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
Department of Chemistry, Graduate School of Pure and Applied Sciences, University of Tsukuba, Tsukuba 305-8571
2
Department of Chemistry, Graduate School of Science, The University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113-0033
1(Received October 15, 2009; CL-090929; E-mail: junji@chem.tsukuba.ac.jp)

PhPh

2X−PhPhPhPhPhPh

NEt2

R2N

R2NX−

NEt2+SPhSM
Two-Electron Oxidation
M = Si, SnPh

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Vol.39 No.1

2010 p.56–57
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January 5, 2010

The Chemical Society of Japan
Published on the web December 12, 2009; doi:10.1246/cl.2010.56

doi:10.1246/cl.2010.56
Published on the web December 12, 2009

56

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
Department of Chemistry, Graduate School of Pure and Applied Sciences, University of Tsukuba, Tsukuba 305-8571
2
Department of Chemistry, Graduate School of Science, The University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113-0033
1(Received October 15, 2009; CL-090929; E-mail: junji@chem.tsukuba.ac.jp)
Dicationic species, naphthalene-1,8-diylbis(diphenylmethylium), successfully promoted oxidative arylation of the sulfur
atom in (4-N,N-dialkylaminophenyl) phenyl sulﬁdes with (4N,N-dialkylaminopheny)silanes, aﬀording triarylsulfonium salts
in good yield.

PhPh2X

PhPhPhPh2e−PhPh12

Scheme 1. Two-electron transfer to dication 1.

Triarylsulfonium salts are highly sensitive to photolytic C­Sbond 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 thepractical role of these salts, synthetic methods to produce them
are still needed. Although a number of synthetic methods foralkylsulfonium salts have been described in the literature,4 thereare few methods on the synthesis of triarylsulfonium salts, andfewer methods starting from diaryl sulﬁdes. Triarylsulfonium
salts are synthesized by the reaction of diaryl sulfoxides witharyl Grignard reagents and silylating reagents,5 but there is thedrawback that excess amounts of both reagents are required.
Diaryliodonium salts6 and aryl formates7 are used for thesynthesis of triarylsulfonium salts directly from diaryl sulﬁdes,albeit in low yield. Thus, triarylsulfonium salt synthesis bydirect arylation of sulﬁdes still remains to be developed.
Recently, we found that naphthalene-1,8-diylbis(diphenylmethylium) (1) acts as an eﬃcient, organic two-electron oxidant.8,9 The dication 1 readily undergoes reduction via electrontransfer with an especially high oxidation potential, comparedwith those of other bis(triarylmethylium)s10 and monotriarylmethyliums.11 In this process, a neutral compound, 1,1,2,2-tetraphenylacenaphthene (2), is formed via C­C 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-dialkylaminophenyl) phenyl sulﬁdes8b have already been accomplished using
dication 1. In the latter case, diaryl sulﬁdes 3 are oxidized togenerate stable radical cation species A.12 We envisaged thatradical 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 oxidative method using dication 1 for the synthesis of triarylsulfonium
salts via arylation of diaryl sulﬁdes with arylsilanes.13First, we screened several aryl metal species that weexpected to act as aryl radical sources under oxidativeconditions. Diaryl sulﬁde 3a was successively treated withdication 1a (X = ClO4) and 4a­4i at ¹78 °C in dichloromethane. The results are summarized in Table 1. Phenyllithium
and phenylmagnesium reagents 4a and 4b aﬀorded triarylsulfonium salt 5a in low yield, along with biphenyl as a major
Chem. Lett. 2010, 39, 56­57−e−R2NR2NAr

SPhR2NSSPhPhA3Ar5

Scheme 2. Triarylsulfonium salt synthesis via oxidative arylation ofsulﬁdes.
Table 1. Oxidative arylation of sulﬁde 3a with arylmetals 4a­4i2 ClO4PhPh
Et2N

PhPh

1a (1.2 equiv)
SPh3aEt2N
ClO4−78 °C, 0.5 h/ CH2Cl2

SPh

ArM
4a−i (1.2 equiv)

Et2N

ClO4
S

−78 °C, 0.5 h
5EntryaArM

Ar

Ph
Yield/%

1PhLi

(4a)

<15 (5a)2PhMgBr(4b)< 7 (5a)3PhSnBu3(4c)004MeO

SnBu3(4d)5Et2N

SnBu3(4e)88 (5b)6Et2N

SiMe3(4f)

77 (5b)78Et2N

SiMe2Ph(4g)80 (5b)73a (5c)

9Et2N

SiPh3(4h)46 (5b)

10Et2NSii -Pr 3(4i)<12 (5b)Using dication 1b (X = OTf) as an oxidant gave triﬂate salt 5c.

product (Entries 1 and 2). Phenyl- and 4-methoxyphenylstannanes 4c and 4d failed to produce sulfonium salts, but 4diethylaminophenylstannane 4e gave the expected sulfonium© 2010 The Chemical Society of Japanwww.csj.jp/journals/chem-lett/57PhPhR2N2 ClO4PhPh

1a (1.2 equiv)

SPhR = Bn (3b)allyl (3c)

−78 °C, 0.5 h / CH2Cl2
NEt2
R2N

NEt2

PhMe2Si4g (1.2 equiv)

S−78 °C, 0.5 hPh

ClO4R = Bn (5d) : 83%
allyl (5e) : 75%

Scheme 3. Oxidative arylation of sulﬁdes 3b and 3c.
R2N

R2N

S

SPh3Ph
Ph

2 X−

NEt2

X5PhPhPh

Ph
Ph−R3SiXPh
Ph

12
NEt2
R3Si

R2N

4−SPh X

6R2N

NEt2
SX−

Ph

SiR37Scheme 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)trimethylsilane (4f) showed similar reactivity. Subsequently, we investigated 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 triarylsulfonium triﬂate 5c in 73% yield (Entry 8). Other oxidizingagents, such as CAN, (p-BrC6H4)3N+●[SbCl6]¹, DDQ, andPhI(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 formation, we surveyed the eﬀect of substituents on the nitrogen atomof sulﬁdes 3, such as benzyl and allyl groups, which can bedeprotected 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 anamino group, which acts as a surrogate for a wide range offunctional groups.
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, 56­57

nium radical cations 7.12 Further oxidation of 7 and eliminationof the silyl group occurs successively to aﬀord triarylsulfonium
salts 5.17
In conclusion, triarylsulfonium salts were readily synthesized 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 thesynthesis of triarylsulfonium salts, such direct arylation of diarylsulﬁdes provides a promising access to triarylsulfonium salts.
References and Notes
1 For a review, see: J. V. Crivello, J. Photopolym. Sci. Technol.
2008, 21, 493.
2 For a review, see: J. V. Crivello, J. Photopolym. Sci. Technol.
2007, 20, 599.
3 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.
4 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. 105­119. b) I. Sokka, A. Fischer, L. Kloo,
Polyhedron 2007, 26, 4893, and references therin.
5 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.
6 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.
7 K. Miyatake, K. Yamamoto, K. Endo, E. Tsuchida, J. Org. Chem.
1998, 63, 7522.
8 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.
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) wasadded 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 wasquenched with saturated aq NaHCO3 and the aqueous layer wasextracted with CH2Cl2. The combined extracts were washed withbrine and dried over Na2SO4. After removal of the solvent, theresidue was puriﬁed by preparative TLC to aﬀord 5b (40 mg,80%) as pale yellow crystals.
15 When 3a was added to the mixture of 4g and 1a, 5b was obtainedin 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 tosulfonium salts 5. Arylsilanes 4 could be oxidized by dication 1 togenerate the corresponding aryl radicals, which would react with 6to give 5. However, we failed to trap the aryl radicals.

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