Interfacial electric field effects on a carbene reaction catalyzed by Rh porphyrins

Article abstract of DOI:10.1021/ja404394z

An intramolecular reaction catalyzed by Rh porphyrins was studied in the presence of interfacial electric fields. 1-Diazo-3,3-dimethyl-5-phenylhex-5-en- 2-one (2) reacts with Rh porphyrins via a putative carbenoid intermediate to form cyclopropanation pro

Full text of DOI:10.1021/ja404394z

Article
pubs.acs.org/JACS
Interfacial Electric Field Effects on a Carbene Reaction Catalyzed by
Rh Porphyrins
Craig F. Gorin, Eugene S. Beh, Quan M. Bui, Graham R. Dick, and Matthew W. Kanan*
Department of Chemistry, Stanford University, Stanford, California 94305, United States
S
* Supporting Information
ABSTRACT: An intramolecular reaction catalyzed by Rh
porphyrins was studied in the presence of interfacial electric
fields. 1-Diazo-3,3-dimethyl-5-phenylhex-5-en-2-one (2) reacts
with Rh porphyrins via a putative carbenoid intermediate to
form cyclopropanation product 3,3-dimethyl-5-
phenylbicyclo[3.1.0]hexan-2-one (3) and insertion product
3,3-dimethyl-2,3-dihydro-[1,1′-biphenyl]-4(1H)-one (4). To
study this reaction in the presence of an interfacial electric
field, Si electrodes coated with thin films of insulating dielectric
layers were used as the opposing walls of a reaction vessel, and
Rh porphyrin catalysts were localized to the dielectric−electrolyte interface. The charge density was varied at the interface by
changing the voltage across the two electrodes. The product ratio was analyzed as a function of the applied voltage and the
surface chemistry of the dielectric layer. In the absence of an applied voltage, the ratio of 3:4 was approximately 10:1. With a
TiO2 surface, application of a voltage induced a Rh porphyrin−TiO2 interaction that resulted in an increase in the 3:4 ratio to a
maximum in which 4 was nearly completely suppressed (>100:1). With an Al2O3 surface or an alkylphosphonate-coated surface,
the voltage caused a decrease in the 3:4 ratio, with a maximum effect of lowering the ratio to 1:2. The voltage-induced decrease in
the 3:4 ratio in the absence of TiO2 was consistent with a field−dipole effect that changed the difference in activation energies for
the product-determining step to favor product 4. Effects were observed for porphyrin catalysts localized to the electrode−
electrolyte interface either through covalent attachment or surface adsorption, enabling the selectivity to be controlled with
unfunctionalized Rh porphyrins. The magnitude of the selectivity change was limited by the maximum interfacial charge density
that could be attained before dielectric breakdown.
to several kcal/mol, an amount sufficient to change the
selectivity by orders of magnitude at room temperature.1−3 In
practice, applying a large electric field to a reaction is
challenging because large voltages applied over macroscopic
distances result in dielectric breakdown. Field strengths on the
order of 0.01−0.1 V/nm can be applied to thin (∼μm) frozen
solutions before breakdown.4 These fields have been shown to
affect the rates of electron transfer reactions for which there are
large changes in dipole moment.5,6
Large electric fields can be applied over molecular length
scales by controlling the placement of ionic and polar
molecules. This concept has been extensively studied in
biological systems. A combination of experimental and
computational studies has indicated that oriented electric fields
on the order of multiple V/nm are created in enzyme active
sites by the placement of polar residues.7−9 These fields may
substantially decrease activation barriers in enzymatic catal-
ysis.10,11 In an example from heterogeneous catalysis, local
electric fields of 1−10 V/nm that arise from proximal cations in
zeolite cavities have been exploited to accelerate photocatalytic
hydrocarbon oxidations.12,13 To exploit field−dipole interac-
INTRODUCTION
■
The development of a selective catalyst for a chemical reaction
requires control over the relative energies of competing
activation barriers. This challenge has typically been addressed
from a structure−activity perspective. Specifically, chemists
have focused on identifying the steric and electronic properties
of the catalyst that result in the lowest barriers for a desired
pathway. A complementary strategy is to change the local
environment in which a reaction takes place. While researchers
commonly screen different solvents to optimize a reaction,
methods to alter the reaction environment independently of
changing the solvent are not generally available. In particular,
there has been very little effort to control the local electrostatic
environment in the vicinity of an active catalyst. A strong local
electric field could in principle change the outcome of a
reaction through field−dipole interactions with the catalyst,
reactants, or solvent molecules surrounding them. If local
electric fields could be applied by adjusting an external
parameter, these effects could be widely exploited to control
selectivity.
The concept of electric field control has been explored
computationally. Density functional theory studies have
concluded that constant electric fields on the order of 1−10
V/nm perturb activation barriers for gas-phase reactions by up
Received: May 2, 2013
© XXXX American Chemical Society
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Figure 1. Parallel plate cell. Cross-sectional illustration of the cell (top) and expanded illustration of the metal oxide−electrolyte interfaces with
localized Rh porphyrin catalyst (bottom) at 0 V (left) and positive voltage (right). Significant V-dependent electric fields are present only at the
interfaces.
tions in the most general manner, however, an easy way of
applying large local electric fields to reaction centers via an
externally adjustable parameter is needed. One possibility is to
exploit the electric fields that are present in the double layer
region at the interface between a polarized electrode and an
electrolyte solution.14 These interfacial fields arise from the
combination of surface charge density on the polarized
electrode and accumulation of oppositely charged ions from
the electrolyte solution. The polarity and magnitude of these
interfacial fields are controlled by the voltage applied to the
electrode. By attaching molecules to electrode surfaces,
previous studies have shown that interfacial electric fields affect
electron transfer rates to enzymes15,16 and the position of a
tautomerization equilibrium of a synthetic molecule.17
Exploiting interfacial electric fields to control selectivity in
catalysis requires confining catalytic reactions to polarized
interfaces and suppressing undesired electron transfer pro-
cesses. We recently developed a reaction vessel, the “parallel
plate cell”, that enables thin-film metal oxide catalysts to be
subjected to interfacial electric fields. Using this cell, we showed
that the selectivity of an Al2O3-catalyzed epoxide rearrangement
changed by up to a factor of 63 as the charge density at the
Al2O3−solution interface was increased.18 The selectivity
changes were consistent with an electrostatic effect on the
competing activation barriers. We hypothesized that a
molecular catalyst could also be subjected to interfacial electric
fields in the parallel plate cell by localizing the catalyst to the
metal oxide−electrolyte interface. The design of a cell suitable
for experiments with a molecular catalyst is shown in Figure 1,
using a Rh porphyrin as an example. Briefly, the parallel plate
cell consists of two heavily doped Si electrodes and a thin
perfluorinated gasket. Each electrode is coated on its polished
side with a thin insulating dielectric. Molecular catalysts are
localized to the interface between the dielectric and the
electrolyte solution through covalent attachment or surface
adsorption. In an assembled cell, the electrodes are pressed
against opposite sides of the gasket, and an electrolyte solution
containing the substrate for a reaction occupies the volume
between the two electrodes. Application of a voltage between
the two electrodes generates electrochemical double layers that
give rise to interfacial electric fields at each electrode−
electrolyte interface.
Here, we describe parallel plate cell reactions to study
electrostatic effects on an intramolecular carbene reaction
catalyzed by Rh porphyrins. Our results demonstrate that the
selectivity of a molecular catalyst can be changed by subjecting
the catalyst to an interfacial electric field.
RESULTS
■
To covalently bind a molecular catalyst to the dielectric−
electrolyte interface in the parallel plate cell, we prepared Rh
tetraarylporphyrin 1, which has four phosphonate esters
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(Scheme 1).19,20 Highly doped Si wafers coated with 45 Å of
atomic layer deposition (ALD)-deposited Al2O3 (“Al2O3/Si
Cells were assembled with one 1/Al2O3/Si electrode, one bare
Al2O3/Si electrode, and 0.5 mM tetrabutylammonium hexa-
fluorophosphate (TBAPF6) in CH2Cl2 between the two
electrodes. The charge density increased in an approximately
linear manner up to 2.6 μC/cm2 at 5 V (Figure 3), beyond
which dielectric breakdown occurred. Similar charge densities
were observed in parallel plate cells with two bare Al2O3/Si
electrodes.
Scheme 1
The reaction chosen for electric field effect studies was a Rh-
catalyzed intramolecular carbene reaction using a diazoketone
as a carbene precursor (Scheme 1).21,22 Carbene reactions are
powerful methods for forming C−C bonds, but controlling
their chemoselectivity is often very challenging.23,24 Diazo-
ketone 2 was selected as a model substrate because it reacts
under mild conditions at low catalyst loadings and forms
predominantly two products. For comparison to subsequent
reactions in the parallel plate cell, the reaction was first
performed in a vial under standard homogeneous conditions. In
a CH2Cl2 solution containing 1 mM diazoketone 2 and 0.5 mM
tetrabutylammonium hexafluorophosphate (TBAPF6), 0.2 mol
% 1 catalyzed the reaction of 2 to form bicyclohexanone 3 and
cyclohexenone 4 in a 9.6:1.0 ratio. A similar ratio was observed
in the absence of electrolyte.
A series of parallel plate cell reactions were performed with
cells comprised of one 1/Al2O3/Si electrode, one bare Al2O3/Si
electrode, and a CH2Cl2 solution containing 1 mM diazoketone
2 and 0.5 mM TBAPF6 (Figure 2). After the parallel plate cell
was assembled, a constant voltage was applied between the
electrodes for 19 h. The cell was subsequently disassembled,
and the product mixture was analyzed by high performance
liquid chromatography (HPLC). Product ratios were analyzed
for voltages ranging from 0 to 5 V (Figure 2). Small leakage
currents of 0−8 nA/cm2 were observed for this voltage range.
Beyond 5 V, leakage currents were much larger and increased
over time, indicating dielectric breakdown. At sub-breakdown
electrodes”) were prepared as described previously.18 1 was
attached to the Al2O3 surface via phosphonate−Al2O3 linkages
by using a variant of available procedures for attaching alkane
phosphonate monolayers to Al2O3 (see the Supporting
Information). The electrode was subsequently sonicated in
CH3OH and THF and soaked in CH2Cl2 to remove multilayers
of 1. X-ray photoelectron spectroscopy (XPS) of the resulting
1-functionalized electrode (“1/Al2O3/Si”, Figure 2) exhibited
the expected Rh, N, and P peaks for 1 in the appropriate ratios
(Figure S2). The Rh:Al ratio was 1:73, which is consistent with
an approximate monolayer coverage. XPS spectra of an
analogous porphyrin-functionalized Al2O3/Si electrode before
and after subjecting it to conditions for a parallel plate cell
reaction (see below) showed no appreciable decrease in surface
coverage (Figures S6, S7), indicating stability of the
phosphonate−Al2O3 linkage.
The voltage-dependent charge densities in the parallel plate
cell were measured by using double-step chronocoulometry.
Figure 2. Schematic of the parallel plate cell reaction with attached 1 (left). Product ratios as a function of voltage for parallel plate cell reactions with
one 1/Al2O3/Si electrode, one bare Al2O3/Si electrode, and a CH2Cl2 solution containing 1 mM diazoketone 2 and 0.5 mM TBAPF6 (right).
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Figure 3. Schematic of electrodes used in parallel plate cell reactions with attached 1 including 1/Al2O3/Si ( ), 1/TiO2/Si (red ), 1/Al2O3/TiO2/
□
□
Si ( ), and 1/TiO2/Al2O3/Si (red ) (top). Chronocoulometric measurement of charge density (σ) at selected voltages in the parallel plate cell
with 0.5 mM TBAPF6 in CH2Cl2 on the various electrodes (bottom left). Product ratios as a function of voltage for parallel plate cell reactions with
one electrode with attached 1, one bare electrode, and a CH2Cl2 solution containing 1 mM diazoketone 2 and 0.5 mM TBAPF6 (bottom right).
voltages, typical conversions after 19 h were 20−30%, and 3
and 4 were the only significant products observed by HPLC. At
0 V, the reaction of 2 formed 3 and 4 in a 10.6:1.0 ratio, similar
to the ratio obtained for the reaction performed in a vial with
homogeneous 1. At |V| < 4.5 V, no effect on the product ratio
was evident. At +4.5 V, however, the 3:4 ratio decreased slightly
to 9.8:1.0. At +5 V, the 3:4 ratio decreased further to 7.2:1.0,
representing a ∼1.5-fold change relative to the reaction at 0 V.
Similar results were observed at −4.5 and −5 V, where the 3:4
ratios were 9.5:1.0 and 6.3:1.0, respectively. Similarity between
selectivity changes at positive and negative voltages was also
observed in our previous study of an Al2O3-catalyzed epoxide
rearrangement reaction in the parallel plate cell.18
Several experiments were performed to assess contributions
of nonelectrostatic factors to the voltage-dependent selectivity
changes in the parallel plate cell (see the Supporting
Information). Parallel plate cell reactions performed with two
bare Al2O3/Si electrodes in the absence of 1 showed no
conversion, even at +5 V. This result indicates that the Al2O3
surface and any electrolytic product generated by the small
leakage currents are incapable of catalyzing the formation of a
carbene from diazoketone 2. Parallel plate cell reactions
performed at +5 V with 1 as described above exhibited product
ratios after 4 h similar to those seen after 19 h, indicating that
the leakage current does not result in the formation of an
electrolytic product that accumulates in the solution and alters
the selectivity of the catalyst. Additionally, when electrodes
functionalized with 1 that had previously been used in parallel
plate cells at +5 V were reused at 0 V with fresh solutions of 2
in the electrolyte, the product ratios for these experiments were
the same as those for a previously unused electrode at 0 V. This
result indicates that the application of a voltage does not lead to
an irreversible redox reaction of the catalyst to generate a
catalytically active species that exhibits different selectivity.
In addition to the applied voltage, the charge density that an
electrode-localized catalyst experiences in the parallel plate cell
depends on the electrical and chemical properties of the
insulating dielectric layer. In an attempt to increase the
magnitude of voltage-dependent selectivity changes, we ex-
plored alternatives to Al2O3. ALD-deposited titania (TiO2) has
a higher dielectric constant than ALD-deposited Al2O3 (32 vs
7).25 We prepared Si wafers coated with 50 Å of ALD-
deposited TiO2. Double-step chronocoulometry with parallel
plate cells assembled from these electrodes (“TiO2/Si”)
exhibited charge densities that increased linearly up to 10.7
μC/cm2 at 4 V, beyond which breakdown occurred (Figure
3). These results suggest that larger interfacial charge densities
are available within the breakdown window for TiO2/Si
electrodes as compared to Al2O3/Si electrodes, although the
extent of oxide charging in the parallel plate cell may differ for
these two interfaces (see Discussion).
1 was attached to a TiO2/Si electrode by using the same
deposition method as for attachment to Al2O3/Si, and the
functionalized electrode (“1/TiO2/Si”) was characterized by
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Figure 4. Schematic of the 1+5/Al2O3/Si and PSS/1+5/Al2O3/Si electrodes (left). Product ratios as a function of voltage for parallel plate cell
■
■
reactions with one electrode with attached 1+5 ( ) or attached PSS/1+5 (red ), one bare electrode, and a CH2Cl2 solution containing 1 mM
diazoketone 2 and 0.5 mM TBAPF6 (right).
XPS. Parallel plate cell reactions were performed in cells
comprised of one 1/TiO2/Si electrode and one bare TiO2/Si
electrode under the conditions described above at voltages
ranging from 0 to 4 V. The product ratio again exhibited
dependence on applied voltage, but in the direction opposite to
that of the Al2O3-coated electrodes (Figure 3). At 0 V, the
rearrangement of 2 proceeded to form 3 and 4 in a 10.1:1.0
ratio, similar to the ratio observed with 1/Al2O3/Si. At |V| < 3
V, no effect on the product ratio was evident. At +3.0 and +3.5
V, the 3:4 ratio increased to 24:1.0 and 75:1.0, respectively. At
+4 V, only product 3 was detectable by HPLC, representing a
3:4 ratio >100:1 and at least a 10-fold change relative to the
reaction at 0 V. Similar selectivity changes in favor of product 3
were observed at negative voltages.
The disparity in the voltage-dependent selectivity changes for
1 on Al2O3 as compared to TiO2 suggests that voltage-
dependent oxide−Rh porphyrin interactions may be operative
in one or both cases. To assess this possibility, we prepared
electrodes coated with oxide stacks.26 Si wafers were coated
with 10 Å of TiO2 atop 40 Å of Al2O3 (“TiO2/Al2O3/Si”) or 10
Å of Al2O3 atop 40 Å of TiO2 (“Al2O3/TiO2/Si”). The
electrodes were functionalized with 1 and characterized as
described above. Double step chronocoulometry measurements
of parallel plate cells with oxide stacks exhibited larger charge
densities than either oxide alone, increasing up to 14 μC/cm2
for Al2O3/TiO2/Si and 13 μC/cm2 for TiO2/Al2O3/Si (Figure
3). A set of parallel plate cell reactions were performed with
cells comprised of a 1/Al2O3/TiO2/Si electrode and an Al2O3/
TiO2/Si electrode under the same conditions described above.
The observed 3:4 ratio at 0 V matched the ratio seen
previously. Application of a voltage induced a change in the
selectivity in the same direction as seen with a 1/Al2O3/Si
electrode, but the magnitude of the change was larger and
required a smaller voltage. The 3:4 ratio reached 4.1:1.0 at +4
V, the maximum voltage before breakdown. A second set of
parallel plate cell reactions were performed with cells comprised
of a 1/TiO2/Al2O3/Si electrode and a bare TiO2/Al2O3/Si
electrode under otherwise identical conditions. The observed
3:4 ratio at 0 V was again the same. Application of a voltage
induced a change in selectivity in the same direction as seen
with a 1/TiO2/Si electrode, but the magnitude of the change
was smaller and required a larger voltage. The 3:4 ratio reached
35:1.0 at +4.5 V, the maximum voltage before breakdown.
These results indicate the direction of the voltage-dependent
selectivity change is dependent on the oxide surface, while the
magnitude depends on the electrical properties of the dielectric
layer.
We hypothesized that greater voltage-dependent charge
densities at the dielectric−electrolyte interface could be
obtained by using electrodes decorated with ionic functionality.
To this end, we codeposited 1 and phosphonate-linked
methylimidazolium bis(trifluoromethanesulfonyl)imide 5 on
Al2O3/Si electrodes to form “1+5/Al2O3/Si” electrodes (Figure
4). XPS indicated approximately a 1:3 ratio of 1:5 on the
surfaces of these electrodes. Parallel plate cell reactions were
performed with cells comprised of a 1+5/Al2O3/Si electrode
and a bare Al2O3/Si electrode under the conditions described
above. At 0 V, the observed 3:4 ratio was 6.6:1.0, significantly
lower than the ratio observed in the absence of 5. This result is
consistent with a perturbed electrostatic environment for 1 in
the vicinity of codeposited 5. Application of a voltage caused a
change in selectivity in the same direction as for electrodes
without 5 but with larger magnitudes. At +4.5 V the ratio was
4.6:1.0, and at +5 V the ratio was 2.6:1.0, representing 1.4- and
2.5-fold changes in selectivity, respectively. Very similar results
were observed at −4.5 and −5 V. In control experiments,
adding 5 to a reaction in a vial catalyzed by 1 had no effect on
the 3:4 ratio.
To further increase the ionic strength of the interfacial
region, we added a polystyrenesulfonate layer to 1+5/Al2O3/Si
electrodes to form “PSS/1+5/Al2O3/Si” electrodes. XPS
indicated that the original triflimide counterions to the
methylimidazolium groups on 1+5/Al2O3/Si electrodes were
exchanged with sulfonate from PSS on PSS/1+5/Al2O3/Si
electrodes (Figure S14). Parallel plate cell reactions were
performed in cells comprised of a PSS/1+5/Al2O3/Si electrode
and a Al2O3/Si electrode under the conditions described above.
Although the addition of the PSS layer decreased the
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Figure 5. Schematic of the parallel plate cell reaction with unattached Rh porphyrin (left). Product ratios as a function of voltage for parallel plate
■
■
□
cell reactions with two Al2O3/Si electrodes ( ) or two TiO2/Si electrodes (red ) or two octylphosphononate/TiO2/Si electrodes (red ) and a
CH2Cl2 solution containing 2 μM 1, 1 mM diazoketone 2, and 0.5 mM TBAPF6 (right). Also shown is the product ratio as a function of voltage for
parallel plate cell reactions with two TiO2/Si electrodes and a CH2Cl2 solution containing 2 μM RhTPP(I)(PPh3), 1 mM diazoketone 2, and 0.5
○
mM TBAPF6 (red ).
conversion of 2, the voltage-dependent selectivity changes were
considerably larger (Figure 4). At 0 V, the 3:4 ratio was 4.4:1.0,
a further decrease from the ratio observed with a 1+5/Al2O3/Si
electrode at 0 V. At −5 V, the ratio was 1.0:1.9, corresponding
to an 8-fold change in selectivity.
oxide surface chemistry. The effect of blocking the oxide surface
with an alkylphosphonate layer was examined. Octylphospho-
nate/Al2O3/Si electrodes were prepared by soaking Al2O3/Si
electrodes in solutions of octylphosphonic acid. Reactions were
performed under the conditions described above with 2 μM
unattached 1 in parallel plate cells comprised of two
octylphosphonate/Al2O3/Si electrodes. Similar voltage-depend-
ent selectivity changes were observed in these reactions as
compared to the reactions with bare Al2O3/Si electrodes,
although the magnitudes were somewhat smaller (see the
Supporting Information). The results suggest that catalyst
localization does not require a bare oxide surface.
A complementary set of parallel plate cell reactions was
performed with unattached 1 and either bare TiO2/Si or
octylphosphonate/TiO2/Si electrodes. With bare TiO2/Si
electrodes and unattached 1, similar voltage-dependent
increases in the 3:4 ratio were observed as compared to
experiments with 1 attached to TiO2. With octylphosphonate/
TiO2/Si electrodes and unattached 1, however, application of a
voltage decreased the 3:4 ratio, mimicking the changes
observed when Al2O3/Si electrodes were used (Figure 5).
Divergent results were also obtained for parallel plate cell
reactions performed with bare TiO2/Si electrodes and either
RhTPP(I) or RhTPP(I)(PPh3) as the catalyst. With RhTPP(I),
the 3:4 ratio increased at +3 and +3.5 V, as seen when
unattached 1 was used as the catalyst. However, with
RhTPP(I)(PPh3), the 3:4 ratio decreased at +3 and +3.5 V,
as seen with unattached 1 when octylphosphonate/TiO2/Si
electrodes were used.
Chemically attaching a catalyst to the oxide surface enforces
its localization to the dielectric−electrolyte interface in the
parallel plate cell. In principle, however, an unattached catalyst
could spontaneously localize to the interface via adsorption on
the surface or by accumulating in the higher ionic strength
solution in the interfacial region. To assess whether voltage-
dependent selectivity changes could be achieved with
unattached catalysts, we performed parallel plate cell reactions
with bare Al2O3/Si electrodes and 2 μM 1 added to the
solution. This concentration corresponds to 50 pmol of 1 per
cm2 of electrode surface, an amount sufficient for approximate
monolayer coverage. Typical conversions after 19 h under these
conditions ranged from 30−50%. At 0 V, the 3:4 ratio was
9.5:1.0, similar to the reaction performed in a vial at higher
catalyst loading. At +4.5 and +5 V, the ratio decreased to
6.6:1.0 and 1.6:1.0, corresponding to 1.4- and 5-fold changes in
selectivity (Figure 5). Similar results were observed with Rh
tetraphenylporphyrin iodide (RhTPP(I)) used as the catalyst
instead of 1. These results suggest that dissolved Rh porphyrins
are able to localize to the electrode−electrolyte interface,27
where they experience the interfacial electrostatic environment
when a voltage is applied. If this model is correct, the voltage-
dependent selectivity changes should be suppressed in the
presence of a molecule that can compete with a Rh porphyrin
for binding to the surface. To test for such an effect, we
repeated the experiments with 2 μM 1 and 50 μM of free base
tetraphenylporphyrin (H2TPP) added to the solution. At +5 V
under these conditions, the 3:4 ratio was 8.9:1.0, very close to
the ratio observed at 0 V.
Additional experiments were performed to assess other
potential effects on the product ratio. Parallel plate cell
reactions performed with any two modified Si electrodes in
the absence of 1 showed no conversion, even at high applied
potential. Parallel plate cell reactions with bare TiO2/Si
electrodes with 2 μM 1 added to the electrolyte were run at
+3.5 V until the current stabilized (5 min), then shorted for the
The use of an unattached catalyst in parallel plate cell
reactions enabled experiments that probed the effect of the
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Scheme 2
remainder of the reaction. The product distribution indicated
no change from 0 V, indicating the proposed 1−TiO2
interaction requires the constant application of voltage. To
test the effect of ionic strength of the solution on selectivity,
reactions were performed in a vial with 1 mM diazoketone 2
and 2 μM 1 in either 50 mM or saturated TBAPF6 in CH2Cl2.
In both cases, 3:4 ratios were obtained that were similar to
those for reactions performed in 0.5 mM TBAPF6. Parallel plate
cell reactions performed with tetraoctylammonium tetrakis[3,5-
3:4 ratio does not require a specific chemical interaction
between the surface and the Rh porphyrin. The selectivity
changes in this direction upon application of a voltage provided
there is no TiO2−Rh porphyrin interaction. This change is
consistent with an electrostatic effect on the reaction in the
polarized interfacial environment, as explained below.
On the basis of previously reported reactions of diazo
substrates with Rh catalysts, a plausible mechanism for the
reaction of 2 is outlined in Scheme 2. The reaction proceeds
through a Rh carbenoid intermediate that either cyclo-
propanates the alkene to form a bicyclohexanone or electro-
philically attacks the alkene to form a zwitterionic intermediate
that subsequently rearranges to the cyclohexenone.30 In a
simple electrostatic model, if reaction of the Rh carbenoid
intermediate were the product-determining step, a local electric
field, E, would change the difference in activation barriers
between the two pathways (ΔΔG⧧) by an amount equal to
E·Δμ, where Δμ is the difference in dipole moments between
the two transition states. We propose that field−dipole
interactions at the polarized interface favor product 4 (lower
the 3:4 ratio) by preferentially stabilizing the activation barrier
for 4 in the product-determining step.
In the parallel plate cell, E arises from the double layer charge
density, charges on the underlying Si electrode that are
balanced by electrolyte ions in the dielectric−electrolyte
interfacial region when a voltage is applied to the cell. The
magnitude and direction of E experienced by a Rh carbenoid
intermediate are expected to fluctuate rapidly relative to the
formation of product as a result of ion motion at the interface
and orientational changes of the intermediate itself. A Rh
carbenoid therefore experiences a distribution of local electric
fields, some of which are aligned to form stabilizing interactions
with a transition state dipole, while others are aligned to form
destabilizing interactions. Stabilizing field−dipole interactions
dominate the effect on selectivity because they accelerate the
product-determining step. Assuming that the transition state for
the formation of the zwitterionic intermediate is more polar
than the transition state for cyclopropanation, field−dipole
interactions favor formation of 4 (the 3:4 ratio decreases)
because larger field−dipole stabilization is attained for the
transition state to 4 than for the transition state to 3.
bis(trifluoromethyl)-phenyl]borate (TOABArF ) as the electro-
4
lyte also demonstrated a change in the product ratio at +5 V to
4.7:1, indicating TBAPF6 is not required for the observed
effects. Finally, mass spectrometry analysis of the reaction
solution after use in a parallel plate cell with bare Al2O3/Si
electrodes and 2 μM RhTPP(I) at 0 and +5 V indicated no
difference in the major catalyst ions present (Figure S15).
DISCUSSION
■
A Rh porphyrin−catalyzed reaction of diazoketone 2 intrinsi-
cally favors cyclopropanation product 3 over insertion product
4 by an approximate ratio of 10:1. The parallel plate cell
experiments demonstrate that when the reaction occurs at the
interface between a dielectric layer and an electrolyte solution,
the selectivity changes when the interface is charged by the
application of a voltage. The direction of the voltage-induced
selectivity change depends on the nature of the dielectric
surface. The use of a dielectric with a TiO2 surface results in an
increase in the 3:4 ratio upon application of a voltage, while the
use of a dielectric with a Al2O3 surface results in a decrease in
the 3:4 ratio. The reactions with unattached Rh porphyrins and
octylphosphonate-coated electrodes shed light on this disparity:
with both coated TiO2 and coated Al2O3 surfaces, application
of a voltage results in a decrease in the 3:4 ratio. The use of
bare TiO2/Si electrodes with a RhTPP(I)(PPh3) catalyst also
results in a decrease of the 3:4 ratio upon application of a
voltage. Together, these results indicate that a voltage-induced
increase in the 3:4 ratio requires a specific chemical interaction
between a TiO2 surface and the Rh porphyrin, which can be
blocked by the presence of an alkylphosphonate layer on TiO2
or a PPh3 ligand on the Rh porphyrin. The effect on the
selectivity is large enough such that at maximum sub-
breakdown voltage, 3 is the exclusively observed product.
Exploiting voltage-induced surface−organometallic catalyst28,29
interactions may be a fruitful strategy for controlling selectivity
in other reactions. In contrast, a voltage-induced decrease in the
The 3:4 ratio decreases at both positive and negative voltages
for 1/Al2O3/Si electrodes. The ratio also decreases at nonzero
voltages when unattached catalysts are used in the parallel plate
cell even though they may adsorb on both the positive and the
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negative electrodes. The electrostatic model described above
can account for this symmetry provided that similar stabilizing
field−dipole interactions are attainable at positive and negative
electrodes.
substrates with a variety of transition metal catalysts, selective
formation of a cyclohexenone product has proven elusive.21
Our results suggest that a Rh porphyrin favors cyclohexenone
product 4 in an appropriate electrostatic environment.
Increasing the preference for 4 beyond the maximum obtained
here (a 3:4 ratio of 1:2) may be possible with the application of
higher electric field strength. Moreover, for this reaction,
voltage-induced selectivity changes in opposite directions are
possible by simply changing the surface chemistry, indicating
that polarized interfaces may be exploited in multiple ways to
access different products. Utilizing other dielectric layers with
enhanced properties should allow for further enhancement of
the effect. In addition, other polarizable interfaces, such as an
immiscible electrolyte−electrolyte interface,32,33 may enable use
of field effects to control selectivity without the limitations of
insulating oxides.
Although E is dynamic on the molecular level, the average
magnitude of E is proportional to the double layer charge
density. If E·Δμ contributes to ΔΔG⧧, the change in product
ratio is expected to exhibit an exponential dependence on the
double layer charge density. Unfortunately, the chemical
complexity of the interface makes it difficult to measure double
layer charge density directly in the parallel plate cell.
Chronocoulometry measures the charge per unit area on the
underlying Si electrode as a function of the voltage (Figure 3).
This measurement does not provide double layer charge
density directly because the charge density on the Si is balanced
by a combination of charges in the double layer and
accumulation of charges in the metal oxide layer. The
amorphous Al2O3 or TiO2 layers in the parallel plate cell can
accumulate positive/negative charges by protonation/deproto-
nation of the oxide itself or coordinated H2O molecules or by
migration of electrolyte ions into the oxide.31 Oxide charging is
evidenced by the relatively slow (several min) discharge
observed upon stepping to 0 V after applying a nonzero
voltage for multiple hours.
Oxide charging provides an explanation for features of the
voltage-dependent selectivity changes in the parallel plate cell
reactions that are otherwise difficult to reconcile with an
electrostatic effect. In all cases, appreciable selectivity changes
are not observed until a threshold voltage is applied despite the
approximately linear increase in charge density with voltage
measured by chronocoulometry. We propose that at subthres-
hold voltages, nearly all of the charge on the Si is balanced by
oxide charging, resulting in very little double layer charge
density and therefore a very small E to perturb the reaction.
After the oxide becomes saturated with charge, additional
voltage results in a significant increase in the double layer
charge density, and effects on the selectivity appear. The
amount of oxide charging at a particular voltage depends on
whether Al2O3, TiO2, or an oxide stack is used. While larger
selectivity changes at lower voltages are observed for 1/Al2O3/
TiO2/Si electrodes as compared to 1/Al2O3/Si electrodes, the
difference is not commensurate with the very large difference in
charge density between the two electrodes (Figure 3). This
result can be explained by a much higher charging capacity for
an Al2O3/TiO2 stack than for Al2O3 alone.
CONCLUSIONS
■
The selectivity of a Rh porphyrin-catalyzed carbene reaction
occurring at a dielectric−electrolyte interface can be changed by
the application of a voltage across the interface. The selectivity
changes are consistent with two distinct phenomena that
depend on the surface chemistry of the dielectric. For a TiO2
surface, a voltage-induced Rh porphyrin−TiO2 interaction
changes the selectivity of the catalyst to favor the cyclo-
propanation product. For other surfaces tested, voltage-induced
interfacial electric fields create field−dipole interactions that
preferentially lower the activation barrier to the insertion
product. The maximum strength of an electric field that can be
applied to a molecular catalyst depends on the interfacial charge
density that can be attained before dielectric breakdown. The
development of interfaces designed for optimal charge density
would further enhance the effects on selectivity.
ASSOCIATED CONTENT
* Supporting Information
■
S
Experimental procedures, characterization data for all com-
pounds, electrode characterization, current density traces,
impedance spectroscopy, mass spectrometry, and additional
control experiments. This material is available free of charge via
the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
mkanan@stanford.edu
■
The maximum electrostatic effect on selectivity that can be
achieved in the parallel plate cell is limited by the maximum
attainable double layer charge density. Oxide charging
attenuates double layer charging at a given voltage, and
dielectric breakdown limits the maximum voltage that can be
applied. Overcoming these limitations will require new designs
for electrode−electrolyte interfaces or alternative interfaces that
can be polarized by the application of a voltage. The larger
selectivity changes observed with 1+5/Al2O3/Si and PSS/1+5/
Al2O3/Si electrodes (Figure 4) suggest that constructing
interfaces with localized ionic functionality is a promising
strategy. The use of alternative dielectrics with higher
capacitance but a lower propensity to accumulate exogenous
charges should enable the attainment of significantly higher
double layer charge densities.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank Stanford University and the Air Force Office of
Scientific Research (FA9550-11-1-0293) for support of this
research. We also thank the Benchmark Stanford Graduate
Fellowship (C.F.G.) and the Althouse Family Stanford
Graduate Fellowship (E.S.B.).
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