Selective catalytic aerobic oxidation of substituted ethylbenzenes under mild conditions

Article abstract of DOI:10.1016/j.molcata.2011.12.009

Ethylbenzene is oxidized to the corresponding hydroperoxide (PEHP) with high selectivity, under mild conditions, by means of a metal-free catalytic system consisting of an aldehyde and N-hydroxyphthalimide (NHPI). The process occurs via a free radical mechanism by in situ generation of the phthalimido-N-oxyl (PINO) radical. The protocol is applied with success on a wide range of substituted ethylbenzenes (ETBs). The competitive experiments carried out on few couples of ETBs revealed a marked polar effect, this proving the key role that PINO plays as real hydrogen abstracting species, at least at low conversion. At higher conversion, the formation of highly reactive OH radicals from PEHP reduces the differences in the reactivity of selected couples of ETBs. The study of the reaction mechanism, including the investigation on aldehyde and catalyst percentage amounts, and temperature and concentration effects, allows to achieve the final PEHPs products with good yields.

Full text of DOI:10.1016/j.molcata.2011.12.009

Journal of Molecular Catalysis A: Chemical 355 (2012) 155–160
Contents lists available at SciVerse ScienceDirect
Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Selective catalytic aerobic oxidation of substituted ethylbenzenes
under mild conditions
Lucio Melone, Simona Prosperini, Cristian Gambarotti, Nadia Pastori, Francesco Recupero, Carlo Punta∗
Department of Chemistry, Materials, and Chemical Engineering “G. Natta” – Politecnico di Milano, P.zza Leonardo da Vinci 32, I-20131 Milano, Italy
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 27 October 2011
Received in revised form 1 December 2011
Accepted 7 December 2011
Available online 16 December 2011
Ethylbenzene is oxidized to the corresponding hydroperoxide (PEHP) with high selectivity, under
mild conditions, by means of
a metal-free catalytic system consisting of an aldehyde and N-
hydroxyphthalimide (NHPI). The process occurs via a free radical mechanism by in situ generation
of the phthalimido-N-oxyl (PINO) radical. The protocol is applied with success on a wide range of
substituted ethylbenzenes (ETBs). The competitive experiments carried out on few couples of ETBs
revealed a marked polar effect, this proving the key role that PINO plays as real hydrogen abstract-
ing species, at least at low conversion. At higher conversion, the formation of highly reactive •OH
radicals from PEHP reduces the differences in the reactivity of selected couples of ETBs. The study of
the reaction mechanism, including the investigation on aldehyde and catalyst percentage amounts,
and temperature and concentration effects, allows to achieve the final PEHPs products with good
yields.
Keywords:
N-Hydroxyphthalimide
Autoxidation
Aldehydes
Ethylbenzene
Hydroperoxide
© 2011 Elsevier B.V. All rights reserved.
1. Introduction
OO
H
HOO
H
Aerobic oxidation of ethylbenzene (ETB) to the corresponding
hydroperoxide (Scheme 1) is becoming increasingly important, due
to the key role that phenyl ethyl hydroperoxide (PEHP) plays as
industrial intermediate for the production of propylene oxide and
styrene monomer (Shell SM/PO Process) [1].
AP PEHP OH
(7)
The industrial autoxidation of ETB is carried out under
metal-free conditions and at 130 ◦C, in order to promote the decom-
position of tiny amounts of (PEHP), which may occur by a thermal
homolytic cleavage of the O O bond (Eq. (1a)) or by a bimolecular
process according to Eq. (1b) [2]. PEHP acts in turn as radical chain
initiator (Eqs. (2) and (3), Scheme 1).
Under these operative conditions, an increase of ETB conversion
would be reflected into a decrease of selectivity into PEHP (Eq. (7)).
For this reasons conversion is kept low (about 13%) to minimize by-
products. The selectivity for ethylbenzene to PEHP is approximately
90%, and the selectivity to MPC and AP is 5–7% [3].
In the last decades several efforts have been devoted to the
development of new catalytic systems with the aim to increase
the conversion of ETB and, at the same time, to provide the cor-
responding hydroperoxide with high selectivity. In 2005, Fierro
and co-workers reported the beneficial effect of the addition of
tiny amounts of alkaline and alkaline-earth metal oxides in the
oxidative batch, in order to neutralize the acidic by-products of the
reaction. As result, selectivity in PEHP increased, but conversion of
ETB was lower than 10% [4].
In recent years, N-hydroxyphthalimide (NHPI) has been
reported as an effective catalyst in many oxidative processes [5].
NHPI acts as a precursor of phthalimido-N-oxyl (PINO) radical,
which is the real hydrogen abstracting species (Eq. (8)). Its acti-
vation may occur by means of transition metal salts [6–8], but the
presence of metallic co-catalysts is detrimental when the final goal
is to achieve hydroperoxides selectively.
∗
Corresponding author. Tel.: +39 0223993026; fax: +39 0223993180.
E-mail address: carlo.punta@polimi.it (C. Punta).
1381-1169/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2011.12.009

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L. Melone et al. / Journal of Molecular Catalysis A: Chemical 355 (2012) 155–160
H
O
O
.
k8 = 2.24 M-1s-1 at 25 °C
N OH
.
O
N
O
O
PINO
NHPI
(8)
In 2003, we reported the metal-free nitric aerobic oxidation
On the bases of our preliminary studies on the aerobic epox-
idation of primary olefins by means of an NHPI/aldehyde system
[18], we have recently reported that cumene (CU) can be selectively
converted to the corresponding hydroperoxide via a free-radical
chain promoted by NHPI and initiated by small amounts of alde-
hydes [19]. Peroxidation of secondary alkylaromatics shows two
main drawbacks if compared with the analogous reaction on CU:
(i) the secondary C H benzylic bond of ETB is less reactive than the
tertiary C H benzylic bond of CU; (ii) PEHP product, which has a
tertiary C H benzylic bond more reactive than the secondary one
of the starting ETB, can further react affording secondary products
[2].
In 1997, Einhorn and co-workers conducted a series of aerobic
oxidations on a wide range of hydrocarbons, combining stoichio-
metric amounts of acetaldehyde (MeCHO) with catalytic quantities
of NHPI [20]. Among the organic substrates, they considered the
oxidation of ETB, obtaining MPC and AP as unique products, while
EBTH was not observed.
of ETB catalyzed by the I2/NHPI system in acetic acid solution,
achieving methyl phenyl carbinol (MPC) through the correspond-
ing acetate in high yields as unique final product [9]. Several
other transition metal-free initiators have been employed in com-
bination with NHPI for the aerobic oxidation of hydrocarbons,
including alkaline-earth chlorides [10], oximes [11], quinones [12],
phenantrolines [13], xanthones [14], quaternary ammonium bro-
mides [15] and nitriles [16]. Nevertheless, when employed for the
catalytic oxidation of ETB, all these systems afforded almost quan-
titatively the corresponding acetophenone (AP).
In 2009, Fierro and co-workers significantly improved their
aforementioned approach by performing the oxidation of ETB to
PEHP in the presence of NHPI and ppm amounts of NaOH [17],
reaching a higher selectivity in PEHP (ca. 80% mol) with a substrate
conversion up to 15%. However, under the reported operating con-
ditions (∼150 ◦C) the organocatalyst undergoes fast decomposition,
this limiting the interest for this approach from an industrial point
of view.
In the present work, which follows a patent application [21],
we exploit the key role that the NHPI/MeCHO system plays in the
autoxidation of ETB and analogous secondary alkylaromatics to
afford the corresponding hydroperoxides in good yields and high
selectivity, under mild conditions.
Initiation
O
Δ
OH
(1a)
(1b)
OOH
2. Experimental
O
2.1. Materials
PEHP
ETB
H2O
All starting materials and catalysts were purchased from com-
mercial suppliers without further purification with the exception
of 1d, which was prepared from the corresponding p-methoxy ace-
tophenone by reduction with N2H4, according to the procedure
reported in the literature [22].
O
OH
(2)
(3)
ETB
2.2. General procedure
MPC
5 mmol of 1a–f and the desired amounts of MeCHO and
NHPI were added to 10 mL of acetonitrile in a 50 mL double-
neck round-bottom flask. The solution was maintained for 6 h
under an atmospheric pressure of O2 and at the temperature of
choice, with magnetic stirring. The oxidation products 2a, 3a–f
and 4a–f were identified by comparison with authentic sam-
ples commercially available. The hydroperoxides 2b [23], 2c, 2d
[23], 2e [24] and 2f [24] were isolated by flash chromatogra-
phy (40–63 ␮m silica gel packing; hexane/ethyl acetate, 9/1),
characterized by H NMR and compared with analytical data
reported in literature. Complete characterization of 2c is reported
below. Conversions and yields were determined by HPLC anal-
ysis (reverse phase column; MeCN/MeOH/H2O, 35/5/60), with
cumyl alcohol added as internal standard, and confirmed by 1H
NMR.
H2O
ETB
Propagation
OH
(4)
(5)
O2
OO
PEHP
ETB
OO
k5 = 0.20 M-1s-1 at 25°C
Termination
4-(1-Hydroperoxyethyl)benzonitrile (2c): 1H NMR (CDCl3)
ı
MPC
O2
(6)
2
(ppm): 1.46 (d, 3H, CH3, J = 6.82 Hz); 2.16 (s, 1H, OOH); 5.11 (q,
3H, CH2, J = 6.82 Hz); 7.48 (d, 2CH Ar, J = 8.12 Hz); 7.67 (d, 2CH Ar,
J = 8.12 Hz). 13C NMR (CDCl3) ı (ppm): 13.5, 76.3, 105.2, 112.0, 120.5,
125.8, 140.8. ESI-MS (m/z): 186 (100), 102.2 (0.045).
O
OO
AP
Scheme 1. Autoxidation mechanism for ETB.

L. Melone et al. / Journal of Molecular Catalysis A: Chemical 355 (2012) 155–160
157
2.3. General procedures for determining the relative and absolute
rate constants of the aerobic oxidations of substituted 1a–f
of MeCHO were sufficient to perform the desired product even at
room temperature and atmospheric pressure. However, the pres-
ence of MeCHO was essential to initiate the free-radical chain (entry
7).
Competitive experiments were run between 1a (2.5 mmol) and
substituted ethylbenzenes 1b–f (2.5 mmol), and a suitable reaction
time (2 h) was utilized to have low conversions of ETBs to the cor-
responding hydroperoxides. The conversions were determined by
GC, using cumyl alcohol as internal standard, and the response
factors were measured by employing authentic samples. Yields
were determined as described before. These competitive kinetics
were utilized to determine the relative rate constants of the aer-
obic oxidations of 1b–f to 2b–f, reported in Table 3. The relative
rate constants were evaluated using Eq. (9), since the absolute rate
constant for the hydrogen atom abstraction from ETB 1a by PINO
radical, which is the rate-determining step of the oxidative process,
had been already evaluated by Lucarini et al. [25] by means of EPR
technique.
Under these very mild conditions, the initiation phase occurs in
a three-step process. In the presence of O2, acetaldehyde is oxidized
to the corresponding acyl peroxyl radical (Eq. (10)), which in turns
may abstracts a hydrogen atom from ETB, NHPI, another molecule
of MeCHO, but also from PEHP and MPC, forming the corresponding
peracetic acid (Eq. (11)). In all of the cases the hydrogen abstrac-
tion by acyl peroxyl radical occurs faster than by an alkyl peroxyl
radical. Peracetic acid may be in turn involved in the generation of
PINO from NHPI, through a molecule-induced homolysis process
(Eq. (12)).
O2
O
O
O
.
O2
O
.
H3C
H
H3C
O
H3C
kx
log(([1b − f ]0 − [2b − f ])/[1b − f ]0)
log(([1a]0 − [2a])/[1a]0)
=
(9)
.
kH
OOH
3. Results and discussion
(10)
(11)
3.1. Effect of initiator
O
O
Our initial experiments were conducted with the aim of repro-
ducing Einhorn’s conditions, that is by operating in the presence
.
H-DONOR
O
O
H3C
O
O
H
O
O
O
.
.
O N
O
CH3
CO2 H2O
O N
H3C
O
H
H
O
O
(12)
of stoichiometric amounts of MeCHO. However, as verified for the
oxidation of cumene [19], also in this case we observed a completely
different behavior with respect to Einhorn’s reported data. In fact,
PEHP resulted the primary product, while small amounts of MPC
were observed, being AP the major by-product (Table 1, entry 1).
Even more interesting, by decreasing the amount of MeCHO, the
conversion also decreased, but the selectivity in PEHP increased up
to 90% (Table 1, entries 2–6). This behavior follows the trend of
the traditional process, but our results showed that tiny amounts
3.2. Effect of catalyst
The key role of NHPI catalysis was evident. In fact, in the absence
of the organocatalyst no reaction occurred under our mild oper-
ating conditions, while an increase in NHPI concentration led to
higher conversions, with still high selectivity in PEHP (Fig. 1).
NHPI is crucial in reducing the concentration of all the fast
terminating oxygen centered radical species deriving from the
autoxidation of both MeCHO and ETB. This role was outlined by the
Table 1
Effect of the amount of MeCHO for the aerobic oxidation of ETB.a
.
OH
O
H
O
O
NHPI/MeCHO
O2, r.T.
+
+
ETB: 1a
MPC
AC
PEHP: 2a
Run
MeCHO (%)
Conversion (%)b
Selectivity (%)b
PEHP
MPC
6
AP
37
23
27
18
10
8
1
2
3
4
5
6
7
100
36
57
90
45
10
5
2.5
–
31
29
26
21
13
–
65
70
82
90
92
–
12
3
–
–
–
–
–
a
5 mmol of ETB in 10 mL of CH3CN were stirred for 6 h at 25 ◦C and atmospheric pressure of O2 in the presence of NHPI (10% mol) and MeCHO (as reported in the table) in
a percent amount with respect to ETB.
b
Conversions and selectivity of the known reaction products were determined by HPLC with cumyl alcohol added as internal standard and confirmed by 1H NMR.

158
L. Melone et al. / Journal of Molecular Catalysis A: Chemical 355 (2012) 155–160
100
80
60
40
20
0
100
90
80
70
60
50
40
30
20
10
0
20
30
40
50
60
T (°C)
0%
5%
10%
15%
20%
25%
% NHPI
Fig. 2. ETB conversion (ꢀ) and yield (ꢁ) and selectivity (ꢂ) in PEHP versus T (◦C). See
Table 1, note (a) for reaction conditions.
Fig. 1. ETB conversion (ꢀ) and yield (ꢁ) and selectivity (ꢂ) in PEHP versus percent
amount of NHPI. See Table 1, note (a) for reaction conditions.
Table 2
Aerobic oxidation of ETBs catalyzed by NHPI/MeCHO system.a
.
experimental results of Table 1. MPC, which under our operating
conditions can only derive by the Russell type termination pro-
cess (Eq. (6)), was generally not observed in the presence of NHPI.
Only with higher quantities of MeCHO tiny amounts of alcohol were
detected, probably due to the presence of a higher concentration of
fast terminating radical species deriving from MeCHO.
HO
O
HOO
O , 1 atm
2
NHPI/MeCHO
X
3a-f
X
X
2a-f
X
1a-f
4a-f
In the catalyzed propagation phase, PINO radical can promote
the hydrogen abstraction from ETB, forming the corresponding
␣-methyl benzyl radical (Eq. (8)). By comparing k5 with k8 we
conclude that Eq. (8) occurs 11 times faster than Eq. (5), this demon-
strating the catalytic role of NHPI. ␣-Methyl benzyl radical in turn
can be trapped by molecular oxygen according to Eq. (4). Once
formed, the peroxyl radical can undergo Russell termination (Eq.
(7)) or promote the hydrogen abstraction from NHPI (Eq. (13)) or
another molecule of ETB (Eq. (5)), present in large excess. If we con-
sider the ratio between k13 and k5 (∼36,000), we can conclude that
NHPI behaves as an ideal H-donor even at very low concentrations,
trapping peroxyl radicals. In this way the Russell termination is
inhibited and the selectivity in PEHP is increased. Thus the presence
of NHPI favorably affects the kinetics of the process by decreasing
the rate of the termination steps [26] and by increasing the rate of
the propagation steps.
1
X
Conversion (%)b
Selectivity (%)b
2
3
4
1a
1b
1c
1d
1e
1f
H
p-Cl
p-CN
p-OCH3
p-CH3
m-CH3
31
33
21
42
36
25
2a: 81
2b: 84
2c: 74
2d: 91
2e: 82
2f: 82
–
–
–
–
–
–
4a: 19
4b: 16
4c: 26
4d: 9
4e: 18
4f: 18
a
See Table 1, note (a). T = 40 ◦C. MeCHO 10%.
See Table 1, note (b).
b
higher for ETB than for CU. As a consequence, for the restricted
range of temperatures here investigated, conversion moved from
26% at 25 ◦C to just 34% at 60 ◦C. Nevertheless, by operating at tem-
peratures higher than 60 ◦C, thermal decomposition of PEHP may
occur, this limiting the selectivity of the process.
The formation of AP as main by-product should be ascribed
to the fast hydrogen abstraction from PEHP by means of peroxyl
radical (Eq. (7)) or of PINO itself (Eq. (14)).
.
O
O
OOH
OO
k
13 = 7.2 103 M-1s-1 at 25 °C
.
OH
O
N
N
O
O
O
(13)
(14)
OOH
O
O
N OH
.
.
N O
OH
O
O
3.3. Effect of temperature
3.4. Effect of substituents
On the contrary of what observed for cumene [19], an increase
of the reaction temperature led just to a poor improvement of
the process efficiency in terms of conversion and yield (Fig. 2).
This behavior should be ascribed to the slightly higher bond
dissociation energy (BDE) of the secondary C H bond for ETB
(85.4 kcal/mol) [27] with respect to the BDE of tertiary C H bond
of CU (84.4 kcal/mol) [28]. This difference determines an activa-
tion energy (Ea) for the reaction of hydrogen abstraction by PINO
With the optimized conditions in hand, we set out to examine
the scope of the reaction for a wider range of substituted ethylben-
zenes (ETBs). The results reported in Table 2 clearly confirm the
general applicability of the process for a wide range of secondary
alkyl aromatics.
Moreover, in order to investigate the polar effect of substituents
on the aromatic ring and to verify the role of PINO as principal

L. Melone et al. / Journal of Molecular Catalysis A: Chemical 355 (2012) 155–160
159
Table 3
Table 4
Absolute rate constants for the H-atom abstraction from ETBs by PINO radical at
Aerobic oxidation of ETB in CH3CN at different concentrations catalyzed by
25 ◦C.
NHPI/MeCHO system.a
X
k (M−1 s−1
)
kX/kH
Run
CH3CN (mL)
T (◦C)
NHPI (%)
PEHPb
H
p-Cl
p-CN
p-OCH3
p-CH3
m-CH3
2.24a
0.16
0.06
11.05
2.76
2.11
1.00
0.072
0.022
4.93
1.23
0.94
Y (%)
S (%)
1
2
3
4
5
6
7
8
10
40
40
70
70
70
40
40
70
70
70
2
12.0
14.3
10.8
15.9
19.5
12.8
12.5
13.4
13.5
28.3
89
90
89
93
89
91
92
91
93
84
10
10
10
10
5
5
5
5
2
5c
2
5
10c
a
From Ref. [25].
2
5c
2
9
5c
1
10d
hydrogen abstracting specie, we have determined the absolute rate
constants by competitive kinetics at shorter reaction times (2 h).
The results are reported in Table 3, while Fig. 3 displays a Hammett
correlation between the results presented in the table and values
of ꢀ.
a
42 mmol of ETB in 10 mL of CH3CN were stirred for 6 h at atmospheric pressure
of O2 in the presence of NHPI and MeCHO (2% with respect to ETB).
b
See Table 1, note (b).
Reaction medium non homogeneous.
36 mmol of cumene were employed instead of ETB.
c
d
For all the substituents reported in Table 3, the effects on the
values of the BDE of their benzylic C H bonds is rather poor and
the polar effect largely prevails over the enthalpic effect, which
leads to a satisfactory Hammett correlation.
The key role of polar effects in the presence of NHPI was
already reported for both the aerobic oxidation of substituted
benzylic alcohols [29,30] and the aerobic oxidation of 1-(4-
methoxyphenyl)ethanol by means of NHPI aryl substituents [30].
The observed behavior has to be related to a more-pronounced
electrophilic character of the PINO radical relative to the peroxyl
radical, which induce a stabilization of the transition state of Eq.
(8) (Fig. 4).
At longer reaction times (6 h) the difference in the reactivity
of selected couple of substituted ethylbenzenes decreases. This
behavior could be ascribed to the higher conversion, which favors
Eqs. (7) and (14), leading to the formation of hydroxyl radicals. The
latter are very reactive, owing to the strong O H bond formed in
the final H2O product, and are not able to discriminate between the
two ETBs in the hydrogen abstraction process, the enthalpic effect
being predominant under these conditions.
3.5. Effect of ETB concentration
Autoxidation of alkylaromatics is usually conducted in the
absence of co-solvents. However, due to its polar nature, NHPI has a
low solubility in alkylbenzenes at the mild temperature employed
in our protocol. Thus, the use of polar solvents such as acetonitrile
or acetone is mandatory in order to dissolve the quantities of NHPI
necessary for observing a good catalytic activity. This aspect could
represent the main limit of the oxidative process here proposed,
if the volumes of solvent were significantly higher than those of
the starting material. For this reason we wanted to investigate the
effect of ETB concentration in order to identify the best operating
conditions.
As shown in Table 4 (entries 1 and 2), in a first step we increased
the amount of ETB up to 42 mmol (∼5 mL), by maintaining the same
volume of acetonitrile (10 mL), with 2% of MeCHO. At 40 ◦C, only
with 2% of NHPI the reaction medium resulted homogeneous. An
increase of temperature had the unique benefit of increasing the
solubility of the catalyst (entries 3–5), reaching a final yield of about
20%, while selectivity was maintained around 90%. We then further
increased the concentration of ETB by decreasing the amount of
co-solvent with a final 1/1 volume ratio between ETB and CH3CN.
The reaction appeared competitive with the classical procedure,
achieving a 13% yield in PEHP with high selectivity. In this case,
an increase of temperature did not provide significant benefits in
terms of final yields, this demonstrating that the 2% of catalyst is
the higher concentration of NHPI reachable under these very mild
conditions.
ρ = −2,31
Log k X/k H
r2 = 0.91
1,00
p-OMe
m-Me
0,00
-1,00
-2,00
p-Me
Optimized conditions were also found for the aerobic oxida-
tion of cumene (CU) (entry 10). As expected for this more reactive
substrate, yields in the corresponding hydroperoxide (CHP) were
higher, even with just 1% of NHPI and a final volume ratio between
CU and CH3CN of 5/2.
NHPI, which remains unchanged during the reaction due to the
low temperature, could be easily recovered at the end of the oxi-
dation by crystallization from the reaction mixture, after removing
the polar solvent by distillation.
p-Cl
p-CN
σ
-0,30
-0,10
0,10
0,30
0,50
0,70
Fig. 3. Hammett correlation for aerobic oxidation of substituted ethylbenzenes. See
Table 3 for reaction conditions. Reaction time: 2 h.
4. Conclusion
O
X
CH2
δ
We have demonstrated that the NHPI/MeCHO catalytic system,
recently reported for the aerobic oxidation of tertiary alkylaro-
matics, is also efficient in the peroxidation of the less reactive
secondary ones. The corresponding hydroperoxides are achieved in
good yields and high selectivity. The protocol of the reaction, opti-
mized for the autoxidation of ETB, has been proved to be suitable
δ
N O
H
C
H
O
Fig. 4. Transition state for Eq. (8).

160
L. Melone et al. / Journal of Molecular Catalysis A: Chemical 355 (2012) 155–160
for a wider range of ethylbenzenes. The polar effect emphasized by
the Hammett correlation clearly shows the key role played by the
PINO radical in the propagation phase of the radical chain. More-
over, the possibility to operate at very high concentration of ETB
in polar solvent, together with the very mild operating conditions,
make the protocol of interest for future industrial applications.
The data herein reported can be considered as a further advance-
ment in the investigation of the NHPI-catalyzed radical oxidation of
alkylaromatics, not only because they are in opposition to the Ein-
horn’s results, but also because they confirm earlier mechanistic
hypothesis like the importance of the equilibrium described in Eq.
(13), and the electrophilic character of PINO compared to peroxyl
radicals.
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Acknowledgements
Financial support from Polimeri Europa S.p.A. is gratefully
acknowledged. We thank MURST for continual support of our free-
radical chemistry (PRIN 2008).
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