Oxidative arylation mediated by naphthalene-1,8-diylbis(diphenylmethylium): Synthetic route to triarylsulfonium salts

Article abstract of DOI:10.1246/cl.2010.56

Dicationic species, naphthalene-1,8-diylbis(diphenylmethylium), successfully promoted oxidative arylation of the sulfur atom in (4-N,N-dialkylaminophenyl) phenyl sulfides with (4-N,N-dialkylaminopheny) silanes, affording triarylsulfonium salts in good yie

Full text of DOI:10.1246/cl.2010.56

doi:10.1246/cl.2010.56
56
Published on the web December 12, 2009
Oxidative Arylation Mediated by Naphthalene-1,8-diylbis(diphenylmethylium):
Synthetic Route to Triarylsulfonium Salts
Hiroyuki Tanabe,1 Takaharu Kawai,2 Terunobu Saitoh,2 and Junji Ichikawa*1
1Department of Chemistry, Graduate School of Pure and Applied Sciences, University of Tsukuba, Tsukuba 305-8571
2Department of Chemistry, Graduate School of Science, The University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113-0033
(Received October 15, 2009; CL-090929; E-mail: junji@chem.tsukuba.ac.jp)
Dicationic species, naphthalene-1,8-diylbis(diphenylmeth-
2 X
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
2e−
ylium), successfully promoted oxidative arylation of the sulfur
atom in (4-N,N-dialkylaminophenyl) phenyl sulfides with (4-
N,N-dialkylaminopheny)silanes, affording triarylsulfonium salts
in good yield.
1
2
Scheme 1. Two-electron transfer to dication 1.
Triarylsulfonium salts are highly sensitive to photolytic C-S
bond cleavage, which causes an efficient proton generation.1
This process allows triarylsulfonium salts to be well utilized as
an efficient photochemical source of Brønsted acids in industrial
processes such as cationic polymerization of alkenes or oxiranes
and hydrolysis in photoresist technology.2,3 In view of the
practical role of these salts, synthetic methods to produce them
are still needed. Although a number of synthetic methods for
alkylsulfonium salts have been described in the literature,4 there
are few methods on the synthesis of triarylsulfonium salts, and
fewer methods starting from diaryl sulfides. Triarylsulfonium
salts are synthesized by the reaction of diaryl sulfoxides with
aryl Grignard reagents and silylating reagents,5 but there is the
drawback that excess amounts of both reagents are required.
Diaryliodonium salts6 and aryl formates7 are used for the
synthesis of triarylsulfonium salts directly from diaryl sulfides,
albeit in low yield. Thus, triarylsulfonium salt synthesis by
direct arylation of sulfides still remains to be developed.
Recently, we found that naphthalene-1,8-diylbis(diphenyl-
methylium) (1) acts as an efficient, organic two-electron oxi-
dant.8,9 The dication 1 readily undergoes reduction via electron
transfer with an especially high oxidation potential, compared
with those of other bis(triarylmethylium)s10 and monotriaryl-
methyliums.11 In this process, a neutral compound, 1,1,2,2-tetra-
phenylacenaphthene (2), is formed via C-C bond formation
between the two carbocationic centers (Scheme 1). Syntheses of
(i) benzidines via self-coupling of N,N-dialkylanilines8a and (ii)
diaryl ethers via arylation of phenols with (4-N,N-dialkylamino-
phenyl) phenyl sulfides8b have already been accomplished using
dication 1. In the latter case, diaryl sulfides 3 are oxidized to
generate stable radical cation species A.12 We envisaged that
radical cations A might be captured by an aryl radical equivalent,
providing an entry to triarylsulfonium salts 5 directly from diaryl
sulfides (Scheme 2). In this communication, we report an oxida-
tive method using dication 1 for the synthesis of triarylsulfonium
salts via arylation of diaryl sulfides with arylsilanes.13
R2N
−e−
Ar
R2N
R2N
Ar
S
SPh
SPh
Ph
A
5
3
Scheme 2. Triarylsulfonium salt synthesis via oxidative arylation of
sulfides.
Table 1. Oxidative arylation of sulfide 3a with arylmetals 4a-4i
2 ClO4
Ph
Ph
Ph
Ph
Et2N
Et2N
Et2N
1a (1.2 equiv)
ClO4
SPh
−78 °C, 0.5 h
/ CH2Cl2
SPh
3a
ArM
ClO4
4a−i (1.2 equiv)
Ar
S
−78 °C, 0.5 h
Ph
5
Entry
ArM
Yield/%
(4a)
1
2
3
PhLi
<15 (5a)
< 7 (5a)
0
PhMgBr
(4b)
(4c)
PhSnBu3
4
5
6
(4d)
(4e)
(4f)
MeO
Et2N
Et2N
SnBu3
0
(5b)
(5b)
SnBu3
SiMe3
88
77
7
8
80 (5b)
73a (5c)
(4g)
Et2N
SiMe2Ph
9
(4h)
(4i)
(5b)
Et2N
Et2N
SiPh3
46
First, we screened several aryl metal species that we
expected to act as aryl radical sources under oxidative
conditions. Diaryl sulfide 3a was successively treated with
dication 1a (X = ClO4) and 4a-4i at ¹78 °C in dichloro-
methane. The results are summarized in Table 1. Phenyllithium
and phenylmagnesium reagents 4a and 4b afforded triarylsulfo-
nium salt 5a in low yield, along with biphenyl as a major
10
Sii-Pr3
<12 (5b)
aUsing dication 1b (X = OTf) as an oxidant gave triflate salt 5c.
product (Entries 1 and 2). Phenyl- and 4-methoxyphenylstan-
nanes 4c and 4d failed to produce sulfonium salts, but 4-
diethylaminophenylstannane 4e gave the expected sulfonium
Chem. Lett. 2010, 39, 56-57
© 2010 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/

57
nium radical cations 7.12 Further oxidation of 7 and elimination
of the silyl group occurs successively to afford triarylsulfonium
salts 5.17
2 ClO4
Ph
Ph
Ph
Ph
In conclusion, triarylsulfonium salts were readily synthe-
sized in good yield from (4-N,N-dialkylaminophenyl) phenyl
sulfides and (4-N,N-diethylaminophenyl)silanes. This oxidative
arylation of sulfides was successfully promoted by dication 1 via
two-electron oxidation. Because there are few methods for the
synthesis of triarylsulfonium salts, such direct arylation of diaryl-
sulfides provides a promising access to triarylsulfonium salts.
R2N
1a (1.2 equiv)
−78 °C, 0.5 h / CH2Cl2
SPh
R = Bn (3b)
allyl (3c)
NEt2
R2N
NEt2
PhMe2Si
4g (1.2 equiv)
S
ClO4
−78 °C, 0.5 h
Ph
References and Notes
R = Bn (5d) : 83%
allyl (5e) : 75%
1
2
3
For a review, see: J. V. Crivello, J. Photopolym. Sci. Technol.
2008, 21, 493.
For a review, see: J. V. Crivello, J. Photopolym. Sci. Technol.
2007, 20, 599.
Scheme 3. Oxidative arylation of sulfides 3b and 3c.
For recent reports, see: a) T. Tsuchimura, K. Shimada, Y. Ishiji, T.
Matsushita, T. Aoai, J. Photopolym. Sci. Technol. 2007, 20, 621.
b) M. Sangermano, M. A. Tasdelen, Y. Yagci, J. Polym. Sci.,
Part A: Polym. Chem. 2007, 45, 4914. c) Y.-Y. Liao, J.-H. Liu,
J. Appl. Polym. Sci. 2008, 109, 3849. d) S. J. Li, Y. He, J. Nie,
J. Appl. Polym. Sci. 2008, 110, 3388.
a) G. C. Banett, in Comprehensive Organic Chemistry, ed. by
D. Barton, W. D. Ollis, Pergamon Press, New York, 1979,
Vol. 3, Chap. 11.5, pp. 105-119. b) I. Sokka, A. Fischer, L. Kloo,
Polyhedron 2007, 26, 4893, and references therin.
a) R. D. Miller, A. F. Renaldo, H. Ito, J. Org. Chem. 1988, 53,
5571. b) S. Imazeki, M. Sumino, K. Fukasawa, M. Ishihara, T.
Akiyama, Synthesis 2004, 1648.
R2N
NEt2
R2N
X
S
SPh
5
3
Ph
2 X−
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
4
−R3SiX
1
2
5
6
NEt2
R3Si
R2N
NEt2
R2N
4
a) J. V. Crivello, J. H. W. Lam, Synth. Commun. 1979, 9, 151.
b) C. O. Yanez, C. D. Andrade, K. D. Belfield, Chem. Commun.
2009, 827.
X−
S
SPh
X−
SiR3
Ph
7
8
K. Miyatake, K. Yamamoto, K. Endo, E. Tsuchida, J. Org. Chem.
1998, 63, 7522.
a) T. Saitoh, S. Yoshida, J. Ichikawa, J. Org. Chem. 2006, 71,
6414. b) T. Saitoh, J. Ichikawa, J. Am. Chem. Soc. 2005, 127,
9696.
6
7
Scheme 4. A plausible mechanism for oxidative arylation on sulfur.
salt 5b in 88% yield (Entry 5), along with 83% yield of 2, which
can be recycled.8a Moreover, (4-diethylaminophenyl)trimethyl-
silane (4f) showed similar reactivity. Subsequently, we inves-
tigated several aminophenylsilanes. (4-Diethylaminophenyl)-
dimethylphenylsilane (4g) reacted with sulfide 3a to give 5b in
80% yield (Entry 7),14,15 while the yields were low in the cases of
the corresponding triphenylsilane 4h and triisopropylsilane 4i
(Entries 9 and 10). In addition, triflate salt 1b (X = OTf) was also
effective as an oxidant for arylation of sulfide 3a to give triaryl-
sulfonium triflate 5c in 73% yield (Entry 8). Other oxidizing
9
H. Wang, F. P. Gabbaï, Org. Lett. 2005, 7, 283.
10 T. Suzuki, E. Ohta, H. Kawai, K. Fujiwara, T. Fukushima, Synlett
2007, 851.
11 N. G. Connelly, W. E. Geiger, Chem. Rev. 1996, 96, 877.
12 The SOMO of the radical cation A (R = Et) has the largest
coefficient on the sulfur atom. See Ref. 8b.
13 For a report on the oxidative coupling of alkyl aryl sulfides, see:
K. Yamamoto, S. Kobayashi, E. Shouji, E. Tsuchida, J. Org.
Chem. 1996, 61, 1912.
14 To a solution of 3a (26 mg, 0.10 mmol) in CH2Cl2 (2.0 mL) was
added 1a (80 mg, 0.12 mmol) at ¹78 °C. After the reaction
mixture was stirred for 0.5 h at ¹78 °C, 4g (35 mg, 0.12 mmol)
was added. After stirring for 0.5 h at ¹78 °C, the reaction was
quenched with saturated aq NaHCO3 and the aqueous layer was
extracted with CH2Cl2. The combined extracts were washed with
brine and dried over Na2SO4. After removal of the solvent, the
residue was purified by preparative TLC to afford 5b (40 mg,
80%) as pale yellow crystals.
agents, such as CAN, (p-BrC6H4)3N+●[SbCl6] , DDQ, and
¹
PhI(OAc)2, gave poor results (0%,16 26%, trace, and 0% yield,
respectively) in this triarylsulfonium salt synthesis with 4g.
To explore the scope of oxidative triarylsulfonium forma-
tion, we surveyed the effect of substituents on the nitrogen atom
of sulfides 3, such as benzyl and allyl groups, which can be
deprotected under reductive conditions. As shown in Scheme 3,
3b and 3c provided 5d and 5e in 83% and 75% yield,
respectively. Deprotection of groups on the nitrogen provides an
amino group, which acts as a surrogate for a wide range of
functional groups.
15 When 3a was added to the mixture of 4g and 1a, 5b was obtained
in 64% yield.
16 The reaction was conducted in DMF at room temperature for 1 h.
17 There is another possible pathway from radical cations 6 to
sulfonium salts 5. Arylsilanes 4 could be oxidized by dication 1 to
generate the corresponding aryl radicals, which would react with 6
to give 5. However, we failed to trap the aryl radicals.
A plausible reaction mechanism for the above-mentioned
arylation of sulfides is shown in Scheme 4. Treatment of diaryl
sulfides 3 with dication 1 generates stable radical cations 6,
which are trapped by silanes 4 on the sulfur atom to give sulfo-
Chem. Lett. 2010, 39, 56-57
© 2010 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/