Sunlight, electrochemistry, and sustainable oxidation reactions

Article abstract of DOI:10.1039/c3gc41650j

Inexpensive, readily available photovoltaic cells have been used to conduct indirect electrochemical oxidation reactions. The reactions retain the efficiency of the solar-electrochemical method while capitalizing on the unique opportunities for selectivity afforded by a chemical oxidant. The versatility of the electrochemical method allowed for the recycling of Os(viii)-, TEMPO-, Ce(iv)-, Pd(ii)-, Ru(viii)-, and Mn(v)-oxidants all with the same very simple reaction apparatus.

Full text of DOI:10.1039/c3gc41650j

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Sunlight, electrochemistry, and sustainable oxidation
reactions†
Cite this: DOI: 10.1039/c3gc41650j
Received 13th August 2013,
Bichlien H. Nguyen, Alison Redden and Kevin D. Moeller*
Accepted 18th September 2013
DOI: 10.1039/c3gc41650j
www.rsc.org/greenchem
Inexpensive, readily available photovoltaic cells have been used to
conduct indirect electrochemical oxidation reactions. The reactions
retain the efficiency of the solar-electrochemical method while
capitalizing on the unique opportunities for selectivity afforded
by a chemical oxidant. The versatility of the electrochemical
method allowed for the recycling of Os(VIII)-, TEMPO-, Ce(IV)-,
Pd(II)-, Ru(VIII)-, and Mn(V)-oxidants all with the same very simple
reaction apparatus.
Oxidation reactions are powerful synthetic tools because they
allow us to selectively increase the functionality of a molecule.
However, they are also inherently problematic from an environ-
mental standpoint. Every stoichiometric oxidation reaction
requires the formation of a stoichiometric reduction product.
If a metal oxidant is used, then the reaction leads to one or
more equivalents of a metal waste product. To circumvent this
problem, efforts to conduct more benign oxidation reactions
typically channel the stoichiometric reduction product to
something that is non-toxic and inexpensive to dispose. For
example, extensive work is being conducted to develop oxygen
Fig. 1 An electrochemical reaction setup with a simple 6 V photovoltaic power
supply is shown. The ammeter is spliced into the line so that the current passed
through the flask can be monitored.
oxidation reactions can be conducted in a manner that con-
sumes only sunlight and produces hydrogen gas as the only
byproduct. The utility of the approach for conducting direct
oxidation reactions is illustrated in Scheme 1 for both an
oxidative cyclization and the functionalization of a proline
1
,2
as a co-oxidant for recycling chemical oxidants. This work
channels the required reduction reaction to the formation of
water. In a similar fashion, electrochemistry can be used to
conduct more benign oxidation reactions by channeling the
6
,7
derivative.
Both of the reactions illustrated in Scheme 1 were run by
passing a constant current of electricity through the electro-
chemical cell. Reactions run in this fashion are extremely
3
reduction reaction to the formation of hydrogen gas.
Recently, we have shown that electrochemical reactions can
be conducted with the use of a simple photovoltaic cell as the
4,5
power supply (Fig. 1).
The positive lead of the photovoltaic cell is connected to
the anode of the reaction flask, and the negative lead is con-
nected through an ammeter in series to the cathode. The
photovoltaic cell is exposed to sunlight, resulting in the
passage of current through the electrolysis flask. In this way,
Department of Chemistry, Washington University in St. Louis, St. Louis, MO 63130,
USA. E-mail: moeller@wustl.edu
†
Electronic supplementary information (ESI) available: A sample experimental
procedure is included for the oxidation reaction along with characterization data
for all new compounds. See DOI: 10.1039/c3gc41650j
Scheme 1
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versatile. During a constant current electrolysis, the potential
at the anode climbs until it matches that of the functional
group with the lowest oxidation potential. The potential at the
anode then holds constant until the majority of the functional
group is consumed. After that point, the potential at the elec-
trode again climbs until it matches that of the functional
group with the second lowest oxidation potential in solution.
By keeping the current density low, the potential at the anode
can be maintained at the level of the first group until 90% or
more of the original functional group is consumed. Substrates
with oxidation potentials of +1.2 V vs. Ag/AgCl (Ep/2) (eqn (1))
or +1.95 V vs. Ag/AgCl (Ep/2) (eqn (2)) can both be oxidized
using the exact same electrochemical apparatus without any
need to manually adjust the potential.
While electrolysis reactions like those illustrated in
Scheme 1 can be very powerful synthetic tools, the same
feature that makes the reactions so versatile with respect to
changing oxidation potentials of the substrates can severely
limit the reactions. For example, if a functional group targeted
for oxidation is not the group in the reaction with the lowest
oxidation potential, then a direct electrolysis approach has no
chance of working. A constant current electrolysis will always
select for the functional group with the lowest oxidation
potential.
Scheme 3
mediator (chemical oxidant). The potential at the anode then
holds constant for the duration of the experiment because the
overall process is catalytic. Since the chemical oxidant is
recycled and never consumed, the oxidation potential at the
anode never climbs. The result is that the same very simple
“
solar-electrochemical” cell used for direct electrolysis reac-
tions should be compatible with the recycling of a wide variety
of chemical oxidants. We report here that this is the case.
Our initial focus was to demonstrate that the indirect
electrolysis reactions already reported in the literature could be
conducted with the use of a simple photovoltaic as the power
supply. To this end, a solar-driven version of the Torii pro-
cedure for conducting electrochemical Sharpless dihydroxyla-
9
tions was examined (Scheme 3). Styrene was introduced
Oxidation reactions that take advantage of chemical
reagents do not have this restriction. A chemical oxidant can
select based on sterics, chirality, binding properties, etc. In
this way, the use of a chemical oxidant offers opportunities for
selectivity in an oxidation reaction that are unavailable to a
direct electrolysis. It was this observation that led to the devel-
into a vial equipped with a reticulated vitreous carbon (RVC)
anode and a carbon rod cathode along with 10 mole% of
potassium ferricyanide, 0.2 mole% of K OsO (OH) , 1 mole%
of (DHQD) PHAL, 3 equiv. of potassium carbonate, and a 1 : 1
2
t-BuOH–H O mixture of solvent. Current (2.5 mA, 2.5 F
mole ) was then passed through the flask using the reaction
setup shown in Fig. 1 to afford a 92% yield of the diol product
in 94% ee. For comparison, the literature yield for the oxi-
dation of styrene with a more traditional electrolysis setup was
2
2
4
2
−
1
8
opment of indirect electrolysis reactions. In an indirect
electrolysis, a chemical oxidant serves as a mediator for the
electrolysis (Scheme 2).
The oxidant formed at the anode oxidizes a substrate in
solution and then is regenerated again at the anode. This
allows the chemical oxidant to be used in a catalytic fashion
while again channeling the necessary reduction reaction
toward the production of hydrogen gas. Hence, the indirect
electrolysis allows one to capitalize on the selectivity of the
chemical reagent while retaining the advantages of the
electrolysis.
Indirect electrolyses often capitalize on constant current
electrolysis conditions. In an indirect oxidation, this means
that the oxidation potential at the surface of the anode again
climbs until it matches that of the most easily oxidized group
in solution. In this case, that group is the reduced form of the
9
5% with a 97% ee. It was clear that the optimized electroche-
mical reaction could be mimicked nicely with the very simple
solar-electrochemical cell, an observation that suggests that
the electrochemical method can be readily adopted by syn-
thetic chemists without any need for specialized equipment.
The exact same reaction setup was also used to run a
mediated anodic oxidation of a sugar substrate. In this case,
the less sterically-hindered primary alcohol of the sugar was
selectively oxidized by using TEMPO as the mediator for the
oxidation (Scheme 4). Once again, the yield of the sunlight-
driven reaction was similar to the yield reported in the litera-
ture for an electrolysis that had utilized more traditional
electrochemical equipment.
1
0
Scheme 2
Scheme 4
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The sunlight-driven reactions can also be used to mimic
more complex electrochemical reactions. In electrolyses that
utilize transition metal mediators, a lack of solubility for the
reduced form of the mediator can represent a significant chal-
lenge. The main issue occurs when the oxidized form of the
mediator encounters the cathode. A reduction at that point
leads to destruction of the oxidant. If the reduced form of the
metal is soluble, then reduction at the cathode is not a major
problem because the mediator will simply be reoxidized at the
anode. The reaction loses current efficiency, but in a sunlight-
driven process this concern is minimal. However, if the
reduced form of the mediator is not soluble, then it is de-
posited on the cathode. This removes the mediator from solu-
tion, a situation that leads to changes in the potential at the
anode and undesired direct oxidation reactions. Two methods
have been employed to address this second scenario. First, the
direction of the current flow in the reaction can be switched.
This change converts the electrode initially used as an anode
to a cathode and the electrode initially used as a cathode to an
anode. For the Ru(VIII)-oxidation shown in Scheme 5, the
optimized literature procedure calls for switching the direction
of the current in the reaction every 30 seconds. Ruthenium
that deposited on the original cathode during the first 30 s is
oxidized when that electrode becomes an anode for the sub-
sequent 30 s. In this way, the mediator is constantly reintro-
duced into the reaction medium. Mimicking the reaction with
a sunlight-driven experiment requires the same switching of
current direction. When the direction of the current was
switched at a rate of once every 30 minutes, the yield of the
reaction with the solar-electrochemical setup was only 40%.
The yield was raised to 82% by switching the direction of the
current every 1 to 2 min. This was done by manually switching
the leads on the setup shown in Fig. 1. Clearly, the reaction
could be further optimized by automating the switching
process so that the 30 s time scale used in the optimized litera-
ture reaction was mimicked more accurately. However, to test
whether an electrochemical reaction in the literature will work
for a specific experiment of interest, this simple method is
very effective.
Scheme 6
Pd(II)-mediated oxidation reactions shown in Scheme 6. Both
reactions were compatible with the use of the simple solar-
electrochemical reaction setup shown in Fig. 1. The electro-
chemical Wacker oxidation (eqn (1)) proceeded in an 81%
isolated yield with the use of a photovoltaic as the power
1
1
12
supply. In a similar fashion, the oxidative Heck reaction
(
eqn (2)) proceeded in a 68% isolated yield with the use of a
13
photovoltaic as the power supply.
In order to further demonstrate the range of reactions that
can be run in a catalytic fashion using the electrochemical
apparatus shown in Fig. 1, both manganese(V)- and cerium(IV)-
mediated electrolyses were conducted (Scheme 7). For Mn(V),
the asymmetric epoxidation of cis-β-methylstyrene was accom-
14
plished. The reaction used Jacobsen’s catalyst as a mediator,
and it afforded a 47–71% yield (NMR) of the product in a 87%
ee. In a similar fashion, the Ce(IV)-mediated deprotection of a
para-methoxybenzyl alcohol proceeded in an 88% yield (NMR)
15
of the product.
For each of the reactions illustrated in Schemes 1–7, the
same very simple sunlight-driven electrochemical apparatus
was used. From direct electrolyses to asymmetric oxidation
reactions, the potential at the working electrode automatically
adjusted to match that of the substrate or mediator. The result
is a very general, readily adoptable method for running sus-
tainable oxidation reactions.
The second approach to circumvent mediator deposition on
the cathode uses a divided cell for the electrolysis. This pre-
vents the chemical oxidant generated at the anode from ever
reaching the cathode. This approach is illustrated with the two
Scheme 5
Scheme 7
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R. D. Little, J. G. Ibanez, A. Palma and R. Vasquez-
Medrano, Green Chem., 2010, 12, 2099.
Acknowledgements
We thank the National Science Foundation (CHE-1151121) and
4 L. A. Anderson, A. Redden and K. D. Moeller, Green Chem.,
2011, 13, 1652.
(CHE-1240194/CenSURF) for their generous support of this
work.
5 The photovoltaic-cells employed in these experiments are
available from Solar-Made.
6
For the specific cyclization reaction shown see: G. Xu and
K. D. Moeller, Org. Lett., 2010, 12, 2590.
References
7
For the oxidation of the t-Boc protected methylproline see:
Y. M. Fobian and K. D. Moeller, Methods Mol. Med., 1999,
23(Peptidomimetic Protocols), 259.
1
For reviews see: (a) A. N. Campbell and S. S. Stahl, Acc.
Chem. Res., 2012, 45(6), 851; (b) R. I. McDonald, G. Liu and
S. S. Stahl, Chem. Rev., 2011, 111(4), 2981;
8 For reviews of indirect electrochemical methods see:
(a) Y. N. Ogivin, M. N. Elinson and G. I. Nikishin, Russ.
Chem. Rev., 2009, 78, 89; (b) J. Simonet and J.-F. Pilard, in
Organic Electrochemistry: Fourth Edition, Revised and
Expanded, ed. H. Lund and O. Hammerich, Marcel Dekker,
New York, 2001, p. 1163; (c) E. Steckhan, Angew. Chem., Int.
Ed. Engl., 1986, 25, 683.
(
c) A. E. Wendlandt, A. M. Suess and S. S. Stahl, Angew.
Chem., Int. Ed., 2011, 50(47), 11062.
2
For selected recent examples see: (a) L. C. John, A. Gunay,
A. J. Wood and M. H. Emmert, Tetrahedron, 2013, 69, 5758;
(
b) A. M. Suess, M. Z. Ertem, C. J. Cramer and S. S. Stahl,
J. Am. Chem. Soc., 2013, 135, 9797; (c) J. Kim and S. S. Stahl,
ACS Catal., 2013, 3, 1652; (d) D. Pun, T. Diao and
9 S. Torii, P. Liu and H. Tanaka, Chem. Lett., 1995, 319.
S. S. Stahl, J. Am. Chem. Soc., 2013, 135, 8213; (e) T. Diao, 10 K. Schnatbaum and H. J. Schäfer, Synthesis, 1999,
D. Pun and S. S. Stahl, J. Am. Chem. Soc., 2013, 135, 8205;
864.
f) A. Rahimi, A. Azarpira, H. Kim, J. Ralph and S. S. Stahl, 11 S. Torri, T. Inokuchi and T. Sugiura, J. Org. Chem., 1986, 51,
(
J. Am. Chem. Soc., 2013, 135, 6415; (g) R. M. Leithall,
155.
V. N. Shetti, S. Maurelli, M. Chiesa, E. Gianotti and R. Raja, 12 T. Inokuchi, L. Ping, F. Hamaue, M. Izawa and S. Torii,
J. Am. Chem. Soc., 2013, 135, 2915; (h) R. J. Angelici, Catal.
Chem. Lett., 1994, 121.
Sci. Technol., 2013, 3, 279; (i) S. Kanbak-Aksu, H. M. Nahid, 13 C. Amatore, C. Cammoun and A. Jutand, Adv. Synth. Catal.,
W. R. Hagen, R. Hollmann, D. Sordi, R. A. Sheldon and
I. W. C. E. Arends, Chem. Commun., 2012, 48, 5745.
For recent reviews of electrochemical reactions see:
2007, 349, 292.
14 H. Tanaka, M. Kuroboshi, H. Takeda, H. Kanada and
S. Torii, J. Electroanal. Chem., 2001, 507, 75.
3
(
a) J. B. Sperry and D. L. Wright, Chem. Soc. Rev., 2006, 35, 15 For the analogous recycling of Ce(IV) at an anode see:
6
05; (b) J. Yoshida, K. Kataoka, R. Horcajada and A. Nagaki,
D. Kesselring, K. Maurer and K. D. Moeller, J. Am. Chem.
Soc., 2008, 130, 11290.
Chem. Rev., 2009, 108, 2265; (c) B. S. Frontana-Uribe,
Green Chem.
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