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References
of 94 and 42%, respectively. We can observe here a
different behavior for the hydrogenation of a conju-
gated (1,3-cyclohexadiene) and non-conjugated system
(cyclohexene). This may occur due to differences in
adsorption strength of substrates on the catalyst sur-
face. Citral, with a conjugated double bond, showed
low reactivity (Table 1, entry 15).
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13. Preparative electrolyses: The controlled current preparative
electrolyses were carried out in a Princeton Applied
Research (PAR) 273A potentiostat/galvanostat, using
undivided cells of 50 mL. Ni foam (or plate) was used as
sacrificial anode (4.0×8.0 cm). The cell potential (cathode
versus anode) may be monitored by using a multimeter. An
iron bar (0.8 dm diameter; 0.1828 dm2 surface) was used
as the working electrode, and may be reused several times,
after cleaning the nickel deposit by polishing. The elec-
trolytic cell was charged with the solvent (H2O/MeOH, 1:1)
and 0.2 M supporting electrolyte. An inert atmosphere (N2)
is necessary when NH4OAc is used as supporting elec-
trolyte to expulse some O2 produced on the anode. A
pre-electrolysis is necessary to deposit nickel on the Fe
electrode; 175 mA dm−2 constant current was applied until
consumption of 58 C. The substrate was added to the
electrochemical system and the electrolysis continued,
following the current density gradient program (it may be
executed manually by calculating the charge passed, Q=
current (A)×time (s)). The reaction may be followed by GC
analysis. At the end, the hydrogenated substrate was
extracted with diethyl ether 3×15 mL, washed with water
and dried with Na2SO4. Gas chromatogram/mass spectra
were taken with a Varian 3380 GC or Finnigan GC-MS
instrument, fitted with a 30 m capillary CP-SPL5CB
Chrompack column, using 60–200°C temperature range
(10°C min−1). Hydrogenation products and reagents were
compared with authentic samples and were confirmed by
GC/MS.
The exchange of supporting electrolyte from NH4OAc
to NH4Cl allowed for lower cell voltages and elimina-
tion of oxygen during electrolysis.14 As a result, higher
yields could be obtained in most cases, along with
higher electrochemical efficiencies. Cyclohexene
remained unreactive (Table 1, entry 2), while ace-
tophenone (Table 1, entry 8) was hydrogenated in a
yield of 85% (33% of electrochemical efficiency). This
may be explained by acetate anion interference on
acetophenone adsorption on the electrode surface.3 2-
Cyclohexen-1-one, benzaldehyde and styrene (Table 1,
entries 4, 6 and 10) were again hydrogenated with
yields over 90%, however, with better electrochemical
efficiencies (82, 50 and 65%, respectively). The yields
of 1,3-cyclohexadiene and trans–trans-2,4-hexadien-1-
ol (Table 1, entries 12 and 14) hydrogenations were
unchanged. In the group of terpenes, a considerable
increase of yield was observed. Citral (Table 1, entry
16) gave citronellol as the principal product of hydro-
genation with 70%, and 84% of electrochemical
efficiency. Linalool and geraniol (Table 1, entries 18
and 20) also were hydrogenated in increased of 93 and
22%, respectively.
The results described herein illustrate ECH selectivity
for various conjugated and non-conjugated double
bonds. Substrates containing a conjugated system are
more easily hydrogenated. NH4Cl supporting elec-
trolyte showed proportionate yield increase for all sub-
strates hydrogenated, diminishing the cell potential
and increasing the electrochemical efficiency in the
major cases. These results show that ECH, improved
by a current density gradient and sacrificial anode of
nickel provides a good tool for hydrogenation of some
classes of organic compounds, without the necessity of
a H2 supply.
Acknowledgements
The authors would like to thank Dr. Flamarion
Borges Diniz for technical support and fruitful discus-
sions, and CNPq, FINEP/CTPETRO and PETRO-
BRAS for financial support.
14. Santana, D. S.; Lima, M. V. F.; Daniel, J. R. R.; Areias,
M. C. C.; Navarro, M. J. Org. Chem., submitted.