Alkyl 2-aryl-2-thioxoethanoates and -thioates: Preparation, dimerization, and EPR spectra of the corresponding radical anions

Article abstract of DOI:10.1080/10426500902855240

The title compounds were prepared by reaction of the corresponding α-halocarboxylic or α-halothiolocarboxylic esters with tetraethylammonium thiosulfate and subsequent treatment of the resulting Bunte salts with sodium hydroxide in a two-phase system. Und

Full text of DOI:10.1080/10426500902855240

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Alkyl 2-Aryl-2-
thioxoethanoates and -
thioates: Preparation,
Dimerization, and EPR Spectra
of the Corresponding Radical
Anions
Jürgen Voss a , Andrzej Sawluk a , Gisela Lange a &
Gunadi Adiwidjaja b
a Department of Chemistry , University of Hamburg ,
Hamburg, Germany
b Department of Geosciences , University of
Hamburg , Hamburg, Germany
Published online: 28 Apr 2009.
To cite this article: Jürgen Voss , Andrzej Sawluk , Gisela Lange & Gunadi Adiwidjaja
(2009) Alkyl 2-Aryl-2-thioxoethanoates and -thioates: Preparation, Dimerization, and
EPR Spectra of the Corresponding Radical Anions , Phosphorus, Sulfur, and Silicon and
the Related Elements, 184:5, 1206-1223, DOI: 10.1080/10426500902855240
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Phosphorus, Sulfur, and Silicon, 184:1206–1223, 2009
Copyright © Taylor & Francis Group, LLC
ISSN: 1042-6507 print / 1563-5325 online
DOI: 10.1080/10426500902855240
Alkyl 2-Aryl-2-thioxoethanoates and -thioates:
Preparation, Dimerization, and EPR Spectra of the
Corresponding Radical Anions1
1
1,∗
Jurgen Voss, Andrzej Sawluk, Gisela Lange,1
¨
and Gunadi Adiwidjaja2
1Department of Chemistry, University of Hamburg, Hamburg, Germany
2Department of Geosciences, University of Hamburg, Hamburg,
Germany
The title compounds were prepared by reaction of the corresponding α-
halocarboxylic or α-halothiolocarboxylic esters with tetraethylammonium thiosul-
fate and subsequent treatment of the resulting Bunte salts with sodium hydroxide
in a two-phase system. Under elimination of sulfite, the deep-blue colored thiones
were generated. These labile compounds could not always be isolated from the
solutions in a pure state. Instead, frequently the [2+2]-dimers (dialkyl 2,4-diaryl-
1,3-dithietane-2,4-carboxylates) or other follow-up products were obtained. Never-
theless, the EPR spectra of the monomer radical anions could be recorded even if so-
lutions of the dimers were reduced by internal electrolysis. Additional EPR spectra
were observed in several cases, which we assign to the radical anions of dialkyl 2,3-
diaryl-2-butene-1,4-dioate radical anions originating from electron induced sulfur
extrusion of the corresponding 1,3-dithietanes. Radical trianions were observed
when a 1,4-benzene-bis-α-thioxoester or -thioxo-thiolester was electroreduced at a
more negative electrode potential.
Keywords Benzo[c]thiophenes; Bunte salts; 1,3-dithietanes; EPR spectroscopy; radical
anions; 2-thioxocarboxylates
Received 28 December 2007; accepted 19 February 2008.
Dedicated to Professor Marian Mikoꢀajczyk, CBMiM PAN in Lꢀ o´dz´, Poland, on the
occasion of his 70th birthday.
The University of Hamburg, the Konrad-Adenauer-Stiftung (gradute scholarship A.
S.), the Deutsche Forschungsgemeinschaft, and the Fonds der Chemischen Industrie
are gratefully acknowledged for their financial support. We thank Prof. Dr. U. Behrens,
University of Hamburg, for his help concerning the X-ray structures.
Address correspondence to Prof. Dr. Ju¨rgen Voss, University of Hamburg, Depart-
ment of Chemistry–Organic Chemistry, Martin-Luther-King-Platz 6, D-20146 Hamburg,
Germany. E-mail: voss@chemie.uni-hamburg.de
∗A. Sawluk passed away on July 29, 1991.
1206

Alkyl 2-Aryl-2-thioxoethanoates and -thioates
1207
INTRODUCTION
We have performed electron paramagnetic resonance (EPR) spec-
troscopic studies on the radical anions of various monothio-α-
dicarbonyl compounds such as alkyl 2-arene-2-oxoethanedithioates,2,3,4
N, N-disubstituted 2-aryl-2-thioxoethanamides,5 and 2-aryl-2-thioxo-
ethanethioamides6 in order to gain insight into the electron distribution
in these interesting species, which exhibit an inherent competition be-
tween a carbonyl group directly linked to a thiocarbonyl group. The
related alkyl 2-aryl-2-thioxoethanoates 2 are not very well known,
since they are considerably more labile than the corresponding stable
N, N-disubstituted amides. According to our earlier results, they ex-
hibit the same marked tendency to dimerize under [2+2]-cycloaddition
and formation of 1,3-dithietanes,7,8 which is observed in the case
of monothiobenzile9 and the monosubstituted N-tert-butyl-2-phenyl-
2-thioxoethanamide.10 Accordingly, they raise difficulties during their
preparation and handling, and we had to find suitable methods for
their preparation that would enable us to generate and study the cor-
responding radical anions by EPR spectroscopy.
PREPARATIONS
In a preliminary note,7 we have described the preparation of ethyl
2-phenyl-2-thioxoethanoate by base-catalyzed cleavage of the cor-
responding BUNTE-salt (organyl thiosulfate). The deep-blue oil
dimerized within a short time even at low temperature or in solution
under formation of the [2+2]-cycloadduct diethyl 2,4-diphenyl-
1,3-dithietane-2,4-dicarboxylate. Its aliphatic counterpart ethyl
3,3-dimethyl-2-thioxobutanoate (2k, see Table I), which Metzner
and coworkers have obtained by reaction of the oxo-ester with hy-
drogen sulfide, is more persistent, but during its preparation the
corresponding dimer 3k is also formed.8,11
We have prepared a series of further Bunte salts 1 from the
corresponding alkyl 2-chloro- or 2-bromo-2-arylethanoates and -
ethanethioates by use of tetraethylammonium thiosulfate, which is sol-
uble in organic solvents and thus more suitable than sodium thiosulfate
(Scheme 1).
Three of the Bunte salts (1g, 1h, and 1i) were isolated as pure
crystalline compounds and were spectroscopically characterized. The
remaining ones were used for the fragmentation experiments without
further purification.
The cleavage reaction (Scheme 2) was performed in a two-phase sol-
vent mixture by stirring an aqueous solution of the organyl thiosulfate

1208
J. Voss et al.
TABLE I EPR Spectroscopic Proton HFS Coupling Constants aH (mT)
µ
and g-Factors of the Radical Anions of α-Thioxo-Esters and
-thioesters p-R1-C6H4-CS-CO-XR2 (2−)
R1
XR2
OMe
OtBu
OCH2-tBu
OMe
OtBu
OEt
SEt
aH2,6
aH3,5
a4H
aRH
g
2
2a
2c
2d
2e
2f
H
H
H
tBu
MeO
NO2
H
0.184
0.200
0.185
0.184
0.190
0.11
0.161
0.235
0.184
0.068
0.075
0.065
0.072
0.070
0.32
0.068
0.235
0.068
0.204
0.216
0.204
—
0.075
—
0.085
0.074
2.01037
a
2.01032
2.01030
0.023b
(1.05)c
0.165
—
—
2.00984
d
a
2g
2h
2i
0.029
2.01098
2.00513
200996
d
CS-CO-OMe
CS-CO-OMe
OMe
OMe
2i3−e
—
0.075
0.020f
0.030
0.075
2j3−e
2k
CS-CO-SEt
SEt
OEt
0.161
–
0.060
–
–
2.01091
2.01045
(0.025)h
g
aNot determined.
baH4−OMe
.
caNNO2
.
dNot resolved.
eRadical trianion, observed at Ered = −2.0 V, see text.
f aH4−COOMe
.
g
2k = tBu-CS-CO-OEt.
haHtBu
.
_
+
NEt4
S
SO3
X
(NEt4)2S2O3
R1
R1
+
_
XR 2
XR 2
NEt4X
O
O
1
X = Cl, Br
1
a
b
c
d
e
f
g
h
R1 =
H
H
H
H
tBu OMe NO2
H
XR2 = OMe OiPr OtBu OCH2tBu OMe OtBu OEt SEt
O
_
SO3
+
NEt4
RX
_
S
1i: XR = OMe
1j: XR = SEt
+
S
S
XR
O3
NEt4
O
SCHEME 1

Alkyl 2-Aryl-2-thioxoethanoates and -thioates
1209
R1
O
XR 2
S
2 x
S
S
NaOH
R1
1
_
XR 2
2X
Na2SO3
R
O
O
2
3
R 1
SCHEME 2
1 with diethyl ether or trichloromethane and sodium hydroxide. In most
cases, the organic phase turned blue and was then separated. Removal
of the ether at low temperature gave the thiones 2 as blue oils or crys-
tals (2f). Most of them dimerized under decolorization within minutes
or after a few hours at 20◦C or even below 0◦C except the crystalline 2f,
which can be stored for a week in the dark at −23◦C. The stabilization of
2f is obviously due to the +M effect of the para-methoxy substituent. We
have recrystallized the resulting colorless solids in several cases and
obtained the pure dialkyl 2,4-diaryl-1,3-dithietane-2,4-dicarboxylates
3b, 3c, 3d, 3e, and 3f and the -bis-thiolocarboxylate 3h.
According to their NMR spectra, several dithietanes were formed
as a mixture of two stereoisomers, the ratios of which depended on the
bulkiness of the alkyl group. The pure trans-stereoisomers of 3b and 3c
could be isolated by fractionizing crystallization. Their configurations
were revealed by X-ray structural analyses. An ORTEP plot of 3b is
shown in Figure 1.
The nitro derivative 2g could not be extracted into the ether phase.
It was generated by stirring a solution of 1g in dry DMF with powdered
sodium hydroxide. The resulting deep-blue solution was directly used
for the EPR measurement. Also the bifunctional Bunte salts dimethyl
benzene-1,4-bis(α-thiosulfatoacetate) (1i) and S, S-diethyl benzene-1,4-
bis(α-thiosulfatoacetothioate) (1j) led to water-soluble products. The
thiones 2i and 2j were therefore generated by fragmentation with pow-
dered sodium hydroxide in dry acetonitrile. The resulting solutions
were directly used for the internal electroreduction and EPR measure-
ment.
Dialkyl α,αꢀ-dibromo-1,2-benzenediacetates (4), on the other hand,
did not even lead to Bunte salts. Instead, they reacted with tetraethyl-
ammonium thiosulfate under formation of the known dimethyl
benzo[c]thiophene-1,3-dicarboxylate (5a)12 and the unknown diethyl

1210
J. Voss et al.
FIGURE 1 ORTEP view of the dithietane 3b. Thermal ellipsoids are drawn
at the 50% probability level. Selected bond lengths (pm) and bond angles (◦):
S1–C1: 1.836, S1–C1ꢀ: 1.839, C1–S1–C1ꢀ: 86.4, S1–C1–S1ꢀ: 93.6.
ester (5b) (Scheme 3), the structure of which was proved by an X-ray
structure analysis (Figure 2).
This result was not unexpected, since 5a is also formed by reaction of
4a with potassium disulfide via sulfur extrusion of the intermediate 3,4-
benzodithiane.12 Probably, mono-Bunte salts are formed from 4 in the
O
OR
O
OR
O
O
OR
Br
Br
_
- H2SO3, -NEt4Br
(NEt4)2S2O3
+
S
SSO3 NEt4
Br
OR
O
O
OR
4a: R = Me
4b: R = Et
OR
5a: R = Me
5b: R = Et
SCHEME 3

Alkyl 2-Aryl-2-thioxoethanoates and -thioates
1211
FIGURE 2 ORTEP view of the benzo[c]thiophene 5b. Thermal ellipsoids are
drawn at the 50% probability level. Selected bond lengths (pm) and bond angles
(◦): S–C1: 1.706, C1–C7a: 1.395, C7–C7a: 1.406, C6–C7: 1.344, C5–C6: 1.431,
C2–S–C3: 92.5, S–C1–C7a: 112.4, C2–C7a–C4a: 111.4.
first step. Subsequently, intramolecular cyclization under elimination
of sulfite and bromide to form 5 occurs (Scheme 3).
EPR SPECTROSCOPIC RESULTS
Radical anions of the α-thioxo-esters and α-thioxo-thioloesters 2 were
generated by internal electroreduction in dry acetontrile or DMF.
Suitable reduction potentials for the single electron transfer steps
were determined during the EPR measurements by controlled po-
tential sweeps at intervals of 50 mV between 0.0 and −2.0 V. Re-
solved EPR spectra were recorded at −35◦C. Experimental EPR spec-
tra of the methyl 2-phenyl-2-thioxophenylethanoate (2a) and ethyl

1212
J. Voss et al.
2-phenyl-2-thioxophenylethanethioate (2h) radical anions along with
the computer simulations are for example shown in Figure 3.
The proton hyperfine structure (HFS) coupling constants aHµ and g-
factors are compiled in Table I. In most cases, a straightforward assign-
ment based on the comparison of substituted derivatives was possible.
In cases where the thiones were not stable enough for isolation, the
blue-colored acetonitrile solutions obtained by stirring of the Bunte
salts 1 with solid sodium hydroxide powder could be directly used for
the electroreductive generation of the radical anions.
The pattern of the proton HFS coupling constants reflects a spin
density distribution in 2·−, which one would expect for arene radical
anions with electron-withdrawing substituents. It can be reproduced
by HMO calculations.13 Application of the McConnell equation, aHµ =
−2.7·ρπµ, gives apHara = 0.212 mT, aH = 0.169 mT, aHmeta = 0.013 mT
in reasonable agreement with the oertxhpoerimental values (Table I). The
ratio aHortho/apHara is close to 1. This is indicative of a particularly strong
spin-attracting effect of the substituent.14 We have observed this effect
in related arene radical anions such as Ph-CO-CS-SR3,4 as well. The
coupling constants aHµ of the aromatic protons in the thiolester 2h·− are
significantly lower compared with the data found for the corresponding
ester radical anions, e.g., 2a·−. This tendency was found in the Ar-CO-
CO-XR (X = O, S) series as well and has been attributed to an increased
spin density in the functional group -CO-CO-SR,4 an argument that
should apply to the -CS-CO-XR functionality too. As usual,4 the protons
of the SEt group in 2h·− exhibit smaller coupling constants compared
with the OEt protons.
The para-nitro substituent of 2g·− totally dominates the spin density
distribution, which therefore completely deviates from 2a·−. The para-
methoxy substituent of 2f·−, on the other hand, does not significantly
disturb the spin density distribution.
In several cases, additional EPR spectra with quite different spec-
troscopic parameters (Table II) appeared at higher fields (g ≈ 2.0036)
when more negative reduction potentials were applied. Figure 4 shows
an example.
The same EPR spectra were also observed when the correspond-
ing dialkyl 2,4-diaryldithietane-1,3-dicarboxylates 3a, 3d, and 3e were
subjected to internal electroreduction. Tentatively, we assign these
EPR spectra to the radical anions of dialkyl 2,3-diarylbut-2-enedioates
(6·−). The low g-factors and small line-widths are indicative of the
absence of sulfur (heavy-atom effect) in the molecules. Furthermore,
HFS couplings with the protons of each two equivalent arene moieties
are observed. This fact implies conjugation between the two arene

Alkyl 2-Aryl-2-thioxoethanoates and -thioates
1213
FIGURE 3 EPR spectra (top: experimental, bottom: simulated) of the radical
anions 2a·− (left) and 2h·− (right).

1214
J. Voss et al.
TABLE II EPR Spectroscopic Proton HFS Coupling Constants aH
µ
(mT) and g-Factors of the Dialkyl But-2-enedioate Radical Anions
R2O−CO−CR1 CR1−CO−OR2 (6·−
)
R1
R2
aH2 ,2
aH3 ,3
aH4 ,4
g
ꢀ
ꢀꢀ
ꢀ
ꢀꢀ
ꢀ
ꢀꢀ
ꢀ
ꢀꢀ
ꢀ
ꢀꢀ
,6 ,6
,5 ,5
6a
6d
6e
6k
Ph
Ph
Me
tBuCH2
Me
0.113
0.114
0.114
0.047
0.047
0.046
0.123
0.121
–
2.00363
2.00363
2.00363
2.00500
4-tBu-C6H4
tBu
Et
0.028a
aaHtBu
rings, which is possible in 6 but not in 3. The alkenes 6 could be
formed by electron-induced sulfur extrusion from the 1,3-dithietanes
3. We assume that first two electrons are transferred to 3 under ring
opening. Elimination of a sulfide anion from the intermediate dianion
would lead to a thiirane. Finally, the well known reductive thiirane
desulfurization15 yields the alkene derivative 6 (Scheme 4).
Interestingly, the EPR spectra of the monomers were also observed
when the dimeric dithietanes were used as starting compounds for
the internal electroreduction. Obviously an equilibrium between the
monomers and the dimers exists in the polar solvent (Scheme 2), or
the radical anions of the latter are cleaved under formation of the
monomers. In fact, the 1,3-dithietanes 3 tend to cleave at elevated
temperatures, which is perceptible by the light-blue color that de-
velops upon warming. Since very low concentrations of paramagnetic
O
_
O
R1
2
_
R1
S
S
S
S
O
R
OR2
2e
_
R2
-S2-
O
2O
R
R1
O
R1
O
3: R1 = Ar, t-Bu
._
O
R1
O
R1
S
2
_
O
R
OR2
3e
-S2-
2 O
2 O
R1
R
R1
6: R1 = Ar, t-Bu
R
O
O
SCHEME 4

Alkyl 2-Aryl-2-thioxoethanoates and -thioates
1215
FIGURE 4 EPR Spectrum (top: experimental, bottom: simulated) of the radi-
cal anion 6a−.

1216
J. Voss et al.
species can be detected by EPR spectroscopy, even trace amounts of the
monomers would lead to the observed result. This observation opens
up the valuable possibility of generating radical anions of the elusive
thione species from the much more stable dimers.
Two different EPR spectra are also observed after electroreduction
of the bis-α-thioxo-ester 2i. First (Ered ≈ −1V) an intense quintet re-
sulting from HFS coupling with four equivalent protons [aHµ (exp.) =
0.250 mT, aHµ (HMO) = 0.227 mT] appears at g = 2.005013. The second,
more complex spectrum with g = 2.00996 is observed at Ered ≈ −2V.
An interpretation of these EPR spectra is not straightforward. Nei-
ther of the two can be assigned to a butenedioate radical anion of type
6. Instead, the quintet may be best assigned to the expected radical
anion 2i·−. Since the spin is symmetrically delocalized over the whole
molecule (Scheme 5) and thus over two thiocarbonyl groups, the g-factor
is lower as compared with the corresponding monofunctional radical
anions of type 2a·−–2h·−, and the four aromatic protons are of course
equivalent. The second spectrum, which is produced at more negative
reduction potential, is tentatively assigned to the radical trianion 2i3−
.
It exhibits an asymmetric spin density distribution, which resembles
the one found in the para-substituted radical monoanion 2e·− (see the
proton HFS coupling constants in Table I). We assume that the spin
label group, in terms of the EPR time scale, is coplanar with the ring,
so that spin delocalization is possible. By contrast, the other, doubly
negatively charged functional group is twisted out of the ring plane
(Scheme 5).
O
O
_
.
S
S
MeO
MeO
_
.
OMe
OMe
O
S
S
_
.
O
O
2i
_
.
S
RX
_
XR
S
_
O
: XR = OMe, 2j .3_
2i .3_
: XR = SEt
SCHEME 5

Alkyl 2-Aryl-2-thioxoethanoates and -thioates
1217
Internal electroreduction (Ered ≈ −2V) of the bis-thiolester 2j re-
sulted in the formation of the radical trianion 2j3−. Its EPR spectrum
strikingly resembles the spectrum of 2h·−, which is explained in the
same way as for 2i3−
.
EPR spectra of radical trianions have also been observed in the
related series of bifunctional α-oxo-thiono- and α-oxo-dithioesters.3
EXPERIMENTAL
Corrected melting points (mp) were determined with an
Electrothermal-Schmelzpunktsapparatur. Boiling points (bp) were
determined during distillation. Thin layer chromatography (TLC) was
performed on Al foils coated with SiO2 F254 (Merck, Darmstadt). The
spots were detected by the extinction of the fluorescence or by spraying
with the iodine/sodium azide reagent.16 Column chromatography (CC)
was performed on Kieselgel 60 (Merck, Darmstadt), 0.063–0.200 mm
(70–230 mesh). Eluents [CH2Cl2, petroleum ether (PE)] were distilled
prior to use. Solvents were purified and dried by standard laboratory
procedures.17 Removal of solvents was performed in a vacuum rotary
evaporator.
IR spectra were measured as KBr pellets or films on Perkin-Elmer
spectrometers 297 and 399. 1H NMR spectra were measured in CDCl3
on Varian T 60 and EM 360, and Bruker WH 270 and EM 400 spec-
trometers. UV/VIS spectra were recorded in methanol (Uvasol) on a
Perkin-Elmer 200 spectrometer. Mass spectra were measured on a Var-
ian MAT CH 7 spectrometer.
EPR spectra were obtained on a Bruker 420s spectrometer (X-band)
equipped with a Bruker Gaussmeter and a microwave counter as
described earlier.4,14,18 The radical anions were generated by inter-
nal electroreduction at room temperature or at −35◦C as described
previously.14,18
X-Ray Structure Analyses
The crystal data of 3b, 3c, and 5b and a summary of the experimental
details are given in Table III. Data collection was performed with a CAD
4 SDP (Enraf Nonius) diffractometer with graphite-monochromated
Cu-Kα radiation (Ni-filter, λ = 1.54184 Å) in the θ/2θ scan mode at
298 K. The structures were solved by the direct method MULTAN.19
All non-hydrogen atoms were localized. After refinement of these pa-
rameters, the hydrogen atoms were localized by differential Fourier
synthesis.20 The final refinement was performed by least squares

1218
J. Voss et al.
TABLE III Crystal Data and Structure Refinement for 3b, 3c, and 5b
Compound
3b
3c
5b
Empirical formula
Formula weight
Crystal system
Space group
a [pm]
C22H24O4S2
416.56
monoclinic
P21/n
870.2 (1)
1973.9 (1)
640.9 (1)
106.35 (1)
1.056 × 109
2
C24H28O4S2
444.62
monoclinic
P21/n
1213.8 (1)
1139.7 (1)
899.5 (1)
105.84 (1)
1.197 × 109
2
C14H14O4S
278.31
orthorhombic
Cmma
696.3 (1)
2204.5 (1)
876.7 (1)
b [pm]
c [pm]
β [◦]
V [pm3]
1.346 × 109
Z
4
ρ
[g cm−3
]
]
1.31
24.1
440
2/70
1.23
21.5
472
2/70
1.37
21.5
584
2/70
calcd.
µ [cm−1
F(000)
θ-limits [◦]
h/k/l-limits
0, 10/0, 24/−7, 7 0, 14/0, 13/−10, 10 0, 8/0, 26/0, 10
Reflections collected (I > 3σ )
Number of parameters
1855
176
1773
193
646
77
I
R-Index
0.035
0.030
0.031
0.030
0.049
0.055
Rw-Index (w = σ−2
)
I
methods. Experimental correction for absorption21 was performed in
addition to the usual Lorentz correction for polarization. Crystallo-
graphic data have been deposited with the Cambridge Crystallographic
Data Centre as supplementary publications No. CCDC-667725 (3a),
CCDC-667726 (3b), and CCDC-667727 (5b). Copies of the data can be
obtained free of charge on application to CCDC, 12 Union Road, Cam-
bridge CB2 IEZ, UK [Fax: (international) +44–1233/336–033; E-Mail:
deposit@ccdc.cam.uk].
Preparations
Ethyl 3,3-dimethyl-2-thioxobutanoate (2k) and diethyl trans-2,4-di-
tert-butyldithietane-1,3-dicarboxylate (3k) were prepared according to
the literature.8,11
Bunte Salts (1): General Procedure
A solution of the corresponding alkyl 2-bromo- or 2-chloro-2-
arylethanoate (4 mmol) and tetraethylammonium thiosulfate22 (4
mmol) was heated to reflux in abs. ethanol (30 mL) under monitor-
ing by TLC. A reaction time of 12 h was applied in cases difficult to

Alkyl 2-Aryl-2-thioxoethanoates and -thioates
1219
survey. Removal of the solvent left the Bunte salts as oils or crystals.
The Bunte salts 1g, 1h, and 1i were purified by crystallization from a
suitable solvent. The other ones were used in the following reactions
without further purification.
Tetraethylammonium S-(Methoxycarbonyl-phenylmethyl)
Thiosulfate (1a)
Salt 1a was obtained from methyl 2-chloro-2-phenylethanoate23 as
a yellow oil.
Tetraethylammonium S-(Isopropyloxycarbonyl-phenylmethyl)
Thiosulfate (1b)
Salt 1b was obtained from isopropyl 2-chloro-2-phenylethanoate23
as a yellow oil.
Tetraethylammonium S-(tert-Butyloxycarbonyl-phenylmethyl)
Thiosulfate (1c)
Salt 1c was obtained from tert-butyl 2-chloro-2-phenylethanoate24
as colorless crystals.
Tetraethylammonium S-(Neopentyloxycarbonyl-
phenylmethyl) Thiosulfate (1d)
Salt 1d was obtained from neopentyl 2-chloro-2-phenylethanoate23
as a yellow oil.
Tetraethylammonium S-[Methoxycarbonyl-(4-tert-
butylphenyl)methyl] Thiosulfate (1e)
Salt 1e was obtained from methyl 2-bromo-2-(4-tert-butylphenyl)
ethanoate25 as a yellow oil.
Tetraethylammonium S-[tert-Butyloxycarbonyl-(4-
methoxyphenyl)methyl] Thiosulfate (1f)
Salt 1f was obtained from tert-butyl 2-bromo-2-(4-methoxyphenyl)
ethanoate26 as a yellow solid.
Tetraethylammonium S-[Methoxycarbonyl-(4-
nitrophenyl)methyl] Thiosulfate (1g)
Salt 1g was obtained from ethyl 2-chloro-2-(4-nitrophenyl)
ethanoate27 as colorless crystals (19%), mp 84–85◦C (EtOAc/hexane).
IR (KBr): ν 1740 (C O), 1530 (NO2), 1350 (NO2), 1230 (C O) cm−1. 1H
NMR (60 MHz): δ = 1.22 (m, 15H, CH3), 3.25 (q, 8H, NCH2), 4.12 (q,
2H, OCH2), 5.28 (s, 1H, CH), 7.64 (d, 2H, 2ꢀ/6ꢀ-H), 8.09 (d, 2H, 3ꢀ/5ꢀ-H).

1220
J. Voss et al.
Anal. Calcd. for C18H30N2O7S2 (450.58): C, 47.98; H, 6.71; N, 6.22; S,
14.23; Found: C, 48.14; H, 6.75; N, 6.21; S, 14.09%.
Tetraethylammonium S-(Ethylthiocarbonyl-phenylmethyl)
Thiosulfate (1h)
The reaction of 2-chloro-2-phenylethanoyl chloride (Merck, Darm-
stadt, 15.0 g, 0.079 mol) with ethanethiol (4.92 g, 0.079 mol) and
triethyl amine (8.02 g, 0.079 mol) in dry diethyl ether (100 mL) ac-
cording to a standard laboratory method17 gave S-ethyl 2-chloro-2-
phenylethanethioate (13.5 g, 80%), which was used without further
purification to yield 1h as hygroscopic colorless crystals (14.9 g, 56%),
mp 85–90◦C (CHCl3/EtOAc). IR (KBr): ν 1680 (C O) cm−1. 1H NMR (60
MHz): δ = 1.20 (m, 15H, CH3), 2.83 (q, 2H, SCH2), 3.20 (q, 8H, NCH2),
5.28 (s, 1H, CH), 7.23 (m, 5H, arom-H). Anal. Calcd. for C18H31NO4S3
(421.65): C, 51.27; H, 7.41; N, 3.32; S, 22.81; Found: C, 51.59; H, 7.51;
N, 3.38; S, 22.41%.
Bis(tetraethylammonium) S,Sꢀ-Benzene-1,4-
bis(methoxycarbonyl-phenylmethyl) Bis(thiosulfate) (1i)
Dimethyl 1,4-benzenediacetate28 was brominated with NBS.17 The
resulting dimethyl α,αꢀ-dibromo-1,4-benzenediacetate was used with-
out further purification to yield 1i as colorless crystals (40%), mp 175–
177◦C (EtOH/Et2O). IR (KBr): ν 1750 (C O), 1230 (C O) cm−1 1H
.
NMR (400 MHz): δ = 1.22 (t, 24H, CH3), 3.25 (q, 16H, NCH2), 3.65 (s,
6H, OCH3), 5.00 (s, 2H, CH), 7.35 (m, 4H, arom-H). Anal. Calcd. for
C28H52N2O10S4 (704.99): C, 47.70; H, 7.43; N, 3.97; S, 18.19; Found: C,
46.30; H, 7.37; N, 4.01; S, 17.63%.
Bis(tetraethylammonium) S,Sꢀ-Benzene-1,4-
bis(ethylthiocarbonyl-phenylmethyl) Bis(thiosulfate) (1j)
1,4-Benzenediacetic acid (Fluka, 2.91 g, 0.015 mol) was treated with
SOCl2 (19.6 g, 0.165mol) and NCS (10.3 g, 0.078 mol).17 Reaction of the
resulting α,αꢀ-dichloro-1,4-benzenediacetyl chloride with ethanethiol
(1.92 g, 0.031ꢀ mol) and NEt3 (3.03 g, 0.030 mol) (see preparation of
1h) led to S, S-diethyl α,αꢀ-dichloro-1,4-benzenediacetothioate (4.00 g,
76%), which was used without further purification to yield 1j as a col-
orless oil.
Bunte Salt Fragmentation: General Procedure
The Bunte salt 1 (1–2 g) was dissolved in a sufficient amount of H2O
(not more than 60 mL). After the addition of diethyl ether (20 mL) and

Alkyl 2-Aryl-2-thioxoethanoates and -thioates
1221
aqueous NaOH (2N, 5 mL), the mixture was vigorously stirred. The
organic phase turned blue to violet and was separated after 15 min.
The extraction was repeated until the ether remained nearly colorless.
In certain cases, further addition of NaOH was appropriate since more
blue product was formed. The ether layer was washed with H2O, dried
over Na2SO4, and vacuum-evaporated at low temperature. The remain-
ing blue-colored oils or crystals were directly used for the generation of
the radical anions by internal electroreduction.
tert-Butyl 2-Phenyl-2-thioxoethanoate (2c)
Thione 2c was obtained from 1c as blue crystals. 1H NMR (60 MHz):
δ = 1.45 [s, 9H, C(CH3)3], 7.2–7.7 (m, 5H, arom-H).
tert-Butyl 2-(4-Methoxyphenyl)-2-thioxoethanoate (2f)
Thione 2f was obtained from 1f with 48% yield as dark-blue crystals.
1H NMR (60 MHz); δ = 1.46 [s, 9H, C(CH3)3], 3.84 (s, 3H, OCH3), 6.9–7.6
(AAꢀBBꢀ-system, 8H, arom-H).
Di-tert-butyl trans-2,4-Diphenyl-1,3-dithietane-2,4-
dicarboxylate (3c)
Yield 48%, mp 164◦C (PE). IR (KBr): ν = 1720 (C O) cm−1. 1H NMR
(60 MHz): δ = 1.37 [s, 18H, C(CH3)3], 7.2–7.7 (m, 10H, arom-H). MS
(70 eV): m/z (%) 343 (9) [M+–CO2tBu], 222 (5) [C6H5CSCO2tBu+], 122
(17), 121 (100) [C6H5CS+], 77 (24) [C6H+]. Anal. Calcd. for C24H28O4S2
(444.61): C, 64.83; H, 6.35; S, 14.42; Fou5nd: C, 64.77; H, 6.43 S, 14.53%.
Dimethyl trans-2,4-Bis(4-tert-butylphenyl)-1,3-
dithietane-2,4-dicarboxylate (3e)
Yield 30%, colorless oil. IR (KBr): ν = 1730 (C O), 1230 (C–O) cm−1
.
1H NMR (60 MHz): δ = 1.32 [s, 18H, C(CH3)3], 3.78 (s, 6H, OCH3),
7.4 (d, 4H, arom-H), 7.58 (d, 4H, arom-H); Anal. Calcd. for C26H32O4S2
(472.67): C, 66.07; H, 6.82; S, 13.57; Found: C, 63.27; H, 6.69; S, 14.15%.
Di-tert-butyl trans-2,4-Bis(4-methoxyphenyl)-1,3-dithietane-
2,4-dicarboxylate (3f)
Yield 48%, mp 153◦C (PE). IR (KBr): ν = 1760 (C O), 1230 (C–
O) cm−1
.
1H NMR (60 MHz): δ = 1.37 [s, 18H, C(CH3)3], 3.84 (s,
6H, OCH3), 6.9–7.6 (A2B2-system, 8H, arom-H). MS (70 eV): m/z (%)
403 (3) [M+–CO2tBu], 252 (10) [CH3OC6H4CSCO2tBu+], 151 (100)
[CH3OC6H4CS+], 107 (2) [CH3OC6H+]. Anal. Calcd. for C26H32O6S2
(504.67): C, 61.88; H, 6.39; S, 12.71; Fo4und: C, 61.68; H, 6.40; S, 12.86%.

1222
J. Voss et al.
S,S-Diethyl trans- and
cis-2,4-Diphenyl-1,3-dithietane-2,4-dicarbothioate (3h)
Yield 60%, mp 107–109◦C (EtOAc/PE). IR (KBr): ν = 1650 (C O),
1
cm−1. H NMR (60 MHz); trans-isomer: δ = 1.20 (t, 6H, CH3), 2.86 (q,
4H, SCH2), 7.4–7.6 (m, 10H, arom-H); cis-isomer: 1.32 (t, 6H, CH3),
2.98 (q, 4H, SCH2), 7.4–7.6 (m, 10H, arom-H). MS (70 eV): m/z (%) 331
(5) [M+–COSEt], 210 (6) [C6H5CSCOSEt+], 121 (100) [C6H5CS+]. Anal.
Calcd. for C26H32O4S2 (420.64): C, 57.11; H, 4.79; S, 30.49; Found: C,
56.85; H, 4.78; S, 30.43%.
Dimethyl Benzo[c]thiophene-1,3-dicarboxylate (5a)
Dimethyl α,αꢀ-dibromo-1,2-benzenediacetate (4a)29 (441 mg, 1.52
mmol) and tetraethylammonium thiosulfate22 (1128 mg, 3.03 mmol)
were refluxed in ethanol (30 mL) overnight. Irrespective of a yellow
precipitate, the solvent was removed. The oily residue was dissolved in
CHCl3/H2O and extracted with another two portions of CHCl3. The ex-
tract was dried over Na2SO4. After removal of the solvent, the residue
was purified by CC (CHCl3) to yield yellow crystals of 5a (230 mg, 61%),
mp 160◦C (CHCl3/PE), lit.12: mp 160◦C. IR (KBr): ν = 1710 (C O), cm−1
.
1H NMR (60 MHz): δ = 4.00 (s, 6H, OCH3), 7,1–8.7 (m, 4H, arom-H).
Anal. Calcd. for C12H10O4S (250.27): C, 57.59; H, 4.03; S, 12.81; Found:
C, 58.09; H, 3.99; S, 13.08%.
Diethyl Benzo[c]thiophene-1,3-dicarboxylate (5b)
Compound 5b was obtained as described for 5a from diethyl
α,αꢀ-dibromo-1,2-benzenediacetate (4b)30 (600 mg, 1.86 mmol) and
tetraethylammonium thiosulfate22 (1400 mg, 3.03 mmol). Yield: 330
mg (63%) yellow crystals, mp 152–153◦C (CHCl3/PE). IR (KBr): ν =
1680 (C = O), cm−1. UV/VIS (MeOH): λmax (log ε) 381 (4.20), 334 (3.96),
1
319 (3.83), 248 (4.66), 219 (4.46), 302 (4.40) nm. H NMR (60 MHz):
δ = 1.43 (t, 6H, CH3), 4.45 (q, 4H, OCH2), 7,1 – 8.7 (m, 4H, arom-H).
Anal. Calcd. for C14H14O4S (278.33): C, 60.42; H, 5.07; S, 11.52; Found:
C, 60.41; H, 5.20; S, 11.46%.
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