Electroreductive additions of aromatic dichalcogenides to fluorinated ethenes

Article abstract of DOI:10.1135/cccc19980378

Intermediates and products formed during the electrochemical reduction of diphenyl disulfide (1) add to chlorotrifluoroethene (3) under the formation of 2-chloro-1,1,2-trifluoroethyl phenyl sulfide (5) and the E/Z isomers of 2-chloro-1,2-difluoroethenyl phenyl sulfide (6). The analogous reaction with 1,2-dichlorodifluoroethene(4) led to a mixture of E/Z isomers of 6. Electrochemically reduced diphenyl diselenide (2) reacted with 3 giving rise to 2-chloro-1,1,2-trifluoroethyl phenyl selenide (7) whereas the reaction with 4 does not proceed. Addition of disulfide 1 to 4 gives evidence of the participation of the radical PhS^?. With diselenide 2, an analogous radical addition reaction was not observed. This is consistent with other experiments suggesting different mechanisms of reductive cleavage of the S-S and Se-Se bonds and the absence of radicals PhSe^? in the latter.

Full text of DOI:10.1135/cccc19980378

378
Dvorak, Neugebauerova, Liska, Ludvik:
ELECTROREDUCTIVE ADDITIONS OF AROMATIC DICHALCOGENIDES
TO FLUORINATED ETHENES
b1
a
b2,
David DVORAK , Eva NEUGEBAUEROVA , Frantisek LISKAa1 and Jiri LUDVIK
*
a Department of Organic Chemistry, Prague Institute of Chemical Technology, 166 28 Prague 6,
Czech Republic; e-mail: 1 frantisek.liska@vscht.cz
b J. Heyrovsky Institute of Physical Chemistry, Academy of Sciences of the Czech Republic,
182 23 Prague 8, Czech Republic; e-mail: 1 david.dvorak@post.cz, 2 ludvik@jh-inst.cas.cz
Received April 28, 1997
Accepted January 27, 1998
Intermediates and products formed during the electrochemical reduction of diphenyl disulfide (1) add
to chlorotrifluoroethene (3) under the formation of 2-chloro-1,1,2-trifluoroethyl phenyl sulfide (5)
and the E/Z isomers of 2-chloro-1,2-difluoroethenyl phenyl sulfide (6). The analogous reaction with
1,2-dichlorodifluoroethene (4) led to a mixture of E/Z isomers of 6. Electrochemically reduced diphenyl
diselenide (2) reacted with 3 giving rise to 2-chloro-1,1,2-trifluoroethyl phenyl selenide (7) whereas
the reaction with 4 does not proceed. Addition of disulfide 1 to 4 gives evidence of the participation
of the radical PhS•. With diselenide 2, an analogous radical addition reaction was not observed. This
is consistent with other experiments suggesting different mechanisms of reductive cleavage of the
S–S and Se–Se bonds and the absence of radicals PhSe• in the latter.
Key words: Electrochemistry; Diphenyl dichalcogenides; Diphenyl disulfide; Diphenyl diselenide;
Chlorofluoroethenes; Electroreductive addition.
When investigating electrochemical reduction of dichalcogenides on mercury elec-
trodes in nonaqueous solutions different reduction patterns are observed1 for diphenyl
disulfide (1) and diphenyl diselenide (2). It has been demonstrated that 1 undergoes a
two-electron reduction by the ECE (electrode–chemical–electrode) mechanism, i.e., the
first electron transfer to 1 is followed by the decay of the formed radical anion to the
phenylsulfanyl radical 1a and anion 1b. Polarographic (cyclic voltammetric) treatment
gives a single irreversible two-electron wave (peak). The existence of radical 1a has
been proved by electrochemically generated luminiscence2 (ECL). Accepting a further
electron, radical 1a transforms to anion 1b (Scheme 1).
Electrochemical reduction of diphenyl diselenide (2) is also associated with the
transfer of two electrons. In contrast to the disulfide case, the reduction of 2 takes place
* The author to whom correspondence should be addressed.
Collect. Czech. Chem. Commun. (Vol. 63) (1998)

Electroreductive Additions
379
in two reversible waves1. The formation of phenylselanyl radical 2a has not been
proved yet, only anion 2b as the final product has been found3 (Scheme 2).
e
+
e
+
Ph
S
S
Ph
Ph S S Ph
PhS
2 PhS
PhS
+
1
1a
1b
1b
SCHEME 1
e
+
PhSe
PhSe
+
2a
2b
Ph Se Se Ph
2e
+
2
2 PhSe
2b
SCHEME 2
The difference in the dichalcogenide reduction pathways is due to the fact that
whereas disulfide 1 (or the product of its interaction with mercury) is adsorbed on the
mercury electrode, diselenide 2 reacts with mercury in nonaqueous medium sponta-
neously4 giving rise to Ph–Se–Hg–Se–Ph, which is in turn reduced to selanyl anion 2b.
(Scheme 3).
fast
Anteceding reaction:
1st wave:
Ph Se Se Ph
Hg
Ph Se Hg Se Ph
+
+ 2e
3
Ph Se Hg Se Ph
Hg
2 Hg(PhSe)
3
+
+ 4e
2nd wave:
2 Hg
2
Hg(PhSe)
6
PhSe
2b
3
+
SCHEME 3
Phenylsulfanyl anion 1b formed during the cathodic reduction can be employed in
alkylation and acylation reactions5, the analogously formed phenylselanyl anion 2b has
been used in alkylations and in ring opening of oxiranes6, in nucleophilic additions to
α,β-unsaturated carbonyl compounds7 or in aromatic substitutions proceeding by the
SRN1 mechanism8,9
Fluorinated alkenes generally undergo additions of compounds of various kinds
under conditions of radical, nucleophilic, electrophilic and cycloaddition reactions10–13
.
.
Chlorotrifluoroethene CFCl=CF2 (3) affords addition products in all of these reaction
types. This compound also enters into addition reactions with ketyl radicals formed by
reduction of acetone at a mercury cathode under acidic conditions14, and the product is
the same as that formed in the photochemical15 and radiation-induced16 addition of pro-
pan-2-ol.
Unlike 3, 1,2-dichlorodifluoroethene (4) is nonreactive in nucleophilic additions; this
fact has been used, for instance, in separation of a mixture17 where symmetric alkene 4
Collect. Czech. Chem. Commun. (Vol. 63) (1998)

380
Dvorak, Neugebauerova, Liska, Ludvik:
was accompanied by its nonsymmetric isomer CF2=CCl2; only the latter undergoes
nucleophilic addition of alcohols.
Regarding organic chalcogen compounds, radical18 and nucleophilic19–21 additions of
alkanethiols and arenethiols to ethene 3 have been examined. In both cases, the tran-
sient alkylsulfanyl radicals RS• or alkylsulfanyl or arylsulfanyl anions (RS–, ArS–) were
added to the CF2 groups of 3. Alkyl or aryl 2-chloro-1,1,2-trifluoroethyl sulfides were
the major products. In addition, telomers have also been isolated from the products of
the photochemically initiated reactions18. Corresponding chlorofluoroethyl phenyl sulfides
and selenides are also formed by the electrophilic addition of chloro(phenyl)sulfanes
and chloro(phenyl)selanes to 3 (ref.22). On the other hand, alkyl 1,2-dichloro-1,2-
difluoroethyl sulfides, formed as sole products in the radiation-initiated addition of
alkanethiols to 4, have been identified and described unambiguously23.
Two goals were pursued in the present work: (i) the synthetic goal, aimed at emplo-
ying the reductive addition of electrochemically generated intermediates to fluoroe-
thenes for the synthesis of fluorinated sulfides and selenides, and (ii) the mechanistic
goal, giving the opportunity to prove the occurrence of radical intermediates chemically
and thus to evidence different pathways of cathodic reduction of dichalcogenides 1 and 2.
Chlorotrifluoroethene (3) and 1,2-dichlorodifluoroethene (4) were chosen as the sub-
strates. Note that as a prerequisite for the reaction to occur, the two dichalcogenides
must be reduced at appreciably more positive potentials than the fluorinated ethenes:
E
1/21 = –0.91 V, E1/22 = –0.32 V (1st wave), or –0.905 V (2nd wave), E1/23 = –1.95 V,
E1/24 = –2.49 V vs saturated calomel electrode24 (SCE). This allows the reagent to be
generated in the mixture selectively, without reducing substrate 3 or 4.
CFCl CF
2
CFCl CFCl
3
4
EXPERIMENTAL
Apparatus and Chemicals
The potentiostatic electrolyses were performed in a special three-compartment electrochemical cell1
with a working volume of 20 ml, enabling voltammetric measurements during the electrolysis. A
stirred mercury pool (mercury for polarography, Merck, Germany) served as the working electrode,
the reference electrode was SCE and the auxiliary electrode was a platinum sheet. Electrochemical
potentiostat PAR Model 173 and 175 interfaced to a Model 178 integrator were employed for the
controlled potential electrolysis and charge consumption monitoring. Electroadditions proceeded in
anhydrous acetonitrile (VEB Apolda, Germany), which was purified by a proven procedure25. Tetra-
ethylammonium perchlorate and tetraethylammonium hexafluorophosphate (Fluka, Switzerland, used
as received), both at a concentration of 0.2 mol l–1, were used as the supporting electrolyte. Oxygen
was removed from the solution by argon purging (Linde). Cyclic voltammetry was performed with a
Metrohm hanging mercury drop electrode (HMDE) in a 5 ml cell (SCE – reference electrode, Pt wire –
auxiliary electrode) using the same potentiostat at velocities 50 to 500 mV s–1
.
Collect. Czech. Chem. Commun. (Vol. 63) (1998)

Electroreductive Additions
381
Diphenyl disulfide and diphenyl diselenide (both Merck, Germany) were used as received, in con-
centrations about 2.10–2 mol l–1. Chlorotrifluoroethene was obtained from Spolchemie Usti nad
Labem, Czech Republic, and 1,2-dichlorodifluoroethene was prepared following ref.24.
TLC analyses were performed on silica gel 60 F254 aluminium sheets (Merck, Germany). NMR
spectra in deuteriochloroform were scanned on a Bruker AM 400 MHz spectrometer; working fre-
quencies for 1H, 19F and 77Se were 400.13, 376.50 and 76.31 MHz, respectively. Tetramethylsilane,
trichlorofluoromethane and diphenyl diselenide served as the respective internal standards. Chemical
shifts (δ-scale) are in ppm, coupling constants (J) in Hz. Infrared spectra were run on a Nicolet FTIR
instrument, wavenumbers are in cm–1. Melting points were not corrected.
Procedure
Electrolyses proceeded roughly to 95% conversion of the starting dichalcogenide (polarographically
monitored). Fluorinated ethene 3 or 4 was fed into the solution in the gaseous state during the elec-
trolysis, acetonitrile was distilled off and diethyl ether was added to the residue. The precipitated
electrolyte was filtered off and the ethereal solution was evaporated to dryness. After dissolution in
hexane, the products were separated by column chromatography on silica gel. The products were
chromatographically homogeneous (TLC, GC) oils.
2-Chloro-1,1,2-trifluoroethyl Phenyl Sulfide (5)
3
3
2
GC: >95%. 19F NMR spectrum: –85.1 ddd, 1 F, J(HF) = 4.6, J(FF) = 19.4, J(FF) = 222.0 (CF-F);
3
3
2
3
–90.3 ddd, 1 F, J(HF) = 7.4, J(FF) = 18.1, J(FF) = 221.3 (CF-F); –148.0 dt, 1 F, J(FF) = 19.5,
2J(HF) = 48.2 (CFHCl).
2-Chloro-1,2-difluoroethenyl Phenyl Sulfide (6)
3
GC: >95% of the E/Z mixture. 1H NMR spectrum: 7.36 m, 3 H (H-2,4,6); 7.45 dd, 2 H, J(HH) =
8.0, 4J(HH) = 1.7 (H-3,5). 19F NMR spectrum, (Z)-isomer: –86.9 d, 1 F, 3J(FFcis) = 17.1 (S-CF);
–112.8 d, 1 F, 3J(FFcis) = 17.1 (=CFCl); (E)-isomer: –104.1 d, 1 F, 3J(FFtrans) = 140.6 (S-CF);
3
–123.7 d, 1 F, J(FFtrans) = 140.7 (=CFCl).
2-Chloro-1,1,2-trifluoroethyl Phenyl Selenide (7)
1
3
3
2
GC: >95%. H NMR spectrum: 6.06 ddd, 1 H, J(HF) = 4.1, J(HF) = 9.0, J(HF) = 48.2 (CHFCl);
7.34–7.45 m, 3 H (H-2,4,6); 7.74 d, 2 H, J(HH) = 7.0 (H-3,5). 19F NMR spectrum: –83.3 ddd, 1 F,
3
3J(HF) = 4.1, 3J(FF) = 20.8, 2J(FF) = 222.5 (CF-F); –89.1 ddd, 1 F, 3J(HF) = 8.4, 3J(FF) = 18.9,
2J(FF) = 222.6 (CF-F); –146.6 dt, 1 F, J(FF) = 19.5, J(HF) = 47.2 (CHFCl). 77Se NMR spectrum:
3
2
2
391.2 t, J(SeF) = 30.7. IR spectrum: 1 044 m, 1 096 s, 1 186 s, 1 271 w, 1 330 w, 1 440 m, 1 478 m,
2 855 w, 2 928 w, 3 012 w, 3 065 w. All the spectra were in accord with the published data (ref.22).
1,2-Bis(phenylselanyl)-2-chloro-1,1,2-trifluoroethane (8)
3
4
GC: >95%. 1H NMR spectrum: 7.38 ddd, 2 H, J(HH) = 7.2, 3J(HH) = 7.0, J(HH) = 1.3 (H-3,5);
3
4
7.46 m, 1 H (H-4); 7.76 dd, 2 H, J(HH) = 7.1, J(HH) = 1.5 (H-2,6). 19F NMR spectrum: –76.7 dd,
1 F, 3J(FF) = 20.3, 2J(FF) = 207.0 (CF-F); –77.5 dd, 1 F, 3J(FF) = 21.9, 2J(FF) = 207.7 (CF-F);
–90.6 t, 1 F, J(FF) = 21.1 (CFCl). 77Se NMR spectrum: 550.9 s (SeCF2); 637.7 d, J(SeF) = 47.3
(SeCFCl). IR spectrum: 1 022 s, 1 063 s, 1 166 s, 1 216 w, 1 305 w, 1 441 s, 1 477 s, 1 579 w, 1 735 w,
2 850 w, 2 928 w, 3 065 w. All the spectra were in accord with the published data (ref.22).
3
2
Collect. Czech. Chem. Commun. (Vol. 63) (1998)

382
Dvorak, Neugebauerova, Liska, Ludvik:
RESULTS
Addition of two dichalcogenides, diphenyl disulfide (1) and diphenyl diselenide (2),
was achieved by electrolysis of their solutions in the presence of chlorotrifluoroethene
(3) or 1,2-dichlorodifluoroethene (4) in acetonitrile at –1.10 V vs SCE. The addition of
1 to 3 gave mainly 2-chloro-1,1,2-trifluoroethyl phenyl sulfide (5) and a mixture of
stereoisomeric (Z)-and (E)-2-chloro-1,2-difluoroethenyl phenyl sulfides (6) with the
E/Z ratio 64 : 36 (determined on the basis of 19F NMR spectra).
The analogous addition of 1 to 4, the used ethene consisting of the E and Z isomers
in the 48 : 52 ratio, also gave a mixture of the E and Z stereoisomers of the sulfide 6
with the E/Z ratio 55 : 45.
CHFCl
S
F
F
CF
S
F
Cl
F
S
2
Cl
5
(E)-6
(Z)-6
A mixture of 2-chloro-1,1,2-trifluoroethyl phenyl selenide (7) and 1,2-bis(phenyl-
selanyl)-2-chloro-1,1,2-trifluoroethane (8) was the major product of the addition of 2 to 3.
The addition of 2 to 4 was conducted analogously, but instead of an addition product,
only the starting diselenide and its complex with mercury having a melting temperature
of 138–142 °C (ref.1) were obtained.
CHFCl
CFCl Se
CF
CF
2
Se
Se
2
7
8
The structure of the reaction products 5–8 reflects the nature of the electrochemically
generated intermediates which add to the double bond of the alkenes and is consistent
with the observed different course of the reduction of the dichalcogenides 1 and 2
themselves. Both radical 1a and anion 1b can participate in the formation of products 5
and 6. Addition of radical 1a to 3 results in the formation of adduct-radical 5a which is
reduced further to carbanion 5b. The latter can also arise in the nucleophilic addition of
anion 1b to 3. Protonation transforms 5b into adduct 5, and the unsaturated sulfide 6
arises from the elimination of the fluoride ion from the neighbouring carbon (Scheme 4).
With 4, we assume that only radical 1a can participate in the formation of sulfide 6:
addition of 1a to 4 gives first adduct-radical 6a, which is reduced at the cathode to
carbanion 6b, and this is in turn transformed to sulfide 6 by elimination of the chloride
ion from the neighbouring carbon (Scheme 5).
The pathway of the reductive addition of 1 to 4 is also supported by cyclic voltam-
metry patterns in dependence on temperature. The shape of the cyclic voltammogram of
Collect. Czech. Chem. Commun. (Vol. 63) (1998)

Electroreductive Additions
383
1 does not change with temperature. The anodic-to-cathodic peak current ratio ipa : ipc
decreases on addition of 4 suggesting a consecutive reaction of intermediate 1a with
Ph
S
CF
CFCl
+ e
1a
3
+
+
2
5a
+
5
H
+
_
1b
Ph
S
CF
CFCl
3
2
5b
(E)/(Z)-6
SCHEME 4
Ph
S
S
CFCl CFCl
+ e
1a
4
+
6a
_
Cl
(E)/(Z)-6
Ph
CFCl CFCl
6b
SCHEME 5
substrate 4. This decrease is low at room temperature because the alkene concentration
is low as well (compound 4 boils at 20 °C). This concentration however, increases by
lowering the temperature to –20 °C, which is accompanied by an additional decrease in
the anodic peak in the cyclic voltammogram (Table I).
The formation of products 7 and 8 is associated with the nucleophilic addition of
phenylselanyl anion 2b to 3. The adduct-anion 7a formed in this step gives selenide 7
by protonation or diselenide 8 by reaction with additional diphenyl diselenide (Scheme 6).
The fact that no addition product is formed in the electroreductive addition of Ph2Se2
(2) to CFCl=CFCl (4) indicates that (in contrast to the Ph2S2 case) in the reduction of 2, the
phenylselanyl radical PhSe• (2a) is not formed as an intermediate, and the only inter-
mediate is anion PhSe– (2b) not reacting with 4.
+
7
8
H
+
_
3
Ph Se CF CFCl
2
+
2b
7a
2
+
_
2b
SCHEME 6
Collect. Czech. Chem. Commun. (Vol. 63) (1998)

384
Dvorak, Neugebauerova, Liska, Ludvik:
The results of electrolyses are given in Table II. The structure of reaction products
1
5–8 was proved by their H, 13C, 19F and 77Se NMR spectra.
CONCLUSIONS
The electroreductive addition of PhSSPh (1) and PhSeSePh (2) to CFCl=CF2 (3) or
CFCl=CFCl (4) was investigated. To identify the nature of the reacting electrochemi-
cally generated intermediates, same reactivity of the two alkenes in radical additions
and different reactivity in nucleophilic additions (where the symmetric CFCl=CFCl is
nonreactive) were utilized. The results give evidence that the electroreduction of the
TABLE I
Peak potentials and anodic-to-cathodic peak current ratios in cyclic voltammetry of PhSSPh (1) and
its mixture with CFCl=CFCl (4)
Compound
Epc, V vs SCE
Epa, V vs SCE
ipa/ipc
Temperature, °C
1
–0.99
–1.05
–0.96
–0.93
–0.88
–0.92
–0.85
–0.775
0.53
0.54
0.40
0.28
20
–20
20
1
1 + 4
1 + 4
–20
TABLE II
Reaction conditions and the yield of cathodic addition of PhSSPh (1) and PhSeSePh (2) to CFCl=CF2
(3) and CFCl=CFCl (4) in acetonitrile/Et4N+ClO−4. The alkenes were fed to the electrolyzed mixture
in the gaseous state
Working
potential
V vs SCE
Dichalcogenide
a
Alkene
napp
Product
Yield, %
mmol
1
3
4
3
4
–1.10
–1.10
–1.10
–1.10
2.09
1.98
2.05
1.99
5
30
32
0.312
6b
1
6c
17
0.325
2
7
8
57
5
1.436
d
d
2
0.440
a napp (Apparent value of n) number of F mol–1 observed in coulometry. b E/Z ratio 64 : 36. c E/Z ratio
55 : 45. d No addition products were observed.
Collect. Czech. Chem. Commun. (Vol. 63) (1998)

Electroreductive Additions
385
disulfide gives rise to radical intermediate 1a, whereas in the electroreduction of 2 the
analogous radical 2a is not involved.
Electroreductive addition of dichalcogenides offers an alternative way of synthesis of
aryl fluoroalkyl sulfides and selenides. The currently established synthetic procedure
for sulfide 5 is base-catalyzed addition of benzenethiol to 3 (ref.20) or the reaction of
sodium benzenethiolate with 1,1,2-trichlorotrifluoroethane26. Sulfide 6 can be prepared
by dehydrofluorination of 5 (ref.27) or by dehalogenation of 1,2-dibromo-2-chloro-1,2-
difluoroethyl phenyl sulfide28. Diselenide 8 has been so far prepared22 by thermally
initiated radical addition of 2 to 3. The electrosynthetic procedure is convenient in that
only the desired reactant is selective generated at the cathode at a certain constant
potential, the reactions proceed under mild conditions (room temperature and atmos-
pheric pressure) and the addition products are relatively easy to isolate from the reac-
tion mixture. In view of the fact that other aromatic and aliphatic diselenides are
reduced electrochemically in a similar way1,4, it is reasonable to assume that such addi-
tions can generally be conducted with other substitution derivatives as well.
The investigation was supported by the Grant Agency of the Czech Republic (Grant No. 203/94/0733).
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