Electrochemical reduction of mono- and dihalothiophenes at carbon cathodes in dimethylformamide. First example of an electrolytically induced halogen dance

Article abstract of DOI:10.1021/jo9613646

Cyclic voltammetry and controlled-potential electrolysis have been employed to probe the electrochemical reduction of a number of mono- and dihalothiophenes at carbon cathodes in dimethylformamide containing tetramethylammonium perchlorate. Reduction of 2

Full text of DOI:10.1021/jo9613646

8074
J . Org. Chem. 1996, 61, 8074-8078
Electr och em ica l Red u ction of Mon o- a n d Dih a loth iop h en es a t
Ca r bon Ca th od es in Dim eth ylfor m a m id e. F ir st Exa m p le of a n
Electr olytica lly In d u ced Ha logen Da n ce
Mohammad S. Mubarak
Department of Chemistry, The University of J ordan, Amman 11942, J ordan
Dennis G. Peters*
Department of Chemistry, Indiana University, Bloomington, Indiana 47405
Received J uly 19, 1996X
Cyclic voltammetry and controlled-potential electrolysis have been employed to probe the
electrochemical reduction of a number of mono- and dihalothiophenes at carbon cathodes in
dimethylformamide containing tetramethylammonium perchlorate. Reduction of 2-bromo-, 3-bro-
mo-, 2-chloro-, 3-chloro-, and 2-iodothiophene gives rise to a single irreversible cyclic voltammetric
wave for each compound that corresponds to the two-electron cleavage of the carbon-halogen bond,
and thiophene is obtained as the only product. Cyclic voltammograms for the reduction of 2,3-
dibromo-, 2,4-dibromo-, 2,5-dibromo-, 3,4-dibromo-, 2-bromo-5-chloro-, and 3-bromo-2-chloro-
thiophene each exhibit a pair of irreversible two-electron waves. Electrolyses of either 2,3-dibromo-
or 2,4-dibromothiophene at potentials corresponding to the first voltammetric wave yield a two-
to-one mixture of 3-bromo- and 3,4-dibromothiophene; under similar conditions, electrolyses of 2,5-
dibromothiophene give a mixture of 2-bromo-, 3-bromo-, and 3,4-dibromothiophene, electrolyses of
2-bromo-5-chlorothiophene afford a mixture of 3-bromo-, 3,4-dibromo-, 3-bromo-2-chloro-, 4-bromo-
2-chloro-, and 2-chlorothiophene, and electrolyses of 3-bromo-2-chlorothiophene yield 2-chloro-
thiophene. Aside from the last result, these product distributions appear to arise from an
electrolytically induced halogen dance. When electrolyses of the dibromothiophenes and of 2-bromo-
5-chloro- and 3-bromo-2-chlorothiophene are performed at potentials that correspond to the second
voltammetric wave, thiophene is the only product obtained.
In tr od u ction
as electrolyte. A paper by Gedye, Sadana, and Leger5
describes the electrochemical reduction of tetrabromo-
thiophene, tetraiodothiophene, 2,3,4-tribromothiophene,
and 3,4-dibromothiophene at mercury, copper, and lead
cathodes in dimethylformamide containing tetraethyl-
ammonium bromide; typical of the behavior of these
compounds is the electrolysis of tetrabromothiophene
which, depending on the choice of potential, could be
converted sequentially into 2,3,4-tribromo-, 3,4-dibromo-,
and 3-bromothiophene. Most recently, procedures for the
selective electrolytic debromination of thiophene deriva-
tives at graphite cathodes have been devised by Dappen-
held and co-workers;6 these investigators were able to
prepare 3-bromo- or 3,4-dibromothiophene from 2,3,4,5-
tetrabromothiophene, 3,4-dibromo-2-chlorothiophene from
2,3,4-tribromo-5-chlorothiophene, and 3-bromo-4-chloro-
thiophene from 2,3,5-tribromo-4-chlorothiophene in good
yield.
There has been considerable interest in base-catalyzed
halogen migrations that involve various halogen-substi-
tuted thiophenes.7-9 An investigation of the use of
potassium amide in liquid ammonia to promote halogen
migrations in 2-bromo-, 2,3-dibromo-, 2,4-dibromo-, and
2,5-dibromothiophene was reported by van der Plas and
co-workers,7 and Gronowitz and colleagues8 have exam-
ined the reaction of methoxide ion with several bromo-
Several publications dealing with the electrochemical
reduction of halogenated thiophenes have appeared in
the literature. In 1963, Brown and Krupski1 examined
the polarographic behavior of eight mono-, di-, tri-, and
tetraiodothiophenes in both dimethylformamide and
2-ethoxyethane containing tetra-n-butylammonium io-
dide, and they tabulated the half-wave potentials for the
reduction of the various carbon-iodine bonds. Shortly
thereafter, Mairanovskii, Barashkova, and Vol’kenshtein2
reported the results of a polarographic study of the entire
nine-member family of mono-, di-, tri-, and tetrabromo-
thiophenes as well as 2-iodothiophene in aqueous ethanol
solutions; for each of these compounds, a diffusion-
controlled reduction wave was observed for the two-
electron cleavage of each carbon-halogen bond, but no
definitive information was provided concerning the prod-
ucts of these reductions. Cyclic voltammetry was em-
ployed by Feldmann and Koberstein3 to probe the three-
step reduction of 2,3,5-tribromothiophene at mercury in
methanol, and Pletcher and Razaq4 have demonstrated
that 3-bromothiophene can be prepared in good yield via
the controlled-potential electrolytic reduction of 2,3,5-
tribromothiophene at lead, mercury, zinc, and graphite
cathodes in dioxane-water containing sodium bromide
(5) Gedye, R. N.; Sadana, Y. N.; Leger, R. Can. J . Chem. 1985, 63,
2669-2672.
X Abstract published in Advance ACS Abstracts, November 1, 1996.
(1) Brown, L. W.; Krupski, E. J . Pharm. Sci. 1963, 52, 55-58.
(2) Mairanovskii, S. G.; Barashkova, N. V.; Vol’kenshtein, Y. B. Sov.
Electrochem. 1965, 1, 60-64.
(6) Dapperheld, S.; Feldhues, M.; Litterer, H.; Sistig, F.; Wegener,
P. Synthesis 1990, 403-405.
(7) van der Plas, H. C.; de Bie, D. A.; Geurtsen, G.; Reinecke, M.
G.; Adickes, H. W. Recl. Trav. Chim. Pays-Bas 1974, 93, 33-36.
(8) Gronowitz, S.; Hallberg, A.; Glennow, C. J . Heterocycl. Chem.
1980, 17, 171-174.
(3) Feldmann, M.; Koberstein, E. Chem.-Tech. (Heidelberg) 1977,
6, 517-521.
(4) Pletcher, D.; Razaq, M. J . Appl. Electrochem. 1980, 10, 575-
582.
(9) Reinecke, M. G. Tetrahedron 1982, 38, 427-498.
S0022-3263(96)01364-3 CCC: $12.00 © 1996 American Chemical Society

Electrolytically Induced Halogen Dance
J . Org. Chem., Vol. 61, No. 23, 1996 8075
iodothiophenes. A review article by Reinecke9 presents
a further discussion of these so-called base-catalyzed
halogen dances of halogenated thiophenes.
Despite all of the earlier electrochemical work, there
appear to have been no previous reports that the halogen
dance can occur when halogenated thiophenes are sub-
jected to electrolytic reduction. In this paper we describe
the electrochemical reduction of a number of mono- and
dihalothiophenes at glassy carbon cathodes in dimethyl-
formamide containing tetramethylammonium perchlor-
ate. Using cyclic voltammetry and controlled-potential
electrolysis, and employing gas chromatography for the
identification and quantitation of products, we have
found that each of the monohalogenated starting materi-
als (2-bromo-, 3-bromo-, 2-chloro-, 3-chloro-, and 2-iodo-
thiophene) undergoes a one-step, two-electron reduction
to form thiophene exclusively. On the other hand,
dihalothiophenes (2,3-dibromo-, 2,4-dibromo-, 2,5-dibro-
mo-, 3,4-dibromo-, 2-bromo-5-chloro-, and 3-bromo-2-
chlorothiophene) are reduced in a stepwise fashion; the
first stage of reduction entails two-electron cleavage of
one carbon-halogen bond (accompanied by an electro-
lytically induced halogen dance) to yield mixtures of
mono- and dihalothiophenes, whereas the second stage
of reduction of a dihalothiophene corresponds to an
overall four-electron process to give thiophene.
F igu r e 1. Cyclic voltammograms recorded at a scan rate of
100 mV s-1 with a circular, planar glassy carbon electrode
(area ) 0.077 cm2) in DMF containing 0.10 M TMAP and (A)
2 mM 3-bromothiophene and (B) 2 mM 2,3-dibromothiophene.
Thiophene, 2-bromothiophene, 3-bromothiophene, 3,4-di-
bromothiophene, 4-bromo-2-chlorothiophene, 3-bromo-2-chlo-
rothiophene, and 2-chlorothiophene were identified by means
of gas chromatography from a comparison of retention times
of suspected products with those of commercially available
authentic compounds. One product of the electrolytic reduc-
tion of 2-bromo-5-chlorothiophene was found to have a chro-
matographic retention time just slightly shorter than that of
3-bromo-2-chlorothiophene; believing that this unknown prod-
uct was most likely to be 4-bromo-2-chlorothiophene, we
acquired mass spectral data at 70 eV for authentic 3-bromo-
2-chlorothiophene and for suspected 4-bromo-2-chlorothiophene.
(a) For 3-bromo-2-chlorothiophene: m/z 200 M+ (30); 198 M+
(100); 196 M+ (73); 163 [M - Cl]+ (9); 161 [M - Cl]+ (9%); 119
[M - Br]+ (11); 117, [M - Br]+ (26); 82 [M - Br - Cl]+ (14).
(b) For 4-bromo-2-chlorothiophene: m/z 200 M+ (30); 198 M+
(100); 196 M+ (75); 163 [M - Cl]+ (27); 161 [M - Cl]+ (26);
119 [M - Br]+ (10); 117 [M - Br]+ (21); 82 [M - Br - Cl]+
(19). This agreement seems to be adequate to confirm the
identity of 4-bromo-2-chlorothiophene. Quantitation of all
electrolysis products was accomplished with the aid of gas
Exp er im en ta l Section
R ea gen t s. Burdick and J ackson “distilled in glass” di-
methylformamide (DMF) was used as received as the solvent,
and tetramethylammonium perchlorate (TMAP) from GFS
Chemicals, Inc., was employed without further purification as
supporting electrolyte. Each of the following halogenated
thiophenes was used as obtained from the Aldrich Chemical
Co.: 2-bromothiophene (98%), 3-bromothiophene (97%), 2-chlo-
rothiophene (96%), 3-chlorothiophene (98%), 2-iodothiophene
(98+%), 2,3-dibromothiophene (98%), 2,5-dibromothiophene
(95%), 3,4-dibromothiophene (99%), and 2-bromo-5-chloro-
thiophene (95%). Thiophene (Eastman Kodak Co., 99%), 2,4-
dibromothiophene (Lancaster, 97%), 3-bromo-2-chlorothiophene
(Acros Organics USA, 97%), and 1,1,1,3,3,3-hexafluoro-2-
propanol (Aldrich, 99+%) were used as received. Deaeration
procedures were carried out with Air Products zero-grade
argon.
Cells, Electr od es, In str u m en ta tion , a n d P r oced u r es.
Details of the cells, instrumentation, and procedures for cyclic
voltammetry10 and controlled-potential electrolysis11,12 are
presented in earlier papers. A short length of 3-mm-diameter
glassy carbon rod (Grade GC-20, Tokai Electrode Manufactur-
ing Co., Tokyo, J apan) was press-fitted into Teflon to give a
circular, planar working electrode with an area of 0.077 cm2
for cyclic voltammetry. Reticulated vitreous carbon disks
(RVC 2X1-100S, Energy Research and Generation, Inc.,
Oakland, CA) were used for controlled-potential electrolyses;
these electrodes (having surface areas estimated to be 200 cm2)
were fabricated, cleaned, and handled according to procedures
given elsewhere.11 All potentials are quoted with respect to a
reference electrode consisting of a cadmium-saturated mercury
amalgam which is in contact with DMF saturated with both
cadmium chloride and sodium chloride; this electrode has a
potential of -0.76 V vs the aqueous saturated calomel elec-
trode at 25 °C.13,14
chromatography, as outlined in a previous publication.12
A
30 m × 0.53 mm capillary column (AT-35, Alltech Associates)
with a stationary phase of poly(phenylmethylsiloxane) was
used. All product yields reported in this paper are based on
gas chromatographic measurements (with n-octane being
added as an electroinactive internal standard to each solution
prior to the start of an electrolysis) and reflect the absolute
percentage of starting material incorporated into a particular
species.
Resu lts a n d Discu ssion
Cyclic Volta m m etr ic Beh a vior of Mon o- a n d Di-
h a loth iop h en es. Figure 1A shows a cyclic voltammo-
gram for the reduction of 3-bromothiophene at a glassy
carbon electrode in DMF containing 0.10 M TMAP. A
single, irreversible wave is observed that corresponds to
the two-electron scission of the carbon-bromine bond to
yield thiophene; this process is proposed on the basis of
the results of controlled-potential electrolyses and on the
identification of thiophene as the only electrolysis prod-
uct, as discussed later. Other monohalogenated thio-
phenes exhibit the same kind of behavior as 3-bromo-
thiophene, and peak potentials for all of the compounds
investigated are listed in Table 1.
(10) Vieira, K. L.; Peters, D. G. J . Electroanal. Chem. Interfacial
Electrochem. 1985, 196, 93-104.
(11) Cleary, J . A.; Mubarak, M. S.; Vieira, K. L.; Anderson, M. R.;
Peters, D. G. J . Electroanal. Chem. Interfacial Electrochem. 1986, 198,
107-124.
(12) Mubarak, M. S.; Nguyen, D. D.; Peters, D. G. J . Org. Chem.
1990, 55, 2648-2652.
(13) Marple, L. W. Anal. Chem. 1967, 39, 844-846.
(14) Manning, C. W.; Purdy, W. C. Anal. Chim. Acta 1970, 51, 124-
126.

8076 J . Org. Chem., Vol. 61, No. 23, 1996
Mubarak and Peters
Ta ble 1. Cyclic Volta m m etr y Da ta for Red u ction of
Mon o- a n d Dih a loth iop h en es a t Gla ssy Ca r bon a
Ta ble 3. Cou lom etr ic Da ta a n d P r od u ct Distr ibu tion s
for Electr olytic Red u ction of 5.0 m M Solu tion s of
Dih a loth iop h en es a t Reticu la ted Vitr eou s Ca r bon in
DMF Con ta in in g 0.10 M TMAP
compd
Ep1, V
Ep2, V
2-bromothiophene
3-bromothiophene
2-chlorothiophene
3-chlorothiophene
-1.57
-1.76
-1.75
-1.84
-0.97
-1.23
-1.28
-1.31
-1.52
-1.34
-1.25
a
Product Distribution,
%
compd
E, V
n
1
2
3
4
5
6
7
2-iodothiophene
2,5-dibromothiophene -1.60 4.04 103
-1.20 1.33
2,3-dibromothiophene
2,4-dibromothiophene
2,5-dibromothiophene
3,4-dibromothiophene
2-bromo-5-chlorothiophene
3-bromo-2-chlorothiophene
-1.75
-1.69
-1.58
-1.74
-1.78
-1.72
63 24 16
-1.20b 1.96
-1.20c 1.99
2
1
92
95
5
2
3,4-dibromothiophene -1.80 3.98 101
2,3-dibromothiophene -1.80 3.91
-1.25 1.32
96
65 36
66 32
aA 2.0 mM concentration of each compound in DMF containing
0.10 M TMAP was used, and the scan rate was 100 mV s-1
2,4-dibromothiophene -1.80 3.81 103
-1.25 1.26
.
2-bromo-5-chlorothio- -1.80 3.92
99
phene
-1.30 1.63
-1.30b 1.76d
-1.30c 1.93
6
3
8
6
79
104
100
Ta ble 2. Cou lom etr ic Da ta a n d P r od u ct Distr ibu tion s
for Electr olytic Red u ction of 5.0 m M Solu tion s of
Mon oh a loth iop h en es a t Reticu la ted Vitr eou s Ca r bon in
DMF Con ta in in g 0.10 M TMAP
3-bromo-2-chlorothio- -1.80 4.04 102
phene
-1.30 1.99
100
compd
E, V
n
thiophene, %
a
1 ) thiophene, 2 ) 2-bromothiophene, 3 ) 3-bromothiophene,
2-bromothiophene
3-bromothiophene
2-chlorothiophene
3-chlorothiophene
2-iodothiophene
-1.60
-1.80
-1.80
-1.90
-1.20
1.99
2.05
1.95
2.03
1.97
97
101
95
102
103
4 ) 3,4-dibromothiophene, 5 ) 4-bromo-2-chlorothiophene, 6 )
b
3-bromo-2-chlorothiophene,
7
) 2-chlorothiophene. 50 mM
1,1,1,3,3,3-hexafluoro-2-propanol was present. c 1 M H2O was
d
present. This n value appears to be artifically low due to an
excessively large background correction.
When a cyclic voltammogram for the reduction of 2,3-
dibromothiophene is recorded (Figure 1B), under condi-
tions that are identical with those of Figure 1A, two
irreversible cathodic waves are seen. We attribute the
first wave to reductive cleavage of the carbon-bromine
bond at the 2-position (to give the 3-bromo-2-thienyl
carbanion that is more stable than the 2-bromo-3-thienyl
carbanion),7,15,16 whereas the second wave corresponds
formally to reduction of the carbon-bromine bond at the
3-position. As revealed in Table 1, it is not surprising
that the peak potential for the second stage of reduction
of 2,3-dibromothiophene (-1.75 V) is very close to the
peak potential for reduction of 3-bromothiophene (-1.76
V). Moreover, owing to the inductive effect of the 3-bromo
substituent of 2,3-dibromothiophene, reduction of the
carbon-bromine bond at the 2-position of 2,3-dibromo-
thiophene is considerably easier than reduction of the
carbon-bromine bond of 2-bromothiophene. Peak po-
tentials for five other dihalothiophenes are included in
Table 1, allowing one to make additional comparisons
among the mono- and dihalothiophenes.
Con tr olled -P oten tia l Electr olyses of Mon o- a n d
Dih a loth iop h en es. Compiled in Table 2 are coulom-
etric data and product distributions for duplicate con-
trolled-potential electrolyses of each of the five mono-
halothiophenes at reticulated vitreous carbon cathodes
in DMF containing 0.10 M TMAP. Coulometric n values
are reproducible to (0.05, and product yields agree to
within (3% absolute. Clearly, the electrolytic reduction
of each monohalogenated compound is a two-electron
process, and thiophene is the only product.
thiophene, when electrolyzed at a potential corresponding
to its second voltammetric wave, undergoes an overall
four-electron reduction to give thiophene as the only
product. However, intriguing results are obtained when
these compounds are electrolyzed in the absence of a
proton donor at potentials that correspond to their first
voltammetric waves. For each compound, except 3-bromo-
2-chlorothiophene, the coulometric n value is significantly
lower than 2, because some of the starting material is
consumed via nonelectrochemical processes (namely, the
halogen dance). When 2,5-dibromothiophene is electro-
lyzed at -1.20 V, three products (2-bromo-, 3-bromo-, and
3,4-dibromothiophene) are obtained. Electrolyses of 2,3-
and 2,4-dibromothiophene at -1.25 V afford a two-to-
one mixture of 3-bromo- and 3,4-dibromothiophene, and
electrochemical reduction of 2-bromo-5-chlorothiophene
at -1.30 V gives rise to five products (2-chloro-, 3-bromo-,
3,4-dibromo-, 4-bromo-2-chloro-, and 3-bromo-2-chloro-
thiophene). When 3-bromo-2-chlorothiophene is reduced
at -1.30 V, the coulometric n value is 2 and the only
product is 2-chlorothiophene, because the carbon-
chlorine bond cannot undergo direct reductive cleavage
at this potential (Table 1). Electrolyses of 3,4-dibromo-
thiophene at potentials corresponding to its first voltam-
metric wave could not be done successfully; because the
two voltammetric waves associated with stepwise reduc-
tion of this compound are not adequately separated
(Table 1), one cannot prevent the occurrence of the second
stage of reduction.
Electrolyses of 2,5-dibromo- and 2-bromo-5-chloro-
thiophene were conducted at potentials corresponding to
their respective first voltammetric waves and in the
presence of an excess of a proton donor (50 mM 1,1,1,3,3,3-
hexafluoro-2-propanol or 1 M water). As revealed in
Table 3, addition of a proton donor causes the n value to
become essentially 2 and the halogen dance that occurs
in the absence of a proton donor is largely blocked. For
2-bromo-5-chlorothiophene, only one product (2-chloro-
thiophene) is obtained; however, for 2,5-dibromothiophene
the principal product is the expected 2-bromothiophene,
Table 3 lists coulometric data and product distributions
for electrolytic reductions of the six dihalothiophenes at
potentials corresponding to either their first or second
voltammetric waves; the reproducibilities of the n values
and product distributions were comparable with those
mentioned in the preceding paragraph. Each dihalo-
(15) Reinecke, M. G.; Hollingworth, T. A. J . Org. Chem. 1972, 37,
4257-4259.
(16) de Bie, D. A.; van der Plas, H. C.; Geurtsen, G.; Nijdam, K.
Recl. Trav. Chim. Pays-Bas 1973, 92, 245-252.

Electrolytically Induced Halogen Dance
J . Org. Chem., Vol. 61, No. 23, 1996 8077
Sch em e 1
Sch em e 2
but thiophene and 3-bromothiophene are formed in very
small amounts.
Mech a n istic F ea tu r es of th e Red u ction of Mon o-
a n d Dih a loth iop h en es. To rationalize the electro-
chemical behavior of the monohalothiophenes, we propose
a mechanistic picture, depicted in Scheme 1, that is
analogous to the well-established set of reactions17 in-
volved in the electrolytic reduction of aryl monohalides.
Using 2-iodothiophene as a representative compound, we
suggest that there is reversible uptake of one electron to
yield a radical-anion intermediate (reaction 1-1), which
subsequently undergoes carbon-halogen bond cleavage
to give a 2-thienyl radical and iodide ion (reaction 1-2).
Once formed, the 2-thienyl radical can accept a second
electron in two ways: it can undergo (a) direct irrevers-
ible reduction at the surface of the cathode (reaction 1-3)
or (b) a reversible solution electron-transfer reaction with
an electrogenerated radical-anion intermediate near the
electrode surface (reaction 1-4). In either case, the
resulting 2-thienyl carbanion is protonated, most prob-
ably by water (present in the solvent-supporting electro-
lyte) but possibly by the tetramethylammonium cation.18
Attempts were made to employ fast-scan cyclic voltam-
metry19 and double-potential-step chronoamperometry19
to measure the rate constant for cleavage of the electro-
generated 2-iodothiophene radical-anion intermediate
(reaction 1-2) and to determine whether the resulting
2-thienyl radical prefers to accept a second electron from
the electrode (reaction 1-3; ECEirr process) or from the
electrogenerated radical-anion intermediate (reaction 1-4;
DISP1 process). Unfortunately, our studies revealed that
2-iodothiophene (as well as other monohalogenated
thiophenes) or the electrogenerated radical-anion inter-
mediate is adsorbed onto glassy carbon (as well as
mercury) electrodes, so that quantitative results were
unattainable. However, we did find that, at cyclic
voltammetric scan rates as slow as 5 to 10 V s-1, a small
anodic current for oxidation of the radical anion of
2-iodothiophene was observable, which indicates that the
lifetime of the radical-anion is in the realm of 100 ms.
To explain the electrolytically induced halogen dance
that takes place when a dihalothiophene is reduced at
potentials corresponding to its first voltammetric wave,
we have constructed Scheme 2, showing a sequence of
reactions pertaining to 2,5-dibromothiophene; analogous
schemes can be written for other dihalothiophenes.
Scheme 2 includes several deprotonation-transbromina-
tion reactions, of the kind invoked by earlier workers,7-9
in which the initially electrogenerated 2-bromo-5-thienyl
(17) Andrieux, C. P.; Blocman, C.; Dumas-Bouchiat, J .-M.; Save´ant,
J .-M. J . Am. Chem. Soc. 1979, 101, 3431-3441.
(18) Vieira, K. L.; Mubarak, M. S.; Peters, D. G. J . Am. Chem. Soc.
1984, 106, 5372-5373.
(19) Andrieux, C. P.; Save´ant, J . M. In Techniques of Chemistry;
Bernasconi, C. F., Ed.; Wiley-Interscience: New York, 1986; Vol. VI,
pp 305-390.

8078 J . Org. Chem., Vol. 61, No. 23, 1996
Mubarak and Peters
anion plays a key role. As a first step (reaction 2-1), we
have the overall two-electron reductive cleavage of 2,5-
dibromothiophene (1) to give the 2-bromo-5-thienyl anion
(2), which in reaction 2-2 accepts a proton from the
medium to yield 2-bromothiophene (3) or in reaction 2-3
accepts a proton from 1 to give 3 and the 2,5-dibromo-
3-thienyl anion (4). Next, in reaction 2-4, 4 and 1 interact
to form 2,3,5-tribromothiophene (5) and 2. Then 5 can
undergo electrolytic reduction (reaction 2-5) to give either
2,4-dibromothiophene (6) or 2,3-dibromothiophene (7),
and each of the last two compounds can be reduced
(reactions 2-6a and 2-6b) to yield 3-bromothiophene (8).
In addition, it is possible for 2 and 6 to interact (reaction
2-7) to give either the 2,4-dibromo-3-thienyl anion (9) and
3 or the 4-bromo-2-thienyl anion (10) and 1. Protonation
of 9 and 10 leads, respectively, to 6 and 8 (reactions 2-8
and 2-9). When 9 interacts with 1 (reaction 2-10), 2,3,4-
tribromothiophene (11) and 2 are produced, and two-
electron reduction of 11 leads in reaction 2-11 to the
formation of 3,4-dibromothiophene (12). It should be
emphasized that for Scheme 2 the source of protons in
reactions 2-5, 2-6a, 2-6b, 2-8, 2-9, and 2-11 can be either
the medium or the starting material itself. Scheme 2
provides a minimal set of reactions that can satisfactorily
account for all of the products obtained from the electro-
chemical reduction of 2,5-dibromothiophene. However,
one of the intermediate species, 2,3,5-tribromothiophene
(5), might also be involved in the halogen dance. Depro-
tonation of 5 by 2 can give rise to the 2,3,5-tribromo-4-
thienyl anion which can be transbrominated by 1 to yield
2,3,4,5-tetrabromothiophene. After it is formed, 2,3,4,5-
tetrabromothiophene could undergo either two-electron
reduction or debromination to yield the 2,3,4-tribromo-
5-thienyl anion, each process being followed by protona-
tion to afford 11 which is subsequently converted elec-
trochemically into 12.
As a separate test of the occurrence of an electrolyti-
cally induced halogen dance, an experiment was per-
formed in which a solution containing 5 mM concentra-
tions of both 2-iodothiophene and 2,5-dibromothiophene
in DMF containing 0.10 M TMAP was electrolyzed at a
potential (-1.00 V) at which only 2-iodothiophene is
directly reducible; the particular aim of this experiment
was to confirm that the 2-thienyl anion electrogenerated
from 2-iodothiophene can induce a halogen dance for 2,5-
dibromothiophene. We obtained an n value of 2.04
(indicating complete two-electron reduction of 2-iodo-
thiopheneswhich correlated with the appearance of
thiophene in 96% yield), and the species emanating from
2,5-dibromothiophene were 2-bromothiophene (48%), 3-bro-
mothiophene (34%), 3,4-dibromothiophene (17%), 2,3-
dibromothiophene (∼1%), and 2,4-dibromothiophene
(∼1%), along with a remaining trace of 2,5-dibromo-
thiophene (∼1%) itself.
In electrolyses in which either 1,1,1,3,3,3-hexafluoro-
2-propanol or water is present during the reduction of
2,5-dibromothiophene (as well as 2-bromo-5-chloro-
thiophene) at potentials corresponding to its first voltam-
metric wave, it is reasonable to conclude that the elec-
trogenerated 2-bromo-5-thienyl carbanion is quickly pro-
tonated by the medium (reaction 2-2) and that its
interaction with the starting material (reaction 2-3) to
initiate the halogen dance is almost completely blocked.
Although the behavior of tri- and tetrahalogenated
thiophenes has not been examined in the present work,
we believe that such compounds will also undergo an
electrolytically induced halogen dance.
J O9613646