Electrocatalytic Dealkylation of Amines Mediated by Ferrocene

Article abstract of DOI:10.1021/acs.organomet.9b00557

The homogeneous catalytic oxidation of dicyclohexylamine (DCHA), N,N-dimethylcyclohexylamine (DMCHA) and N,N-dicyclohexylmethylamine (DCHMA) has been investigated in the presence of electrochemically generated ferrocenium ions as the catalyst. Mechanistic details for this electrocatalytic process have been scrutinized with the use of cyclic voltammetry, bulk electrolysis, and digital simulations techniques. A one-electron catalytic process between ferrocene and the respective amines was observed. The products obtained from bulk electrolysis were isolated and identified by FTIR, ¹H and ¹³C NMR spectroscopy, and mass spectrometry. Both DCHMA and DMCHA proceed to yield a secondary amine product by the elimination of one methyl group. In the absence of this group, as in the case of DCHA, the cycloalkyl group is then eliminated. The catalytic efficiency and the second-order rate constants were estimated and found to follow the order DCHA ≤ DMCHA < DCHMA. The results presented in this work should open up a new avenue to achieve simple, low-cost, and efficient amine oxidation, which could be useful in several areas of chemistry.

Full text of DOI:10.1021/acs.organomet.9b00557

Article
pubs.acs.org/Organometallics
Cite This: Organometallics XXXX, XXX, XXX−XXX
Electrocatalytic Dealkylation of Amines Mediated by Ferrocene
Angel A. J. Torriero,* Joanne Morda, and Jessica Saw
School of Life and Environmental Sciences, Deakin University, Burwood, Victoria 3125, Australia
*
S Supporting Information
ABSTRACT: The homogeneous catalytic oxidation of dicyclohexylamine (DCHA), N,N-dimethylcyclohexylamine (DMCHA)
and N,N-dicyclohexylmethylamine (DCHMA) has been investigated in the presence of electrochemically generated
ferrocenium ions as the catalyst. Mechanistic details for this electrocatalytic process have been scrutinized with the use of
cyclic voltammetry, bulk electrolysis, and digital simulations techniques. A one-electron catalytic process between ferrocene and
1
the respective amines was observed. The products obtained from bulk electrolysis were isolated and identified by FTIR, H and
1
3
C NMR spectroscopy, and mass spectrometry. Both DCHMA and DMCHA proceed to yield a secondary amine product by
the elimination of one methyl group. In the absence of this group, as in the case of DCHA, the cycloalkyl group is then
eliminated. The catalytic efficiency and the second-order rate constants were estimated and found to follow the order DCHA ≪
DMCHA < DCHMA. The results presented in this work should open up a new avenue to achieve simple, low-cost, and efficient
amine oxidation, which could be useful in several areas of chemistry.
INTRODUCTION
Electrochemical oxidation of aliphatic amines at conven-
tional electrodes, such as glassy carbon (GC) and platinum,
produces more unusable intermediates than in the case of
aromatic amines due to the lack of delocalization of the radical
cation charge generated on the nitrogen atom. Moreover, the
study of these processes is complicated due to the surface
fouling of the working electrode by oxidation products
■
Amines are a family of chemical compounds that share as a
common feature the presence of at least one sp -hybridized
3
nitrogen atom forming three covalent bonds to other atoms. It
also contains a lone pair of electrons in the remaining
unbounded hybrid orbital. This last set of electrons is
responsible for the chemical and electrochemical reactivity
observed. Amines participate in many important biological and
chemical reactions and hence are of interest in the food,
corrosion, agrochemicals, pharmaceutical, and other indus-
generally observed (most commonly with oxidation products
8,9
of primary amines).
The need to apply very positive
potentials also represents a problem. The potential for
oxidation is dependent on the structure of the aliphatic
amine, with primary amines (1.4−2.0 V vs NHE) being more
difficult to oxidize than secondary or tertiary (1.0−1.3 V vs
1
,2
tries.
The direct electrochemical oxidation of aromatic amines has
been extensively investigated, as the generated intermediate in
these reactions has a tendency to polymerize to yield useful
10−12
NHE) ones.
The catalytic and electrocatalytic oxidation of aliphatic
3
conductive polymers as the final product. Nonetheless, the
amines with reversible redox couple acting as a mediator has
1
3−15
electrochemical oxidation also may produce dimers as the final
product in the great majority of the cases through the
formation of C−C bonds (benzidines), C−N bonds (phenyl-
been much less investigated.
However, it has the
Received: August 16, 2019
1
,4−7
amines), or N−N bonds (hydrazines).
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.organomet.9b00557
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Article
ox
advantage to decrease the overpotential needed for the
oxidation of amines, as well as to minimize the surface fouling
calculated from the average of the oxidation (E ) and
p
red
ox
red
reduction (Ep ) peak potentials (E = 1/2(E + Ep )), are
m
p
1
5
0/+
effect on the working electrodes. This correspondingly
increases the sensitivity and enhances the reliability and
reproducibility of the obtained data. As discussed by Torriero
et al., an ideal catalyst (or mediator) needs to have a standard
reversible potential which is less positive than the oxidation
potential of the substrate, exhibits fast electron-transfer
kinetics, and is stable in both the oxidized and reduced form
0.509 and 0.548 V vs DmFc redox couple in acetonitrile and
dichloromethane containing 0.1 M [Bu N][PF ] as the
4
6
15
supporting electrolyte, respectively.
The CVs for the direct oxidation of DCHMA (Figure 1A,
curve b), DMCHA (Figure 1B, curve b), and DCHA (Figure
1C, curve b) on a glassy carbon electrode were also recorded in
acetonitrile solution containing 0.1 M [Bu N][PF ] as the
4
6
15,16
toward the species present in the reaction media.
Effective
supporting electrolyte. An oxidation peak at respectively
0
/+
mediators who meet these requirements are based on
ruthenium complexes, quinone, and ferrocene (Fc) and their
derivatives, either homogeneously dispersed in the solution or
immobilized in a monolayer or multilayer configuration onto
around 0.734, 0.871, and 1.175 V vs DmFc for DCHMA,
DMCHA, and DCHA (Table 1) were observed when the
potential is scanned in the positive direction. When the
potential scan direction is reversed, no complementary
reduction peak is observed over the potential window and at
the different scan rates studied (from 0.02 to 2 V s ). These
voltammograms are typical for charge transfer processes
coupled with fast irreversible chemical reactions. The direct
electrochemical oxidation of related secondary and tertiary
amines has been reported by Ross and Mann et al. to proceed
12,13,15,17−19
the electrode surface.
Although it is generally
−
1
understood that the oxidation products depend on the type of
oxidant, the reaction medium and the nature of the aliphatic
group attached to the nitrogen atom, little is known about the
products obtained from the catalytic oxidation of aliphatic
amines. Generally, primary amines and symmetric secondaries
21
15,20
22,23
and tertiary aliphatic amines have been studied.
However,
via a one-electron oxidation process.
mechanism will be discussed later.
The reaction
only the work of Torriero et al. provides a comprehensive
electrocatalytic analysis for the oxidation of propylamine,
diethylamine, pyrrolidine, and triethylamine using the electro-
+
The oxidation of Fc to the corresponding Fc , in the
presence of the different cyclohexylamines, was also studied.
Figure 1 provides examples of catalytic voltammograms
observed upon addition of 15 mM DCHMA (Figure 1A,
curve c), 110 mM DMCHA (Figure 1B, curve c) and 400 mM
DCHA (Figure 1C, curve c). It is observed that the Fc
oxidation current intensity was substantially increased, while its
reversible behavior changed to a chemically irreversible one.
Given that the irreversible peak potentials for the direct
oxidation of these amines at the GC electrode surface are more
anodic respect to the Fc oxidation potential (Figures 1, curve b
and Table 1), the increase in the anodic current on CVs c is
attributed to the regeneration of ferrocene resulting from the
+
15
chemically generated ferrocenium (Fc ) ion as the catalyst.
In the present work, mechanistic details of the homogeneous
catalytic oxidation of dicyclohexylamine (DCHA), N,N-
dimethylcyclohexylamine (DMCHA), and N,N-dicyclohexyl-
methylamine (DCHMA, Scheme 1) in acetonitrile and
Scheme 1. Structure of the Redox Catalyst and Amines
under Investigation
+
reaction of Fc with the respective amines, following a classical
15,24
electrochemical catalytic (EC’) mechanism.
Consequently,
the current detected, I, is the sum of two contributions: (i) the
diffusion-controlled current (I ) of Fc in the absence of amine-
d
15
containing compound and (ii) the catalytic current (IL).
Figure 2A shows an example of CVs of 0.75 mM Fc in
CH CN (0.1 M [Bu N][PF ]) in the absence and presence of
3
4
6
increasing concentrations of DMCHA (see Figure S1 for
DCHMA and DCHA behavior). The addition of amines
triggers an increase in the catalytic current. Figure 2B shows
the variations of the respective limiting current as a function of
the concentration of the amine. The catalytic current is
dichloromethane containing 0.1 M [Bu N][PF ] as the
4
6
supporting electrolyte have been investigated by voltammetric
and related techniques when the electrochemically generated
ferrocenium ion is used as the catalyst. The catalytic oxidation
normalized toward I . It is observed that at a given scan rate,
d
I /I increases linearly as a consequence of increasing the
L
d
rate constant, k , the catalytic efficiency, and the influences of
concentration of the amine up to 15, 70, and 120 mM for
DCHMA, DMCHA and DCHA, respectively, deviating from
the linearity at higher concentrations. Simultaneously, the
voltammograms change from a peak-shape to sigmoidal-shaped
f
the experimental conditions on the catalytic oxidation current
are discussed. The catalytic oxidation mechanisms for the
different amines are also discussed in detail.
curves, where the half-wave potential, E1/2, for the sigmoidal
RESULTS AND DISCUSSION
0/+
■
curve coincides with the E value for the Fc couple. Due to
m
Curves a in Figure 1A−C represent the cyclic voltammograms
DCHA solubility limitations, this change was only observed at
−
1
(
CVs) of ferrocene (Fc) in acetonitrile solution containing 0.1
scan rates lower than 20 mV s (Figure S1). Meanwhile, the
shape of the Fc voltammograms loses its S-shape at high
DCHMA concentrations at the expense of a current slope that
appears at high potential values. The origin of this slope on
raising the DCHMA concentration is related to the direct
oxidation of the amine. Thereby, the observed current at high
amine concentrations (≥10 mM DCHMA) is a summary of
M [Bu N][PF ] as the supporting electrolyte. The voltam-
metric properties in this and dichloromethane solvent systems
are consistent with a reversible one-electron redox process to
form ferrocenium ion (Fc ), as previously described.
similar redox behavior can also be observed in a very large
number of solvent systems. The Fc midpoint potentials (Em)
4
6
+
15,16
A
16
B
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Figure 1. CVs obtained with a scan rate of 0.1 V s−1 at a glassy carbon electrode (1.0 mm diameter) for the oxidation of 0.75 mM Fc (A, B, and C,
a), 4.3 mM DCHMA (A, b), 8.3 mM DMCHA (B, b), 5.2 mM DCHA (C, b), and 0.75 mM Fc in the presence of 15 mM DCHMA (A, c), 110
mM DMCHA (B, c), and 400 mM DCHA (C, c) in 0.1 M [Bu N][PF ] acetonitrile. T = 21 ± 1 °C.
4
6
1
5
Table 1
created. From Figure 2A, it is possible to see that the excess
factor, γ = c */c *, increases from 1 to 147 (c * and c * are
a
ΔE (V vs DmFc0/+)
b
kf (M−1 s−1
c
A
Fc
A
Fc
amine
IL/γId
)
the concentrations of amine and ferrocene, respectively).
When the γ values are large enough for the consumption of
amine to be negligible, the classical sigmoidal-shaped catalytic
DCHMA
DMCHA
DCHA
0.734
0.871
1.175
1.023
0.048
0.004
460
55
1.6
response is observed with I following the relationship
L
a
DCHMA = N,N-dicyclohexylmethylamine, DMCHA = N,N-
b
dimethylcyclohexylamine, DCHA = dicyclohexylamine. Difference
between the DmFc midpoint potential and the irreversible oxidation
peak potential for the designated amine compounds. Values obtained
by comparison of digital simulations of CVs and experimental data
over a wide range of scan rates, Fc concentrations, and amine
concentrations.
IL = FAc*Fc
*
D k c
Fc f A
c
where A, F, k , and D are the electrode area, Faraday constant,
f
Fc
rate constant for the forward reaction, and diffusion coefficient
of Fc, respectively. Thus, a purely kinetically controlled
condition is achieved when the homogeneous kinetics is the
rate-limiting step. Conversely, for small values of γ or large
both the previously mentioned catalytic process and the direct
DCHMA oxidation. It is hypothesized that electrode fouling
products of the direct oxidation of DCHMA could be
responsible for the limiting behavior observed (Figure 2B).
Figure 2C presents the voltammograms of Fc in the
presence of an excess of DMCHA at different scan rates. As
previously discussed, a pure catalytic behavior with voltammo-
grams close to S-shapes and limiting current independent of
scan rates, mainly in the range of 0.01−0.1 V s , can only be
observed when the substrate is in large excess respect to the
catalyst and the catalysis is not very fast so that an amine
concentration gradient across the reaction layer is not being
values of k , the consumption of amine is faster than its
f
diffusion from the bulk solution to the electrode, with its
diffusion now being the rate-limiting step. Under these
conditions, a peak is detected when the potential is scanned
in the positive direction and superimposed onto the sigmoidal-
shaped component. The effects of these two parameters are
observed in the voltammetric behavior of DCHMA (Figure
S1).
−
1
To obtain further insights into the catalytic reaction
mechanism, the apparent number of electrons (napp
)
exchanged during the controlled-potential bulk electrolysis of
0.75 mM Fc in the absence and presence of cyclohexylamine-
C
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Article
Figure 2. (A) CVs obtained for 0.75 mM Fc in acetonitrile (0.1 M [Bu N][PF ]) at a glassy carbon electrode (d = 1.0 mm) over the potential
4
6
−
1
region where the Fc oxidation occurs with the addition of 0.0, 20, 40, 70, and 110 mM DMCHA at a scan rate of 0.1 V s . (B) Variation of the
catalytic current (I /I ) with the concentration of added DMCHA, DCHMA, and DCHA obtained at a scan rate of 0.1 V s . (C) CVs obtained for
.75 mM Fc in acetonitrile (0.1 M Bu NPF ) in the presence of 20 mM DMCHA at scan rates of 0.02, 0.05, 0.1, 0.2, 0.3, 0.5, and 0.7 V s .
−
1
L
d
−
1
0
4 6
containing compounds was determined in CH CN and
catalytic reaction. This is in agreement with our previous
observations using propylamine, diethylamine, pyrrolidine, and
triethylamines.
3
CH Cl containing 0.1 M [Bu N][PF ] as the supporting
2
2
4
6
15
electrolyte. To this end, the same procedure as that described
before was implemented, which implies the selection of the
electrolysis potential so that only the catalytic oxidation
CVs of a mixture of 0.5 mM Fc and 2.0 mM DMCHA in
acetonitrile (0.1 M [Bu N][PF ]) before (Figure 3, curve a)
4
6
15
process takes place. Also, the current obtained was recorded
as a function of time until it decayed to about 2% of the initial
value. Under the same conditions, controlled potential
electrolysis experiments were undertaken in the absence of
Fc and cyclohexylamines. The coulombs obtained in the last
experiment were subtracted to the former one to obtain the Fc
faradaic component. As expected, the total coulombs gave napp
and after (Figure 3, curve b) exhaustive controlled-potential
bulk electrolysis was also obtained. The fact that neither
0/+
DMCHA (oxidation process at around 0.871 V vs DmFc in
Figure 3, curve a) nor its oxidation product (a secondary
amine) formed during the catalytic reaction is detected by
cyclic voltammetry at the end of the electrolysis experiment
indicates that they are protonated and present as quaternary
ammonium ions at the end of this experiment. The same result
was observed during the BE of DCHMA and DCHA (Figure
S2). This is in agreement with the observations of Adenier et
al. during their studies on the direct electrooxidation of
=
1.0 ± 0.1 electrons per molecule of Fc in the absence of
amine. However, napp values of 2.1 ± 0.1 and 2.7 ± 0.2
electrons per molecule of Fc were observed for bulk
electrolysis experiments containing 1:1 and 1:2 Fc/amine
ratio, respectively, which implies that one electron per
+
8
molecule of cyclohexylamine is transferred to Fc during the
amines.
D
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Article
react with 4 to yield formaldehyde and protonated secondary
amine 5. This protonation yields an electrochemically inactive
ammonium ion, justifying the observed consumption of one
electron per starting Fc molecule.
Figure 3. CVs obtained before (a) and after (b) exhaustive controlled
0
/+
potential electrolysis at 0.6 V vs DmFc for 0.5 mM Fc in the
presence of 2.0 mM DMCHA in acetonitrile (0.1 M [Bu N][PF ]).
4
6
−
1
Scan rate = 0.1 V s , T = 21 ± 1 °C.
To identify the reaction products, controlled-potential bulk
electrolyses of DMCHA, DCHMA, and DCHA were carried
out in dichloromethane containing 0.1 M [Bu N][PF ] as the
4
6
supporting electrolyte. At the end of the process, the products
were extracted with 3 × 10 mL of Milli-Q water, then treated
with a 40% NaOH aqueous solution to deprotonate the amines
and extracted again with 3 × 10 mL CH Cl . The organic layer
2
2
was then dried and evaporated under vacuum. The oxidation
products for DMCHA, DCHMA, and DCHA were identified
as N-methylcyclohexylamine, N,N-dicyclohexylamine, and
cyclohexylamine, respectively.
Taking into consideration all the previously mentioned
results, it is possible to propose that the electrochemical
oxidation of ferrocene to ferrocenium (eq 1) triggers the
reaction sequence shown in eqs 2−6 (the global unbalanced
reaction is represented in eq 7). In this mechanism, the
dicyclohexylmethylamines transfers one electron to Fc+ to
regenerate Fc (EC′ mechanism) and affords radical cation 1
(eq 2), which can deprotonate following two possible paths to
give a radical. One pathway is the formation of cyclohexyl
radical 2 (eq 3), and the second option is the formation of
methylene radical 3 (eq 4). These radicals (2 or 3, depending
on the amine under study) may be involved in a
disproportionation process to yield the starting amine and 4
Global reaction:
(eq 5). From the BE results of DMCHA and DCHMA, it is
possible to observe that the formation of radical 3 is
preferential over that of radical 2. However, in the absence
of methyl groups, the formation of radical 2 occurs, as
observable from the BE of DCHA. This may be explained by
2
considering that planarity at iminium intermediate 4 (sp -
hybridized carbon) is required (eq 5), which will be favored in
the methyl group rather than in the cyclohexyl functional
group. It seems worthwhile to point out that in the case of
radical 3, an enamine intermediate is not possible to be
formed. Nevertheless, the dealkylation process still takes place,
and the secondary amine is obtained as the reaction product.
This is in agreement with observations formulated by Ross
The second-order rate constants, k , for homogeneous
electron-transfer reactions between Fc and the amines (eq
2) under study were estimated by comparison of experimental
f
23
+
with similar tertiary amines. Water molecules, present in the
organic solvent or added during the product extraction, can
E
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−
1
Figure 4. Comparison of experimental (−) and simulated (○) CVs obtained with a scan rate of 0.02 V s at a glassy carbon electrode (1.0 mm
diameter) for the oxidation of (A) 0.75 mM Fc in acetonitrile (0.1 M [Bu N][PF ]) inal the presence of 20 mM DMCHA and (B) 0.85 mM Fc in
4
6
2
−1
5
3
the presence of 2.9 mM DCHMA. Simulation parameters: T = 294 K, A = 0.72 mm , α = 0.5, k° = 1 cm s , K = 1 × 10 , K
= 1 × 10 , k
eq
eq‑DISP
f‑DISP
(
DMCHA) = 10 M− s , k
1
−1
(DCHMA) = 2 M s , E ° = 0.5 V, C = 0.40 μF, D = 2.3 × 10 cm s .
−1 −1
−5
2
−1
f‑DISP
f
dl
Fc
CVs with digitally simulated ones obtained by using the
mechanism outlined in eqs 1, 2 and 5. Figures 4 and S3 show
MATERIALS AND METHODS
■
Reagents and Chemicals. Ferrocene (Fc, 98%, Sigma-Aldrich),
N,N-dimethylcyclohexylamine (DMCHA, 99%, Sigma-Aldrich),
dicyclohexylamine (DCHA, 99%, Sigma-Aldrich), N,N-dicyclohex-
ylmethylamine (DCHMA, 97%, Sigma-Aldrich), tetrabutylammonium
hexafluorophosphate (Bu NPF , ≥99%, Sigma-Aldrich), acetonitrile
−
1
some examples obtained at 20 mVs . The simulations provide
excellent agreement with experimental observations made over
a wide range of catalyst concentrations (0.2−10 mM) and
amine concentrations (0.20−15.0, 1.0−110, and 1.0−400 mM
for DCHMA, DMCHA, and DCHA, respectively), when the
4
6
(CH
CN, ≥99.9%, Merck), and dichloromethane (CH Cl , ≥99.8%,
2 2
3
rate constants of 460, 55, and 1.6 M−
1
s
−1
are employed,
Merck) were used as received from the manufacturer.
Instrumentation and Procedures. Electrochemical experiments
were carried out in a standard three-electrode arrangement with an
respectively (Table 1). Determinations of the rate constants
were also performed at different scan rates (from 0.02 to 0.7 V
s−1) to minimize the interference of natural convection in the
electrochemical response, which is normally observed at scan
Interface1000 electrochemical workstation (Gamry, USA). A+
platinum wire was used as a counter-electrode, and an Ag/Ag
(
0.01 M AgNO , 0.1 M [Bu N][PF ]) double-junction reference
3 4 6
−
1
rates ≤0.05 V s . Significantly, k values decrease in the order
f
electrode and an Ag/AgCl (CH Cl , 0.1 M [Bu N][PF ]) single-
2 2 4 6
DCHMA > DMCHA > DCHA, which is in agreement with
junction quasi-reference electrode were used for studies in acetonitrile
and dichloromethane, respectively. Nevertheless, all potential values
are quoted relative to the decamethylferrocene/decamethyl-
the catalytic efficiency trend (I /γI ) observed (Table 1).
L
d
0
/+
ferrocenium (DmFc ) potential scale, which was used before and
after each experiment to monitor any possible drift of the reference
electrode potential. A glassy-carbon (GC) working electrode with a
CONCLUSIONS
■
This work demonstrates that ferrocene can catalyze the
homogeneous oxidation of amine in acetonitrile and dichloro-
methane containing 0.1 M [Bu N][PF ] as the supporting
2
diameter of 1.0 mm and an effective area of 0.72 mm was used for
cyclic voltammetric experiments. The area of the working electrode
was determined from CVs using the peak current derived as a
function of the square root of scan rate from the oxidation of a 1.0
mM Fc in CH CN (0.1 M Bu NPF ) solution (Fc diffusion
4
6
electrolyte after its initial electrochemical oxidation. It was
possible to propose an oxidation mechanism to account for the
product formation by using cyclic voltammetry, bulk
electrolysis, and digital simulations techniques in association
with the respective product isolation. The catalytic reaction
between ferrocenium and the secondary and tertiary amines
followed a one-electron-oxidation mechanism with a consid-
erable recovery of the catalyst at the end of the experiment.
Both DCHMA and DMCHA proceed to yield a secondary
amine product by the elimination of a methyl group. In
absence of this alkyl group, as in the case of DCHA, the
cycloalkyl group is then eliminated. The catalytic efficiency and
the second-order rate constant for this last reaction is
significantly slower than that observed for DCHMA and
DMCHA and found to follow the order DCHA ≪ DMCHA <
DCHMA. The results presented in this work should open up a
new avenue to achieve simple, low-cost, and efficient amine
oxidation, which could be useful in several areas of chemistry.
3
4
6
−
5
2
−1
coefficient = 2.3 × 10 cm s ) degassed with N and with the
2
25
application of the Randles−Sevcik relationship. Before each
voltammetric experiment, the working electrode was polished with
.3 μm alumina slurry (Buehler, Lake Bluff, IL) on a clean polishing
cloth (Buehler). After polishing, the electrode was rinsed with distilled
water and acetone, and then dried with nitrogen gas.
Controlled-potential bulk electrolyses (BE) were performed both
in acetonitrile and dichloromethane under nitrogen atmosphere
99.999%, BOC Australia) using a reticulated glassy carbon-working
electrode, the previously mentioned reference electrodes, and a
platinum-gauze counter electrode separated from the test solution by
a fine-porosity glass frit. Dichloromethane was used as a solvent
during the BE product identification.
All voltammetric experiments were carried out at ambient
temperature (22 ± 1 °C). No IRu compensation was performed
during the voltammetric experiments. The commercially available
software package DigiElch was used to simulate the voltammetric
0
(
F
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Article
responses. LC-MS experiments were conducted using an LCMS-8040
LC/MS/MS System (Shimadzu). The mass spectrometer was
operated in positive mode at a collision energy of −20 V. Nuclear
magnetic resonance experiments were conducted using a Spinsolve 43
MHz Benchtop NMR Spectrometer (Magritek) and Fourier trans-
form infrared spectroscopy experiments were conducted using an
ALPHA FT-IR Spectrometer (Bruker).
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1
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1
m, 542 w, 428 w. H NMR (CDCl with TMS, 43 MHz; Figures S4)
3
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3
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13
CH ). C NMR (CDCl with TMS, 10 MHz; Figures S5) δ : 58.55
2
3
C
(
2
9
CH), 33.62 (NCH ), 33.33 (2 × NCHCH ), 26.39 (CH CH CH ),
3 2 2 2 2
5.09 (2 × CHCH CH ). LC-MS: m/z (relative intensity): 113 (14),
2 2
(8) Adenier, A.; Chehimi, M. M.; Gallardo, I.; Pinson, J.; Vila, N.
5 (2), 85 (7), 81 (14), 77 (11), 70 (3), 68 (27), 66 (6), 58 (2), 57
Electrochemical Oxidation of Aliphatic Amines and Their Attachment
to Carbon and Metal Surfaces. Langmuir 2004, 20, 8243−8253.
(9) Deinhammer, R. S.; Ho, M.; Anderegg, J. W.; Porter, M. D.
Electrochemical oxidation of amine-containing compounds: a route to
the surface modification of glassy carbon electrodes. Langmuir 1994,
(
20), 56 (13), 55 (1), 45 (5), 43 (100), 39 (13).
Dicyclohexylamine. Molar mass: 181 g/mol. Yield 48%. IR (neat):
920 s, 2851 s, 1448 m, 1126 m, 887 m, 703 m. H NMR (CDCl
1
2
3
with TMS, 43 MHz; Figures S6) δ : 2.50 (m, 2H, 2 × NCH), 2.17−
H
1
3
0
.41 (m, br, 21H, NH, 10 × CH ). C NMR (CDCl with TMS, 10
2
3
1
(
0, 1306−13.
MHz; Figures S7) δ : 53.19 (2 × NCH), 34.65 (4 × NCHCH ),
C
2
10) Steckhan, E. Anodic Oxidation of Nitrogen-Containing
2
6.53 (2 × CH CH CH ), 25.53 (4 × CHCH CH ). LC-MS: m/z
2 2 2 2 2
Compounds. In Organic Electrochemistry; Lund, H., Hammerich, O.,
Eds.; Marcel Dekker: New York, 2001.
(relative intensity): 183 (6), 182 (87), 167 (20), 165 (1), 164 (3),
1
55 (7), 151 (3), 141 (16), 138 (6), 137 (6), 119 (7), 103 (7), 100
(11) Smith, P. J.; Mann, C. K. Electrochemical dealkylation of
(
(
15), 99 (5), 98 (10), 97 (18), 91 (3), 87 (1), 86 (1), 85 (3), 83
100), 82 (4), 80 (3), 75 (3), 73 (9), 59 (3), 55 (5), 45 (3), 30 (5),
aliphatic amines. J. Org. Chem. 1969, 34, 1821−1826.
2
3 (3), 19 (1), 18 (1).
Cyclohexylamine. Molar mass: 99 g/mol. Yield 42%. IR (neat):
366 w, 3278 w, 2924 s, 2854 s, 2750 w, 1606 w, 1460 m, 1370 w,
(12) Barnes, K. K.; Mann, C. K. Electrochemical oxidation of
primary aliphatic amines. J. Org. Chem. 1967, 32, 1474−1479.
(13) Largeron, M.; Neudorffer, A.; Benoit, M.; Fleury, M.-B.
Oxidation of primary aliphatic amines catalyzed by an electro-
generated quinonoid species: A small molecule mimic of amine
oxidases. Proc. Electrochem. Soc. 2003, 2003−12, 185−188.
3
1
1
149 w, 1090 w, 896 m, 829 m, 777 w, 554 w, 466 w. H NMR
(
1
CDCl with TMS, 43 MHz; Figures S8) δ : 2.49 (m, br, H, NCH),
3
H
1
3
.85 to 1.07 (m, br, CH ), 1.15 (s, NH ). C NMR (CDCl with
2 2 3
TMS, 10 MHz; Figures S9) δ : 50.72 (NCH), 37.23 (2 × CHCH ),
(14) Regino, M. C. S.; Brajter-Toth, A. Real time characterization of
catalysis by online electrochemistry/mass spectrometry. Investigation
of quinone electrocatalysis of amine oxidation. Electroanalysis 1999,
C
2
2
6.01 (CH CH CH ), 25.47 (2 × CHCH CH ). LC-MS: m/z
2 2 2 2 2
(
(
relative intensity): 99 (60), 82 (18), 81 (7), 79 (42), 77 (8), 55
26), 54 (14), 53 (26), 45 (7), 43 (100), 40 (8), 39 (10), 27 (8).
1
(
1, 374−379.
15) Torriero, A. A. J.; Shiddiky, M. J. A.; Burgar, I.; Bond, A. M.
ASSOCIATED CONTENT
Supporting Information
The Supporting Information is available free of charge on the
Homogeneous electron-transfer reaction between electrochemically
generated ferrocenium ions and amine containing compounds.
Organometallics 2013, 32, 5731−5739.
■
*
S
.
(16) Torriero, A. A. J., Ed. Electrochemistry in Ionic Liquids. Vol. 1:
ACS Publications website at DOI: 10.1021/acs.organo-
met.9b00557.
Fundamentals. Springer: Switzerland, 2015.
17) Shiddiky, M. J. A.; Torriero, A. A. J.; Zeng, Z.; Spiccia, L.;
(
CVs over the potential region where the Fc oxidation
occurs with the addition of different concentrations of
DCHMA and DCHA, CVs before and after exhaustive
controlled potential electrolysis of Fc in the presence of
DCHMA and DCHA, comparison of experimental and
Bond, A. M. Highly Selective and Sensitive DNA Assay Based on
Electrocatalytic Oxidation of Ferrocene Bearing Zinc(II)-Cyclen
Complexes with Diethylamine. J. Am. Chem. Soc. 2010, 132,
10053−10063.
(18) Gao, Z.-N.; Zhang, J.; Liu, W.-Y. Electrocatalytic oxidation of
N-acetyl-L-cysteine by acetylferrocene at glassy carbon electrode. J.
Electroanal. Chem. 2005, 580, 9−16.
1
13
simulated CVs, H and C NMR spectra of products
PDF)
(
(19) Togni, A.; Hayashi, T. Ferrocenes: Homogeneous Catalysis,
Organic Synthesis, Materials Science; VCH: Weinheim, 2006.
(20) Kumar, C. H. V.; Shivananda, K. N.; Jagadeesh, R. V.; Raju, C.
N. Ruthenium complex catalyzed oxidative conversion of aliphatic
amines to carboxylic acids using bromamine-T: Kinetic and
mechanistic study. J. Mol. Catal. A: Chem. 2009, 311, 23−28.
AUTHOR INFORMATION
Corresponding Author
Tel.: +61 3 9244 6897. E-mail: angel.torriero@deakin.edu.au.
■
*
ORCID
(
21) Bard, A. J.; Faulkner, L. R. Electrochemical Methods:
Fundamentals and Applications; Wiley: New York, 2001.
22) Ross, S. D. The mechanism of anodic dealkylation of aliphatic
amines in acetonitrile. Tetrahedron Lett. 1973, 14, 1237−1240.
23) Portis, L. C.; Bhat, V. V.; Mann, C. K. Electrochemical
Angel A. J. Torriero: 0000-0001-8616-365X
Notes
The authors declare no competing financial interest.
(
(
REFERENCES
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Electrochemistry: An Electrochemical Approach to Electron Transfer
Chemistry; John Wiley & Sons, Inc., 2006.
■
(
1) Lund, H.; Hammerich, O. Organic Electrochemistry, 4th ed.;
Marcel Dekker: New York, 2001.
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Cambridge University Press: Cambridge, U.K., 2004.
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(
(
(25) Torriero, A. A. J.; Siriwardana, A. I.; Bond, A. M.; Burgar, I. M.;
Dunlop, N. F.; Deacon, G. B.; MacFarlane, D. R. Physical and
G
DOI: 10.1021/acs.organomet.9b00557
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
Electrochemical Properties of Thioether-Functionalized Ionic Liquids.
J. Phys. Chem. B 2009, 113, 11222−11231.
H
DOI: 10.1021/acs.organomet.9b00557
Organometallics XXXX, XXX, XXX−XXX