Hydroperoxidation of tertiary alkylaromatics catalyzed by N-hydroxyphthalimide and aldehydes under mild conditions

Article abstract of DOI:10.1002/adsc.201000786

A metal-free catalytic system consisting of an aldehyde and N-hydroxyphthalimide (NHPI) for the selective oxidation of tertiary alkylaromatics with molecular oxygen has been developed. Cumene was oxidized efficiently to the corresponding hydroperoxide under mild conditions. The molecule-induced homolysis between peracids generated in situ and NHPI ensured the formation of the phthalimide N-oxyl (PINO) radical even at room temperature. Investigations on aldehyde, solvent and temperature effects allowed us to achieve good conversions with high selectivity in hydroperoxide. The optimized procedure was successfully extended to phenylcyclohexane, a valuable alternative for the production of phenol. The mechanism is discussed in detail.

Full text of DOI:10.1002/adsc.201000786

FULL PAPERS
DOI: 10.1002/adsc.201000786
Hydroperoxidation of Tertiary Alkylaromatics Catalyzed By
N-Hydroxyphthalimide and Aldehydes under Mild Conditions
Lucio Melone,a Cristian Gambarotti,a Simona Prosperini,a Nadia Pastori,a
Francesco Recupero,a,* and Carlo Puntaa,*
a
Dipartimento di Chimica, Materiali e Ingegneria Chimica “G. Natta”, Politecnico di Milano, Sezione Chimica, Via
Mancinelli 7, I-20131 Milano, Italy
Fax : (+39)-02-2399-3180; phone: (+39)-02-2399-3026 (C.P.), (+39)-02-2399-3007 (F.R.); e-mail: carlo.punta@polimi.it or
francesco.recupero@polimi.it
Received: October 18, 2010
The authors dedicate this article to Prof. Francesco Minisci on the occasion of his 80th birthday.
Abstract: A metal-free catalytic system consisting of Investigations on aldehyde, solvent and temperature
an aldehyde and N-hydroxyphthalimide (NHPI) for effects allowed us to achieve good conversions with
the selective oxidation of tertiary alkylaromatics high selectivity in hydroperoxide. The optimized pro-
with molecular oxygen has been developed. Cumene cedure was successfully extended to phenylcyclohex-
was oxidized efficiently to the corresponding hydro- ane, a valuable alternative for the production of
peroxide under mild conditions. The molecule-in- phenol. The mechanism is discussed in detail.
duced homolysis between peracids generated in situ
and NHPI ensured the formation of the phthalimide Keywords: autoxidation; co-oxidation; cumene; hy-
N-oxyl (PINO) radical even at room temperature. droperoxides; N-hydroxyphthalimide
Introduction
(AP), which is the main by-product at these relatively
high temperatures, and cumyl alcohol (CA).
The aerobic oxidation of hydrocarbons is a major
Hence, at temperatures lower than 1008C, the non-
issue in the chemical industry.[1] In a generic process, catalyzed oxidation of cumene is too slow; upon in-
catalysis by transition metal salts is usually required creasing the temperature, the conversion increases
in order to activate the molecular oxygen.[2] However, but the selectivity decreases. For these reasons, many
if the target compound is a hydroperoxide, the pres- efforts have been devoted over the years for the de-
ence of metal salts should be avoided, as they easily velopment of new catalytic systems in order to afford
decompose the desired product according to a fast the desired hydroperoxides with higher selectivity at
redox reaction.[3]
lower temperatures.
The main process oriented to the production of a
In the last decade, a new derivative, N-hydroxyph-
hydroperoxide consists in the autoxidation of cumene thalimide (NHPI), has been reported as an effective
À
to cumene hydroperoxide (CH), which is then decom- catalyst for C H activation by hydrogen abstraction
posed by means of acidic catalysis to afford phenol (Scheme 2).[5]
and acetone, according to the well-known Hock pro-
Due to its unique behaviour and to its general effi-
ciency, NHPI has attracted increasing interest, both in
cess (Scheme 1).[4]
Nowadays, about 95% of phenol produced world- academia and in industry.[6,7] It acts as a precursor of
wide is manufactured following this procedure.
The free-radical autoxidation step of the Hock pro-
cess is carried out under metal-free conditions and at
temperatures higher than 1108C, in order to promote
the homolytic thermal decomposition of tiny amounts
of CH, which acts in turn as radical chain initiator.
However, the selectivity in the hydroperoxide de-
creases to the extent in which the CH itself acts as ini-
tiator, as its decomposition produces acetophenone Scheme 1. The Hock process.
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Lucio Melone et al.
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the organic substrates, they considered the oxidation
of cumene, obtaining a moderate conversion (37%)
only after 72 h, and the formation of acetophenone
(AP) as a unique product, while CH was not ob-
served.
On the bases of our previous studies on NHPI reac-
tivity, we were encouraged to investigate this specific
reaction and its potentialities in greater depth. Hence,
the present work, which follows the granting of two
Scheme 2. Structures of NHPI and PINO.
the phthalimide N-oxyl (PINO) radical, which is the patents,[21] is addressed to the investigation of the key
effective abstracting species in all of the free radical role played by the NHPI/aldehyde catalytic system in
processes mediated by this N-hydroxy derivative.[5,8]
the autoxidation of tertiary alkylaromatics, being es-
A few years ago we reported that an NHPI/Co(II) pecially aimed at the investigation of the best reaction
catalytic system could efficiently promote the selec- conditions to achieve good conversions and high se-
tive aerobic oxidation of diisopropyl aromatic com- lectivity in hydroperoxides.
pounds to the corresponding tertiary alcohols in high
yields.[9] Very recently Orlinska has reported a de-
´
tailed investigation of the catalytic system consisting Results and Discussion
of NHPI in combination with various Cu(II), Co(II)
and Mn(II) salts for the oxidation of cumene with The results related to the optimization of the catalytic
oxygen.[10] However, as mentioned before, in order to system for the aerobic oxidation of cumene are re-
directly obtain the desired hydroperoxides, the pres- ported in Table 1.
ence of transition metal salts is generally detrimental.
Initial experiments, conducted in the presence of
For this reason, in the last decade several procedures stoichiometric amounts of MeCHO, afforded conver-
were reported for the aerobic oxidation of hydrocar- sions analogous to those reported by Einhorn et al.
bons using NHPI as organocatalyst in combination
with initiators such as alkaline earth chlorides,[11]
oximes,[12] quinones,[13] phenantrolines,[14] xanthones in
Table 1. Optimization of the NHPI/MeCHO catalytic system
for the aerobic oxidation of cumene.[a]
combination with tetramethylammonium chloride,[15]
quaternary ammonium bromides[16] and a,a-azobisiso-
Run MeCHO [%] NHPI [%] Conv. [%][b]
Selectivity
[%][b]
butyronitrile.[17] Nevertheless, all these processes re-
quired relatively high temperatures (808C), affording
the corresponding carbonylic and carboxylic deriva-
tives as major products and no or limited amounts of
hydroperoxides.
CH CA AP
1
2
3
4
5
6
7
8
100
50
25
10
5
10
10
10
10
10
10
20
1
35
34
38
36
23
–
69
5
–
63 35
74 25
80 19
81 18
89 11
2
1
1
1
–
–
1
6
–
1
2
3
In this context, in 2006 we reported a new and ef-
fective metal-free aerobic epoxidation of olefins cata-
lyzed by NHPI,[18] using stoichiometric amounts of
acetaldehyde (MeCHO) as co-oxidant, both to pro-
mote the in situ generation of PINO and as source of
the epoxidizing agent (Scheme 3).
Aldehyde autoxidation rates are very high even at
room temperature and atmospheric pressure and this
property has been often used to promote the so-
called “co-oxidation” processes, in which a mixture of
an aldehyde and another less reactive organic sub-
strate is submitted to molecular oxygen.[19]
–
–
–
10
10
10
–
77 22
73 21
9
–
–
–
10[c]
11[d]
10
10
10
62
62
49
82 17
79 19
78 19
–
12[e] 10
[a]
5 mmol of cumene in 10 mL of acetonitrile were stirred
for 6 h at 258C and atmospheric pressure of O2 in the
presence of NHPI/MeCHO catalytic system, in the per-
cent amount with respect to cumene as reported in the
Table.
In 1997, Einhorn and co-workers conducted a series
of aerobic oxidations on a wide range of hydrocar-
bons,[20] combining stoichiometric amounts of
MeCHO with catalytic quantities of NHPI. Among
[b]
Conversions and selectivity of the known reaction prod-
ucts were determined by HPLC with 2-phenylethanol
1
added as internal standard and confirmed by H NMR.
[c]
[d]
[e]
0.25 mmol of m-CPBA were added in place of acetalde-
hyde.
0.25 mmol of m-CPBA were diluted in 2 mL of acetoni-
trile and added dropwise in 3 h.
0.025 mmol of m-CPBA were added to the reaction mix-
ture.
Scheme 3. Aerobic epoxidation of olefins catalyzed by the
NHPI/MeCHO catalytic system.
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Hydroperoxidation of Tertiary Alkylaromatics Catalyzed By N-Hydroxyphthalimide
methyl peroxyl radicals, generated in situ according to
the mechanisms which will be discussed later on.
Moreover, peracetic acid, generated in solution, could
be consumed via a non-radical process, reacting with
another molecule of MeCHO, present at high concen-
tration, and forming acetic acid.
By further decreasing the amount of aldehyde,
yields were lowered again (run 5), while in the ab-
sence of the initiator no conversion was observed
(run 6), thus confirming the key role played by
MeCHO in the reaction mechanism.
The effect of amount of NHPI was also considered.
As expected, an increase in organocatalyst concentra-
tion led to higher conversions with still a good selec-
tivity in CH (run 7). On the contrary, by decreasing
the amount of NHPI, conversions fell down (run 8),
while in the absence of the N-hydroxy derivative no
trace of products was observed (run 9).
&
^
Figure 1. Cumene conversion ( ) and yield in CH ( ), CA
~
*
( ) and AP ( ) versus percent amount of acetaldehyde.
According to our interpretation, in an initiation
(35%), but in a shorter time (5 h) and with opposite step the acetaldehyde is oxidized to the corresponding
selectivity (run 1). Indeed, only traces of acetophe- acyl peroxyl radical which, in turn, may abstract a hy-
none were detected, the hydroperoxide and the alco- drogen atom from cumene, NHPI or another mole-
hol being the main products, in a ratio of 2:1, respec- cule of MeCHO, respectively (Scheme 4). In all the
tively.
cases the abstraction occurs faster than by an alkyl
Moreover, in contrast to the co-oxidative mecha- peroxyl radical for both polar and enthalpic reasons
nism explanation reported by Einhorn, we believed (BDERCOOO H =ca. 93 kcalmolÀ1;
BDEROO
=
À
H
À
that, for this specific reaction, aldehyde had the ca. 88 kcalmolÀ1).
unique role of initiator of the radical chain, causing
Peracetic acid, formed in situ according to Eqs. (1)–
the formation of PINO. If this had been true, lower (3) of Scheme 4, may in turn be involved in the gener-
quantities of MeCHO could be sufficient to promote ation of PINO from NHPI, through a molecule-in-
the process, making it interesting from an applicative duced homolysis mechanism, which leads to the for-
point of view.
mation of two radicals (Scheme 5). Our group has al-
To verify this hypothesis and improve the efficiency ready reported experimental evidence of the interven-
of the protocol, we progressively reduced the amount tion of this process by combining m-chloroperbenzoic
of MeCHO in the reaction medium (entries 2–5). In acid (m-CPBA) with NHPI in a neutral deoxygenated
doing so, we were able not only to achieve similar solvent.[18,22] The generation in situ of the PINO radi-
conversions, but also to increase the CH selectivity up cal was detected by EPR spectroscopy, while experi-
to 80%, simply by operating with 10% of acetalde- ments conducted in benzene, which acted as a radical
hyde with respect to starting cumene (Figure 1).
trapping source, demonstrated the formation of m-
The lower selectivity observed in the presence of chlorobenzenecarboxyl radical and of the correspond-
stoichiometric amounts of MeCHO could be ascribed ing decarboxylated m-chlorophenyl radical. Under
to the formation of high concentrations of fast termi- these mild conditions, the simple thermal homolysis
nating radical species, such as acetyl peroxyl and of the peracid could not occur. On the contrary, the
Scheme 4. Hydrogen abstraction by acetyl peroxyl radical.
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Lucio Melone et al.
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Scheme 5. Molecule-induced homolysis of NHPI with peracetic acid.
À
low BDE value of the O H bond for NHPI (88.1 kcal donors. By comparing k4 and k6 we can conclude that
molÀ1)[8] suggested that peracids could induce homoly- PINO abstracts the hydrogen from cumene 18 times
sis of the N-hydroxy derivative.
faster than the cumyl peroxyl radical [Scheme 6, Eqs.
Thus, we wanted to verify the intervention of this (4) and (6), respectively], which justifies the faster au-
mechanism in our process by conducting experiments toxidation occurring in the presence of the organoca-
in the presence of m-CPBA in place of acetaldehyde talyst, if compared with the non-catalysed process.
as radical initiator. Indeed, we succeeded in increas- Under the same reaction conditions, cumyl peroxyl
ing the conversion up to ca. 60%, while maintaining radical abstracts the hydrogen from NHPI about 4000
the high selectivity in CH (run 10). Further attempts times faster than from cumene (value determined by
based on the dropwise addition of the peracid over considering both the ratio between k7 and k6 and the
time did not afford significative improvements ratio between the concentrations of NHPI and
(run 11), while the combination of MeCHO with cumene). The faster reaction between cumyl peroxyl
lower amounts of m-CPBA led to lower conversions radical and NHPI dramatically decreases the station-
in comparison with the NHPI/peracid system ary concentration of the peroxyl radicals, increasing
(run 12).
the free-radical chain length and allowing the achieve-
Once formed, PINO is able to promote a free-radi- ment of higher selectivity in CH.
cal chain according to Scheme 6. The nitroxyl radical
To understand various aldehyde effects, both ali-
can abstract a hydrogen from cumene [Eq. (4)], gen- phatic and aromatic aldehydes were used to promote
erating the corresponding cumyl radical. The latter the oxidative process (Table 2).
undergoes fast addition to oxygen, leading to the for-
When operating in the presence of benzaldehyde,
mation of the cumyl peroxyl radical [Eq. (5)], which lower conversions were observed. The propagation
in turn can promote the hydrogen abstraction from rates for free-radical chain processes of autoxidation
NHPI [Eq. (6)] or cumene, present in large excess involving aldehydes are generally comparable. Never-
[Eq. (7)].
theless, the initiation step for aromatic aldehydes is
The relative rates[8] referred to these hydrogen ab- generally slower. The reasons of this difference are
À
straction reactions depend on the absolute kinetic mainly due to polar effects, the BDE of the C H
constants and on the concentration of the different H- bond being independent of the aldehyde structure.
Scheme 6. Propagation mechanism leading to the formation of CH.
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Hydroperoxidation of Tertiary Alkylaromatics Catalyzed By N-Hydroxyphthalimide
Table 2. Aerobic oxidation of cumene catalyzed by NHPI in
the presence of different aldehydes.[a]
Run
RCHO
Conv. [%][b]
Selectivity [%][b]
CH
CA
AP
1
2
MeCHO
PhCHO
EtCHO
(Et)2CHCHO
(Et)2CHCHO
(Me)3CCHO
(Me)3CCHO
36
25
41
40
69
62
77
81
93
85
86
71
58
52
18
7
1
–
1
–
2
5
5
3
14
14
27
37
43
4[c]
5
6c
7
[a]
The reaction conditions are those reported in Table 1,
run 4; see also Table 1, note[a]
.
[b]
[c]
See Table 1, note[b]
Reaction time 2 h.
.
Other primary aldehydes like propionaldehyde
(EtCHO, run 3) acted similarly to MeCHO, while in
the presence of secondary and tertiary aldehydes the
reactions ran faster (Figure 2). In particular, 2-ethyl-
butyraldehyde (Et)2CHCHO afforded conversions
and selectivity analogous to those observed with
MeCHO in just 2 h (run 4), while for longer reaction
times conversion increased but selectivity decreased
(run 5), probably due to the higher concentration of
peroxyl radicals in solution, which favours fast termi-
nation. Pivalaldehyde [(Me)3CCHO] led to conver-
sions even higher with respect to (Et)2CHCHO, but
with poor selectivity in CH (entries 6 and 7). The
faster kinetics in the presence of secondary and terti-
ary aldehydes and the poor selectivity with
(Me)3CCHO could be due to the fact that, being
more electron-rich than primary ones, they are easily
oxidized under our reaction conditions and undergo
fast decarbonylation.[23]
However, from an industrial application point of
view, MeCHO still remains the aldehyde of choice
due to its low commercial value and the limited mass
loss. Thus, other parameters were considered, in order
to increase the efficiency of the process.
For this reason, different solvents were used as re-
action media to study their effects and establish the
most convenient one (Table 3). Acetonitrile still re-
sulted as the best choice, combining good conversions
and high selectivity in CH. Low yields observed in
the presence of t-BuOH were probably due to the
protic nature of the solvent, which is able to undergo
hydrogen bonding with NHPI, inhibiting the hydro-
gen abstraction propagation step of the process. On
the contrary, dimethyl carbonate afforded high con-
versions but a poor selectivity in CH, while in the
&
^
Figure 2. Cumene conversion ( ) and yield in CH ( ), CA
~
*
( ) and AP ( ) versus reaction time for the aerobic oxida-
tion catalyzed by NHPI in the presence of acetaldehyde
(A), 2-ethylbutyraldehyde (B) and pivalaldehyde (C).
presence of acetone both lower conversions and creasing the temperature from 25 to 458C in the pres-
slightly lower selectivity were achieved. ence of acetaldehyde, we were able to reach a conver-
Finally, we wanted to investigate the temperature sion limit value of 65%, while, when operating with
effect on our process (Table 4). By progressively in- m-CPBA in the same range of temperatures, we did
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Table 3. Aerobic oxidation of cumene catalyzed by NHPI in
the presence of different solvents.
Solvent
Methoda Conv. [%][b]
Selectivity [%][b]
CH
CA
AP
MeCN
MeCN
t-BuOH
t-BuOH
ACHTUNGTRENNUNG
A
B
A
B
A
B
A
B
36
62
11
21
84
67
27
45
81
82
82
78
40
66
72
80
18
17
16
16
56
32
24
17
1
1
2
6
4
2
4
3
ACHTUNGTRENNUNG
(Me)2CO
(Me)2CO
[a]
The reaction conditions are those reported in Table 1,
run 4 (Method A) and run 10 (Method B); see also
Table 1, note[a]
.
[b]
See Table 1, note[b]
.
Table 4. Temperature effect on the aerobic oxidation of
cumene catalyzed by NHPI.
Run T [8C] Method[a] Conv. [%][b]
Selectivity [%][b]
CH
CA
AP
1
2
3
4
5
6
7
25
25
35
35
45
45
45
A
B
A
B
36
62
55
67
65
67
86
81
82
72
80
73
77
70
18
17
26
19
25
21
26
1
1
2
1
2
2
4
&
^
Figure 3. Cumene conversion ( ) and yield in CH ( ), CA
A
B
~
*
( ) and AP ( ) versus reaction time for the aerobic oxida-
tion catalyzed by NHPI in the presence of acetaldehyde (A)
and m-CPBA (B).
A[c]
[a]
[b]
[c]
See Table 3, note[a]
.
.
See Table 1, note[b]
in the dehydration to methylstyrene and subsequent
hydrogenation, or transformed into phenol as well by
H2O2 and acid catalysis.[25] Thus, under our optimized
20% of NHPI was employed.
not observe any significant differences in cumene con- conditions, we were able to perform good conversions
sumption. of cumene under very mild conditions, with selectivity
In both the cases the processes seem to verge to a in CH+CA>99%.
limiting value of conversion (faster with m-CPBA), as
The main drawback concerning the oxidation of
also confirmed by the kinetics reported in Figure 3, cumene for the production of phenol via the Hock
which show a gradual slowdown of the reactions. This process is the overproduction of acetone (Scheme 1).
behaviour could be ascribed to the reversibility of Eq. The demand for phenol, in fact, is growing more rap-
(7). The increase of CH during the course of the reac- idly than that for acetone and consequently the pro-
tion shifts Eq. (7) to the left.[24] This determines a duction of acetone exceeds the market requirrement.
higher concentration of peroxyl radicals, inducing For this reason, different approaches have been pro-
higher termination rates, and at the same time a posed to overcome this limitation. One of the most
lower concentration of PINO, with a consequent over- promising solutions consists in substituting the isopro-
all decrease of the reaction rate. A higher termination pyl group of cumene with different alkyl groups, in
rate also means a lower selectivity due to the forma- order to produce, in the last step of the process, indus-
tion of the alcohol CA parallel with CH (Scheme 7). trially interesting ketones.
Furthermore, cumyl alkoxyl radical may undergo b-
Phenylcyclohexane (PC) was mainly considered for
scission affording the AP as an undesired by-product. this purpose,[17c] as it affords cyclohexanone as co-
This process is favoured at higher temperatures product, an important precursor for the synthesis of
(Table 4) and for this reason we limited the tempera- different substrates like adipic acid, e-caprolactone
ture range of investigation. Indeed, AP represents the and e-caprolactam. Moreover, cyclohexanone can be
real waste product of the process, as CA can be re- also directly converted to phenol by dehydrogena-
converted to cumene by a two-step process consisting tion.[26] Phenylcyclohexane is produced by acid-cata-
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Hydroperoxidation of Tertiary Alkylaromatics Catalyzed By N-Hydroxyphthalimide
Scheme 7. Termination mechanism.
Experimental Section
All starting materials and catalysts were purchased from
commercial suppliers and used without further purification.
General Procedure
5 mmol of cumene 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 main-
tained for 6 h under an atmospheric pressure of O2 and at
the temperature of choice, with magnetic stirring. The oxida-
tion products deriving from cumene (CH, CA and AP) were
identified by comparison with authentic samples commer-
cially available. The hydroperoxide of PC was isolated by
flash chromatography (40–63 mm silica gel packing; hexane/
ethyl acetate, 9/1) and characterized by H NMR by compari-
son with a sample prepared from the corresponding com-
mercial alcohol using H2O2 and H2SO4, according to a pro-
cedure reported in literarure.[17c,27]
Scheme 8. Aerobic oxidation of phenylcyclohexane in the
presence of NHPI and MeCHO (i) or m-CPBA (ii).
lyzed alkylation of benzene in the presence of cyclo-
hexene or by direct reductive alkylation of benzene,
which is partially converted to cyclohexene in situ.
By applying the present MeCHO/NHPI catalytic
system for the aerobic oxidation of PC, under our op-
timized mild conditions (Scheme 8), it was possible to
achieve good conversions (ca. 40%) with a high selec-
tivity in the corresponding hydroperoxide, while,
when operating in the presence of m-CPBA, conver-
sion increased but selectivity declined.
Conversions and yields were determined by HPLC analy-
sis (reverse phase column; MeCN/MeOH/H2O, 35/5/60),
with 2-phenylethanol added as internal standard, and con-
firmed by 1H NMR.
Acknowledgements
Financial support from Polimeri Europa S.p.A., MURST
(PRIN 2006 and 2008) and Politecnico di Milano is grateful-
ly acknowledged.
Conclusions
References
We have investigated a novel catalytic system consist-
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of cumene. The molecule-induced homolysis between
NHPI and the peracid generated in situ leads to the
formation of PINO radical under very mild conditions
in the absence of transition metals, allowing us to
apply this catalytic system for the selective synthesis
of cumyl hydroperoxide. A detailed optimization of
the reaction conditions allowed us to extend the pro-
cedure to phenylcyclohexane, which can be consid-
ered as another intriguing precursor of phenol with
the concomitant production of cyclohexanone. This
new route towards the selective synthesis of tertiary
hydroperoxides provides an attractive and cheaper al-
ternative for the production of phenol with respect to
the classical autoxidative process which requires high
temperatures.
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