The N-Hydroxyphthalimide catalyzed oxidation of cyclohexene to cyclohexenyl hydroperoxide: Reasons for deactivation and stability of the catalyst

Article abstract of DOI:10.1016/j.catcom.2018.06.010

Selective oxidation of cyclohexene to cyclohexenyl hydroperoxide catalyzed by N-hydroxyphthalimide (NHPI) was studied. Kinetic observations and ¹³C and ¹H NMR spectroscopy showed that accumulation of cyclohexenyl hydroperoxide was ac

Full text of DOI:10.1016/j.catcom.2018.06.010

Catalysis Communications 114 (2018) 84–88
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Catalysis Communications
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Short communication
The N-Hydroxyphthalimide catalyzed oxidation of cyclohexene to
cyclohexenyl hydroperoxide: Reasons for deactivation and stability of the
catalyst
⁎
N.I. Kuznetsova , L.I. Kuznetsova, O.A. Yakovina, D.E. Babushkin, B.S. Bal'zhinimaev
Boreskov Institute of Catalysis SB RAS, Russia
A R T I C L E I N F O
A B S T R A C T
Keywords:
Oxidation
N-Hydroxyphthalimide
Cyclohexene
Oxygen
Selective oxidation of cyclohexene to cyclohexenyl hydroperoxide catalyzed by N-hydroxyphthalimide (NHPI)
1
3
1
was studied. Kinetic observations and C and H NMR spectroscopy showed that accumulation of cyclohexenyl
hydroperoxide was accompanied by deactivation of the catalyst. A mechanism of the deactivation through
phthalimido N-oxyl radical (PINO) addition to vinyl position of olefin was suggested. Responsible for the de-
6
activation, PINO-C H10-OOH hydroperoxide adducts were detected in the reaction conditions, and their mole-
Hydroperoxide
cular structure was elucidated by DFT technique. The mechanism explained an unexpected experimental effect,
that is, lowering the oxygen partial pressure improved the stability of NHPI without decrease in production of
cyclohexenyl hydroperoxide.
1. Introduction
catalyst, it forms 2-cyclohexen-1-yl hydroperoxide (CHHP), but the
selectivity was as low as 50% at 6% conversion of cyclohexene [18].
Promising innovations in aerobic liquid-phase oxidation of hydro-
Better selectivity to CHHP (80%) was achieved when the oxidation was
carried out with 2,2′-azobis-(2-methylpropionitrile) initiator [19].
Catalysis by NHPI is often resulted in formation of stable products,
predominantly, allylic ketone. Ishii first performed the aerobic oxida-
tion of cyclohexene with NHPI. 2-Cyclohexen-1-one and 2-cyclohexen-
1-ol with overall selectivity of 43% were obtained at 70 °C in benzo-
nitrile solution [1]. Later, NHPI was used together with anthraquinone
[20] or more reactive 2,3-dichloro-5,6-dicyano-benzoquinone [21]
promoters. CHHP was in both cases obtained as a minor product to-
gether with main product cyclohexen-1-one. In a recent study [22],
NHPI catalyzed oxidation of cyclohexene to CHHP was performed at
carbons are currently associated with the application of organocata-
lysts. N-hydroxyphthalimide (NHPI) is an important and widely used
member of this family, which is capable of catalyzing numerous free-
radical oxidations [1–8]. The free-radical oxidation of hydrocarbons is
initiated by hydrogen abstraction from NHPI to generate reactive
phthalimido N-oxyl radical (PINO) (Reaction (1)). Reactions (2)–(4)
form a catalytic cycle in which a hydrocarbon is converted to hydro-
peroxide.
NHPI → PINO
(1)
(2)
(3)
(4)
PINO + HR → NHPI + R
35 °С. The oxidation was very slow, but substantially accelerated by
UV-irradiation and addition of NI (naphthalene monoimide). Due to the
photo-activation, the catalyst worked at the low temperature which in
turn provided stability of CHHP. We tried to obtain CHHP from cy-
clohexene under conventional catalysis by NHPI and in the absence of
any additives and metal components. Another issue that received little
attention before was possible deactivation of NHPI during the oxidation
of olefins [6]. In particular, nothing is known about stability of NHPI
under oxidation of cyclohexene. Thus, the present study was aimed to
understand the details of the catalytic action of NHPI in cyclohexene to
CHHP oxidation, establish the mechanism of the catalyst deactivation,
and recommend conditions for its better stability.
R + O
2
→ ROO
ROO + NHPI → ROOH + PINO
The organic hydroperoxides are highly demanded reagents for
synthesis of fine organics [9, 10]. Sheldon [11, 12] and other researches
[
13–16] achieved excellent results in the preparation of hydroperoxides
derived from alkylbenzenes in the presence of NHPI catalyst. Similar
information regarding olefins is not very extensive. The aerobic radical
oxidation of olefins is usually directed to allylic position to form allyl
hydroperoxides as primary products. Cyclohexene is the most suscep-
tible to the allylic oxidation among light olefin [17]. In the absence of
⁎
Corresponding author.
E-mail address: kuznina@catalysis.ru (N.I. Kuznetsova).
https://doi.org/10.1016/j.catcom.2018.06.010
Received 12 April 2018; Received in revised form 24 May 2018; Accepted 13 June 2018
Available online 15 June 2018
1566-7367/ © 2018 Elsevier B.V. All rights reserved.

N.I. Kuznetsova et al.
Catalysis Communications 114 (2018) 84–88
2
. Experimental
appeared as the products of reduction of cyclohexane dihydroperoxide
and 2-cyclohexene dihydroperoxide. In most oxidation experiments,
2–
2.1. Oxidation experiments
iodometric titration gave slightly more O
2
groups than combined
amount of hydroxyls in 2-cyclohexen-1-ol and other hydroxyl com-
pounds determined by GC after the treatment of the reaction solutions
Oxidation of cyclohexene in acetonitrile solution of NHPI was per-
formed at 1 bar of O
2
in a glass reactor (V 20 mL, 2.5 mL of solution), or
with Ph
3
P. This small excess could be attributed to dimer and polymer
P to hardly detectable
at variable pressures of air in titanium reactor (V 300 mL, 50 mL of
solution). For convenience, all data were normalized to 2.5 mL of the
reaction solution.
peroxides [18] which were converted by Ph
3
high-boiling compounds. Thus, at the conversion of cyclohexene
around 30%, CHHP was obtained with selectivity of 89% (Run 2 in
Table 1). These conversion/selectivity parameters are comparable with
those achieved in synthesis of other hydroperoxides. For instance, the
NHPI catalyzed oxidation of hydrocarbons gave selectivity in the range
of 90–96% at a conversion of 22–35% for alkylbenzenes (sec-bu-
tylbenzene, cyclohexylbenzene, cumene) [14, 15], and 40% selectivity
at 16% conversion in case of isobutane [23].
2.2. Analysis of the reaction solution
Cyclohexene and oxidation products were analyzed by GC and
GCMS after treatment of the solution with triphenylphosphine (see
Supporting Information). O
2–
2
groups in the products were additionally
determined by iodometric titration. Quantitative analysis of NHPI was
made by HPLC. Transformation of NHPI in the reaction solution was
investigated by H, C and N NMR.
3
.2. Transformation of the NHPI catalyst
1
13
15
HPLC analysis of the reaction solutions showed that 15 to 60% of
The experimental technique is detailed in Supplementary material.
the initial amount of NHPI was lost for the reaction time (Table 1),
probably, due to transformation into inactive species during catalysis.
The gradual degradation of the catalyst caused slowing down in time of
the oxygen consumption, as shown by the curves in Fig.1. (The time-
varying concentration of cyclohexene and oxidation products in the
reaction solution could also contribute to the slowing of the reaction [6,
3. Results and discussion
3.1. Products of cyclohexene oxidation
Typical time dependencies of oxygen consumption during cyclo-
1
6].) The increase of initial NHPI resulted in an increase of cyclohexene
hexene oxidation in a glass reactor (curves 1 and 2) and in titanium
reactor (curve 3) are presented in Fig.1. The curves 1 and 2 showed an
induction period that was shorter in case of addition of a small amount
of CHHP initiator. When the oxidation was conducted in the titanium
reactor (curve 3), it was difficult to follow initial oxygen consumption
because the reactor was heated to the reaction temperature for ap-
proximately 10 min. Most likely, the induction period was short or
absent due to possible initiation of the radical Reaction (1) on metal
parts of the reactor.
conversion (from 12 to 31%) and yield of CHHP (from 10 to 26%, No 2
and 4–8 in Table 1). Amount of NHPI transformed during the reaction
was the greater, the larger was initial NHPI, and the transformation
decreased with a decrease in cyclohexene loading from 4.5 to 2.4 mmol
and oxygen partial pressure from 1 to 0.5 bar (Fig. 2).
To follow the catalytic reaction and transformation of the catalyst in
time, we performed the oxidation in more severe than usual conditions
using higher temperature and oxygen pressure (Runs 10, 11 in Table 1).
Intensive oxygen consumption began immediately as soon as the re-
actor was heated to 70 °C, whereas after 2.5 h a gradual deceleration of
the oxidation process became remarkable (Fig. 3, curves 10, 11). HPLC
analysis of NHPI showed that the rapid oxygen consumption for oxi-
dation of cyclohexene within the first two hours was accompanied by
intensive loss of NHPI (Fig.3, curve NHPI). During this period, 40% of
NHPI disappeared, and then the degradation continued slowly.
Table 1 shows the results of cyclohexene oxidation. In Runs 1, 2 and
3, conversion of cyclohexene for 7–8 h was in the range of 29–31%. The
reductive treatment of the reaction solution with Ph P converted the
3
main oxidation product CHHP to 2-cyclohexen-1-ol. After the treat-
ment, GC determined 2-cyclohexen-1-ol corresponded to CHHP yield of
1
1
8, 26 and 20% in Runs 1, 2, 3, respectively. In addition, 2-cyclohexen-
-one (2.4%), cyclohexene oxide (0.2%), 2-cyclohexene-1,4-diols
(
0.3%, cis- and trans-), 1,4- and 1,2 -cyclohexanediols (0.4%) were
3.3. NMR study of the NHPI transformation
detected by GC in the reaction solution treated with Ph P. Dialcohols
3
Deactivation of NHPI in homogeneous reactions is sometimes as-
sociated with hydrolysis to form phthalic acid. However, we did not
observe formation of a precipitate of poorly soluble in CH CN phthalic
3
acid. No phthalic acid was also detected in the reaction solution by
NMR. Another transformation responsible for deactivation of NHPI
could be recombination of the reactive PINO radicals to inactive dimers
and trimers [8], or irreversible binding of the PINO radicals by any
radical acceptor. The experimentally observed correlation of NHPI de-
gradation not only with initial NHPI, but also with oxygen and cyclo-
hexene concentrations (Fig. 2) indicated both oxygen and cyclohexene
to be involved in deactivation of NHPI. In attempt to understand a
nature of this phenomenon, we considered all possible interactions of
NHPI and olefins other than allylic oxidation. In the presence of strong
oxidants, NHPI formed PINO that reacted with olefins and cycloolefins
by abstraction of an allylic H-atom. In the absence of molecular oxygen,
the allyl radical attached another PINO radical to form monosubstituted
allylic products [24, 25]. Addition of two PINO moieties across double
bond of the olefins occurred under non-radical interaction assisted by
Pb(IV). Due to anaerobic conditions, allylic oxidation was excluded in
both cases.
Fig. 1. Gas consumption versus time. Numbers near the curves correspond to
runs in Table 1: O 1 bar (1, 2) or 4.6 bar (3), addition of CHHP 50 μmol for (2).
Another type of interaction represented by radical addition of tert-
RO• and ROO• to the double bond [26] was also realized by PINO
2
85

N.I. Kuznetsova et al.
Catalysis Communications 114 (2018) 84–88
Table 1
Results of oxidation of cyclohexene.a
Initia-torb
T o
C
PO2 bar
Reaction time min
С
10 converted mmol
O
2
2−
mmol
CHHP mmol
Amount of NHPI, μmol
Run No
6
H
Initial
After reaction
1
2
3
4
5
6
7
8
9
−
+
−
+
+
+
+
+
−
−
−
−
+
60
60
60
60
60
60
60
60
60
70
70
70
60
1
1
4.6
1
1
1
1
1
1
4.8
2.4
0.6
0.5
453
480
420
465
453
455
453
453
480
410
420
420
453
1.1
0.91
1.16
1.19
0.43
0.62
0.76
0.97
1.22
0.70
1.76
1.91
2.02
1.24
0.82
1.16
0.98
0.43
0.58
0.70
0.90
1.15
0.60
1.48
1.58
1.84
1.18
190
176
184
4
35
65
120
255
218
184
184
184
176
155
142
67
2.5
25
53
99
218
205
30
1.30
1.40
0.55
0.74
0.90
1.04
1.40
1.20
2.06
2.02
2.34
1.50
1
1
1
1
0
1
2
3
69
136
154
a
C
6
H
10 4.5 mmol (1–2, 4–8 and 13) or 2.5 mmol (9), or 4.9 mmol (3 and 10–12), CH
3
CN 2 mL.
b
CHHP 50 μmol (0.1 mL CH CN solution containing 50 μmol CHHP and 3.8 μmol NHPI).
3
radical. In the presence of oxygen, the addition of PINO to vinyl posi-
tion of olefin was followed by addition of oxygen molecule to adjacent
carbon to produce keto- and hydroxyl-N-alkoxyphthalimides. This pe-
culiar dioxygenation was described for styrenes, enynes [27–30] and 2-
norbornene [31] the last one being converted to N-(2-hydro-
peroxybicyclo[2.2.1]heptan-2-yloxy)phthalimide. Being promising re-
agents for use in organic synthesis, the products of the dioxygenation
are catalytically inactive due to substantial stability, and their forma-
tion is able to deactivate the NHPI catalyst. Cyclohexene is less sus-
ceptible to the radical addition to the double bond than norbornene and
styrenes. Nevertheless, it was proven the reaction to occur also with
cyclohexene and be responsible for deactivation of NHPI in the system
under study. Whereas products of PINO addition to the reactive olefins
were earlier obtained under specific synthetic conditions, we observed
1
formation of 1,2-adduct to cyclohexene under catalytic conditions.
H
NMR spectrum of the reaction solution contained signals of hydroper-
oxide (ca. 10 ppm) and aromatic protons (7.6–7.9 ppm) referred to
PINO to cyclohexene adduct (Fig. S10). In fact, we detected formation
of two isomeric adducts a nature of those was investigated by careful
NMR analysis of the reaction solution after removal of volatile com-
ponents. Measured by NMR double resonance technique, the value of
Fig. 2. NHPI transformed vs. its initial amount. The reaction conditions are
given in Table 1. Appropriate curves of O
Fig. S5.
2
consumption in time are shown in
15
spin-spin interaction of N nucleus with one of the protons of cyclo-
hexane ring (J 3N H = 2.0 Hz and 3.0 Hz) was characteristic for NeH in-
teraction through two or three single bonds. That was consistent with
connection of the cyclohexane ring with a chain of NeO heteroatoms.
1
13
In addition, the chemical shifts of H and C of the CeH groups bonded
to heteroatoms indicated formation of H-C-O fragments. Thereby, both
characteristics confirmed the presence of H-C-O-N group in the struc-
ture of adducts. The second O-bonded carbon of cyclohexane ring had
13
C signal shifted to a weak field relative to corresponding alcohol,
which indicated hydroperoxide nature of oxygen. The assignment of the
adducts to hydroperoxides was also chemically confirmed in special
experiment by their reduction with d -DMSO to corresponding alco-
6
hols. We observed a mixture of trans-(A) and cis-(B) isomeric 1,2-ad-
ducts with a molar ratio estimated as 3:1 (Scheme 1). The isomers were
distinguished by different space positions of substituents in cyclohexane
ring. Assignment of NMR signals to each of the isomers was based on
3
characteristic values of spin-spin interaction constants JHH in the cy-
3
3
clohexane ring, the typical values of which are Jax-ax = 6–14 Hz, Jax-
eq = 0–5 Hz, J 3e q-eq = 0–5 Hz. Н NMR spectrum of the trans-1,2-adduct
was in agreement with the thermodynamically most stable armchair
disubstituted eq,eq-conformation. DFT calculations gave at least 4 dif-
ferent conformations of C-O-O-H moiety, two of them corresponded to
the most energetically stable isomers. Among them, isomer (A) (Scheme
1
Fig. 3. Time dependency of oxygen consumption and NHPI disappearance (in
Run 11). The procedure of Table 1 was utilized.
1
) was chosen as a preferable due to better agreement of calculated and
1
13
experimental H and C spectral parameters of cyclohexane ring. For
86

N.I. Kuznetsova et al.
Catalysis Communications 114 (2018) 84–88
Scheme 1. Molecular structures of isomeric 1,2-adducts according to NMR study.
cis-1,2-adduct several conformations are possible. Since only one of two
cis-substituents (-OOH or -ON <) can be in equatorial position, at least
two isomers (ax,eq- and eq,ax-) should be considered. In addition,
various conformations of C-O-O-H moiety gave several local minima for
each of these isomers. DFT calculations determined structure (B) as
energetically preferable. The energetically favorable equatorial position
of –OOH group stabilized this conformer via 2.1 Å hydrogen bond
probably, associated with various reactivity of the radical intermediates
(depicted in Scheme 2) with respect to molecular oxygen. The rate of
CHHP formation was independent on oxygen concentration, which in-
dicated rapid interaction of O
2
with C
6
H
9
cyclohexenyl radical. In
10 radical was slow, and a
contrast, analogous O addition to PINO-C
2
6
H
reverse reaction of detachment of PINO was possible at a low oxygen
concentration. A similar effect of oxygen on competing reactions of
radicals was observed in epoxidation of pinene [32]. The improvement
of the oxidation at a low oxygen pressure was also observed at lower
temperature. Comparison of the oxidations at 1 and 0.5 atm of O2 at
60 °C (Runs 7 and 13 in Table 1) showed 1.5 times slower degradation
of NHPI and increased production of CHHP in diluted oxygen. This
indicated the oxygen pressure to be able to control stability of NHPI and
its efficiency in oxidation of cyclohexene. In Run No 13, Table 1, for
instance, the molar ratio of produced CHHP to decomposed NHPI was
over 50. Thus, at low oxygen pressure the synthesis of CHHP can be
realized with a minimum loss of NHPI, the bulk of which could be se-
parated from the reaction solution by well-known methods, e.g., de-
scribed in reference [33], and reused.
(
shown as dotted line in Scheme 1). On the contrary, none of the al-
ternative ax-OOH, eq-ON isomers was able to form the hydrogen bond.
1
13
The informative regions of H and C NMR spectra of adducts (A) and
(
B) are shown in Fig. S7.
Estimated from NMR spectra, total amount of the isomers (A) and
B) was consistent with degradation of NHPI determined by HPLC. No
(
other compounds produced from NHPI were detected by NMR.
3.4. Improvement of NHPI stability by reducing oxygen pressure
The key step of the NHPI catalyzed oxidation is the interaction of
PINO with cyclohexene resulted in cleavage of allyl CeH bond to form
cyclohexenyl radical (reaction (2)). Detection of the PINO-C 10-OOH
6
H
hydroperoxide adducts depicted in Scheme 1 indicated that addition of
PINO to vinyl position of cyclohexene competed against the catalytic
route, as shown in Scheme 2. Both allyl cleavage and vinyl addition
steps were followed by addition of molecular oxygen to the radical
intermediates. The competing reactions of PINO explain why intensive
degradation of NHPI coincides in time with the most intensive catalysis
when a large part of NHPI is converted to reactive PINO radical (Fig.3).
Nevertheless, we found that relative rates of cyclohexene conversion to
4
. Conclusions
It was found that deactivation of the NHPI catalyst occurred in
parallel with oxidation of cyclohexene to CHHP. The deactivation was
caused by trapping of PINO radicals by a double bond of cyclohexene to
form stable and catalytically inactive PINO-C H10-OOH hydroperoxide
6
adducts. The adducts have been for the first time observed and struc-
turally characterized by NMR in the reaction solution. A reaction
scheme was suggested which included competing reactions of cyclo-
hexene with PINO toward cleavage of allylic CeH bond or addition to
C]C double bond. Different susceptibility of resulting cyclohexenyl
and cyclohexene-to-PINO radicals to further oxygenation afforded the
opportunity to control efficiency of the catalytic performance by
oxygen pressure. As experimentally shown, lowering the oxygen partial
pressure was effective to minimize the deactivation of the NHPI catalyst
without deceleration of the cyclohexene to CHHP oxidation and even
with several percent increase in production of CHHP.
6
CHHP or PINO-C H10-OOH adducts were effectively controlled by
concentration of oxygen or its partial pressure. Deactivation of the
catalyst was revealed to become slower under lowering oxygen partial
pressure, as seen from data of Table 1, Runs 10–12. When oxygen
pressure was changed from 4.8 to 2.6 and 0.6 atm, degradation of NHPI
for 7 h reaction time decreased from 84 to 63 and further to 26%. At the
same time, oxygen consumption and production of CHHP did not de-
crease, but even showed some increase at the low oxygen concentration
(
Table 1 and Fig.3), due to the less intensive deactivation of the cata-
lyst. The positive effect of low oxygen pressure on stability of NHPI is,
Scheme 2. Competing routes toward CHHP formation and catalyst deactivation.
87

N.I. Kuznetsova et al.
Catalysis Communications 114 (2018) 84–88
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