Oxidative coupling of methyl phenyl sulfide via sulfonium formation using an oxovanadium complex

Full text of DOI:10.1021/jo951490u

1912
J . Org. Chem. 1996, 61, 1912-1913
methane.4 In the presence of VO(acac)2, the reaction is
accompanied by quantitative oxygen uptake.5 For each
mole of oxygen consumed, 2.0 mol of product is generated.
The combination of 500 MHz COSY 1H- and 13C-NMR
and IR spectroscopy permitted identification of the sul-
fonium structure.6 In the NMR spectra, signals
(1H-NMR: 3.65 ppm; 13C-NMR: 28.4 ppm) attributed to
the adjacent methyl group appeared at lower field than
those due to the neutral methylthio group (2.45 and 14.6
ppm, respectively).
Oxid a tive Cou p lin g of Meth yl P h en yl
Su lfid e via Su lfon iu m F or m a tion Usin g a n
Oxova n a d iu m Com p lex
Kimihisa Yamamoto,† Shintaro Kobayashi,
Eiichi Shouji, and Eishun Tsuchida*
Department of Polymer Chemistry, Advanced Research
Center for Science and Engineering, Waseda University,
Tokyo 169, J apan
The demethylation (eq 1) of methylphenyl-4-(meth-
ylthio)phenylsulfonium perchlorate (2 g, 5.7 mmol) was
carried out in pyridine (15 mL) at reflux temperature for
1 h. The demethylation proceeds quantitatively, result-
ing in the formation of methyl 4-(phenylthio)phenyl
sulfide (1.27 g) in 96% yield.7 After the oxidative
coupling, the reaction mixture was neutralized with
excess pyridine (100 mL) and then refluxed for 1 h. This
one-pot synthesis resulted in the same product (yield
98%) (Table 1). However, the oxidative coupling of
cyclopropyl phenyl sulfide did not proceed, presumably
because of elimination of a labile methyne proton after
the oxidation. Diphenyl sulfide was not oxidized, pre-
sumably due to its higher oxidation potential (1.7 V). The
alkyl-substituted arylmethyl sulfides with a lower oxida-
tion peak potential also produced the corresponding aryl
sulfides. The electron-donating alkyl substituents pro-
mote the coupling reaction because the reaction proceeds
through electrophilic substitution.
Methyl phenyl sulfide shows an irreversible oxidation
peak potential of 1.5 V (vs Ag/AgCl) (Figure 1). The
isolated sulfonium cation (methylphenyl-4-(methylthio)-
phenylsulfonium perchlorate) is oxidized at a higher
potential of 1.9 V.8 Incorporation of the sulfonium group
into the phenylene ring results in an increase in the
oxidation potential due to electron withdrawal. It was
concluded that the dimerized sulfonium cation was not
oxidized by the oxovanadium catalyst because oxygen
Received August 14, 1995
There is considerable interest in direct oxidation as a
means of synthesizing sulfoxides. Numerous catalysts
have been utilized as oxygenase-like models1 in the
formation of sulfoxides by the oxidation of sulfide com-
pounds with oxygen. The mechanism likely involves a
one-electron transfer process.
Traditionally, sulfonium compounds have been syn-
thesized2 with low conversion by the alkylation of the
sulfides with iodoalkanes in the presence of an equimolar
amount of silver salt, by the protonation of sulfoxides,
and by the treatment of alkyl sulfoxides with alkyl
Grignard reagents.
Recently, we reported that N,N'-ethylenebis(salicylide-
neaminato)oxovanadium undergoes a two-electron trans-
fer at 0.5 V (vs Ag/AgCl) through disproportionation in
the presence of acids.3 Oxovanadium acetylacetonate
(VO(acac)2) could be employed as a two-electron mediator
in the oxidation of an aryl sulfide (ca. 1.0-2.0 V vs Ag/
AgCl) because of its suitably high redox potential (1.1 V
vs Ag/AgCl).
In this paper, we report that unsymmetric arylalkyl-
sulfonium salts can be synthesized by the oxygen-
mediated oxidative coupling of the arylsulfides catalyzed
by an oxovanadium complex (eq 1). Of note is the use of
(4) A typical procedure for this catalytic reaction is as follows: VO-
(acac)2 (0.066 g, 5 mmol/L), trifluoromethanesulfonic acid (0.075 g, 10
mmol/L), trifluoroacetic anhydride (2.1 g, 0.2 mol/L), and tetrabutyl-
ammonium perchlorate (3.42 g, 0.2 mol/L) were dissolved in CH2Cl2
(40 mL) in a closed vessel maintained at 20 °C. The mixture was stirred
for 1 h under
a dry argon atmosphere before the reaction. The
atmosphere in the vessel was then replaced with oxygen. A solution
of methyl phenyl sulfide (1.24 g, 0.2 mol/L) in CH2Cl2 (10 mL) was
the abundant, inexpensive oxidant, oxygen, as a raw
material. The reaction involves not only a new catalytic
system for oxidation of sulfides by an electron-transfer
analogous to that of an oxidase model but also the direct
formation of unsymmetric aryl sulfides via sulfonium
cations. We believe that a two-electron transfer relay
system from the sulfide to oxygen leads to oxidative
coupling yielding the sulfonium compounds without
sulfoxide or sulfone formation.
The oxidative coupling of an aryl sulfide with oxygen
to form the corresponding sulfonium cation by an electron
transfer can be catalyzed with VO(acac)2 in the presence
of tetrabutylammonium perchlorate, trifluoromethane-
sulfonic acid, or trifluoroacetic anhydride in dichloro-
added. The reaction was carried out for 40
h under an oxygen
atmosphere with constant stirring at 20 °C. Methyl phenyl sulfide and
methyl phenyl sulfoxide were not detected in the reaction mixture by
gas chromatography. The mixture was poured into ether (300 mL) to
precipitate the product. Methylphenyl-4-(methylthio)phenylsulfonium
perchlorate was isolated after extraction of the precipitate with CH2-
Cl2 (3.2 g, 94%).
(5) Oxygen consumption was determined as follows: The vanadyl
catalyst and acids were dissolved in CH2Cl2 in
a closed vessel
maintained at 20 °C and were mixed for 1 h under a dry argon
atmosphere until a steady state was reached. A similar vessel equipped
with a manometer and a burette was connected to it with a tube. The
atmosphere in the vessel was replaced by oxygen. The whole instru-
ment was kept airtight. A solution of aryl sulfide in CH2Cl2 was added.
Tetrachloroethane was carefully added from a burette to the second
vessel to keep the pressure at 1 atm. Oxygen consumption was
measured from the net amount of the tetrachloroethane added during
the reaction.
† PREST, J RDC-Investigator 1992-1994.
(1) Riley, D. P.; Smith, M. R.; Correa, P. E. J . Am. Chem Soc. 1988,
110, 177.
(2) Roush, D. M.; Price, E. M.; Templeton, L. K.; Templeton, D. H.;
Heathcock, C. H. J . Am. Chem. Soc. 1979, 101, 2971. Yamamto, K.;
Shouji, E.; Nishide, H.; Tsuchida E. J . Am. Chem. Soc. 1993, 115, 5819.
Anderson, K. K.; Caret, R. L.; Karup-Nielsen, I. J . Am. Chem. Soc.,
1974, 96, 8020.
(3) Yamamoto, K.; Oyaizu, K.; Iwasaki, N.; Tsuchida, E. Chem. Lett.
1993 1223. Tsuchida, E.; Yamamoto, K.; Oyaizu, K.; Iwasaki, N.;
Anson, F. C. Inorg. Chem. 1994, 33. 1056.
(6) Methylphenyl-4-(methylthio)phenylsulfonium perchlorate: IR
(KBr, cm-1) 3002, 2922, 1088, 812, 743, 680, 625; 13C-NMR (CDCl3,
500 MHz 119.7, 126.4, 127.2, 129.5, 130.2, 131.4, 134.6, 148.9 14.6,
28.4; 1H-NMR (CDCl3, 500 MHz 7.36, 7.58, 7.63, 7.79, 7.63 (9H, m);
2.45, 3.65 (6H). Anal. Calcd for C14H15S2ClO4: C, 48.48; H, 4.36.
Found: C, 48. 40; H, 4.38.
(7) Methyl 4-thiophenoxyphenyl sulfide: IR (KBr, cm-1) 3056, 2919,
1580, 1476, 1437, 810, 739, 689; 1H-NMR (500 MHz, CDCl3) 7.29-
7.01 (9H, m); 2.28 (3H); 13C-NMR (500 MHz, CDCl3) 126.7, 127.2, 129.1
130.2, 131.4, 132.2, 136.7, 138.2, 15.6. Anal. Calcd for C13H12S2: C,
67.2; H, 5.21. Found: C, 66.8; H, 5.28.
0022-3263/96/1961-1912$12.00/0 © 1996 American Chemical Society

Communications
J . Org. Chem., Vol. 61, No. 6, 1996 1913
Ta ble 1. Oxid a tive Cou p lin g of Th ioa n isol Der iva tives
Ca ta lyzed by VO(a ca c)2 F ollow ed by Dem eth yla th ion
F igu r e 1. Cyclic voltammetry of methyl phenyl sulfide (2
mmol/L) (a), methylphenyl-4-(methylthio)phenyl sulfonium
perchlorate (2 mmol/L) in CH2Cl2 (b), and VO(acac)2 (2 mmol/
L) under anaerobic conditions in CH2Cl2 in the presence of
trifluoromethanesulfonic acid (10 mmol/L) and trifluoroacetic
anhydride (0.2 mol/L) (c). Scanning rate 50 mV/s, Pt electrode
(28.3 mm2).
consumption was not observed in the same reaction of
the sulfonium compound. The redox potential (E1/2) of
VO(acac)2, which is ascribed to the redox couple between
V(IV)/V(V), is 1.1 V in dichloromethane. In an anaerobic
acidic environment, two reversible redox couples were
observed at very close potentials (1.4 and 1.6 V) via cyclic
voltammetry and can be assigned to V(III)/V(IV) and
V(IV)/V(V), respectively.9 This result supports the idea
that a two-electron transfer is facilitated due to the small
potential separation between the two redox couples.10
The oxidative coupling of methyl phenyl sulfide with
oxygen occurs through two redox couples that bridge
the large potential gap between oxygen and aryl sul-
fides. The oxovanadium catalyst acts as an excellent
electron mediator in the catalytic reaction. The catalyst
is activated by oxidation with oxygen to a high-valent
Sch em e 1
vanadyl(V) species. The resulting active cationic spe-
cies11 of methyl phenyl sulfide derived from its oxidation
by the activated vanadyl catalyst reacts electrophilically
with the benzene ring of the sulfide to form the dimerized
sulfonium cation accompanied by proton elimination. The
resulting vanadium(III) species is reoxidized to vanadyl-
(V) by oxygen (Scheme 1).
(8) Electrochemical measurements were carried out in
a two-
compartment cell kept under a dry argon atmosphere. A platinum disk
(28.3 mm2) and platinum wire (φ ) 0.2 mm, 200 mm) were used as
working and auxiliary electrodes, respectively. The reference electrode
consisted of an Ag/AgCl electrode in CH2Cl2 solution containing 0.1
mol/L tetrabutylammonium tetrafluoroborate. The electrode potential
was determined at 0.21 V vs NHE by adjustment with a ferrocene-
ferrocenium(+) redox couple. Cyclic voltammograms were measured
in CH2Cl2 solution containing 0.1 M TBABF4.
(9) Bonadis, J . A.; Butler, W. M.; Percoraro, V. L.; Carrano, C. Inorg.
Chem. 1987, 26, 1218. Bonadis et al. have reported the redox properties
of VO(salen) in an acidic atmosphere and provided evidence for what
they considered to be the disproportionation of VO(salen) to VO(salen)+-
(V) and a vanadium(III) species. Yamamoto, K.; Tsuchida, E.; Nishide,
H.; J ikei, M.; Oyaizu, K. Macromolecules 1993, 26, 3432.
Ack n ow led gm en t. This work was partially sup-
ported by a Grant-in-Aid for Development Scientific
Research (No. 04555223, 1992-1994) and Scientific
Research (Nos. 07651006, 07215282, 1995-1996) from
the Ministry of Education, Science and Culture J apan,
and The Fundation “Hattri-houkokai”.
J O951490U
(10) Rechardson, D. E.; Taube, H. Inorg. Chem. 1981, 20, 1278. M
- e ) M+:E1°, M+ - e ) M2+:E2o, [M+]/[M][M2+] ) exp[(E2 - E1o)-
(11) It is presumed that the reaction proceeds via formation of a
vanadium-sulfide dication complex through a two-electron transfer.
However, the structure of active sulfide species has not yet been
identified.
o
o
25.69] at 298 K. On the basis of this equation decrease of the (E2
-
E1o) results in a decrease of [M+]. This means that the two-electron
transfer is facilitated at the small potential separation.