Synthesis of unsymmetrical polyfluorinated diaryl sulfides by rhodium-catalyzed aryl exchange reaction

Article abstract of DOI:10.1016/j.tetlet.2013.06.021

RhH(PPh₃)₄ and 1,2-bis(diphenylphosphino)benzene (dppBz) catalyze the aryl exchange reaction of polyfluorinated diaryl sulfides. By employing these catalysts, unsymmetrical polyfluorinated diaryl sulfides are synthesized by the react

Full text of DOI:10.1016/j.tetlet.2013.06.021

Tetrahedron Letters 54 (2013) 4327–4329
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Tetrahedron Letters
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Synthesis of unsymmetrical polyfluorinated diaryl sulfides
by rhodium-catalyzed aryl exchange reaction
⇑
⇑
Mieko Arisawa , Takuya Ichikawa, Masahiko Yamaguchi
Department of Organic Chemistry, Graduate School of Pharmaceutical Sciences, Tohoku University, Aoba, Sendai 980-8578, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
RhH(PPh3)4 and 1,2-bis(diphenylphosphino)benzene (dppBz) catalyze the aryl exchange reaction of poly-
fluorinated diaryl sulfides. By employing these catalysts, unsymmetrical polyfluorinated diaryl sulfides
are synthesized by the reaction of symmetrical polyfluorinated diaryl sulfides and substituted pentaflu-
orobenzenes in the presence of triisopropylsilane.
Received 25 April 2013
Revised 31 May 2013
Accepted 6 June 2013
Available online 13 June 2013
Ó 2013 Elsevier Ltd. All rights reserved.
Keywords:
Aryl exchange reaction
Unsymmetrical polyfluorinated diaryl
sulfides
Symmetrical polyfluorinated diaryl sulfides
Rhodium catalyst
Pentafluorobenzenes
Polyfluorinated diaryl sulfides are interesting compounds that
exhibit unusual properties. For example, the nickel complex of
bis(pentafluorophenyl)sulfide is more stable than that of di(4-
methoxyphenyl) sulfide.1 As related substances, fluorinated
poly(arylene sulfide)s have attracted much attention as novel
materials.2 However, the synthesis of polyfluorinated diaryl sul-
fides is not straightforward. Although, these sulfides can be synthe-
sized by the reaction of polyfluoroaryl thiolates and aryl halides,
their synthesis is often accompanied by polymerization, because
the thiolates are unstable under basic conditions.3 To control the
reactivity of the thiolates, copper, nickel, and silver polyfluoroaryl
thiolates having suitable stability and affinity with sulfur are used.4
Alternatively, the following reactions of polyfluoroaryl metal
compounds and sulfur reagents are used: the reaction of sulfur
dichloride with pentafluorophenyllithium or pentafluorophenyl-
magnesium bromide,5 the reaction of sulfur with polyfluoro-
diarylmercury at 250 °C,6 the reaction of sulfur with
iodopentafluorobenzene without a solvent at 230 °C,7 and the reac-
tion of sulfur, chloropentafluorobenzene, and copper at 350 °C.8 All
these methods require stoichiometric amounts of metal reagents.
Also note that only a few scattered examples of the synthesis of
unsymmetrical polyfluorinated diaryl sulfides have been
reported.3b,c,4c
Described in this study is the rhodium-catalyzed synthesis of
unsymmetrical polyfluorinated diaryl sulfides by the reaction of
symmetrical diaryl sulfides and substituted pentafluorobenzenes
in the presence of triisopropylsilane (Scheme 1). In this reaction,
the C–S bond of the unsymmetrical polyfluorinated diaryl sulfides
is formed by the aryl exchange reaction of symmetrical polyfluori-
nated diaryl sulfides and aryl fluorides. The advantage of this
method is that neutral polyfluorinated diaryl sulfides can be used
as the thiolating reagent without using large amounts of metal re-
agents. It should also be emphasized that such an aryl exchange
reaction of diaryl sulfides was not known before.
Previously, we reported the rhodium-catalyzed synthesis of
symmetrical polyfluorinated diaryl sulfides from substituted pen-
tafluorobenzenes and sulfur.9 In this study, we utilize the products
obtained by this method to synthesize unsymmetrical diaryl sul-
fides. The overall reactions provide various unsymmetrical diaryl
sulfides from aryl fluorides and sulfur.
⇑
Corresponding authors. Tel.: +81 22 795 6814; fax: +81 22 795 6811 (M.A.); tel.:
+81 22 795 6812; fax: +81 22 795 6811 (M.Y.).
E-mail addresses: arisawa@m.tohoku.ac.jp (M. Arisawa), yama@m.tohoku.ac.jp
(M. Yamaguchi).
Scheme 1.
0040-4039/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2013.06.021

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M. Arisawa et al. / Tetrahedron Letters 54 (2013) 4327–4329
The study was initiated on the basis of the finding that a rhodium
(10 mol %), and triisopropylsilane (1 equiv) in refluxing THF for
6 h, 1b was obtained in 19% yield. Analysis on the reaction course
also indicated that 2ab was initially formed, which was followed
by the formation of 1b: The yields of 2ab/1b were 31%/5% (1 h),
43%/8% (2 h), 54%/19% (4 h), and 51%/21% (6 h). Thus, 1b was
formed from 1a by the aryl exchange twice with 3b, and the tetra-
fluorobenzoylphenyl group of 2ab had sufficient reactivity to un-
dergo the second aryl exchange.11 When 3 equiv of 3b was used
over 1a, the yield of 1b increased to 43%.
Other solvents could be used, and DMF at 80 °C (2ab and 1b,
46% and 21%, respectively), and chlorobenzene at reflux (28% and
23%) gave comparable results. The efficiencies of the dppBz ligand
(34% and 53%) were observed to be higher than those of dppe (48%
and 20%), 1,2-bis{di(4-methoxyphenyl)phosphino}ethane (46%
and 11%), and (Ph2PCH2CH2)2PPh (49% and 7%); however, no reac-
tion occurred using cis-1,2-bis(diphenylphosphino)ethylene
(dppv), 1,2-bis(dicyclohexylphosphino)benzene (dcyBz), or dppf.
The silanes exhibiting activity were n-Bu3SiH (yield of 2ab and
1b, 34% and 53%, respectively), Ph3SiH (36% and 52%), Ph2MeSiH
(29% and 54%), PhMe2SiH (25% and 57%).
complex catalyzes the cleavage and exchange reaction of two sym-
metric diaryl sulfides. When bis(4-benzoyl-2,3,5,6-tetrafluorophenyl)
sulfide 1a and bis(4-phenylthio-2,3,5,6-tetrafluorophenyl) sulfide 1b
(1 equiv) were reacted in refluxing chlorobenzene for 12 h in the pres-
ence of RhH(PPh3)4 (10 mol %), 1,2-bis(diphenylphosphino)benzene
(dppBz, 20 mol %), 1-(4-phenylthio-2,3,5,6-tetrafluorophenylthio)-
4-benzoyl-2,3,5,6-tetrafluorobenzene 2ab (40%) was obtained
with the recovery of 1a (44%) and 1b (36%) (Scheme 2). The
reverse reaction of 2ab to form 1a and 1b proceeded under the
same rhodium-catalyzed conditions. No reaction occurred in the
absence of the rhodium complex. The rhodium complex cleaved
and exchanged the arylthio group of 1a and 1b under equilibrium.
It was then considered that the reaction of a diaryl sulfide with
a substituted pentafluorobenzene would give an unsymmetrical
diaryl sulfide, and for an efficient conversion, the resulting aryl
fluoride should be reduced to the unreactive hydrogenated form
with the concomitant formation of silyl fluoride.
When 1a was reacted with phenylthiopentafluorobenzene 3b (1
equiv) in the presence of RhH(PPh3)4 (5 mol %), dppBz (10 mol %),
and triisopropylsilane (1 equiv) in refluxing THF for 6 h, 2ab was
obtained in 51% yield, which was accompanied by 4-benzoyl-
2,3,5,6-tetrafluorobenzene 4 (74%) and 4-phenylthio-2,3,5,6-tetra-
fluorobenzene (18%) (Scheme 3). The added triisopropylsilane was
converted to triisopropylsilyl fluoride in 57% yield, which was con-
firmed by 19F NMR d À185.8 (singlet).10 No reaction occurred with-
out triisopropylsilane. Note that the symmetric sulfide 1b was also
obtained in 21% yield, which should be formed by the reaction of
the products 2ab with 3b. Accordingly, when 2ab was reacted with
3b (1 equiv) in the presence of RhH(PPh3)4 (5 mol %), dppBz
In the presence of RhH(PPh3)4 (5 mol %), dppBz (10 mol %), and
triisopropylsilane (1 equiv), pentafluorobenzenes having benzoyl,
Table 1
Synthesis of unsymmetrical polyfluorinated diaryl sulfides by aryl exchange reaction.
Entry
R
R’
Yield of 2xy/%
Yield of 1y/%
1
2
3
PhCO (1a)
PhCO (1a)
PhCO (1a)
PhCO (1a)
PhCO (1a)
MeCO (1c)
PhS (1b)
PhS (3b)
51 (2ab)
53 (2ad)
39 (2ae)
41 (2ac)
40 (2af)
34 (2bc)
32 (2ab)
48 (2be)
21 (1b)
20 (1d)
11 (1e)
16 (1c)
4 (1f)
5 (1b)
28 (1a)
21 (1e)
4-TolS (3d)
4-MeOC6H4S (3e)
MeCO (3c)
Ph2N (3f)
PhS (3b)
PhCO (3a)
4a
5b
6
7
8
PhS (1b)
4-MeOC6H4S (3e)
a
RhH(PPh3)4 (10 mol %) and dppBz (20 mol %) were used.
Compound 3f (3 equiv) was reacted in DMF at 80 °C.
b
Scheme 2.
Scheme 3.
Figure 1.

M. Arisawa et al. / Tetrahedron Letters 54 (2013) 4327–4329
4329
acetyl, phenylthio, p-tolylthio, p-methoxyphenylthio, and diphen-
ylamino groups reacted with 1 to give the unsymmetrical sulfide
2, which was accompanied by the formation of approximately
20% of the symmetrical sulfide (Table 1). All these reactions pro-
ceeded at the p-position of pentafluorobenzenes.12 The reaction
of (diphenylamino)pentafluorobenzene 3f (3 equiv) with 1a gave
2af in 40% yield in DMF at 80 °C for 6 h. No reaction occurred in
THF. The reaction of bis(4-cyano-2,3,5,6-tetrafluorophenyl) sulfide
and bis(4-nitro-2,3,5,6-tetrafluorophenyl) sulfide with 3b did not
occur. Unsymmetrical polyfluorinated diaryl sulfides were ob-
tained from symmetrical diaryl sulfides using substituted pentaflu-
orobenzenes as the arylating reagent.
(22.4 mg, 57%), 4-benzoyl-2,3,5,6-tetrafluorobenzene 4 (47.0 mg,
74%), and 4-phenylthio-2,3,5,6-tetrafluorobenzene (11.9 mg, 18%).
Acknowledgments
This work was supported by a Grant-in-Aid for Scientific Re-
search (No. 21229001). M.A. expresses her appreciation to the
financial supports from the Grant-in-Aid for Scientific Research
(Nos. 22689001 and 25109503), and also to the Asahi Glass
Foundation.
Supplementary data
A possible mechanism of the reaction is as follows (Fig. 1). The
high-oxidation-state rhodium complex
A is formed by the
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.tetlet.2013.
06.021.
oxidative addition of polyfluorinated diaryl sulfide and substituted
pentafluorobenzene. Then, the unsymmetrical diaryl sulfide is lib-
erated as the product by reductive elimination. The aryl group in
the resulting aryl rhodium fluoride B is reduced to 4-substituted-
2,3,5,6-tetrafluorobenzene by triisopropylsilane. The rhodium
catalyst is regenerated, which is accompanied by the formation
of triisopropylsilyl fluoride. Alternatively, reaction mechanism
containing Rh(I)/Rh(III) cycles via transmetallation may be possi-
ble. Recently, the formation of silyl sulfides by the palladium-nano-
particle-mediated reaction of diaryl sulfides and silanes has been
reported.13 Since no reaction of 1a and triisopropylsilane occurred
under the present rhodium catalyzed condition, the formation of
silyl sulfide intermediates may not be likely in the present reac-
tion. The rhodium complex should directly activate polyfluorinated
diaryl sulfides14 and pentafluorobenzenes.15
In summary, unsymmetrical polyfluorinated diaryl sulfides
were synthesized by the reaction of symmetrical polyfluorinated
diaryl sulfides with substituted pentafluorobenzenes in the pres-
ence of triisopropylsilane. This is a novel aryl exchange method
for the synthesis of diaryl sulfides. This method has an advantage
in that various unsymmetrical polyfluorinated diaryl sulfides can
be obtained from a single symmetrical diaryl sulfide via aryl ex-
change with aryl halides.
References and notes
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11. A reviewer noted that 1b may be formed by the dispropotionation reaction of
2ab (Scheme 3) by analogy to the reaction in Scheme 2. The reaction conditions
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Scheme 2. When 2ab was reacted in the presence of RhH(PPh3)4 (5 mol %),
dppBz (10 mol %), and i-Pr3SiH (1 equiv) in refluxing THF for 6 h, 1b was not
detected. The results indicated that 2ab did not undergo disproportion to 1a
and 1b under the condition of Scheme 3. Therefore, it was concluded that 1b
was formed from 1a by the aryl exchange twice with 3b.
Typical experimental procedures
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Ichikawa, T.; Yamaguchi, M. Tetrahedron 2011, 67, 7846.
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placed 1a (0.25 mmol, 134.6 mg), phenylthiopentafluorobenzene
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bis(diphenylphosphino)benzene (10 mol %, 11.2 mg), and triiso-
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propylsilane (0.25 mmol, 52.2 lL) in THF (0.25 mL) under an argon
atmosphere, and the solution was heated at reflux for 6 h. Then, the
solvent was removed under reduced pressure, and the residue was
purified by flash column chromatography on silica gel giving 2ab
(68.7 mg, 51%), 1b (8.5 mg, 21%), triisopropylsilyl fluoride