R. Du et al.
MolecularCatalysis467(2019)24–29
Table 1
Aerobic oxidation of KA oil to caprolactone under various conditions.a
Entry Cata./mol
2Initial
1Initial
:
MeCN/g 1 Conv./%c 2 Sel./
3 Sel./
%
%
%
Scheme 1. Aerobic oxidation of KA oil to caprolactone.
1
2
3
5
1.5
1.5
5
5
30
42
27
28
44
–
75
0
2
14
2
11
30
36
18
–
48
17
–
98
90
78
67
73
11
41
66
–
alcohols catalyzed by NHPI had been studied in an earlier work by Ishii
[32]. This new strategy has been receiving much attention over the past
years, however, it also leaves room for improvement.
3
10
1.5
5
4
5
6
7
8
9
15
5
5
1.5
0
1
5
5
5
5
Inspired by Ishii and coworkers, herein we report a new strategy
which is easy to conduct in one pot to produce caprolactone from KA oil
using molecular oxygen (Scheme 1). A good selectivity of caprolactone
can be readily reached under mild reaction conditions. The facile and
high active catalyst was composed of NHPI and CAN which is abundant
on earth [33]. To our surprise, the CAN played a bifunctional role in
this reaction, which catalyzed not only the cyclohexanol oxidation as a
radical initiator but also the Baeyer-Villiger oxidation as a Lewis acid.
The mechanism research also revealed a unique synergistic catalysis
effect originated from a byproduct and cerium.
5
2
10
5
1.5
1.5
1.5
1.5
0
10
35
5
34
36
52
13
5
a
General conditions, cyclohexanol (1) 10 mmol, 1,1,1,3,3,3-hexafluoro-2-
propanol (HFIP) 30 g, O2 1 atm, 45℃, 600 rpm, reaction was monitored by GC,
biphenyl was used as the internal standard compound.
b
N-hydroxyphthalimide and cerium ammonium nitrate, based on cyclo-
hexanol(1).
c
2. Experimental
1 Conv. = (n1, initial − n1, end) / n1, initial × 100%.
2 Sel. = (n2, end − n2, initial) / (n1, initial − n1, end) × 100%.
3 Sel. = n3, end / (n1, initial − n1, end) × 100%.
25 h.
37 h.
14 h.
50 h.
HFIP 0 g.
Reaction did not occur with 5 mol% catalyst.
Cyclohexanol 0 mmol.
Conversion of cyclohexanone.
Selectivity of oxepane-2,7-dione.
d
e
2.1. Materials
f
g
NHPI (98.5%), Ce(NH4)2(NO3)6 (98%), Ce(NO3)3·6H2O (99.99%),
HFIP (98%), biphenyl (99%), p-toluenesulfonic acid monohydrate (p-
TsOH·H2O, 98%), cyclohexanol (99%) and cyclohexanone (99%), 6-
hydroxyhexanoic acid (97%) were purchased from Energy Chemical.
Acetonitrile (CH3CN, 99%), hydrogen peroxide aqueous solution (30%)
were purchased from Sinopharm Chemical Reagent Co., Ltd. All the
reagents were used directly without further purification. The ultrapure
water was produced by Master-S UV laboratory water purification
system.
h
i
j
k
l
m
n
o
Based on cyclohexanone conversion.
500 MHz and 13C NMR at 125 MHz. Elemental analysis result was ob-
tained by Elementar Vario MICRO cube. A portable pH meter was used
to test the pH variation of 6-hydroxyhexanoic acid with cerium nitrate.
2.2. Oxidation
The aerobic oxidation proceeded in a 100 mL three-necked round
bottom flask with a magnetic stirring bar in it. After the reagents were
added, the flask was purged with pure oxygen for three times by a
membrane pump, after which it was heated to 45℃ by oil bath. The
reaction was monitored by a Shimadzu gas chromatography instrument
(GC-2010) equipped with a capillary column (HP-1, 30 m length,
0.25 mm diameter, 0.25 μm film) and a FID detector. Biphenyl was used
as the internal standard compound. For the reaction of hydrogen per-
oxide and cyclohexanone, a 25 mL single-necked flask was used while
other conditions remained unchanged. GC–MS tests were conducted
with a Shimadzu GCMS-QP2010 instrument. For the reaction exploring
water content effect, one equivalent water (compared to cyclohexanol)
was added at certain time, the other conditions remained unchanged.
Spiro-bisperoxide (7,8,15,16-tetraoxadispiro[5.2.5.2]hexadecane) was
prepared according to the paper [38] demonstration. The rearrange-
ment of spiro-bisperoxide proceeded in a 25 mL single-necked flask
with 5 mol% catalyst added.
3. Results and discussion
3.1. Aerobic oxidation of KA oil to caprolactone with in situ produced
hydrogen peroxide
KA oil is composed of cyclohexanone and cyclohexanol which is
generated from aerobic oxidation of cyclohexane [35,36]. Herein, cy-
clohexanone and cyclohexanol were mixed at certain percentage to
simulate the KA oil [1] for the discussion of ratio effect. We attempted
to couple the H2O2 preparation from aerobic oxidation of cyclohexanol
and the H2O2 utilization for Baeyer-Villiger oxidation of cyclohexanone
in one pot. Table 1 presents the results for aerobic oxidation of KA oil
under various conditions (Scheme 1). As the reaction progressed, the
chemical substances showed a decrease of cyclohexanol (1) along with
the increase of cyclohexanone (2) and caprolactone (3). Under the
optimized conditions (Entry 2), almost quantitive caprolactone was
obtained until 34% cyclohexanol was oxidized. Thereafter, cyclohex-
anone started to increase (Sel.% = 2%) while the selectivity of capro-
lactone holds to 90% based on a 46% conversion of cyclohexanol at
37 h. The oxidization of cyclohexanol became slow and caprolactone
selectivity dropped significantly in the later stage (see supplementary
material). Beside the Baeyer-Villiger oxidation with the in situ produced
hydrogen peroxide, the cyclohexanone was also oxidized by the mole-
cular oxygen. In the absence of cyclohexanol (Entry 11), no hydrogen
peroxide was generated, cyclohexanone was converted to caprolactone
at a low selectivity catalyzed, and oxepane-2,7-dione (Sel.% = 20%)
was recognized as the main byproduct (Scheme 2).
2.3. Characterization
The hydrogen peroxide measurement was performed according to
previous paper [34]. The Fourier Transform Infrared Spectroscopy
(FTIR) was conducted with ThermoFisher Scientific NICOLET iS10 in-
strument. Solid sample was tested as KBr pellets while liquid sample
was tested as a film on the CaF2 salt tablet. The electron paramagnetic
resonance (EPR) spectrum was recorded by a Bruker cw-EPR spectro-
meter (A300) at X band. The modulation frequency was 100 KHz and
power of microwave was 2 mw. The nuclear magnetic resonance (NMR)
tests were conducted on Bruker AVANCE III 500 instrument, 1H NMR at
As a result of the unwanted reaction, the selectivity of caprolactone
25