H. Liu et al. / Journal of Catalysis 346 (2017) 170–179
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Scheme 1. The Cu-catalyzed C(CO)–C(a) bond cleavage of ketones.
catalyst, which is easily reached in laboratory, cheap to use in
industry and safe to handle. The copper catalyst is highly effective
for the oxidative cleavage of aryl ketones, aliphatic ketones as well
as sustainable lignin b–O–4 ketones to acids in acetonitrile. Mech-
anistic studies show that ketone is activated by Cu(NO3)2 catalyst
2.2.3. The oxidation of aliphatic methyl ketones
The procedures and amounts of materials were the same as
depicted above. After reactions, the mixture was diluted with
0.005 M H2SO4 aqueous solution to certain volume. The products
were identified and quantified using High Performance Liquid
Chromatography (HPLC) of an Agilent 1260 Infinity System
equipped with an Hi-plex H column (8 um, 300 ⁄ 7.7 mm), using
external standard method with RID and UV (210 nm) detectors.
For the oxidation of acetone, 2-pentanone and 2-heptanone, the
product yield was quantified using HPLC (data shown in
Scheme S1).
via
a single electron transfer process, with the following
multiple-step oxidation and CAC cleavage on copper site giving
the final acid.
2. Experimental
moles of certain carboxylic acid product
Yield of acid ¼
2.1. Materials
moles of substrate
All reagents were of analytical-reagent grade purchased from
Aladdin, Alfa, and J&K Chemicals, and were used without further
purification.
moles of carbon atoms in acid product
YC ¼ Yield of carbon ¼
total moles of carbon atoms in substrate
TYC means the total yield of carbon for all the carboxylic acid
products.
2.2. Experimental procedures
2.3. The analytical procedure
2.2.1. The oxidation of acetophenone and substituted aryl ketones
The catalytic reactions were performed in a 15-mL autoclave
reactor with an internal Teflon insert. Typically, 0.5 mmol of
ketone, 0.1 mmol of copper salt and 2 mL of solvent were added
into the reactor. Then, the reactor was charged with 0.6 MPa O2
and heated to 120 °C under magnetic stirring. After cooling to the
room temperature, the reaction mixture was diluted with 4 mL
methanol before analysis. The products were identified and quan-
tified using gas chromatography-mass spectrometry (GC–MS) and
an Agilent 7890A/5975C instrument equipped with an HP-5 MS
column (30 m in length, 0.25 mm in diameter). p-Xylene was used
as the internal standard.
2.3.1. The detection of reaction compounds
For aryl ketones, the carboxylic acid products were esterified to
the methyl esters before analysis using GC–MS. After cooling to
room temperature, excess amount of absolute methanol and cat-
alytic amount of H2SO4 were added and refluxed for 6 h, and then
the esterified sample was analyzed and quantified using GC–MS
and GC. The typical GC traces of the product analysis (oxidation
of acetophenone for 3 h) are shown in Figs. S1 and S2. As for the
isolated yield, after the autoclave was cooled, aqueous HCl solution
(0.1 M) was added into the system. The precipitated solids were
washed with cold water for three times and then vacuum dried
for 10 h at 50 °C. If no obvious solid appeared after acidification,
the mixture was extracted with ethyl acetate (3 ꢀ 20 mL) and the
combined organic layers were rotary evaporated to remove ethyl
acetate. Solid products obtained were vacuum dried for 10 h at
60 °C.
2.2.2. The oxidation of b–O–4 and b–1 lignin model ketones procedure
for preparation of 2-phenoxy-1-phenylethanone (4a)
2-Phenoxy-1-phenylethanone was prepared by the literature
procedures [38,39]. A 350 mL pressure bottle was charged with
phenol (6.9 g, 73 mmol) and K2CO3 (10.4 g, 75 mmol) in acetone
(150 mL) in Ar atmosphere and stirred at room temperature (RT)
for 30 min. To this solution, 2-bromoacetophenone (14.0 g,
70 mmol) was added, and the resulting suspension was stirred at
RT for 16 h, after which the suspension was filtered and concen-
trated in cacuo. The solid was dissolved in ethyl acetate and
washed with NaOH aqueous (5%, 30 mL) and water (30 mL). The
organic phase was dried over anhydrous Na2SO4. The crude pro-
duct was recrystallized from ethanol to give 2-phenoxy-1-
phenylethanone as a white solid in 87% yield. Spectral data were
in accordance with those previously reported. For the other meth-
oxyl substituted 2-phenoxy-1-phenylethanone, the preparation
procedure is the same as described above, except for using differ-
ent stating materials.
2.3.2. Spectroscopic measurements
Electron paramagnetic resonance (EPR) spectra were taken on a
Bruker spectrometer in the X-band at 77 K with a field modulation
of 100 kHz. The microwave frequency was maintained at 9.4 GHz.
The prepared solutions of indicated components were taken out
into a small capillary tube and preserved in liquid nitrogen for
low temperature EPR examination.
Gas products analysis was conducted in a U-type quartz tube
connected to a Mass spectrometer (GSD320 Thermostar). 1H
NMR and 13C NMR spectra of isolated products were recorded with
a Bruker 400 M spectrometer while the NMR spectra of mixtures
for intermediates during reaction were recorded on Bruker 700 M
spectrometer.
The procedure and product detection methods for the oxidation
reactions were unchanged, while the amount of substrate was
reduced to 0.1 mmol, with the molar ratio of catalysts and sub-
strates consistent with the optimized conditions.
2.3.3. Cyclic voltammetry
ACHI 650D Bipotentiostat (Shanghai Chenhua) was used for the
electrochemical measurements in a conventional three-electrode