Beilstein J. Org. Chem. 2015, 11, 280–287.
chemical oxidants. In all cases, the use of constant current elec- equipped with a RVC anode and platinum wire cathode using
trolysis conditions allows the potential at the anode surface to two of the three necks of the flask. The photovoltaic system was
be adjusted to that of the substrate. Hence, the same experi- inserted in series with the reaction flask along with an ammeter
mental protocol can be used for each reaction.
to monitor the current. The reaction was carried out at a
constant current of 8.0 mA until the desired amount of charge
It should be noted, that in many ways, the use of a simple was passed. The crude mixture was washed with water and then
tion as illustrated in Figure 1 is a gimmick. For a more complex and purified by silica gel chromatography.
system or large-scale electrolysis, a more sophisticated photo-
Representative procedure for solar-driven
voltaic array would be used to harvest enough energy to run a
standard potentiostat. This would result in a far more selective
electrolysis since the current passed through the reaction could (Scheme 6)
be carefully controlled and the efficiency of the electrochem- A flame-dried three-necked round-bottomed flask was charged
ical process optimized. However, the use of a simple photo- under argon with 1 equiv of o-phenylenediamine (0.31 mmol,
voltaic device to drive the reactions does highlight two key 33.8 mg), 1.1 equiv of 3-nitrobenzaldehyde (0.34 mmol,
points. First, the reactions illustrate how electrochemistry can 52 mg), 20 mol % CAN (31 mg), and LiClO4 (0.1 M, 127 mg)
be used to expand the growing area of visible-light-driven in 12 mL of THF/MeOH 5:1. A RVC anode and a carbon rod
chemistry to include electron-transfer reactions in molecules cathode were inserted into the flask. A constant current of 8 mA
and reaction systems that have no internal chromophore. was supplied (either via a potentiostat or a photovoltaic cell)
Second, the reactions illustrate how simple sustainable electro- until 2.3 F/mol of charge had passed. After the electrolysis, the
chemical methods can be employed. This is particularly impor- contents of the flask were extracted with EtOAc, washed with
tant since the larger synthetic community is often hesitant to brine, and dried with MgSO4. The product was purified by
adopt electrochemical reactions. This hesitation frequently has column chromatography (EtOAc/hexanes 1:1) to give the benz-
its origins in the perception that electrochemical reactions imidazole product.
require the use of sophisticated and expensive equipment. The
reaction setup shown in Figure 1b demonstrates that this found in the original publications for the reactions as cited in
perception is not accurate. Any electrochemical reaction in the the main text. The modification of these reactions to the solar-
literature can be mimicked satisfactorily with only a small driven versions were carried out according to the general infor-
investment of time and money.
mation and example solar-driven procedures provided above.
Experimental
Acknowledgements
General information
We thank the National Science Foundation (CHE-1151121,
Electrolysis reactions were performed using a photovoltaic cell CBET-1262176) and (CHE-1240194/CenSURF) for their
and a light source (direct sunlight or a compact fluorescent bulb generous support of our work.
(
hydrophonic, full spectrum, 60 W, 5500K)) with an ammeter
and an optional coulometer connected in series. The output References
voltage of the photovoltaic cells varied from 6–35 V depending
on the light intensity, which was varied to control the current
output. For reactions requiring higher current, a Topray solar
2
.
.
3
Frontana-Uribe, B. A.; Little, R. D.; Ibanez, J. G.; Palma, A.;
(
Figure 1a). Alternatively, several 6 V solar photovoltaic cells
4.
See for a description of electrochemical methods specifically targeted
to a synthetic audience.
The enol ether substrate was dissolved in anhydrous MeOH
5
.
.
(
0.03 M) with lithium perchlorate (0.03 M, 1.0 equiv) in a
See for a recent review of indirect electrochemical methods.
6
flame-dried three-necked round-bottomed flask at room
temperature under an argon atmosphere. The flask was
286