Electrochemical performance of ABNO for oxidation of secondary alcohols in acetonitrile solution

Article abstract of DOI:10.3390/molecules24010100

The ketones was successfully prepared from secondary alcohols using 9-azabicyclo[3.3.1]nonane-N-oxyl (ABNO) as the catalyst and 2,6-lutidine as the base in acetonitrile solution. The electrochemical activity of ABNO for oxidation of 1-phenylethanol was investigated by cyclic voltammetry, in situ Fourier transform infrared spectroscopy (FTIR) and constant current electrolysis experiments. The resulting cyclic voltammetry indicated that ABNO exhibited much higher electrochemical activity when compared with 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) under the similar conditions. A reasonable reaction mechanism of the electrocatalytic oxidation of 1-phenylethanol to acetophenone was proposed. In addition, a series of secondary alcohols could be converted to the corresponding ketones at room temperature in 80-95% isolated yields.

Full text of DOI:10.3390/molecules24010100

molecules
Article
Electrochemical Performance of ABNO for Oxidation
of Secondary Alcohols in Acetonitrile Solution
Pengfei Niu 1,2 , Xin Liu 1,2, Zhenlu Shen 1,* and Meichao Li 1,2,
*
1
College of Chemical Engineering, Zhejiang University of Technology, Hangzhou 310032, China;
niu_pengfei@yeah.net (P.N.); lx370682@163.com (X.L.)
Research Center of Analysis and Measurement, Zhejiang University of Technology, Hangzhou 310032, China
Correspondence: zhenlushen@zjut.edu.cn (Z.S.); limc@zjut.edu.cn (M.L.);
Tel.: +86-571-8832-0416 (Z.S.); +86-571-8832-0568 (M.L.)
2
*
ꢀꢁꢂꢀꢃꢄꢅꢆꢇ
ꢀꢁꢂꢃꢄꢅꢆ
Received: 2 December 2018; Accepted: 27 December 2018; Published: 28 December 2018
Abstract:
The ketones was successfully prepared from secondary alcohols using
9-azabicyclo[3.3.1]nonane-N-oxyl (ABNO) as the catalyst and 2,6-lutidine as the base in acetonitrile
solution. The electrochemical activity of ABNO for oxidation of 1-phenylethanol was investigated
by cyclic voltammetry, in situ Fourier transform infrared spectroscopy (FTIR) and constant current
electrolysis experiments. The resulting cyclic voltammetry indicated that ABNO exhibited much
higher electrochemical activity when compared with 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO)
under the similar conditions. A reasonable reaction mechanism of the electrocatalytic oxidation of
1-phenylethanol to acetophenone was proposed. In addition, a series of secondary alcohols could be
converted to the corresponding ketones at room temperature in 80–95% isolated yields.
Keywords: electrochemical; ABNO; secondary alcohols; in situ FTIR; ketones
1. Introduction
The carbonyl compounds are important intermediates for synthesis of fine chemicals, such as
fragrances, pharmaceuticals, and food additives, and the worldwide demand for carbonyl compounds
is increasing every year [
oxidation of the corresponding primary and secondary alcohols is one of the most fundamental
functional group transformations in organic chemistry [ ]. The stoichiometric or excess amounts of
oxidants containing chromium reagents, manganese reagents, or hypervalent iodine are often required
among the traditional protocols [ 10]. Unfortunately, these oxidants would generate equivalent
wastes and pose a risk to the environment [11 12]. It is well known that green chemistry is a crucial
1–3]. Presently, the preparation of aldehydes and ketones by selective
4–7
8–
,
principle of chemical research and its application in organic synthesis has attracted more and more
attention [13]. From both environmentally benign and sustainable viewpoints, it is a great desire to
develop a green and efficient catalyst for the synthesis of the corresponding carbonyl compounds.
Recently, a stable nitroxyl radical 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) has been
successfully applied to synthesize aldehydes, nitriles, and imines because of its cleaness and
inexpensiveness [14
have been widely used for the synthesis of aldehydes from alcohols [17
–
16]. Several combinations of TEMPO and transition metals, especially Fe and Cu,
29]. Despite of using molecular
–
oxygen as the terminal oxidant, the transition-metal catalyst still exists in these processes. Subsequently,
our group developed several catalytic oxidation systems for synthesizing the carbonyl compounds
without transition metals, such as TEMPO/HBr/tert-butyl nitrite (TBN)/O2, TEMPO/TBN/O2,
TEMPO/2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ)/TBN/O2, etc [30–32]. Although a lot
of meaningful works about the preparation of aldehydes from primary alcohols catalyzed by TEMPO
were studied both in homogeneous and heterogeneous conditions, due to the high steric hindrance
Molecules 2019, 24, 100; doi:10.3390/molecules24010100
www.mdpi.com/journal/molecules

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arising from four methyl group in TEMPO, the synthesis of ketones from secondary alcohols is still a
serious challenge [33–36].
Subsequently, several less hindered nitroxyl radicals, such as 2-azaadamantan-N-oxyl (AZADO),
1-Me-AZADO, and 5-F-AZADO, have been utilized by Iwabuchi and co-workers [37–39]. Owing to
their less steric hindrance around the reaction center, AZADO and its derivatives exhibited high activity
at a low loading for the preparation of carbonyl compounds from selective oxidation of high hindered
alcohols as compared with TEMPO [40]. However, AZADOs should be synthesized through long and
complicated processes. Another nitroxyl radical, 9-azabicyclo[3.3.1]nonane-N-oxyl (ABNO) has the
similar activity with AZADOs and it could be prepared via only four steps [41]. It put forward the
possibility to prepare the corresponding carbonyl compounds from primary and secondary alcohols
with ABNO as the catalyst under mild conditions. Previously, the ABNO/TBN/O2 system for the
aerobic oxidation of secondary alcohols in water has been developed [42]. This reaction should be
performed at 80 ◦C under 0.3–0.5 Mpa of oxygen.
As we all know, the electrochemical method in synthesis possesses the advantages of friendly
environment, low cost, and high atom utilization, paving a new way for production of many
fine chemicals [43–46]. In 2012, Raja et al. disclosed an electrochemical method to achieve the
preparation of aromatic aldehydes with sodium nitrate as an effective redox mediator in biphasic
medium [47]. However, the primary alcohols substituted with strong electron-withdrawing group,
such as -nitro and some of secondary alcohols gave low product yields. Lately, we prepared a
series of TEMPO-modified polymer electrodes, which could be used successfully for the selective
oxidation of alcohols to aldehydes [48–50]. It is well known that the detailed information of the
electrochemical behaviour of organic compounds is crucial for electrochemical synthesis. Stahl’s
group reported that 4-acetamido-TEMPO (ACT) possessed of the superior activity than AZADO and
ABNO at high pH with electrocatalytic studies about the nitroxyl/oxoammonium redox potential [51].
In continuation with our previous work about the synthesis of aldehydes from alcohols, we try to
study the electrochemical performance of ABNO as the catalyst for the selective oxidation of secondary
alcohols in the presence of 2,6-lutidine in acetonitrile solution. In this work, the electrochemical
behaviour of ABNO for the oxidation of 1-phenylethanol to acetophenone on the Pt electrode was
studied to a better understanding of the catalytic properties (Scheme 1).
Scheme 1. The electrochemical synthesis of ketones from secondary alcohols.
2. Results and Discussion
2.1. Cyclic Voltammetric Study for Oxidation of 1-Phenylethanol
The cyclic voltammograms for the oxidation of 1-phenylethanol in 0.1 M NaClO4-CH3CN solution
using ABNO as the catalyst were shown in Figure 1. We first started out investigation in 0.1 M
NaClO4-CH3CN solution containing 1-phenylethanol (1.0 mmol) and 2,6-lutidine (1.0 mmol) at
Pt electrode (Figure 1a). No obvious reaction peaks appeared in the range of scanning potential
between −0.1 and 0.8 V. As shown in Figure 1b, a pair of reversible redox peaks was well centered at
about 0.25 V in the presence of ABNO (0.1 mmol). The oxidation peak at 0.3 V corresponded to the
one-electron oxidation of nitroxyl radical to oxoammonium ion (ABNO+) rather than the response
of 1-phenylethanol or 2,6-lutidine [51]. By the addition of 1-phenylethanol, the reaction was so weak

Molecules 2019, 24, 100
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that we could not detect the obvious change about the oxidation peak current (Figure 1c). However,
when the base 2,6-lutidine was added to the reaction solution, the apparent increase of oxidation peak
current from 1.37 mA to 1.51 mA was observed (Figure 1d). It certified that the active oxoammonium
cations reacted with 1-phenylethanol on the surface of the working electrode and 2,6-lutidine was
beneficial to facilitate the oxidation of 1-phenylethanol [52].
0.15
d
c
0.10
b
0.05
a
0.00
-0.05
-0.2
0.0
0.2
0.4
0.6
0.8
E / V ( vs. Ag / Ag+)
Figure 1. Cyclic voltammograms of Pt electrode in 0.1 M NaClO4-CH3CN solution with (
1-phenylethanol (1.0 mmol) and 2,6-lutidine (1.0 mmol); ( ) active oxoammonium cations (ABNO)
(0.1 mmol); ( ) ABNO (0.1 mmol) and 1-phenylethanol (1.0 mmol); and, (
a)
b
c
d
) ABNO (0.1 mmol),
−1
1-phenylethanol (1.0 mmol) and 2,6-lutidine (1.0 mmol) at the scan rate of 50 mV·s
.
For comparison, other experiments for oxidation of 1-phenylethanol in 0.1 M NaClO4-CH3CN
solution using TEMPO as the catalyst under the similar conditions have been conducted. As shown
in Figure 2, the redox peaks at about 0.34 and 0.22 V could be seen in the presence of TEMPO
(0.1 mmol), which were corresponded to one electron transfer from TEMPO to TEMPO+
[53].
When 1-phenylethanol and 2,6-lutidine were added to the above reaction solution sequentially,
the corresponding cyclic voltammograms were almost overlapped and were omitted in Figure 2 hence.
0.15
0.10
0.05
0.00
-0.05
-0.10
-0.2
0.0
0.2
0.4
0.6
0.8
E / V ( vs. Ag / Ag+)
Figure 2.
Cyclic voltammogram of Pt electrode in 0.1 M NaClO4-CH3CN solution with
−1
2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) (0.1 mmol) at the scan rate of 50 mV·s
.
To futher investigate the electrochemical behavior of ABNO for 1-phenylethanol oxidation,
the cyclic voltammograms with various scan rates were studied and shown in Figure 3A. Due to the
high solution resistance in CH3CN solution, the peak-to-peak separation between the oxidation and
reduction peaks was about 100 mV at the scan rate of 50 mV s
−1, which was slightly more than the
·
59 mV separation [54]. Nevertheless, with the increasing of scanning rate, there was a less shift in the
oxidation peak, suggesting that the electrochemical reaction of ABNO still had good reversibility [55].

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The excellent linear correlation between the oxidation peak current (Ip) and the square root of scan
rate (Figure 3B) exhibited that the electrode reaction was also a diffusion-controlled process, which
provided further evidence for reacting on the surface of working electrode [56].
50 mV.s-1
0.28
0.3
100 mV.s-1
A
B
150 mV.s-1
200 mV.s-1
250 mV.s-1
0.26
0.24
0.22
0.20
0.18
0.16
0.14
0.2
0.1
2
R =0.9996
0.0
-0.1
-0.2
6
8
10
1/2
12
1/2 -1/2
14
16
-0.2
0.0
0.2
0.4
0.6
0.8
E / V ( vs. Ag / Ag+)
v
/mV ⋅s
Figure 3.
(A) Cyclic voltammograms for the oxidation of 1-phenylethanol (1.0 mmol) with ABNO
(
0.1 mmol) and 2,6-lutidine (1.0 mmol) in 0.1 M NaClO4-CH3CN solution at various scan rates.
(B) Linear plot between the oxidation peak current (Ip) and the square root of scan rate.
2.2. In situ FTIR Spectroscopic Analysis
In situ FTIR spectroscopy is one of the most useful techniques in electrochemistry today, and it can
be used to study the organic reaction at the electrode surface. Figure 4 showed the in situ FTIR spectra
that were obtained during the oxidation of 1-phenylethanol in the presence of ABNO and 2,6-lutidine
in 0.1 M NaClO4-CH3CN solution at the scanning potentials varying from 0 to 800 mV. When the
sample potential reached to 200 mV, some weak bands emerged gradually. With the increasing of
potential, the corresponding infrared spectra signals could be seen clearly at approximately 400 mV.
The downward bands at 1646, 1629, and 1587 cm−1 were contributed to ring stretching vibration of
2,6-lutidinium cation [57]. The band at 1176 cm−1 was related to C-H in-plane bending of 2,6-lutidinium
cation [58
,
59]. It confirmed that 2,6-lutidine got a hydrogen proton to become 2,6-lutidinium cation
61]. Meanwhile, three negative-going bands at 1685, 1360,
during the electrocatalytic process [60
,
and 1267 cm−1 were detected, which were attributed to C=O stretching vibration, C-H deformation
vibration in the methyl group, and skeleton vibration of acetophenone, respectively [62]. Moreover, a
weak but important upward band of N-O stretching vibration, showing the participation of ABNO in
this r−eaction, was observed at 1369 cm−1
[
49]. The other band located at 1129 cm−1 was assigned to
ClO4 ions of supporting electrolyte [63
,64]. However, with the interference of the nearby signal peaks
at about 1280 and 1627 cm−1, the ring stretching modes of 2,6-lutidinium cation and N+=O stretching
vibration of oxoammonium ion could not be obviously found [48]. From the above results, we could
get the conclusion that 1-phenylethanol was oxidized to acetophenone with ABNO as the catalyst and
2,6-lutidine as the base.
To further observe the spectral changes with increasing time, the in situ time-resolved FTIR spectra
experiment was carried out during the oxidation of 1-phenylethanol on Pt disk electrode with ABNO
as the catalyst at 400 mV. The resulting spectra were displayed in Figure 5. The seven negative signals
at 1685, 1360, and 1267 cm−1 and at 1645, 1629, 1587, and 1176 cm−1, illustrated to acetophenone and
2,6-lutidinium cation, respectively, could still be seen clearly [58,59,62]. The intensity of these bands
significantly increased with the increasing of time. Meanwhile, no new bands emerged, which showed
that no new reaction occurred on the electrode surface.

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Figure 4. In situ FTIR spectra collected on Pt disk electrode during the oxidation of 1-phenylethanol
(1.0 mmol) in the presence of ABNO (0.1 mmol) and 2,6-lutidine (1.0 mmol) in 0.1 M NaClO4-CH3CN
solution in a short time interval 10 s.
Figure 5. In situ time-resolved FTIR spectra collected on Pt disk electrode during the oxidation of
1-phenylethanol (1.0 mmol) in the presence of ABNO (0.1 mmol) and 2,6-lutidine (1.0 mmol) in 0.1 M
NaClO4-CH3CN solution at 400 mV.
For comparision, the in situ FTIR spectra collected during the oxidation of 1-phenylethanol in the
presence of 2,6-lutidine with TEMPO as the catalyst and without any catalyst in 0.1 M NaClO4-CH3CN
solution under the similar conditions were obtained. The spectra at 400 mV were put together and
presented in Figure 6. As shown in Figure 6a, no additional bands were observed except for the band
of supporting electrolyte at Pt electrode without any catalyst. In addition, with the addition of TEMPO
as the catalyst in the reaction solution, all of the characteristic bands changed weakly (Figure 6b).
Only with ABNO as the catalyst in the reaction solution, the intensity of the bands enhanced markedly,
which was identify with cyclic voltammograms results (Figure 6c). Thus, ABNO could act as an efficient
catalyst for the oxidation of secondary alcohols instead of TEMPO under the same conditions [65,66].

Molecules 2019, 24, 100
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Figure 6. Comparison of in situ FTIR spectra collected on Pt disk electrode during the oxidation
of 1-phenylethanol (1.0 mmol) in the presence of 2,6-lutidine (1.0 mmol) at 400 mV. ( ) without any
catalyst, (b) with TEMPO (0.1 mmol) as the catalyst, (c) with ABNO (0.1 mmol) as the catalyst.
a
Based on the above observation and relevant literatures, a plausible mechanism for the
oxidation of 1-phenylethanol was presented in Scheme 2. The catalyst ABNO could be oxidized
to generate the active oxoammonium cations (ABNO+) via one electronic transfer process
on the anode electrode [36
acetophenone and 9-hydroxy-9-azabicyclo[3.3.1]nonane (ABNOH), accompanied by one hydrogen
proton generation [42,68] Meanwhile, ABNOH was regenerated to ABNO [51 69]. The base
,
67]. Subsequently, 1-phenylethanol reacted with ABNO+ to give
.
,
2,6-lutidine got the hydrogen proton to form 2,6-lutidinium cation during the oxidation reaction [48].
In addition, H2 was generated as the side product on the cathode electrode [54].
Scheme 2. A plausible mechanism for the oxidation of 1-phenylethanol in the presence of ABNO and
2,6-lutidine (M).
2.3. Preparation Electrolysis
Inspired by these results, the scope of this electrochemical method for the oxidation of secondary
alcohols was explored. As shown in Table 1, 1-phenylethanol could be converted completely to
acetophenone in 98% GC internal standard yield (entry 1). Due to the volatility of acetophenone
during the process of purification and concentration, the isolated yield was relatively low (80%).
Meanwhile, we tried to calculate the faradaic efficiency in this electrochemical process. The faradaic

Molecules 2019, 24, 100
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efficiencies with 1-phenylethanol were about 81.4% at 6 h, with 92% conversion and 99% selectivity.
With the reaction proceeding, the concentration of the substrate was decreased and the reaction slowed
down gradually. The faradaic efficiency was about 61.8% at 8.5 h. Similarly, a series of secondary
benzyl alcohols with both electro-donating and electron-withdrawing substituents in phenyl group
were provided in good to excellent isolated yields (entires 2–11). For example, four secondary benzylic
alcohols with electro-donating group were converted into the corresponding ketones with good
isolated yields under the standard conditions (entries 5–8). Meanwhile, halide substituents products
such as chloro-acetophenone (o-, m-, and p-), 4-bromo-acetophenone and 4-fluoro-acetophenone were
all obtained in excellent isolated yields (entries 2–4,10,11). Furthermore, 1-(4-nitrophenyl)ethanol could
afford the target product efficiently (entry 9). When 1-phenylpropan-1-ol was chosen as the substrate,
a full conversion with 88% isolated yield could be achieved (entry 12). Other excellent results were
also obtained, including 1-phenyl-1-butanol and 1-(p-tolyl)propan-1-ol (entries 13 and 14). Notably,
the conversion of 2-methyl-1-phenylpropan-1-ol could reach to 98% by prolonging the reaction time
to 11 h (entry 15). The selectivity to 2-methylpropiophenone was 92% and the main by-product
was benzaldehyde. Next, the polycyclic and heterocyclic substrates were smoothly converted to
the corresponding ketones with satisfying isolated yields (entries 16–18). In order to explore the
applicability of the protocol for unactivated secondary aliphatic alcohols, cycloheptanol and 2-octanol
were tested. Fortunately, the high GC yields of their corresponding ketones were received in 98%
and 90%, respectively (entries 19 and 20).
Table 1. Electrochemical conversion of various secondary alcohols to ketones a.
Conversion b (%)
Selectivity b (%)
Entry
Substrate
Product
Yield c (%)
OH
O
1
>99
>99
80 (98)
OH
2
3
>99
>99
98
97
94
88
Cl
O
Cl
O
OH
Cl
4
5
>99
>99
96
97
87
92
Cl
OH
O
6
>99
97
93
OH
O
7
8
>99
>99
98
97
95
89
OH
O
O
O
9
>99
96
90

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Table 1. Cont.
Conversion b (%)
Selectivity b (%)
Entry
Substrate
Product
Yield c (%)
10
>99
97
85
11
12
>99
99
97
99
98
97
92
99
92
88
92
93
89
90
O
13
98
14
>99
98
15 d
16
>99
OH
17
18
19
>99
>99
99
99
95
80
99
92
99
98
98 e
90 e
20
a
Electrolytic conditions: substrate (1.0 mmol), ABNO (0.1 mmol), 2,6-lutidine (1.0 mmol), 0.1 M NaClO4-CH3CN
solution (15 mL), undivided cell, constant current 10 mA, electrolysis time 8.5 h, room temperature. b Determined by
c
GC with peak area normalization method. Isolated yield. Values in parentheses were determined by GC internal
standard method. Electrolysis time 11 h. Yields were calculated based on the GC conversion and selectivity.
d
e
3. Materials and Methods
3.1. Catalyst Preparation and Reagents
ABNO was synthesized in our laboratory according to the available procedures [41]. Other
chemicals and solvents were purchased from the supplier and were used as received. In this work,
the racemic secondary alcohols were used.
3.2. Cyclic Voltammetry Study
The cyclic voltammetric study was carried out by using CHI620B electrochemical workstation
(CH Instrument Inc., Austin, TX, USA) with an “L” type Pt electrode as the working electrode. A big
square platinum sheet (1.5 cm in length) and a Ag/Ag+ electrode (0.1 M AgNO3 in acetonitrile)
were employed as the counter electrode and the reference one, respectively. The experiments were
performed in 0.1 M NaClO4-CH3CN solution at the scan rate of 50 mV
·
s
−1 at room temperature. All of
the potentials in this article were referred to the Ag/Ag+ electrode (0.1 M AgNO3 in acetonitrile).

Molecules 2019, 24, 100
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3.3. In Situ FTIR Spectroscopic Study
The in situ infrared spectroscopy experiments were performed on 263 A Potentiostat/Galvanostat
and Nicolet 670 FTIR spectrometer (Thermo Fisher Nicolet, Waltham, MA, USA), equipped with a
refrigerated MCT-A detector and KBr beam splitter. The disk electrode of Pt (0.6 cm in diameter) was
used as the working electrode. A three-electrode spectro-electrochemical cell equipped with CaF2
window as the IR window was used for collecting the interferograms. Each single-beam spectrum was
−1
collected and two hundred interferograms were coadded at a spectral resolution of 8 cm
. The sample
potentials varied in a stepwise fashion from 0 to 800 mV and the reference potential was fixed at
−100 mV [70–72].
3.4. Preparation Electrolysis Experiments
The preparative electrolysis experiments were conducted with in an undivided cell containing
0.1 M NaClO4-CH3CN solution (15 mL), alcohol substrate (1.0 mmol), ABNO (0.1 mmol),
and 2,6-lutidine (1.0 mmol) at a constant current of 10.0 mA with moderate magnetic stirring for
8.5 h in the atmosphere. Two square platinum sheets were employed as the anode and cathode,
respectively. The electrolytic reaction was monitored by gas chromatography (GC) on a GC-2010
system (Shimadzu, Kyoto, Japan) equipped with a SH-Rtx-Was polar column and a flame ionization
detector (FID). Both the injector and detector were maintained at 220 ◦C, the carrier gas is nitrogen,
◦
and the flow rate is 1.2 mL/min. The initial oven temperature of 100 C was held for 2 min and
◦
◦
then ramped up at 15 C per min to 220 C. This final temperature was held for 8 min. After the
reaction was finished, the resulting mixture was concentrated in a rotary evaporator (Heidolph,
Schwabach, Germany) and purified by column chromatography on silica gel using petroleum and
ethyl acetate 15:1) as eluent to afford the products. The products were confirmed by GC-MS, 1H-NMR,
and 13C-NMR. NMR spectroscopy was carried out on a Bruker Avance III spectrometer (Bruker,
Fällanden, Switzerland). The GC-MS analysis was measured on Thermo Trace ISQ instrument (Thermo
Fisher Nicolet, Waltham, MA, USA) with TG 5MS capillary column.
1
Acetophenone (colorless oil, yield 80%): H-NMR (500 MHz, CDCl3)
δ
7.94–7.92 (m, 2H), 7.55–7.51
(m,1H), 7.44–7.41 (m, 2H), 2.57 (s, 3H). 13C-NMR (125 MHz, CDCl3)
δ 198.0, 136.9, 132.9, 128.4, 128.1,
26.4. GC-MS (EI): m/z: 120.14 [M+].
1
1-(4-Chlorophenyl)ethanone (colorless oil, yield 94%): H-NMR (500 MHz, CDCl3)
δ
7.89–7.87 (m, 2H),
7.42–7.41 (m, 2H), 2.57 (s, 3H). 13C-NMR (125 MHz, CDCl3)
δ 196.7, 139.5, 135.4, 129.7, 128.8, 26.5.
GC-MS (EI): m/z: 154.03 [M+].
1
1-(3-Chlorophenyl)ethanone (colorless oil, yield 88%): H-NMR (500 MHz, CDCl3)
δ
7.92–7.91 (m, 1H),
7.83–7.81 (m, 1H), 7.54–7.51 (m, 1H), 7.42–7.39 (m, 1H), 2.59 (s, 3H). 13C-NMR (125 MHz, CDCl3)
δ 196.6, 138.6, 134.9, 133.0, 129.9, 128.3, 126.4, 26.6. GC-MS (EI): m/z: 154.17 [M+].
1
1-(2-Chlorophenyl)ethanone (colorless oil, yield 87%): H-NMR (500 MHz, CDCl3)
δ
7.56–7.54 (m, 1H),
200.4, 139.1, 132.0,
7.43–7.37 (m, 2H), 7.34–7.31 (m, 1H), 2.65 (s, 3H). 13C-NMR (125 MHz, CDCl3)
131.3, 130.6, 129.4, 126.9, 30.7. GC-MS (EI): m/z: 154.11 [M+].
δ
1
1-(4-Fluorophenyl)ethanone (colorless oil, yield 85%): H-NMR (500 MHz, CDCl3)
δ
8.00–7.96 (m, 2H),
7.15–7.10 (m, 2H), 2.58 (s, 3H). 13C-NMR (125 MHz, CDCl3)
δ 196.4, 165.8 (d, J = 254.8 Hz), 133.6 (d,
J = 2.4 Hz), 130.9 (d, J = 9.3 Hz), 115.6 (d, J = 21.9 Hz), 26.5. GC-MS (EI): m/z: 138.06 [M+].
1
1-(4-Bromophenyl)ethanone (white solid, yield 92%): H-NMR (500 MHz, CDCl3)
δ
7.83–7.80 (m, 2H),
7.61–7.59 (m, 2H), 2.58 (s, 3H). 13C-NMR (125 MHz, CDCl3)
δ 196.9, 135.8, 131.9, 129.8, 128.3, 26.5.
GC-MS (EI): m/z: 197.98, 200.04 [M+].
1
1-(4-Methylphenyl)ethanone (colorless oil, yield 92%): H-NMR (500 MHz, CDCl3)
δ
7.86–7.84(m, 2H),
7.28–7.24 (m, 2H), 2.56 (s, 3H), 2.40 (s, 3H). 13C-NMR (125 MHz, CDCl3)
δ 197.7, 143.8, 134.7, 129.2,
128.4, 26.4, 21.5. GC-MS (EI): m/z: 134.15 [M+].

Molecules 2019, 24, 100
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1
1-(3-Methylphenyl)ethanone (colorless oil, yield 93%): H-NMR (500 MHz, CDCl3)
δ
7.77–7.74(m, 2H),
7.38–7.33 (m, 2H), 2.59 (s, 3H), 2.41 (s, 3H). 13C-NMR (125 MHz, CDCl3)
128.7, 128.4, 125.5, 26.6, 21.2. GC-MS (EI): m/z: 134.10 [M+].
δ 198.3, 138.3, 137.1, 133.8,
1
1-(2-Methylphenyl)ethanone (colorless oil, yield 95%): H-NMR (500 MHz, CDCl3)
δ
7.71–7.69(m, 1H),
7.40–7.36 (m, 1H), 7.28–7.24 (m, 2H), 2.58 (s, 3H), 2.54 (s, 3H). 13C-NMR (125 MHz, CDCl3)
138.3, 137.6, 132.0, 131.5, 129.3, 125.6, 29.5, 21.5. GC-MS (EI): m/z: 134.09 [M+].
δ 201.7,
1
1-(4-Methoxyphenyl)ethanone (colorless oil, yield 89%): H-NMR (500 MHz, CDCl3)
δ
7.94–7.91 (m, 2H),
6.94–6.91 (m, 2H), 3.86 (s, 3H), 2.55 (s, 3H). 13C-NMR (125 MHz, CDCl3)
δ 196.7, 163.5, 130.5, 130.3,
113.6, 55.4, 26.3. GC-MS (EI): m/z: 150.14 [M+].
1
1-(4-Nitrophenyl)ethanone (yellow crystal powder, yield 90%): H-NMR (500 MHz, CDCl3)
δ
8.28–8.27
(m, 2H),
δ
8.11–80.08 (m, 2H),
δ
2.66 (s, 3H); 13C-NMR (125 MHz, CDCl3)
δ
196.3, 150.3, 141.3, 129.2,
123.7, 26.9. GC-MS (EI): m/z: 165.05 [M+].
1
Propiophenone (colorless oil, yield 88%): H-NMR (500 MHz, CDCl3)
δ 7.99–7.97 (m, 2H), 7.58–7.55(m,
1H), 7.49–7.45 (m, 2H), 3.04–3.00 (m, 2H), 1.24 (t, J = 7.2 Hz, 3H). 13C-NMR (125 MHz, CDCl3)
137.0, 132.9, 128.6, 128.0, 31.8, 8.3. GC-MS (EI): m/z: 134.16 [M+].
δ 200.9,
1
Phenylbutanone (colorless oil, yield 92%): H-NMR (500 MHz, CDCl3)
δ 7.95–7.93 (m, 2H), 7.54–7.50(m,
1H), 7.44–7.41 (m, 2H), 2.94–2.91 (m, 2H), 1.79–1.72 (m, 2H), 0.99 (t, J = 7.4 Hz, 3H). 13C-NMR (125 MHz,
CDCl3) δ 200.2, 137.0, 132.7, 128.4, 127.9, 40.3, 17.6, 13.7. GC-MS (EI): m/z: 148.17 [M+].
2-Methylpropiophenone (colorless oil, yield 89%): 1H-NMR (500 MHz, CDCl3)
δ 7.98–7.96 (m, 2H),
7.58–7.54 (m, 1H), 7.49–7.46 (m, 2H), 3.60–3.54 (m, 1H), 1.23 (d, J = 6.9 Hz, 6H). 13C-NMR (125 MHz
CDCl3) δ 204.5, 136.3, 132.8, 128.6, 128.3, 35.3, 19.1. GC-MS (EI): m/z: 148.18 [M+].
,
1-(4-Methylphenyl)propan-2-one (colorless oil, yield 93%): 1H-NMR (500 MHz, CDCl3)
δ 7.87 (d,
J = 8.2 Hz, 2H), 7.28–7.25 (m, 2H), 3.00–2.96 (m, 2H), 2.41 (s, 3H), 1.22 (t, J = 7.3 Hz, 3H). 13C-NMR
(125 MHz, CDCl3) δ 200.5, 143.5, 134.4, 129.2, 128.1, 31.6, 21.5, 8.3. GC-MS (EI): m/z: 148.16 [M+].
1
1-Tetralone (white solid, yield 90%): H-NMR (500 MHz, CDCl3)
δ 8.04–8.03 (m, 1H), 7.48–7.45 (m, 1H),
7.30 (t, J = 7.5 Hz, 1H), 7.25 (d, J = 7.6 Hz, 1H), 2.97 (t, J = 6.1 Hz, 2H), 2.67–2.64 (m, 2H), 2.17–2.12 (m,
2H). 13C-NMR (125 MHz, CDCl3)
(EI): m/z: 146.17 [M+].
δ
198.3, 144.4, 133.3, 132.6, 128.7, 127.1, 126.6, 39.1, 29.7, 23.2. GC-MS
1
Benzophenone (white solid, yield 95%): H-NMR (500 MHz, CDCl3)
J = 7.4 Hz, 2H), 7.49 (t, J = 7.6 Hz, 4H). 13C-NMR (125 MHz, CDCl3)
GC-MS (EI): m/z: 182.00 [M+].
δ
δ
7.82 (d, J = 7.4 Hz, 4H), 7.60 (t,
196.7, 137.6, 132.4, 130.0, 128.2.
1
1-(Thiophen-2-yl)ethanone (yellow oil, yield 80%): H-NMR (500 MHz, CDCl3)
7.63–7.62 (m, 1H), 7.12–7.10 (m, 1H), 2.55 (s, 3H). 13C-NMR (125 MHz, CDCl3)
132.4, 128.0, 26.8. GC-MS (EI): m/z: 125.97 [M+].
δ
δ
7.69–7.68 (m, 1H),
190.6, 144.5, 133.7,
1
Cycloheptanone (colorless oil, yield 57%): H-NMR (500 MHz, CDCl3)
δ 2.49–2.47 (m, 4H), 1.70–1.64 (m,
8H); 13C-NMR (125 MHz, CDCl3) δ 215.3, 43.8, 30.4, 24.3. GC-MS (EI): m/z: 112.11 [M+].
4. Conclusions
In conclusion, the electrochemical behaviour of the nitroxyl radical ABNO for the oxidation
of 1-phenylethanol in acetonitrile solution have been studied. The electrochemical measurements
revealed that ABNO showed reversible redox behavior and it had lower potential than TEMPO.
The oxoammonium ion (ABNO+) was generated by single-electron oxidation. According to the in situ
FTIR spectra results, the base 2,6-lutidine received a hydrogen proton to become 2,6-lutidinium cation,
which was a crucial process during the synthesis of acetophenone from 1-phenylethanol. Under mild

Molecules 2019, 24, 100
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reaction conditions, a variety of substrates, including aromatic, heteroaromatic, and aliphatic secondary
alcohols could be converted to the corresponding ketones with good to excellent isolated yields.
Author Contributions: Funding acquisition, M.L.; Investigation, P.N. and X.L.; Supervision, Z.S. and M.L.;
Writing–original draft, P.N.; Writing–review & editing, Z.S. and M.L.
Funding: This research was funded by the National Natural Science Foundations of China (21773211 and
21776260), and the Natural Science Foundation of Zhejiang Province (LY17B060007).
Conflicts of Interest: The authors declare no conflict of interest.
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