for
C13H16OSSi (MC), 248.0691, found 248.0706.
4.3.3. 2,5-Dimethyl-3-(trimethylsilyloxy)thiophene (3d).
IR (neat) 2962, 1578, 1254 and 845 cmK1 1H NMR
;
(CDCl3) d 6.26 (s, 1H, 4-position in thiophene), 2.34 (s, 3H,
Me), 2.17 (s, 3H, Me), 0.22 [s, 9H, Si(CH3)3]; 13C NMR
(CDCl3) d 146.7, 133.0, 120.1, 115.6, 15.8, 10.8, 0.17; MS
(EI) m/z (relative intensity): 200 (MC, 100), 185 (MCKMe,
52), 111 (19), 73 (Me3SiC, 44); HRMS (EI) calcd for
C9H16OSSi (MC), 200.0691, found 200.0699.
4.3.4. 3,4-Bis(trimethylsilyloxy)thiophene (3f). IR (neat)
2960, 1493, 1254 and 845 cmK1; 1H NMR (CDCl3) d 6.26
(s, 2H, 2- and 5-position in thiophene), 0.26 [s, 18H,
Si(CH3)3]; 13C NMR (CDCl3) d 144.4, 103.0, 0.18; MS (EI)
m/z (relative intensity): 260 (MC, 43), 245 (MCKMe, 14),
73 (Me3SiC, 100); HRMS (EI) calcd for C10H20O2SSi2
(MC), 260.0723 found 260.0748.
4.3. General procedure for the reaction of dienol silyl
ether with elemental sulfur
4.3.5. 4-Trimethylsilyloxy-3H,6H-1,2-dithiine (4e). 1H
NMR (C6D6, 270 MHz) d 5.02–4.98 (tt, JZ4.6, 1.4 Hz,
1H, C]CH), 3.07–3.02 (m, 4H, C]CH–CH2 and
CH2–C]CH), 0.09 [s, 9H, Si(CH3)3]; 13C NMR (C6D6,
68 MHz) d 149.7, 104.9, 33.3, 31.5, 0.90; MS (EI) m/z (%)
206 (MC, 87), 143 (23), 142 (MCK2S, 68), 127 (100), 85
(18), 75 (54), 73 (Me3SiC, 54); HRMS (EI) m/z calcd for
C7H14OS2Si 206.0255, found 206.0253.
˚
Molecular sieves 4A (50.0 mg) were placed in a 10 mL
glass tube, which was flame-dried under reduced pressure.
Elemental sulfur (48.1 mg, 1.50 mmol), dienol silyl ether 2
(0.300 mmol) and benzene (3 mL) were added to the tube
and the solution was freeze-dried twice. The glass tube was
then sealed and the mixture was heated at 180 8C until the
amount of thiophene derivative reached a constant value.
Reactions were monitored by 1H NMR using benzene-d6 as
a solvent in one-fifth scale of the procedure described above
with mesitylene as an internal standard. The yields of
thiophene derivatives 3 listed in Tables 1 and 2 were
determined by 1H NMR of the crude reaction mixture based
on the added mesitylene. The reaction mixture was
concentrated in vacuo and the residue was purified by
column chromatography on dried neutral silica gel, to afford
thiophene 3. Cyclic polysulfides 4e and 5e were also
produced in the reaction of dienol silyl ether 2e. Thiophenes
3a,5 3e13, 3h14 and 2,5-diphenyl-3-hydroxythiophene5 are
known compounds.
4.3.6. 1,2,3,4-Tetrathia-6-trimethylsilyloxy-6-cyclo-
octene (5e). 1H NMR (C6D6, 270 MHz) d 4.61 (t, JZ
9.0 Hz, 1H, C]CH), 3.07 (s, 2H, CH2–C]CH), 2.97 (d,
JZ9.0 Hz, 2H, C]CH–CH2), 0.21 [s, 9H, Si(CH3)3]; 13C
NMR (C6D6, 68 MHz) d 154.2, 103.8, 41.0, 35.6, 0.66; MS
(EI) m/z (%) 270 (MC, 16), 206 (MCK2S, 100), 173 (21),
159 (16), 142 (MCK4S, 59), 127 (58), 75 (39), 73
(Me3SiC, 40); HRMS (EI) m/z calcd for C7H14OS4Si
269.9697, found 269.9698.
Acknowledgements
4.3.1. 2-Methyl-5-phenyl-3-(trimethylsilyloxy)thiophene
The work was partially supported by a Grant-in-Aid for
Scientific Research from the Ministry of Education, Culture,
Sports, Science and Technology, Japan. One of the authors
(Y.K.) expresses his special thanks for the 21st century
center of excellence (21COE) program, ‘Creation of
Integrated EcoChemistry of Osaka University’.
1
(3b). IR (neat) 2958, 1568, 1502, 1252 and 847 cmK1; H
NMR (CDCl3) d 7.51 (d, 2H, JZ7.3 Hz, o-aromatic H at
thiphene-C5), 7.35 (dd, 2H, JZ7.3 Hz, m-aromatic H), 7.25
(d, 1H, JZ7.3 Hz, p-aromatic H), 6.82 (s, 1H, 4-position in
thiophene), 2.27 (s, 3H, Me), 0.28 [s, 9H, Si(CH3)3]; 13C
NMR (CDCl3) d 148.1, 136.9, 134.5, 128.7, 126.9, 124.7,
118.6, 117.8, 11.3, 0.35; MS (EI) m/z (relative intensity):
262 (MC, 54), 247 (MCKMe, 15), 173 (10), 73 (Me3SiC,
100), 59 (96); HRMS (EI) calcd for C10H18OSSi (MC)
262.0848, found 262.0846.
References and notes
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