A novel approach to the practical synthesis of sulfides: An InBr 3-Et3SiH catalytic system promoted the direct reductive sulfidation of acetais with disulfides

Article abstract of DOI:10.1002/ejoc.200900566

We have demonstrated a facile and direct synthesis of sulfide derivatives using acetais and ketals, derived, from aromatic/ conjugated, aldehydes and aromatic ketones, with disulfides and the InBr₃-Et₃SiH reducing system., We also succeeded in developing an unprecedented one-pot preparation of an aliphatic sulfide from a disulfide and an aliphatic acetal. (Wiley-VCH Verlag GmbH & Co. KGaA.

Full text of DOI:10.1002/ejoc.200900566

FULL PAPER
DOI: 10.1002/ejoc.200900566
A Novel Approach to the Practical Synthesis of Sulfides: An InBr3–Et3SiH
Catalytic System Promoted the Direct Reductive Sulfidation of Acetals with
Disulfides
Norio Sakai,*[a] Kohei Moritaka,[a] and Takeo Konakahara[a]
Keywords: Sulfides / Reduction / Acetals / Indium / Silanes
We have demonstrated a facile and direct synthesis of sulfide
derivatives using acetals and ketals, derived from aromatic/
conjugated aldehydes and aromatic ketones, with disulfides
and the InBr3–Et3SiH reducing system. We also succeeded
in developing an unprecedented one-pot preparation of an
aliphatic sulfide from a disulfide and an aliphatic acetal.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2009)
Introduction
As sulfide skeletons are ubiquitous in natural products
and in biologically active substances,[1] there have been nu-
merous attempts to prepare them in a facile and practical
way.[2] Typical approaches have included the reduction of
sulfones or sulfoxides,[3] a Williamson-type thioether syn-
thesis from thiols and alcohols in the presence of a base,[4]
and an Ullmann-type reaction of thiols with organic halides
Scheme 1.
Previously, we reported that an InBr3–Et3SiH reducing
using metals such as copper,[5] palladium,[6] and nickel.[7] In system effectively promotes the reduction of the acetyl
this context, for the last two decades, extensive work has group of propargylic acetates, as well as the direct and selec-
been dedicated to developing a direct reductive preparation tive reduction of the carbonyl group in esters and amides,
of sulfides from thiols and either aldehydes or ketones using which directly produces the corresponding ethers and
a combined method with an appropriate catalyst and a re- amines, respectively.[10,11] During ongoing studies on the se-
ducing reagent (Path a, Scheme 1). For example, Kikugawa lective reduction of carbonyl compounds and related com-
reported the preparation of sulfides from either aldehydes pounds using our standard InBr3–Et3SiH reducing system,
or ketones with thiols in CF3CO2H using a pyridine–BH3 a member of our group found that this reducing catalytic
reducing system.[8] Also, Olah and co-workers demon- system promoted a new C–S bond formation in acetals (ket-
strated the synthesis of sulfides from carbonyl compounds als), which were relatively inactive in common organic reac-
via an O,S-acetal intermediate using a BF3·H2O–Et3SiH re- tions, with fewer odorous disulfides than found when using
ducing system.[9] However, these contributions generally thiols, and which led to the direct formation of sulfide de-
have several disadvantages, including harsh reaction condi- rivatives. As far as can be ascertained, a direct skeletal
tions owing to the use of a strong acid, a flammable rea- transformation from acetals and disulfides to a sulfide skel-
gent, and a thiol that has an unpleasant odor. There have eton accompanied by the cleavage of two C–O bonds has
also been reports of the use of more than a stoichiometric not been reported (Path b, Scheme 1). Herein, we detail the
amount of catalyst. Therefore, the development of a conve- results of a novel method for the synthesis of sulfide deriva-
nient, efficient, and easy-to-handle preparation of sulfides tives using the InBr3–Et3SiH catalytic system.
from carbonyl compounds is a priority.
Results and Discussion
[a] Department of Pure and Applied Chemistry, Faculty of Science
and Technology, Tokyo University of Science (RIKADAI),
To find the optimal sulfidation conditions, the use of
other solvents and 2 equiv. of the silane was first investi-
gated using benzaldehyde dimethyl acetal (1a) and diphenyl
disulfide (2a) as the model reaction. The results are listed
Noda, Chiba 278-8510, Japan
Fax: +81-4-7123-9890
E-mail: sakachem@rs.noda.tus.ac.jp
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.200900566.
Eur. J. Org. Chem. 2009, 4123–4127
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N. Sakai, K. Moritaka, T. Konakahara
FULL PAPER
in Table 1. For example, the use of polar solvents such as Table 2. Synthesis of sulfide derivatives 3–14 from acetals and di-
sulfides.
THF, MeCN, and DMF did not produce desired sulfide
product 3 (Table 1, runs 2–4). On the other hand, a nonpo-
lar solvent, toluene, had a relatively good effect (Table 1,
run 5). When the reaction temperature was raised to 100 °C,
the yield slightly increased and the reduction was completed
within 1 h (Table 1, run 6). Moreover, when the amount of
Et3SiH was increased to four equivalents with respect to
disulfide 2, the yield improved dramatically to 83%
(Table 1, run 7). Needless to say, without InBr3, the desired
reductive sulfidation did not take place, but led to the re-
covery of the starting materials (Table 1, run 8). Thus, we
found that heating the reagents at 100 °C in a toluene solu-
tion with 5 mol-% of InBr3 and 4 equiv. of Et3SiH per ace-
tal in the presence of a disulfide were the optimal condi-
tions.
Table 1. Optimization of the conditions for sulfidation.
Run
Et3SiH
[equiv.]
Solvent Temperature
Time
[h]
Yield[a]
[%]
[°C]
The reactions of ketal derivatives 15a–e with disulfides 2
also gave the desired aryl or alkyl benzyl sulfides 16–20 in
excellent yields (Table 3). Neither the type of substituent on
the benzene ring of ketal 15 nor the substituted group on
the disulfide had any effect on the product yields. In ad-
dition, the reaction could be applied to the sulfidation of
trimethyl orthoformate (21), which produced O,S-acetal 22
in 88% yield (Scheme 2).
1
2
3
4
5
6
2
2
2
2
2
2
4
2
CHCl3
THF
60
60
60
60
60
100
100
100
18
18
18
18
18
1
63
n.d.
trace
n.d.
50
61
83
MeCN
DMF
PhMe
PhMe
PhMe
PhMe
7
1
1
8[b]
n.r.
[a] NMR yield. [b] Reaction was carried out without InBr3.
Table 3. Synthesis of sulfide derivatives 16–20 from ketals and di-
sulfides.
By using the optimal reaction conditions we investigated
the scope and limitation of the reductive sulfidation of a
variety of acetals 1a–i. The results are listed in Table 2. For
example, when sulfidation of benzaldehyde dimethyl acetal
with aromatic disulfides, such as diphenyl disulfide (2a) and
di-p-tolyl disulfide (2b) was performed under the optimal
conditions, the corresponding sulfide derivatives 3 and 4
were obtained in good yields. On the other hand, employ-
ment of di-tert-butyl disulfide (2c) produced expected sul-
fide 5 in only a 48% yield, which seemed to be caused by
the steric effect of the bulky tert-butyl groups. When di-n-
butyl disulfide (2d) was used as the starting material, de-
sired sulfide 6 was produced in a moderate yield. In general,
reactions of benzaldehyde dimethyl acetal derivatives 1b–g
with either an electron-donating substituent, such as a
methyl or methoxy group, or with an electron-withdrawing
substituent, such as a fluorine or chlorine atom, produced
expected sulfides 7–12 in good-to-excellent yields. The reac-
Unfortunately, with the exception of a benzyl-type acetal,
tion of acetal 1h derived from the conjugated aldehyde, cin- the expected formation of an aliphatic aromatic sulfide did
namaldehyde also produced sulfide 13 in a practical yield. not occur when the reaction was applied to the sulfidation
The reaction of acetal 1i bearing a furan moiety with di-n- of aliphatic acetal 23. Instead, the disappearance of both
butyl disulfide gave sulfide 14 in only 34% yield.
starting materials and the formation of unknown compli-
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Novel Approach to the Practical Synthesis of Sulfides
ether 24.[15] This result suggests the existence of another
reaction path involving the reductive deoxygenation of an
S,O-acetal with a silane, as proposed by several other
groups.[9c,16]
Scheme 2. Sulfidation of orthoformate derivative 21.
cated products were observed (Scheme 3). Hence, when the
sulfidation path for the reaction of aromatic acetal 1 and
di-p-tolyl disulfide (2b) was reinvestigated in detail, the for-
mation of both benzyl methyl ether (64% NMR yield) and
p-toluenethiol (84% GC yield) was confirmed under our
conditions.[12] Moreover, when a mixture of benzyl methyl
ether with p-toluenethiol was treated with the reducing rea-
gents, expected sulfide 4 was obtained in 67% yield (see
Table 2).[13] Consequently, as the results show in Scheme 4,
with an aromatic acetal and a disulfide, sulfidation proceeds
entirely by trans-sulfidation of the benzyl methyl ether de-
rivative and the thiol generated in situ by the InBr3–Et3SiH
reducing system.
Scheme 5. One-pot preparation of a dialkyl sulfide.
Conclusions
We have demonstrated the facile reductive synthesis of a
variety of sulfide derivatives from acetals (or ketals), which
were derived from aromatic or conjugated aldehydes and
relatively easy-to-handle disulfides using the InBr3–Et3SiH
reduction system. In particular, we found that this reaction
occurs by the trans-sulfidation of a benzyl methyl ether
with an in situ formed thiol. We have also developed an
unprecedented preparation of a dialkyl sulfide from a linear
aliphatic acetal and an aliphatic disulfide.
Scheme 3. Attempt at the direct sulfidation of an aliphatic acetal.
Experimental Section
General Methods: Chloroform and MeCN were distilled from P2O5.
THF and PhMe were distilled from sodium benzophenone. Other
organic solvents were dried and distilled prior to use. InBr3 and
triethylsilane were commercially available and were used without
further purification. All reactions were carried out under an atmo-
1
sphere of nitrogen unless otherwise noted. H NMR spectra were
measured at 500 and 300 MHz using tetramethylsilane as an in-
ternal standard. 13C NMR spectra were measured at 125 and
75 MHz using the center peak of chloroform (δ =77.0 ppm) as the
internal standard. High-resolution mass spectra were measured by
using NBA (3-nitrobenzyl alcohol) as the matrix. Benzaldehyde di-
methyl acetal (1a), diphenyl disulfide (2a), di-n-butyl disulfide (2d),
and trimethoxymethane (21) were commercially available and used
without further purification. Other acetals were prepared from the
corresponding aldehydes or ketones and alcohols according to a
previously reported procedure, and were identified by their 1H and
13C NMR (JEOL ECP-500) and mass spectra (JEOL MS-700
MStation).
Scheme 4. Reaction path for sulfidation using a benzylic acetal and
a disulfide.
Therefore, our original plan, the direct synthesis of a
thioether from the corresponding aliphatic acetal, had to be
shifted to the development of a stepwise procedure. After a
great deal of experimentation, we managed to succeed in
the development of a novel stepwise procedure as follows.
A mixture of di-n-butyl disulfide (2d), InBr3, and Et3SiH
was initially heated without a solvent at 100 °C for 1 h. Ali-
phatic acetal 23 was then added to the resultant solution
containing the in situ generated thiol with further heating
at 60 °C for 1 h, which finally produced desired dialkyl sul-
fide 24 in 48% yield along with thioacetal 25 (Scheme 5).[14]
To confirm the sulfidation path, isolated dithioacetal 25
was treated with the standard InBr3/Et3SiH reducing sys-
General Procedure for the Preparation of Acetals
Method A:[17a] A methanol solution (10 mL) of trimethyl orthofor-
mate (2.1 g, 20 mmol) and Yb(OTf)3 (62 mg, 0.10 mmol) was
stirred at room temperature under N2, followed by the addition of
an aldehyde (10 mmol). After stirring at room temperature for 2 h,
the reaction mixture was poured into a mixture of saturated
tem but unfortunately it did not form expected dialkyl thio- NaHCO3 aqueous solution (5 mL) and diethyl ether (15 mL). The
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N. Sakai, K. Moritaka, T. Konakahara
FULL PAPER
ether layer was extracted, dried with anhydrous Na2SO4, and evap-
orated under reduced pressure. The crude product was distilled un-
der reduced pressure with a bulb-to-bulb distillation apparatus to
give the corresponding acetal.
(quint., J = 7.5 Hz, 2 H, CH2), 2.31 (s, 3 H, Ar-CH3), 2.38 (t, J =
7.5 Hz, 2 H, CH2), 5.11 (s, 1 H, Ph-CH-S), 7.10 (t, J = 8 Hz, 2 H,
Ar-H), 7.20 (t, J = 8 Hz, 1 H, Ar-H), 7.25–7.31 (m, 4 H, Ar-H),
7.41 (d, J = 7.5 Hz, 2 H, Ar-H) ppm. 13C NMR (125 MHz,
CDCl3): δ = 13.6, 20.9, 22.0, 31.1, 31.9, 53.8, 126.9, 128.1, 128.2,
128.4, 129.1, 136.6, 138.6, 141.8 ppm. MS (EI): m/z (%) = 270
[M]+, 181 (100). C18H22S (270.43): calcd. C 79.94, H 8.20; found C
80.18, H 8.46.
Method B:[17b]
A CH2Cl2 solution (40 mL) of an aldehyde
(10 mmol) and trimethyl orthoformate (2.1 g, 20 mmol) was stirred
at room temperature under N2, followed by the addition of In-
(OTf)3 (28 mg, 0.050 mmol). After stirring at room temperature for
3 min, a further portion of In(OTf)3 (28 mg, 0.050 mmol) was
added, and the reaction mixture was stirred for another 30 min.
The reaction mixture was then poured into a mixture of a saturated
NaHCO3 aqueous solution (5 mL) and diethyl ether (15 mL). The
ether layer was extracted, dried with anhydrous Na2SO4, and evap-
orated under reduced pressure. The crude product was distilled un-
der reduced pressure with a bulb-to-bulb distillation apparatus to
give the corresponding acetal.
Procedure for the Stepwise Synthesis of Dialkyl Sulfide 24: Di-n-
butyl disulfide (2d; 0.3 mmol), InBr3 (10.6 mg, 0.0030 mmol), and
Et3SiH (200 µL, 2.0 mmol) were successively added to a screw-
capped vial under N2. The vial was sealed with a cap containing a
PTFE septum. During heating of the reaction mixture at 100 °C
(bath temperature) the reaction was monitored by TLC until con-
sumption of the starting disulfide. After 1 h, 3-phenylpropylalde-
hyde dimethyl acetal (23; 0.3 mmol) was added to the resulting mix-
ture, and the vial was reheated at 100 °C for 1 h. The reaction was
monitored by TLC until consumption of the substrates. A saturated
NaHCO3 aqueous solution (3 mL) was then added to quench the
reaction. The aqueous layer was extracted with CH2Cl2 (15 mL)
and the combined organic phases were dried with anhydrous
Na2SO4, filtered, and then evaporated under reduced pressure. The
crude product was purified by a preparative TLC (SiO2/hexane)
and a further GPC separation (CHCl3) to give the corresponding
sulfide 24 and thioacetal 25.
4-Methylbenzophenone Dimethyl Acetal (15e). Method A: Yield:
1
1.45 g (60%), colorless oil. H NMR (300 MHz, CDCl3): δ = 2.29
(s, 3 H, Ar-CH3), 3.11 (s, 6 H, O-CH3), 7.09 (d, J = 7.8 Hz, 2 H,
Ar-H), 7.17–7.22 (m, 1 H, Ar-H), 7.25–7.30 (m, 2 H, Ar-H), 7.37
(d, J = 7.8 Hz, 2 H, Ar-H), 7.47–7.51 (m, 2 H, Ar-H) ppm. 13C
NMR (75 MHz, CDCl3): δ = 21.1, 49.2, 102.9, 126.7, 126.8, 127.3,
127.9, 128.7, 137.0, 139.5, 142.6 ppm. MS (EI): m/z (%)= 242 211
(100) [M]+. C16H18O2 (242.31): calcd. C 79.31, H 7.49; found C
79.05, H 7.80.
Supporting Information (see footnote on the first page of this arti-
cle): Details of experimental procedures and spectroscopic data for
the known compounds.
General Procedure for the Synthesis of Sulfides: A magnetic stirrer
bar, acetal (0.50 mmol), disulfide (0.25 mmol), InBr3 (8.9 mg,
0.0025 mmol), and Et3SiH (330 µL, 2.0 mmol) were successively
added to a freshly distilled toluene solution (0.5 mL) in a screw-
capped vial under N2. The vial was sealed with a cap containing a
PTFE septum. The reaction mixture was stirred at 100 °C (bath
temperature) and monitored by TLC until the starting disulfide
was consumed. Saturated NaHCO3 aqueous solution (3 mL) was
then added to quench the reaction. The aqueous layer was ex-
tracted with CH2Cl2 (15 mL) and the combined organic phases
were dried with anhydrous Na2SO4, filtered, and then evaporated
under reduced pressure. The crude product was purified by prepar-
ative TLC (SiO2/hexane) to give the corresponding sulfide.
Acknowledgments
This work was partially supported by the fund for the “High-Tech
Research Center” Project of Private Universities, a matching sub-
sidy from the Ministry of Education, Culture, Sports, Science and
Technology (MEXT) (2000–2004 and 2005–2007), and by a grant
from the Japan Private School Promotion Foundation (2008). The
authors thank Shin-Etsu Chemical Co., Ltd., for the gift of trieth-
ylsilane (Et3SiH).
1-[(Butylthio)methyl]naphthalene (12): Yield: 114 mg (99%), yellow
oil. 1H NMR (500 MHz, CDCl3): δ = 0.88 (t, J = 7.5 Hz, 3 H,
CH3-CH2), 1.37 (sext., J = 7.5 Hz, 2 H, CH2), 1.57 (quint., J =
7.5 Hz, 2 H, CH2), 2.47 (t, J = 7.5 Hz, 2 H, CH2), 4.15 (s, 2H Ar-
CH2-S), 7.36–7.37 (m, 2 H, Ar-H), 7.48 (t, J = 7.5 Hz, 1 H, Ar-H),
7.53 (t, J = 7.5 Hz, 1 H, Ar-H), 7.75 (m, 1 H, Ar-H), 7.84 (d, J =
8.5 Hz, 1 H, Ar-H), 8.15 (d, J = 8.5 Hz, 1 H, Ar-H) ppm. 13C
NMR (125 MHz, CDCl3): δ = 13.7, 22.0, 31.4, 31.8, 34.1, 124.0,
125.0, 125.7, 126.0, 126.9, 127.9, 128.7, 131.4, 134.0, 134.1 ppm.
MS (EI): m/z (%) = 230 (100) [M]+. HRMS (FAB): calcd. for
C15H19S [M + H]+ 231.1207; found 231.1236.
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Received: May 22, 2009
Published Online: July 9, 2009
Eur. J. Org. Chem. 2009, 4123–4127
© 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
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