Oxidative coupling of diaryldisulfides by MoCl5 to thianthrenes

Article abstract of DOI:10.1002/chem.200902466

"Chemical Equation Presented" Oxidative C-S coupling: The oxidative coupling reaction of 1,2-diaryldisulfides by using MoCl₅ or mixtures thereof with TiCl₄ gives fast and efficient access to dibenzo[b,e] [1,4]dithiins such as that depicted (thianthrenes).

Full text of DOI:10.1002/chem.200902466

COMMUNICATION
DOI: 10.1002/chem.200902466
Oxidative Coupling of Diaryldisulfides by MoCl5 to Thianthrenes
Anke Spurg,[a] Gregor Schnakenburg,[b] and Siegfried R. Waldvogel*[a]
Biaryls are unique molecular moieties appear frequently
in natural products[1] where they exhibit an important role
as well in molecular catalysis[2] as in material sciences.[3]
Consequently, powerful methods for their syntheses have
been established including transition-metal-catalyzed cross-
coupling reactions.[4] Recently, dehydrohalogenation reac-
tions for the installation of a biaryl bond became popular in
which at least one reaction partner does not require a leav-
ing functionality.[5] Dehydrodimerization reactions are only
encountered when the substrates are sufficiently electron
rich.[6] The methodology is well elaborated for phenolic sub-
strates resulting in 2,2’-dihydroxybiaryls.[7] While for the oxi-
dative coupling of phenols numerous protocols are reported
the thio congener is not known. The oxidative treatment of
thiophenols will under all conditions immediately result in
the formation of diaryl disulfides. However, these can serve
Scheme 1. Potential oxidation products of diaryldisulfides (2).
as appropriately protected thiols and moreover tethered
substrates. The oxidative treatment of 2 will either provide
ternatively, acceptable yields are obtained when toxic orga-
notin and mercury intermediates are employed.[15]
dibenzo
G
Here, we describe the oxidative treatment of diaryldisul-
fides (2) with molybdenum pentachloride (Scheme 1). This
particular oxidant was chosen in our group since we already
had some experience with this powerful reagent in the oxi-
dative arylation reaction.[16–18] MoCl5 represents a one-elec-
tron oxidant,[19] which is compatible with a variety of func-
tional groups.[20,21] A detailed comparative study of King
et al. supports the potential of MoCl5 as useful reagent in
Scholl-type reactions.[22] The performance is similar to that
cyclic architectures[8] or dibenzo
AHCTUNGTRENNUNG
The latter are known as thianthrenes and have received sig-
nificant attention as oxidized functional p systems.[9] Upon
oxidation to the radical cation and dication, 4 undergoes a
conformational and color change.[9] Consequently, 4 was in-
corporated in supramolecular architectures to modulate op-
tical properties.[10] Thianthrenes 4 are usually synthesized in
moderate to low yields using strong cationic media from dia-
ryldisulfides,[11] harsh reaction conditions,[12] highly toxic re-
action media[13] or decomposition of benzothiadiazins.[14] Al-
of phenyliodineACTHNUTRGNEUNG
(III)-bistrifluoroacetate (PIFA).[23] For oxi-
dative cyclization reactions Lewis acidic additives can be
beneficial since they act as HCl trapping agents.[24]
The required starting materials disulfides can either be
purchased or are accessible by various standard oxida-
tions,[25] Among those, the treatment with iodine/pyridine
turned out to be the most practical. Only the optimized re-
action conditions for the oxidative coupling products are de-
picted (Table 1). The times and amounts of reagents are cru-
cial. Subjecting unsubstituted diphenyldisulfide to MoCl5 or
to oxidative mixtures with TiCl4 only traces of the desired
cyclized compound were found, whereas 3,3’-dimethoxydi-
phenyl disulfide (2a) provides the known thianthrene deriv-
ative 4a[26] in almost quantitative yield (Table 1, entry 1).
[a] A. Spurg, Prof. Dr. S. R. Waldvogel
Kekulꢀ Institute for Organic Chemistry and Biochemistry
Bonn University, Gerhard-Domagk-Strasse 1, 53121 Bonn (Germany)
Fax : (+49)228-739608
E-mail: waldvogel@uni-bonn.de
[b] Dr. G. Schnakenburg
X-ray analysis department, Institute for Inorganic Chemistry, Bonn
University
Gerhard-Domagk-Strasse 1, 53121 Bonn (Germany)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.200902466.
Chem. Eur. J. 2009, 15, 13313 – 13317
ꢁ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
13313

Table 1. Oxidative coupling reaction of 1,2-diaryldisulfides.
Since the analytical data do not match with litera-
ture[26]
a
clear assignment between the isomeric
Entry Disulfide 2
[1,4]-Dithiin 4
Conditions[a] Yield
[%]
dibenzoAHCTUNETGRG[NNUN c,e] [1,2]-dithiin 3a and thianthrene 4a was
difficult. The molecular structure of the product could
be verified by X-ray analysis of a suitable single crys-
tal leading unequivocally to the thianthrene 4a
(Figure 1).
MoCl5
(2 equiv),
90min
1
97
À
No indication for the oxidative C C-bond forma-
tion was found. The strong oxidizing power and elec-
trophilic nature of MoCl5 promotes the installation of
sulphur arene bonds. The activating effect of the me-
À
MoCl5
(2 equiv),
90min
thoxy groups para to the freshly installed C S bonds
leads to the sole product 4a.
2
85
When simple methyl groups are in positions 3 and
3’ of the diaryldisulfide, an isomeric mixture of dime-
thylthianthrenes is obtained in good yield. The ratio
of 4b, 4b’ and 4b’’ is caused by statistics and the steri-
cal demand in the product. Therefore, less than 10%
yield of the 1,8-dimethyl derivative 4b’’ are found.
The isomeric mixture turned out to be inseparable by
preparative chromatography (entry 2). In order to
avoid these mixtures, the corresponding tetramethyl
substrate 2c was subjected to the oxidative treatment.
A uniform product 4c was obtained in 45% yield
when applying a mixture of excess MoCl5 and TiCl4
for a short time. Other reaction conditions rendered
less product or decomposition (entry 3). However, in
order to verify the molecular structure of 4c, an X-ray
analysis of a suitable single crystal was carried out
(Figure 2). As known the thianthrene scaffold is
folded along the S,S axis and the resulting butterfly
conformers interconvert by a flip–flap mechanism.[29]
Both thianthrenes 4a and 4c show a folding angle of
49.14(5) and 45.21(6)8, respectively.[30]
MoCl5
(3 equiv)
TiCl4
(3.3 equiv),
15min
3
4
45
64
MoCl5
(4 equiv)
TiCl4
(4.4 equiv),
60min
MoCl5
(3 equiv)
TiCl4
(3.3 equiv),
30min
5
77
MoCl5
(4 equiv)
TiCl4
(4.4 equiv),
30min
The conversion of the tetrachlorodiphenyldisulfide
(2d) requires longer reaction times as well as larger
excess of reagent mixture to form 1,3,6,8-tetrachloro-
thianthrene (4d) in 64% yield (entry 4). The molybde-
num pentachloride mediated cyclization can be also
applied to dinaphthyldisulfides. Subjecting 2,2’-dinaph-
thyldisulfide (2e) to the protocol for 30 min results in
6
50
77% yield of the dibenzoACTHNUTRGNEU[GN a,h]thianthrene (4e). Specif-
ic attention was paid to the ethoxycarbonylmethyl
moiety for the protection of phenolic hydroxyl groups.
This particular group can be efficiently removed
under mild conditions.[31,32] Furthermore, this substruc-
ture allows the preferential coordination of the Lewis
acidic MoCl5 reagent and modulation of the reaction
pathway.[33] The dehydrodimerization reaction of 2 f
leads to an isomeric mixture of 4 f and 4 f’ in a total
yield of 50%. Wherein product 4 f’ represents the
minor component (entry 6). Electron-releasing sub-
stituents such as methoxy exhibit a strong activating
effect for the oxidative coupling in the para position.
The coordination of Lewis acidic metal fragments to
the alkoxycarbonylmethyloxy moiety diminishes the
electron-releasing quality of that particular substituent
MoCl5
(4 equiv)
TiCl4
(4.4 equiv),
45min
7
50
[a] The reaction is generally performed in dichloromethane at room temperature.
13314
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ꢁ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2009, 15, 13313 – 13317

Oxidative Coupling of Diaryldisulfides
COMMUNICATION
matographically pure 7 to the dehydrodimerization protocol
involving MoCl5 and TiCl4 in dichloromethane results in a
mixture of three products in a statistical manner which were
univocally identified by GC-MS techniques (Scheme 3). The
product mixtures consist of 1,2-diphenyldisulfide (8) which
is almost inert to the oxidative treatment and the desired
compound 9 with two different aryl moieties as well as the
2,7-dimethoxythianthrene (4a). The exchange of aryl sys-
tems reveals that during the reaction process the [(ArS)3]+
species might be involved. Such cationic intermediates are
known as mediators from the cationic pool method.[37,38]
Figure 1. Molecular structure of 4a by X-ray analysis.
Cationic 1,2-diaryldisulfides intermediates undergo usually
[39]
À
C S-bond formations leading to thianthrene architectures.
Scheme 3. Synthesis of non symmetric thianthrene by oxidation of mixed
1,2-diaryldisulfides.
Figure 2. Molecular structure by X-ray analysis 4c.
and therefore the ortho coupling is partially observed
(entry 6). Applying the established conditions to the substi-
tuted 2,2’-dinaphthyldisulfide 2g leads exclusively to a less
symmetric coupling product 4g, wherein the positions 1 and
3’ of the naphthalenes undergo sulphur substitution
(entry 7). This unusual regioselectivity of the oxidative cou-
pling is difficult to explain.
Increasing electron density should facilitate the oxidative
coupling process. For that purpose the substrate tetrame-
thoxybiphenyldisulfide (5) was prepared. Dehydrodimeriza-
tion conditions did not yield the corresponding thianthrene
architecture but led to the known 2,3,6,7-tetramethoxydi-
benzothiophene (6)[34] (Scheme 2). The high electron density
promotes the extrusion of a sulphur by the strongly thiophil-
ic molybdenum reagent. However, partial desulfuration of
thianthrenes is known under harsh conditions.[35]
In conclusion, the oxidative treatment of 1,2-diaryldisul-
fides using MoCl5 or mixtures thereof with TiCl4 gives a
quick access to thianthrenes in moderate to excellent yields.
Due to cationic exchange of the aryl moiety the cross-cou-
pling of mixed disulfides does not proceed cleanly. More
moderate reaction conditions for this particular transforma-
tion are currently under investigation and will be reported
in due course.
Experimental Section
General procedure for the oxidative coupling reaction: Diphenyldisulfide
(0.28 mmol) was dissolved in dry dichloromethane under argon, titanium
tetrachloride was added first if required and molybdenum pentachloride
as a solid. The reaction mixture was stirred under inert conditions at
room temperature the given time. For workup water (50 mL) was added
and the aqueous layer was extracted with ethyl acetate (3ꢂ50 mL). The
combined organic layer was washed with brine, dried with anhydrous
magnesium sulfate and concentrated under reduced pressure. Purification
was performed by column chromatography on silica gel using mixtures of
cyclohexane/ethyl acetate as eluents.
For a fast construction of non-symmetrically substituted
thianthrenes a cross-coupling of different aryl sulfanyl units
seems to be attractive. The preparation of the mixed 1,2-dia-
ryldisulfides can be performed by standard procedures.[36]
The protocol exploiting phenyl sulfanylchloride turned out
to be most practical in the synthesis of 7.[36c] Subjecting chro-
X-ray crystal structure analysis for 4a: Crystals were obtained by evapo-
ration of a CH2Cl2/heptane solution at ambient conditions as colorless,
plate-like crystals. Formula C14H12O2S2, M=276.37, 0.24ꢂ0.22ꢂ0.12 mm,
a=8.2311(4), b=18.8729(10), c=8.1475(4) ꢃ, a=b=g=908, V=
1265.67(11) ꢃ3, 1calcd =1.450 gcmÀ3, m=0.410 mmÀ1, “Multi-Scan” absorp-
tion correction was applied, Z=4, orthorhombic, space group Pca21, l=
0.71073 ꢃ, T=123(2) K, w and f scans, 8605 reflections collected, 2814
independent (Rint =0.0587), completeness to q=28.008: 99.6%, 165 re-
fined parameters, R1 =0.0351, wR2 (all data)=0.0794, largest difference
peak and hole: 0.277, and À0.307 eꢃÀ3. CCDC 746442 contains the sup-
plementary crystallographic data for this paper. These data can be ob-
tained free of charge from The Cambridge Crystallographic Data Centre
via www.ccdc.cam.ac.uk/data_request/cif.
Scheme 2. Reaction pathway of very electron-rich 1,2-diaryldisulfides.
Chem. Eur. J. 2009, 15, 13313 – 13317
ꢁ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
13315

S. R. Waldvogel et al.
Dimethylthianthrenes (4b, 4b’, 4b’’): 3,3’-Dimethyldiphenyldisulfide (2b)
(0.100 g, 0.41 mmol) and molybdenum pentachloride (0.225 g, 0.82 mmol)
were reacted in dichloromethane (10 mL). Dimethylthianthrenes (4b,
4b’, 4b’’) were obtained as light yellow paste (0.086 g, 85%). 1H NMR
(400 MHz, CDCl3, 208C): d = 2.29–2.35 (m, 6H, CH3), 7.01–7.36 ppm
(m, 6H, Ar-H); 13C NMR (4b’’) (100 MHz, CDCl3, 208C): d = 20.52,
20.88, 126.65, 127.52, 128.01, 128.99, 131.31, 131.51, 132.49, 133.97, 134.49,
135.30, 135.91, 136.97 ppm; MS (ESI, 10 eV): m/z (%): 244 (100) [M+
[C6H3O]+; HRMS: m/z: calcd for C20H20O6S2: 420.0701; found: 420.0699
[M]+.
2,9-Di(ethoxycarbonylmethyl)dibenzoACTHNUGRTNEUNG[a,i]thianthrene (4g): 6,6’-Di(eth-
oxycarbonylmethoxy)dinaphthyl-2,2’-disulfide (2g) (0.040 g, 0.08 mmol),
titanium tetrachloride (0.04 mL, 0.067 g, 0.35 mmol) and molybdenum
pentachloride (0.087 g, 0.32 mmol) were treated in dichloromethane
(5 mL). 2,9-Di(ethoxycarbonylmethyl)-dibenzoACTHNUTRGENUG[N a,i]thianthrene (4g) was
obtained as yellow solid (0.019 g, 50%). M.p. 1158C; 1H NMR
(400 MHz, CDCl3, 208C): d = 1.28–1.31 (t, 6H, CH3), 4.26–4.32 (q, 4H,
CH2CH3), 4.73 (s, 4H, CH2CO), 7.07 (d, 2H, 1-H, 10-H), 7.32–7.35, 7.34–
7.35 (2ꢂdd, 2ꢂ1H, 3-H, 8-H), 7.54–7.62 (m, 4H, 6-H, 11-H, 13-H, 14-H),
8.41–8.43, 8.58–8.60 ppm (2ꢂd, 2ꢂ1H, 4-H, 7-H); 13C NMR (100 MHz,
Na] +, 229 (6) [MÀCH3]+, 211 (72) [MÀSÀH]+, 197 (7) [MÀSÀCH3]+,
C
178 (8) [MÀ2SÀ2H]+, 122 (6) [C7H6S]+, 91 (5) [C7H7]+, 77 (4) [C6H5]+;
HRMS (ESI, 10 eV): m/z: calcd for C14H12S2Na: 244.0380, found
244.0386 [M+Na]+.
CDCl3, 208C): d
= 14.15, 61.49, 65.47, 65.48, 107.74, 107.80, 119.45,
1,3,6,8-Tetramethylthianthrene (4c): 3,3’,5,5’-Tetramethyldiphenyldisul-
fide (2c) (0.200 g, 0.73 mmol), titanium tetrachloride (0.28 mL, 0.458 g,
2.41 mmol) and molybdenum pentachloride (0.598 g, 2.19 mmol) were
treated in dichloromethane (10 mL). 1,3,6,8-Tetramethylthianthrene (4c)
was obtained as light yellow solid (0.091 g, 45%). M.p. 99–1018C;
1H NMR (300 MHz, CDCl3, 208C): d = 2.28 (t, 6H, CH3), 2.45 (t, 6H,
CH3), 6.95 (s, 2H, 2-H, 7-H), 7.21 ppm (s, 2H, 4-H, 9-H); 13C NMR
(75 MHz, CDCl3, 208C): d = 20.67, 20.78, 127.33, 129.96, 132.39, 135.26,
136.82, 136.88 ppm; MS (EI, 70 eV): m/z (%): 272 (100) [M]+, 239 (54)
[MÀSÀH]+, 225 (8) [MÀSÀCH3]+, 208 (4) [MÀ2S]+, 136 (8) [SC6H2-
119.55, 126.38, 126.4, 126.64, 126.79, 126.84, 127.20, 127.99, 128.37, 131.18,
132.27, 133.57, 133.92, 133.95, 134.43, 156.07, 156.13, 169.57 ppm; MS
(ESI,
10 eV):
m/z
(%):
543
(72)
[M+Na]+,
354
(84)
[M+NaÀCH2CO2C2H5ÀOCH2CO2C2H5+H]+,
230
(100)
[MÀC2H5CO2CH2OC10H5SÀC2H6]+; HRMS (ESI, 10 eV): m/z: calcd for
C28H24O6S2Na: 543.0912, found 543.0907 [M+Na]+.
ACHTUNGTRENNUNG
(CH3)2]+; HRMS: m/z: calcd for C16H16S: 272.0693; found: 272.0696
[M]+.
Acknowledgements
X-ray crystal structure analysis for 4c: Crystals were obtained by evapo-
ration of a CH2Cl2/heptane solution at ambient conditions as translucent
light yellow plate-like specimen. Formula C16H16S2, M=272.41, 0.08ꢂ
0.30ꢂ0.50 mm3, a=8.1850(3), b=8.2397(3), c=11.4911(7) ꢃ, a=
Financial support by the Fonds der Chemischen Industrie is highly appre-
ciated. A donation of MoCl5 by H. C. Starck was very helpful.
92.483(4), b=96.527(3), g=117.855(2)8, V=676.63(5) ꢃ3, 1calcd
=
À
Keywords: C C coupling
molybdenum · sulfur
· cyclization · heterocycles ·
1.337 gcmÀ3, m=0.372 mmÀ1, “Multi-Scan” absorption correction was ap-
¯
plied, Z=2, triclinic, space group P1, l=0.71073 ꢃ, T=100(2) K, w and
f scans, 22635 reflections collected, 3236 independent (Rint =0.0537),
completeness to q=27.998: 99.3%, 167 refined parameters, R1 =0.039,
wR2 (all data)=0.1058, largest difference peak and hole: 0.425, and
À0.354 eꢃÀ3. CCDC 746443 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge from The
Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_
request/cif.
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1,3,6,8-Tetrachlorothianthrene (4d): 3,3’,5,5’-Tetrachlorodiphenyldisulfide
(2d) (0.100 g, 0.28 mmol), titanium tetrachloride (0.14 mL, 0.234 g,
1.12 mmol) and molybdendum pentachloride (0.307 g, 1.12 mmol) were
reacted in dichloromethane (10 mL). 1,3,6,8-Tetrachlorothianthrene (4d)
was obtained as yellow solid (0.062 g, 64%). M.p. 2228C; 1H NMR
(300 MHz, [D8]-toluene, 208C): d = 6.76–6.77 (d, 2H; 2-H, 7-H, 4J=
4
2.1 Hz), 6.83–6.84 ppm (d, 2H, 4-H, 9-H, J=2.1 Hz); 13C NMR (75 MHz,
[D8]-toluene, 208C):
d
=
127.70, 129.07, 133.35, 133.80, 134.39,
137.39 ppm; MS (EI, 70 eV): m/z (%): 356 (70), 354 (100), 352 (30) [M]+,
322 (10), 320 (32), 318 (30) [MÀCl]+, 286 (18), 284 (36) [MÀ2Cl]+, 250
(8) [MÀ2ClÀSÀH]+, 215 (2) [MÀ3ClÀS]+, 177 (10) [C6H2Cl2S]+, 142
(15) [C6H3ClS]+, 112 (9) [C4S2]+, 69 (11) [C3HS]+, 57 (22) [C2HS]+, 44
(13) [CS]+; HRMS (ESI, 10 eV): m/z: calcd for C12H4Cl4S2: 351.8508;
found: 351.8508.
Di(ethoxycarbonylmethoxy)thianthrenes (4 f, 4 f’): 3,3’-Di(ethoxycarbo-
nylmethoxy) diphenyldisulfide (2 f) (0.046 g, 0.11 mmol), titanium tetra-
chloride (0.06 mL, 0.092 g, 0.48 mmol) and molybdenum pentachloride
(0.119 g, 0.44 mmol) were reacted in dichloromethane (7 mL). Di(ethoxy-
carbonylmethoxy)thianthrenes (4 f, 4 f’) were obtained as yellow oil
(0.023 g, 50%). 1H NMR (4 f) (500 MHz, CDCl3, 208C): d = 1.27–1.31
(t, 6H, CH2CH3, 3J=7.1 Hz), 4.24–4.29 (q, 4H, CH2CH3, 3J=7.1 Hz),
4.59 (s, 4H, CH2CO2), 6.79–6.82 (dd, 2H, 3-H, 8-H, 3J=8.6 Hz, 4J=
2.7 Hz), 7.04–7.05 (d, 2H, 1-H, 6-H, 4J=2.7 Hz), 7.34–7.37 ppm (d, 2H,
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=
14.14, 61.50, 65.60, 114.55, 114.90, 127.42, 129.43, 137.96, 157.67,
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168.41 ppm; MS (EI, 70 eV): m/z (%): 420 (30) [M]+, 330 (4) [MÀ2-
A
E
ACHTUNGTRENNUNG
213 (10) [MÀ2(OCH2CO2C2H5)ÀH]+, 182 (10) [MÀ2(CH2CO2C2H5)
À2S]+, 165 (25) [MÀCH2CO2C2H5ÀOCH2CO2C2H5À2SÀH]+, 91 (100)
13316
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Chem. Eur. J. 2009, 15, 13313 – 13317

Oxidative Coupling of Diaryldisulfides
COMMUNICATION
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Received: September 7, 2009
Published online: November 9, 2009
Chem. Eur. J. 2009, 15, 13313 – 13317
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