Rhodium-Catalyzed Isomerization and Alkyne Exchange Reactions of 1,4-Dithiins via the 1,2-Ethenedithiolato Rhodium Complex

Article abstract of DOI:10.1021/acs.organomet.8b00498

Rhodium-catalyzed isomerization and alkyne exchange reactions of 1,4-dithiines occurred by cleavage of two C-S bonds. The 2,5- and 2,6-disubstituted 1,4-dithiins underwent isomerization reactions in toluene at 110 °C, providing equilibrium mixtures of isomers. At 150 °C, the reaction of 1,4-dithiins and dimethyl acetylenedicarboxylate gave unsymmetric 2,3-di(methoxycarbonyl)-1,4-dithiins and 2,3-di(methoxycarbonyl)thiophenes, the latter of which were formed by desulfurization of the 1,4-dithiins. A related reaction of the alkyne and a 1,2-dithiete gave a 2,3-di(methoxycarbonyl)thiophene. These reactions are considered to involve 1,2-ethenedithiolato rhodium intermediates.

Full text of DOI:10.1021/acs.organomet.8b00498

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Cite This: Organometallics XXXX, XXX, XXX−XXX
Rhodium-Catalyzed Isomerization and Alkyne Exchange Reactions
of 1,4-Dithiins via the 1,2-Ethenedithiolato Rhodium Complex
Mieko Arisawa,* Kyosuke Sawahata, Takuya Ichikawa, and Masahiko Yamaguchi*
Department of Organic Chemistry, Graduate School of Pharmaceutical Sciences, Tohoku University, Aoba, Sendai 980-8578, Japan
S
* Supporting Information
ABSTRACT: Rhodium-catalyzed isomerization and alkyne
exchange reactions of 1,4-dithiines occurred by cleavage of two
C−S bonds. The 2,5- and 2,6-disubstituted 1,4-dithiins under-
went isomerization reactions in toluene at 110 °C, providing
equilibrium mixtures of isomers. At 150 °C, the reaction of 1,4-
dithiins and dimethyl acetylenedicarboxylate gave unsymmetric
2,3-di(methoxycarbonyl)-1,4-dithiins and 2,3-di-
(methoxycarbonyl)thiophenes, the latter of which were formed
by desulfurization of the 1,4-dithiins. A related reaction of the
alkyne and a 1,2-dithiete gave a 2,3-di(methoxycarbonyl)-
thiophene. These reactions are considered to involve 1,2-
ethenedithiolato rhodium intermediates.
Scheme 1. Isomerization and Exchange Reactions of 1,4-
Dithiins via the 1,2-Ethenedithiolato Rhodium Intermediate
INTRODUCTION
■
1,4-Dithiines make up an interesting group of six-membered
heterocyclic compounds containing two sulfur atoms and two
CC bonds with nonaromatic nonplanar structures.1 The
compounds, especially those not condensed with aromatic
groups, exhibit notable chemical properties because of their
nonaromatic nature and the presence of two reactive sulfur
atoms. Theoretical and experimental studies have been
conducted on the chemical reactivity of 1,4-dithiines to
provide radical cations.2 An interesting reaction is sulfur
extrusion by 1,4-dithiins under heating to provide thiophenes,
where two C−S bonds on a single sulfur atom were cleaved
with concomitant formation of a C−C bond.3 For example,
2,5-diphenyl-1,4-dithiin extruded a sulfur atom at 190 °C to
form 2,4-diphenylthiophene.
We considered that the use of transition-metal catalysis on
the reaction of 1,4-dithiins would provide a new methodology
for synthesizing organosulfur compounds.4 Described here are
rhodium-catalyzed isomerization and alkyne exchange reac-
tions of 1,4-dithiins by the cleavage of two C−S bonds
(Scheme 1). The isomerization reaction between 2,5- and 2,6-
disubstituted 1,4-dithiins provided equilibrium mixtures of
isomers in statistical yields.5 The alkyne exchange reactions of
2,5- and 2,6-disubstituted 1,4-dithiins with dimethyl acetyle-
nedicarboxylate gave 5-substituted 2,3-di(methoxycarbonyl)-
1,4-dithiin and/or 2,3-di(methoxycarbonyl)thiophenes. The
reaction of dimethyl acetylenedicarboxylate and a 1,2-dithiete
gave a 2,3-di(methoxycarbonyl)thiophene.
[CpRh(S2C2Z2), where Z = CO2Me] was proposed to be an
intermediate in the catalytic formation of 2,3,4,5-tetra-
(methoxycarbonyl)thiophene from dimethyl acetylenedicar-
boxylate and sulfur.6a The work presented here describes
notable catalytic reactivity of 1,2-ethenedithiolato rhodium
complexes.
RESULTS AND DISCUSSION
■
The isomerization reaction of 2,5- and 2,6-disubstituted 1,4-
dithiins was examined under rhodium-catalyzed conditions.
When 2,6-di(tert-butyl)-1,4-dithiin 1a was treated with
dimethyl acetylenedicarboxylate 3 (3 equiv) in the presence
of RhH(dppe)2 [10 mol %; dppe
= 1,2-bis-
(diphenylphosphino)ethane] in refluxing toluene (bp 110
°C) for 24 h, 2,5-di(tert-butyl)-1,4-dithiin 2a (48%) was
obtained with recovery of 1a (51%) (Scheme 2). 1a and 2a
These rhodium-catalyzed reactions are considered to involve
1,2-ethenedithiolato rhodium intermediates. Studies of the
catalytic reaction of the 1,2-ethenedithiolato rhodium complex
have been limited.6 The η5-cyclopentadienyl (1,2-dimethox-
ycarbonyl-1,2-ethylenedithiolato-S,S)rhodium complex
Received: July 16, 2018
© XXXX American Chemical Society
A
DOI: 10.1021/acs.organomet.8b00498
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a mixture of 1b and 1c with 44% and 43% yields along with
recovered 2b and 2c. The crossover product 2-(4-methox-
yphenyl)-6-phenyl-1,4-dithiin was not detected. Then, the
isomerization is an intramolecular process. This reaction is a
convenient method for obtaining substituted 1,4-dithiins.
A possible mechanism of the isomerization reaction is as
follows (Scheme 4). The phosphine ligand in RhH(dppe)2 is
Scheme 2. Isomerization of 2,5- and 2,6-Di(tert-butyl)-1,4-
dithiins
Scheme 4. Possible Mechanism of Isomerization of 1,4-
Dithiins
were readily separated by silica gel chromatography. No
reaction occurred in the absence of RhH(dppe)2 or 3. The
yield of 2a was 19% at 6 h. When the molar ratio of 3 to 1a was
reduced from 3 equiv to 2, 1, and 0.5 equiv, the yield of 2a
decreased from 48% to 40%, 38%, and 15%, respectively.
Although a stoichiometric amount of 3 was required for higher
chemical yields, 3 was recovered quantitatively. The reverse
reaction of 2a under the same reaction conditions gave 1a in
43% yield with recovery of 2a (50%) (Scheme 2), which
indicated the equiliblium nature of the reaction. The
isomerization reaction occurred under rhodium-catalyzed
conditions without desulfurization to form thiophenes. The
results suggest the formation of 1,2-ethenedithiolato rhodium
intermediate 4.
Several 2,6-diaryl-1,4-dithiins (1b and 1c) were isomerized
to the corresponding 2,5-dialkyl-1,4-dithiins (2b and 2c,
respectively) in refluxing toluene (Scheme 3). 2,6-Diaryl-1,4-
exchanged with 3. A 1,4-dithiin coordinates through two sulfur
atoms to form intermediate A with release of alkyne 3. Then,
oxidative addition provides 1,2-ethenedithiolato rhodium
intermediate B,6 and the coordinated alkyne isomerizes from
B to B′. Subsequently, reductive elimination provides an
isomeric 1,4-dithiin via formation of A′, and the coordination
of 3 regenerates the rhodium(I) catalyst.
Scheme 3. Isomerization of Various 1,4-Dithiins
Unsymmetric 1,4-dithiins were formed by a rhodium-
catalyzed alkyne exchange reaction, which was conducted
under forced conditions using excess dimethyl acetylenedi-
carboxylate 3 without a solvent. When 2,6-di(tert-butyl)-1,4-
dithiin 1a was reacted with 3 (20 equiv) in the presence of
RhH(dppe)2 (10 mol %) at 150 °C for 6 h, 5-(tert-butyl)-2,3-
di(methoxycarbonyl)-1,4-dithiin 7 (17%), 5-(tert-butyl)-2,3-
di(methoxycarbonyl)thiophene 8 (25%), 4-(tert-butyl)-2,3-
di(methoxycarbonyl)thiophene 9 (14%), and 2,3,4,5-tetra-
(methoxycarbonyl)thiophene 10 (16%) were obtained with
recovery of 1a (24%) and 2a (23%) (Scheme 5). The total
yield of alkyne exchange products 7−9, which contained the
Scheme 5. Alkyne Exchange of 2,6-Di(tert-butyl)-1,4-
dithiins
dithiins (1d and 1e) also underwent the isomerization giving
2,5-dialkyl-1,4-dithiins (2d and 2e, respectively). These 2,6-
disubstituted 1,4-dithiins 1 and 2,5-disubstituted 1,4-dithiins 2
were readily separated by silica gel chromatography. The
structure of the isomerized 1,4-dithiins was determined by
comparison with the synthesized authentic compounds.7 The
substituent on 1,4-dithiins did not significantly affect the
reaction. Unsymmetric 2-(tert-butyl)-5-(4-tolyl)-1,4-dithiin
(5) isomerized to 2-(tert-butyl)-6-(4-tolyl)-1,4-dithiin (6)
with a 50% yield. A crossover reaction of 2b and 2c provided
B
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1,2-dicarboxylate moiety, reached 56%. An independent
experiment confirmed thermal extrusion of a sulfur atom
from 7 at 150 °C for 3 h without a catalyst.3b Reaction of 8 and
9 with sulfur did not provide 7 in the presence of RhH(dppe)2,
which indicated that the formation of 7 is irreversible. From
these results, compounds 7−9 are thought to be the alkyne
exchange products, which are derived from the cleavage of two
C−S bonds in 1,4-dithiin 1a followed by addition of 3. It was
then considered that the alkyne exchange reaction occurred via
1,2-ethenedithiolato rhodium intermediate 11. The formation
of 10 was due to the trapping of the eliminated sulfur atom
derived from 1,4-dithiin 7 by two molecules of 3 (also see
Scheme 7).
Scheme 7. Alkyne Exchange of 2,5-Diphenyl-1,4-dithiins
The reaction of 2,6-di(tert-butyl)-1,4-dithiin 2a and 3 gave 7
(16%), 8 (20%), 9 (12%), and 10 (15%), which were
accompanied by the recovery of 1a (20%) and 2a (22%)
(Scheme 6). The total yield of alkyne exchange products 7−9
Scheme 6. Alkyne Exchange of 2,5-Di(tert-butyl)-1,4-dithiin
Figure 1. ORTEP views of 13b and 13c with thermal ellipsoids drawn
at the 50% probability level.
reached 48%. The formation of 7 from 1a and 2a in
comparable yields is consistent with the involvement of 1,2-
ethenedithiolato rhodium intermediate 11 (Schemes 5 and 6).
Because no desulfurization products derived from 1a and 2a
were observed, the desulfurization of 7 to thiophenes 8 and 9
appears to be faster than the desulfurization of 1a and 2a to
2,4-di(tert-butyl)thiophene 12. In accordance with this, heating
of a mixture of 2a and 3 at 150 °C for 6 h in the absence of
RhH(dppe)2 gave desulfurization product 12 in only 32%
yield. When the alkyne exchange reaction of 2a was conducted
with 4-tolylacetylene, 1,2-diphenylacetylene, 2-propynoic acid
methyl ester, or 1-decyne instead of 3, no reaction other than
desulfurization occurred.
intermediate, which is formed by trapping the sulfur atom
eliminated from 2b.4
2,6-Diphenyl-1,4-dithiin 1b also gave 13b (58%), which was
accompanied by 14b (38%) and 10 (50%) (Scheme 8).
Scheme 8. Alkyne Exchange of 2,6-Diphenyl-1,4-dithiins
1,4-Dithiins with two aryl groups also underwent an alkyne
exchange reaction, which showed a stronger tendency to form
thiophenes. The rhodium-catalyzed reaction of 2,5-diphenyl-
1,4-dithiin 2b and 3 at 150 °C gave 2,3-di(methoxycarbonyl)-
5-phenylthiophene 13b (55%), which was accompanied by
1,2-di(4-methoxyphenyl)-4,5-diphenylbenzene 14b (41%) and
2,3,4,5-tetra(methoxycarbonyl)thiophene 10 (55%) (Scheme
7). The structure of 13b was determined by X-ray crystal
structure analysis (Figure 1). Unsymmetric thiophene 13b was
formed by alkyne exchange of 2b and 3 followed by
desulfurization, and no corresponding dithiin was obtained.
14b was formed by the rhodium-catalyzed trimerization
reaction of 3 and phenylacetylene, which was extruded from
2b. Because 10 was not formed by the reaction of 13b and 3,
the formation of 10 was due to trapping of the eliminated
sulfur atom derived from the 1,4-dithiins by two molecules of
3. The formation of 10 likely proceeds via the dithiorhodium
Isomeric dithiins 1b and 2b provided the same alkyne-
exchanged thiophene 13b (Schemes 7 and 8), which is again
consistent with the formation of 1,2-ethenedithiolato rhodium
intermediate 15. The reaction of 2,5-di(4-methoxyphenyl)-1,4-
dithiin 1c and 3 gave 2-(4-methoxyphenyl)-4,5-di-
(methoxycarbonyl)thiophene 13c (43%), which was accom-
panied by 14c (44%) and 10 (38%) (Scheme 8). The structure
of 13c was determined by X-ray crystal structure analysis
(Figure 1).
The alkyne exchange reaction was summarized as follows.
(i) The isomerization and alkyne exchange occurred via a
similar mechanism. (ii) Desulfurization could occur at a higher
temperature. (iii) These reactions could be explained by the
intermediacy of the 1,2-dithiolato complex.
C
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To further examine the formation of the 1,2-ethenedithio-
lato rhodium intermediate, the rhodium-catalyzed reaction of
1,2-dithiete 16 and alkyne 3 was conducted (Scheme 9).
Scheme 10. Synthesis of 13b Using the 1,2-Ethenedithiolato
Rhodium Complex
Scheme 9. Reaction of 1,2-Dithiete 16 with 3
benzene derivatives. (iv) The reaction of a 1,2-dithiete and 3
gives a thiophene. (v) The stoichiometric reaction of a 1,2-
ethenedithiolato rhodium complex and an alkyne gives a
thiophene. A possible mechanism of the alkyne exchange
reaction of a 1,4-dithiin follows (Scheme 11). The phosphine
Recently, we have reported the synthesis of 1,3,2-dithiaphosp-
holes from 1,2-dithietes, which was considered to involve a 1,2-
ethenedithiolato rhodium intermediate.8 When 3-(tert-butyl)-
4-phenyl-1,2-dithiete 169 was reacted with excess 3 (20 equiv)
in the presence of RhH(dppe)2 (10 mol %) at 130 °C for 6 h,
5-(tert-butyl)-2,3-di(methoxycarbonyl)-4-phenylthiophene 17
(60%) was obtained, which was accompanied by 10 (60%)
(Scheme 9). The structure of 17 was determined by X-ray
crystal structure analysis (Figure 2). No isomeric 4-(tert-
Scheme 11. Possible Mechanism of the Alkyne Exchange
Reaction of 1,4-Dithiins
Figure 2. ORTEP view of 17 with thermal ellipsoids drawn at the
50% probability level.
butyl)-2,3-di(methoxycarbonyl)-5-phenylthiophene was ob-
tained. 6-(tert-Butyl)-2,3-di(methoxycarbonyl)-5-phenyl-1,4-
dithiin was not detected. RhH(dppe)2 and 3 were essential
for the formation of thiophenes 17 and 10, and no reaction
occurred in their absence. When the reaction temperature was
decreased to 100 °C, the yields of 17 and 10 decreased to 20%
and 30%, respectively. It is considered that 1,2-ethenedithio-
lato rhodium intermediate 18 was formed.
These rhodium-catalyzed reactions provided moderate
yields of unsymmetric 5-alkyl/aryl-2,3-di(methoxycarbonyl)-
thiophenes 8, 9, 13b, 13c, and 17, which have rarely been
synthesized.10
The reactivity of the 1,2-ethenedithiolato rhodium inter-
mediate was also examined using 19 [CpRh(S2C2Z2), where Z
= CO2Me].6a The reaction of 19 in refluxing phenylacetylene
(bp 143 °C) gave unsymmetric thiophene 13b (41%), which
was accompanied by 14b (14%) and 10 (22%) (Scheme 10).
Thiophene 13b was obtained by the stoichiometric reaction of
1,2-ethenedithiolato rhodium 19 and an alkyne.
The formation of 1,2-ethenedithiolato rhodium intermedi-
ates by the cleavage of two C−S bonds in 1,4-dithiins is
supported by the following observations. (i) Isomerization
between 2,5- and 2,6-disubstituted 1,4-dithiins occurs. (ii)
Alkyne-exchanged 1,4-dithiin and/or thiophenes are obtained
by the reaction of 1,4-dithiins and 3. (iii) Liberated alkynes
derived from 1,4-dithiins are captured by trimerization to form
ligand in RhH(dppe)2 is exchanged with 3, and a 1,4-dithiin
coordinates through two sulfur atoms to form intermediate A
with release of alkyne 3. Then, oxidative addition provides an
isomeric mixture of 1,2-ethenedithiolato rhodium intermedi-
ates B and B′. The alkyne exchange in B and B′ with 3 forms
another 1,2-ethenedithiolato rhodium intermediate C. Sub-
sequently, reductive elimination provides a 1,4-dithiin via
formation of D, and the coordination of 3 regenerates the
rhodium(I) catalyst.
CONCLUSIONS
■
In summary, rhodium-catalyzed isomerization and alkyne
exchange reactions involving the cleavage of two C−S bonds
of 1,4-dithiins are developed. The reactions are novel synthetic
methods for substituted 1,4-dithiins and 2,3-di-
(methylcarbonyl)thiophenes. These reactions involve forma-
tion of 1,2-ethenedithiolato rhodium intermediates from 1,4-
dithiins, which can be used for the synthesis of various
organosulfur compounds.
D
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= 2.8 Hz), 2.02−2.06 (12H, m), 6.15 (2H, s); 13C NMR (100 MHz,
CDCl3) δ 28.5, 36.6, 40.2, 41.9, 117.2, 152.1; IR (KBr) 2901, 2845,
1552, 1446 cm−1; MS (EI) m/z 384 (M+, 100%), 135 (M+ − 249,
87%); HRMS calcd for C24H32S2 384.1945, found 384.1940.
2,5-Di(1-adamantyl)-1,4-dithiin (2d). Colorless solid [96.6 mg,
EXPERIMENTAL SECTION
■
1H and 13C nuclear magnetic resonance (NMR) spectra were
recorded on a Varian Mercury instrument (400 MHz), and
tetramethylsilane was used as a standard. Infrared (IR) spectra were
recorded on a JASCO FT/IR-410 spectrophotomer. Melting points
were determined with a Yanagimoto micro melting point apparatus
without correction. High- and low-resolution mass spectra were
recorded on a JEOL JMS-DX-303, a JEOL JMS-700, or a JMS-
T100GC spectrometer. X-ray diffraction data were recorded on a
Rigaku R-AXIIS RAPID instrument. Kanto Chemical. Co. Inc. silica
gel 60 (63−210 μm) was employed for flash column chromatography.
Starting materials 1a−1c, 2a−2e, and 5 were synthesized by literature
methods.5,7 1d and 1e were synthesized by the rhodium-catalyzed
isomerization of 2d and 2e presented here.
1
50%, Rf = 0.22 (hexane)]; mp 164.5−166.0 °C (hexane); H NMR
(400 MHz, CDCl3) δ 1.64−1.73 (12H, m), 1.81 (12H, d, J = 2.8 Hz),
2.01−2.04 (12H, m), 6.00 (2H, s); 13C NMR (100 MHz, CDCl3) δ
28.5, 36.6, 39.7, 41.8, 115.1, 154.7; IR (KBr) 2901, 2846, 1548, 1446
cm−1; MS (EI) m/z 384 (M+, 97%), 135 (M+ − 249, 100%); HRMS
calcd for C24H32S2 384.1945, found 384.1937.
2,6-Di(2-methylpropyl)-1,4-dithiin (1e). Yellow oil [recovery of
1e, 57.5 mg, 50%, Rf = 0.50 (hexane)]; 1H NMR (400 MHz, CDCl3)
δ 0.88 (12H, d, J = 6.4 Hz), 1.91 (2H, septet, J = 6.8 Hz), 2.22 (4H,
d, J = 6.8, 0.4 Hz), 5.91 (2H, s); 13C NMR (100 MHz, CDCl3) δ
22.1, 27.4, 45.5, 116.2, 139.7; IR (neat) 2954, 2925, 2868, 1464, 1385
cm−1; MS (EI) m/z 228 (M+, 100%), 185 (M+ − 43, 29%); HRMS
calcd for C12H20S2 228.1006, found 228.1005.
2,5-Di(2-methylpropyl)-1,4-dithiin (2e). Yellow oil [53.7 mg, 47%,
Rf = 0.53 (hexane)]; 1H NMR (400 MHz, CDCl3) δ 0.89 (12H, d, J =
6.8 Hz), 1.90 (2H, septet, J = 6.8 Hz), 2.21 (4H, dd, J = 6.8, 0.4 Hz),
5.93 (2H, s); 13C NMR (100 MHz, CDCl3) δ 22.1, 27.3, 46.0, 116.4,
138.9; IR (neat) 2954, 2925, 2868, 1566, 1464 cm−1; MS (EI) m/z
228 (M+, 100%), 185 (M+ − 43, 33%); HRMS calcd for C12H20S2
228.1006, found 228.1000.
Typical Procedures for Isomerization of 1a to 2a. In a two-
neck flask were placed RhH(dppe)2 [45.0 mg, 10 mol %; dppe = 1,2-
bis(diphenylphosphino)ethane], dimethyl acetylenedicarboxylate (1.5
mmol, 184.2 μL), and 2,6-di(1,1-dimethylethyl)-1,4-dithiin 1a (0.5
mmol, 114.0 mg) in toluene (0.5 mL) under an argon atomosphere,
and the mixture was heated at reflux for 24 h. The solvent was
removed under reduced pressure, and the residue was purified by flash
column chromatography on silica gel giving 2,5-di(1,1-dimethyleth-
yl)-1,4-dithiin 2a [54.9 mg, 48%, Rf = 0.43 (hexane)] and 1a [59.6
mg, 51%, Rf = 0.38 (hexane)]. 2a:5 colorless solid; mp 81.0−82.5 °C
1
(hexane); H NMR (400 MHz, CDCl3) δ 1.20 (18H, s), 6.08 (2H,
2-(1,1-Dimethylethyl)-5-(4-tolyl)-1,4-dithiin (5). Orange solid
[recovery of 5, 62.4 mg, 48%, Rf = 0.32 (20/1 hexane/toluene)];
s); 13C NMR (100 MHz, CDCl3) δ 29.7, 37.9, 115.2, 154.0; IR (KBr)
2964, 1458, 1361 cm−1; MS (EI) m/z 228 (M+, 100%), 157 (M+ −
71, 57%); HRMS calcd for C12H20S2 228.1006, found 228.0981. 1a:5
1
mp 47.0−48.0 °C (hexane); H NMR (400 MHz, CDCl3) δ 1.24
(9H, s), 2.34 (3H, s), 6.20 (1H, d, J = 0.4 Hz), 6.45 (1H, d, J = 0.4
Hz), 7.14 (2H, d, J = 8.4, 0.8 Hz), 7.48 (2H, d, J = 8.4 Hz); 13C NMR
(100 MHz, CDCl3) δ 21.2, 29.7, 38.1, 114.8, 117.0, 126.9, 129.2,
133.9, 138.5, 142.3, 152.9; IR (KBr) 3025, 2964, 1541, 1503, 1456
cm−1; MS (EI) m/z 262 (M+, 100%), 247 (M+ − 15, 49%); HRMS
calcd for C16H12S2 262.0850, found 262.0838.
1
pale yellow oil; H NMR (400 MHz, CDCl3) δ 1.23 (18H, s), 6.20
(2H, s); 13C NMR (100 MHz, CDCl3) δ 29.8, 38.5, 116.7, 151.9; IR
(neat) 2964, 2868, 1459, 1239 cm−1; MS (EI) m/z 228 (M+, 100%),
157 (M+ − 71, 94%); HRMS calcd for C12H20S2 228.1006, found
228.1014.
2,6-Diphenyl-1,4-dithiin (1b). Orange solid [recovery of 1b, 64.3
2-(1,1-Dimethylethyl)-6-(4-tolyl)-1,4-dithiin (6). Yellow oil [65.8
1
mg, 48%, Rf = 0.70 (4/1 hexane/ethyl acetate)]; mp 63.0−64.5 °C
mg, 50%, Rf = 0.29 (20/1 hexane/toluene)]; H NMR (400 MHz,
(hexane) (lit.7a 62.0−63.0 °C); H NMR (400 MHz, acetone-d6) δ
1
CDCl3) δ 1.28 (9h, s), 2.35 (3H, s), 6.20 (1H, s), 6.55 (1H, s), 7.14
(2H, d, J = 8.0 Hz), 7.49 (2H, d, J = 8.4 Hz); 13C NMR (100 MHz,
CDCl3) δ 21.2, 29.8, 38.6, 115.9, 119.1, 126.9, 129.2, 134.6, 138.4,
140.4, 151.5; IR (neat) 3024, 2965, 1547, 1505, 1460 cm−1; MS (EI)
m/z 262 (M+, 84%), 206 (M+ − 56, 100%); HRMS calcd for
C15H18S2 262.0850, found 262.0866.
6.83 (2H, s), 7.35−7.43 (6H, m), 7.72 (4H, dd, J = 6.8, 1.6 Hz); 13C
NMR (100 MHz, acetone-d6) δ 119.9, 127.6, 129.5, 129.6, 137.8,
138.8; IR (KBr) 3027, 1533, 1488 cm−1; MS (EI) m/z 268 (M+,
100%), 121 (M+ − 147, 33%); HRMS calcd for C16H12S2 268.0380,
found 268.0394.
2,5-Diphenyl-1,4-dithiin (2b). Yellow solid [67.1 mg, 50%, Rf =
0.72 (4/1 hexane/ethyl acetate)]; mp 117.5−119.0 °C (hexane)
Typical Procedures for Alkyne Exchange Reaction of 1a
with 3. In a two-neck flask were placed RhH(dppe)2 [45.0 mg, 10
mol %; dppe = 1,2-bis(diphenylphosphino)ethane], 2,6-di(1,1-
dimethylethyl)-1,4-dithiin 1a (0.5 mmol, 114.0 mg), and dimethyl
acetylenedicarboxylate 3 (10.0 mmol, 1.23 μL) under an argon
atomosphere, and the mixture was heated at 150 °C for 6 h. The
solvent was removed under reduced pressure, and the residue was
purified by flash column chromatography on silica gel giving 5-(tert-
butyl)-2,3-di(methoxycarbonyl)-1,4-dithiin 7 [24.0 mg, 17%, Rf =
0.37 (4/1 hexane/ethyl acetate)], 5-(tert-butyl)-2,3-di-
(methoxycarbonyl)thiophene 8 [32.3 mg, 25%, Rf = 0.37 (4/1
hexane/ethyl acetate)], 4-(tert-butyl)-2,3-di(methoxycarbonyl)-
thiophene 9 [17.7 mg, 14%, Rf = 0.33 (4/1 hexane/ethyl acetate)],
and 2,3,4,5-tetra(methoxycarbonyl)thiophene 10 [24.9 mg, 16%, Rf =
0.15 (4/1 hexane/ethyl acetate)] with recovery of 1a (27.4 mg, 24%)
and 2a (26.2 mg, 23%). Then, 7 and 8 were separated by recycling gel
[lit.11 116.5−118.0 °C (methanol)]; H NMR (400 MHz, CDCl3) δ
1
6.57 (2H, s), 7.32−7.38 (6H, m), 7.60 (4H, dd, J = 8.4, 1.6 Hz); 13C
NMR (100 MHz, CDCl3) δ 117.3, 127.1, 128.6, 128.7, 136.5, 140.6;
IR (KBr) 3027, 1542, 1481, 1441 cm−1; MS (EI) m/z 268 (M+,
100%), 121 (M+ − 147, 38%); HRMS calcd for C16H12S2 268.0380,
found 268.0367.
2,6-Di(4-methoxyphenyl)-1,4-dithiin (1c).12 Yellow solid [recov-
ery of 1c, 77.1 mg, 47%, Rf = 0.50 (4/1 hexane/ethyl acetate)]; mp
124.0−125.5 °C (hexane) (lit.12 125.5−127.5 °C); H NMR (400
1
MHz, CDCl3) δ 3.83 (6H, s), 6.42 (2H, s), 6.89 (4H, d, J = 8.8 Hz),
7.58 (4H, d, J = 8.8 Hz); 13C NMR (100 MHz, CDCl3) δ 55.4, 113.9,
116.3, 128.3, 129.7, 139.4, 159.9; IR (KBr) 2927, 1604, 1505, 1253,
1175, 1032 cm−1; MS (EI) m/z 328 (M+, 100%), 313 (M+ − 15,
37%); HRMS calcd for C18H16O2S2 328.0592, found 328.0600.
2,5-Di(4-methoxyphenyl)-1,4-dithiin (2c). Yellow solid [83.5 mg,
51%, Rf = 0.52 (4/1 hexane/ethyl acetate)]; mp 172.5−173.5 °C
(hexane) [lit.3c 167.0−169.0 °C (methylene choloride/petroleum
permeation chromatography. 7:13 colorless oil; H NMR (400 MHz,
1
CDCl3) δ 1.20 (9H, s), 3.85 (3H, s), 3.89 (3H, s), 6.09 (1H, s); 13C
NMR (100 MHz, CDCl3) δ 32.1, 35.0, 52.5, 52.6, 123.4, 129.8, 136.9,
161.6, 163.2, 165.0; IR (neat) 2925, 1731, 1464, 1261, 1075 cm−1;
MS (EI) m/z 288 (M+, 100%), 57 (M+ − 231, 71%); HRMS calcd for
1
ether)]; H NMR (400 MHz, CDCl3) δ 3.83 (6H, s), 6.45 (2H, s),
6.88 (4H, d, J = 8.8 Hz), 7.55 (4H, d, J = 8.8 Hz); 13C NMR (100
MHz, CDCl3) δ 55.3, 113.9, 115.2, 128.5, 129.2, 141.1, 160.0; IR
(KBr) 2958, 1601, 1502, 1252, 1175, 1028, 785 cm−1; MS (EI) m/z
328 (M+, 100%), 313 (M+ − 15, 28%); HRMS calcd for C18H16O2S2
328.0592, found 328.0587.
1
C12H16O4S2 288.0490, found 288.0487. 8: pale yellow oil; H NMR
(400 MHz, CDCl3) δ 1.32 (9H, s), 3.85 (3H, s), 3.94 (3H, s), 7.23
(1H, s); 13C NMR (100 MHz, CDCl3) δ 30.7, 34.5, 52.4, 52.9, 126.0,
131.3, 138.9, 150.8, 161.6, 168.2; IR (neat) 2924, 2854, 1741, 1457,
1242, 1022 cm−1; MS (EI) m/z 256 (M+, 16%), 209 (M+ − 47,
100%); HRMS calcd for C12H16O4S 256.0769, found 256.0761. 9:
2,6-Di(1-adamantyl)-1,4-dithiin (1d). Colorless solid [recovery of
1d, 89.8 mg, 47%, Rf = 0.20 (hexane)]; mp 213.0−214.0 °C (hexane);
1H NMR (400 MHz, CDCl3) δ 1.66−1.74 (12H, m), 1.83 (12H, d, J
1
pale yellow oil; H NMR (400 MHz, CDCl3) δ 1.39 (9H, s), 3.86
E
DOI: 10.1021/acs.organomet.8b00498
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
(3H, s), 3.90 (3H, s), 7.02 (1H, s); 13C NMR (100 MHz, CDCl3) δ
32.0, 35.0, 52.5, 52.6, 123.4, 129.8, 136.9, 161.6, 163.2, 165.0; IR
(neat) 2957, 1723, 1452, 1246, 1092, 802 cm−1; MS (EI) m/z 256
(M+, 18%), 241 (M+ − 15, 100%); HRMS calcd for C12H16O4S
256.0769, found 256.0781. 10:14 colorless solid; mp 126.5−127.5 °C
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.8b00498.
■
S
(hexane) (lit.14 129.3−131.1 °C); H NMR (400 MHz, CDCl3) δ
1
3.921 (6H, s), 3.923 (6H, s); 13C NMR (100 MHz, CDCl3) δ 53.1,
53.3, 136.0, 137.0, 160.2, 162.8; IR (KBr) 2960, 1733, 1437, 1230
cm−1; MS (EI) m/z 316 (M+, 16%), 285 (M+ − 31, 100%); HRMS
calcd for C12H12O8S 316.0253, found 316.0259.
Desulfurization of 7 to 8 and 9. In a two-neck flask was placed
5-(tert-butyl)-2,3-di(methoxycarbonyl)-1,4-dithiin 7 (20.2 mg, 0.07
mmol) in 1,2-dichlorobenzene (0.5 mL) under an argon atomo-
sphere, and the mixture was heated at 150 °C for 3 h. The solvent was
removed under reduced pressure, and the residue was purified by flash
column chromatography on silica gel giving 5-(tert-butyl)-2,3-
di(methoxycarbonyl)thiophene 8 (9.7 mg, 54%) and 4-(tert-butyl)-
2,3-di(methoxycarbonyl)thiophene 9 (6.5 mg, 36%).
NMR spectra and crystallographic details (PDF)
Accession Codes
CCDC 1527675, 1559628, and 1560074 contain the
supplementary crystallographic data for this paper. These
data can be obtained free of charge via www.ccdc.cam.ac.uk/
data_request/cif, or by emailing data_request@ccdc.cam.ac.
uk, or by contacting The Cambridge Crystallographic Data
Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44
1223 336033.
AUTHOR INFORMATION
Corresponding Authors
*E-mail: arisawa@m.tohoku.ac.jp.
*E-mail: yama@m.tohoku.ac.jp.
■
2,4-Di(tert-butyl)thiophene (12). Colorless oil [0.5 mmol scale,
1
31.0 mg, 32%, Rf = 0.45 (hexane)]; H NMR (400 MHz, CDCl3) δ
1.27 (9H, s), 1.37 (9H, s), 6.72 (1H, d, J = 1.6 Hz), 6.77 (1H, d, J =
1.6 Hz); 13C NMR (100 MHz, CDCl3) δ 31.2, 32.5, 33.5, 34.5, 114.3,
120.6, 152.4, 157.0; IR (neat) 2964, 1462, 1364, 1249 cm−1; MS (EI)
m/z 196 (M+, 20%), 181 (M+ − 15, 100%); HRMS calcd for C12H2S
196.1286, found 196.1299.
5-Phenyl-2,3-thiophenedicarboxylic Acid 2,3-Dimethyl Ester
(13b). Colorless solid [76.2 mg, 55%, Rf = 0.33 (4/1 hexane/ethyl
acetate)]; mp 92.0−93.0 °C (pentane); 1H NMR (400 MHz, CDCl3)
δ 3.91 (3H, s), 3.94 (3H, s), 7.38−7.45 (3H, m), 7.46 (1H, s), 7.61
(2H, dd, J = 6.8, 2.4 Hz); 13C NMR (100 MHz, CDCl3) δ 52.67,
52.69, 124.30, 126.2, 129.2, 131.6, 132.4, 138.0, 147.4, 149.1, 161.4,
164.5; IR (KBr) 2951, 1728, 1442, 1249 cm−1; MS (EI) m/z 276
(M+, 90%), 245 (M+ − 31, 100%); HRMS calcd for C14H12O4S
276.0456, found 276.0458.
ORCID
Mieko Arisawa: 0000-0002-9561-2653
Masahiko Yamaguchi: 0000-0003-3965-7598
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This research was supported by the Platform Project for
Supporting Drug Discovery and Life Science Research from
AMED under Grant JP18am0101100, JSPS KAKENHI Grants
17K19112 and 15H00911, the Tohoku University Center for
Gender Equality Promotion (TUMUG), and the Morinomiya-
ko Project for Empowering Women in Research.
1,2-Diphenyl-4,5-di(methoxycarbonyl)benzene (14b).15 Color-
less solid [35.4 mg, 41% based on phenyl aetylene, Rf = 0.35 (4/1
hexane/ethyl acetate)]; mp 107.0−108.0 °C (hexane); 1H NMR (400
MHz, CDCl3) δ 3.94 (6H, s), 7.12−7.15 (4H, m), 7.23−7.25 (6H,
m), 7.79 (2H, s); 13C NMR (100 MHz, CDCl3) δ 52.7, 127.4, 128.1,
129.6, 130.7, 131.2, 139.5, 143.4, 167.9; IR (KBr) 2922, 1455, 805,
751 cm−1; MS (EI) m/z 346 (M+, 100%), 315 (M+ − 31, 74%);
HRMS calcd for C22H18O4 346.1205, found 346.1205.
REFERENCES
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5-(4-Methoxyphenyl)-2,3-thiophenedicarboxylic Acid 2,3-Di-
methyl Ester (13c). Yellow solid [88.3 mg, 58%, Rf = 0.26 (4/1
hexane/ethyl acetate)]; mp 109.5−111.0 °C (hexane); 1H NMR (400
MHz, CDCl3) δ 3.85 (3H, s), 3.90 (3H, s), 3.94 (3H, s), 6.94 (2H, d,
J = 9.2 Hz), 7.34 (1H, s), 7.54 (2H, d, J = 8.8 Hz); 13C NMR (100
MHz, CDCl3) δ 52.6, 52.7, 55.4, 114.6, 123.2, 125.2, 127.6, 130.3,
138.2, 149.3, 160.5, 161.4, 164.7; IR (KBr) 2955, 2839, 1713, 1438,
1257, 1088, 1029 cm−1; MS (EI) m/z 306 (M+, 100%), 275 (M+ −
31, 54%); HRMS calcd for C15H14O5S 306.0562, found 306.0562.
1,2-Di(4-methoxyphenyl)-4,5-di(methoxycarbonyl)benzene
(14c). Colorless solid [38.7 mg, 38% based on 4-methoxyphenyl
acetylene, Rf = 0.33 (4/1 hexane/ethyl acetate)]; mp 120.5−122.0 °C
(hexane); 1H NMR (400 MHz, CDCl3) δ 3.79 (6H, s), 3.93 (6H, s),
6.79 (4H, d, J = 8.4 Hz), 7.07 (4H, d, J = 8.4 Hz), 7.74 (2H, s); 13C
NMR (100 MHz, CDCl3) δ 52.7, 55.2, 113.6, 130.3, 130.7, 131.2,
132.0, 142.9, 158.9, 168.0; IR (KBr) 2953, 2840, 1726, 1608, 1517,
1249, 1134, 835 cm−1; MS (EI) m/z 406 (M+, 100%), 375 (M+ − 31,
20%); HRMS calcd for C24H22O6 406.1416, found 406.1406.
5-tert-Butyl-4-phenyl-2,3-thiophenedicarboxylic Acid 2,3-Di-
methyl Ester (17). Colorless solid [0.25 mmol scale, 50.0 mg, 60%,
Rf = 0.30 (4/1 hexane/ethyl acetate)]; mp 81.0−83.0 °C (hexane);
1H NMR (400 MHz, acetone-d6) δ 1.24 (9H, s), 3.49 (3H, s), 3.83
(3H, s), 7.24−7.26 (2H, m), 7.37−7.40 (3H, m); 13C NMR (100
MHz, acetone-d6) δ 32.3, 36.8, 52.3, 52.6, 124.6, 128.5, 128.9, 131.4,
136.2, 138.2, 143.2, 159.2, 161.7, 165.7; IR (KBr) 2954, 1741, 1718,
1442, 1289 cm−1; MS (EI) m/z 332 (M+, 29%), 285 (M+ − 47,
100%); HRMS calcd for C18H20O4S 332.1082, found 332.1088.
F
DOI: 10.1021/acs.organomet.8b00498
Organometallics XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.organomet.8b00498
Organometallics XXXX, XXX, XXX−XXX