Journal of The Electrochemical Society, 161 (14) G122-G127 (2014)
G123
research in this area has appeared. A short list of seminal work19–26
on this topic is cited at the end of this paper; these references provide
additional avenues to the literature. A relatively recent paper27 from
our laboratory includes a reasonably comprehensive bibliography of
other contributions in this field. One of the major attributes of silver
is its superiority to both carbon and mercury for the electrochemical
reduction of halogenated organic compounds. Reductive cleavage of
a carbon–halogen bond at a silver electrode can occur at potentials
as much as 1 V more positive than the same process at a carbon or
mercury cathode.
prior electrolysis. Furthermore, a thin layer of silver oxide may be
formed during the use of an electrode, causing it to be partially de-
activated; however, in practice, a brief cathodic polarization of the
electrode after being introduced into the air-free electrolysis cell can
be used to activate the electrode completely.
Serving as the auxiliary (counter) electrode was a graphite rod
immersed in a DMF–0.10 M TMABF4 solution, separated from the
cathode compartment by a sintered glass disk backed by a methyl
cellulose–DMF–0.10 M TMABF4 plug. All potentials in the present
work are given with respect to a reference electrode consisting of a
cadmium-saturated mercury amalgam in contact with DMF saturated
with both cadmium chloride and sodium chloride; this electrode has a
potential of –0.76 V vs. an aqueous saturated calomel electrode (SCE)
In the present work, we have employed cyclic voltammetry and
controlled-potential (bulk) electrolysis to examine the direct reduc-
tions of methyl 1-bromomethyl-2-oxocyclopentane-1-carboxylate (1)
and ethyl 1-bromomethyl-2-oxocyclohexane-1-carboxylate (2) at sil-
ver cathodes in DMF containing 0.10 M TMABF4 as supporting elec-
trolyte. Ring-expanded products arising from the electrochemical re-
duction of 1 and 2 have been identified, characterized, and quantitated
with the aid of gas chromatography (GC), gas chromatography–mass
spectrometry (GC–MS), high-resolution mass spectrometry (HRMS),
Syntheses of methyl 1-bromomethyl-2-oxocyclopentane-1-
carboxylate (1) and ethyl 1-bromomethyl-2-oxocyclohexane-1-
carboxylate (2).— We prepared methyl 1-bromomethyl-2-
oxocyclopentane-1-carboxylate (1) and ethyl 1-bromomethyl-
1
and H NMR spectrometry. For each starting compound, we found
2-oxocyclohexane-1-carboxylate (2) according to
a published
that four different electrolysis products are formed: (a) the desired
ring-expanded product, (b) a dehalogenated product, (c) a dimeric
species, and (d) a product, with an extended ester moiety, that has not
been previously reported in an electrochemical study. To account for
the formation of the four products, a set of pathways has been pro-
posed that involves both radical and carbanion intermediates arising
from electroreductive scission of the carbon–bromine bond of each
substrate.
procedure by Dowd and Choi,3 with some modification, that
involves the addition of a solution of 12.9 g (90 mmol) of methyl
2-oxocyclopentane-1-carboxylate or 15.1 g (90 mmol) of ethyl
2-oxocyclohexane-1-carboxylate in 60 mL of dry tetrahydrofuran
slowly to a stirred suspension of 3.8 g (108 mmol) of sodium
hydride in 150 mL of dry tetrahydrofuran containing 19.4 g of
hexamethylphosphoramide at room temperature under argon. After
each mixture was stirred for 1 h, 52 g (300 mmol) of dibromomethane
was added, and the solution was heated at reflux temperature for 10 h.
Then the mixture was poured into a separatory funnel containing 600
mL of diethyl ether; the organic layer was washed with five 100-mL
portions of water, dried over anhydrous potassium carbonate, filtered,
and concentrated. Finally, the residue was vacuum distilled to afford
the desired product as a colorless oil; boiling points (0.1 mm) for 1
and 2 were 110–112◦C and 117–119◦C, respectively. For each of
Experimental
Reagents.— Each of the following compounds (with purity
indicated in parentheses) was purchased from Aldrich Chemi-
cal Company and was used without further purification: methyl
2-oxocyclopentane-1-carboxylate (95%), ethyl 2-oxocyclohexane-
1-carboxylate (95%), hexamethylphosphoramide (99%), sodium
hydride (60% suspension in mineral oil), dibromomethane (99%), tri-
n-butyltin hydride (97%), 2,2ꢀ-azobis(2-methylpropionitrile) (AIBN,
98%), iodomethane (99.5%), ethyl 2-cyclohexanoneacetate (97%,
ethyl 2-(2-oxocyclohexyl)acetate, 5b), ethyl bromoacetate (98%), n-
dodecane (99%), and deuterium oxide (D2O, 99%). Solutions for
all electrochemical experiments were deaerated with zero-grade ar-
gon (Air Products). Dimethylformamide (DMF, 99.9%, EMD Chem-
icals) was employed without further purification as the solvent for
all electrochemical experiments. Tetramethylammonium tetrafluoro-
borate (TMABF4, >99%, GFS Chemicals), used as the supporting
electrolyte, was recrystallized from water–methanol and stored in a
vacuum oven at 70–80◦C prior to use.
1
the two compounds, mass and H NMR spectra were in agreement
with previously published data.3 Our mass spectrometric data for
these two substrates are as follows: (a) for 1, m/z (70 eV) 205 [M –
OCH3]+ (3%); 203 [M – OCH3]+ (3%); 155 [M – Br]+ (100%); 123
[M – Br – CH3OH]+ (53%); 95 [M – Br – CO – CH3OH]+ (53%);
HRMS (ESI) m/z: calcd. for C8H12BrO3 [M + H]+ 234.9970, found
234.9967; (b) for 2, m/z (70 eV) 219 [M – OC2H5]+ (8%); 217 [M –
OC2H5]+ (8%); 183 [M – Br]+ (100%); 137 [M – Br – C2H5OH]+
(58%); 109 [M – Br – CO – C2H5OH]+ (90%); HRMS (ESI) m/z:
calcd. for C10H15BrNaO3 [M + Na]+ 285.0102, found 285.0096.
Syntheses of methyl 3-oxocyclohexane-1-carboxylate (3a) and
ethyl 3-oxocycloheptane-1-carboxylate (3b).— Preparation of the ti-
tle compounds was accomplished via a slightly modified version of
the procedure outlined by Dowd and Choi.3 To begin, we added AIBN
(0.042 g, 0.24 mmol) to a stirred mixture of either 1 or 2 (2.6 mmol) in
300 mL of dry benzene containing tri-n-butyltin hydride (0.70 g, 2.4
mmol), and the resulting solution was refluxed at atmospheric pressure
for 24 h. After the mixture was cooled to room temperature, the sol-
vent was removed under reduced pressure to afford the crude product
as a yellow oil, which was dissolved in dichloromethane (150 mL) and
washed with five 20-mL portions of 10% aqueous potassium fluoride
solution. Then the organic phase was dried over anhydrous sodium
sulfate, filtered, and concentrated under reduced pressure, and the re-
maining oil was transferred onto a silica-gel column which was eluted
with 2:1 hexane–ethyl acetate to afford each of the desired products.
Purities of 3a and 3b were checked with the aid of GC and TLC. For
each of the two title compounds, mass and 1H NMR spectra were in
agreement with previously published data.3 Mass spectral data for the
two species are as follows: (a) for 3a, m/z (70 eV) 156 [M]+ (27%);
125 [M – OCH3]+ (10%); 97 [M – CO – OCH3]+ (100%); HRMS
(ESI) m/z: calcd. for C8H12O3 [M]+ 156.0786, found 156.0780; (b)
for 3b, m/z (70 eV) 184 [M]+ (47%); 155 [M – C2H5]+ (12%); 139
Electrodes, cells, and instrumentation.— Details and pertinent lit-
erature concerning the electrodes, cells, and instrumentation for cyclic
voltammetry and controlled-potential (bulk) electrolysis can be found
in earlier publications.28–30 Cyclic voltammetry was performed with
a planar, circular silver working electrode (with a geometric area of
0.071 cm2) constructed from 3.0-mm diameter rod (99.9%, Alfa Ae-
sar), as described elsewhere.30 Working electrodes, with surface areas
of approximately 20 cm2, for controlled-potential (bulk) electrolyses
were fabricated from silver gauze (Alfa Aesar, 99.9%, 20 mesh woven
from 0.356-mm diameter wire).30
We have found that proper pretreatment of silver gauze electrodes
is important to ensure reproducible and complete reduction of a start-
ing material. Accordingly, prior to each electrolysis, the silver gauze
electrode was fully submerged in a room-temperature aqueous slurry
(suspension) of solid sodium bicarbonate and subjected to ultrasoni-
cation for 30 min. Then the cathode was rinsed thoroughly in distilled,
deionized water to remove sodium bicarbonate (and any impurities)
from the electrode surface and then placed in an oven set at 180◦C and
at atmospheric pressure for 20 min. Without this treatment, the elec-
trode surface may be contaminated with adsorbed substances from a
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