Solvent-free oxidation of cumene by molecular oxygen catalyzed by cobalt salen-type complexes

Article abstract of DOI:10.1007/s11164-011-0425-5

Co(salen)-type [where salen = di-(salicylal)-ethylenediimine] complexes were shown to be efficient catalysts in the oxidation of 2-phenylpropane (cumene) by dioxygen primarily to 2-phenyl-2-propanol (cumyl alcohol), 2-phe-nylpropene (a-methylstyrene), and 1-phenylethanone (acetophenone) applying ¹H NMR spectroscopy and gas chromatography-mass spectrometry (GC-MS). The effect of substitution on the ligand was also monitored in both oxygen-absorption and the catalytic reaction. Based on these results, the trend observed for the production of a-methylstyrene and cumyl alcohol were parallel to dioxygen uptake by the catalyst in neat cumene, while acetophenone productions obeyed a non-linear trend. The best selectivity for the reaction in terms of acetophenone production was observed for the complex with the least oxygen-absorption feature. The intermediate of the reaction, LCo(III)-OOcumyl (where L = salen) complex, was synthesized and characterized by IR, ¹H NMR spectroscopy as well as elemental analysis, and its reactivity in the present catalytic reaction was also studied. A series of experiments were performed to propose a mechanism for the reaction on the basis of the product distributions in the reaction mixture. Springer Science+Business Media B.V. 2011.

Full text of DOI:10.1007/s11164-011-0425-5

Res Chem Intermed (2012) 38:871–883
DOI 10.1007/s11164-011-0425-5
Solvent-free oxidation of cumene by molecular oxygen
catalyzed by cobalt salen-type complexes
•
B. Ghanbari S. R. Ferdosi H. Tafazolian
•
Received: 16 July 2011 / Accepted: 17 October 2011 / Published online: 10 November 2011
Ó Springer Science+Business Media B.V. 2011
Abstract Co(salen)-type [where salen = di-(salicylal)-ethylenediimine] com-
plexes were shown to be efficient catalysts in the oxidation of 2-phenylpropane
(cumene) by dioxygen primarily to 2-phenyl-2-propanol (cumyl alcohol), 2-phe-
nylpropene (a-methylstyrene), and 1-phenylethanone (acetophenone) applying
1H NMR spectroscopy and gas chromatography–mass spectrometry (GC–MS). The
effect of substitution on the ligand was also monitored in both oxygen-absorption
and the catalytic reaction. Based on these results, the trend observed for the pro-
duction of a-methylstyrene and cumyl alcohol were parallel to dioxygen uptake by
the catalyst in neat cumene, while acetophenone productions obeyed a non-linear
trend. The best selectivity for the reaction in terms of acetophenone production was
observed for the complex with the least oxygen-absorption feature. The interme-
diate of the reaction, LCo(III)-OOcumyl (where L = salen) complex, was synthe-
sized and characterized by IR, 1H NMR spectroscopy as well as elemental analysis,
and its reactivity in the present catalytic reaction was also studied. A series of
experiments were performed to propose a mechanism for the reaction on the basis of
the product distributions in the reaction mixture.
Keywords Schiff base ꢀ Catalyst ꢀ Cumene ꢀ Dioxygen ꢀ Oxidation ꢀ Cobalt(II)
Introduction
Development of catalysts that carry out the oxidation process under mild condition
and with inexpensive and environmentally benign oxidants has become an
important goal in such pursuits as pharmaceuticals, fragrances, and flavors [1, 2].
B. Ghanbari (&) ꢀ S. R. Ferdosi ꢀ H. Tafazolian
Department of Chemistry, Sharif University of Technology, PO Box 11155-3516, Tehran, Iran
e-mail: ghanbari@sharif.edu
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B. Ghanbari et al.
Most recently, the transition metal-based oxidation of hydrocarbons is being more
industrially exploited in particular for the synthesis of cumyl hydroperoxide (CHP),
phC(CH3)2OOH (precursor of phenol), which utilizes metal-based catalytic
oxidation of hydrocarbons [3]. The dominant position of molecular oxygen as
the oxidant for bulk chemical oxy-functionalizations is because it is the only
economically, and environmentally friendly, feasible oxidant for large-scale
processing [4]. Basically, emulsion oxidation of alkylaromatic compounds appeared
to be more efficient for the production of hydroperoxides [5] The first paper devoted
to emulsion oxidation of cumene appeared in 1950 by Armstrong et al. [6]. The
kinetics of the emulsion oxidation of cumene was intensively studied by Kucher
et al. [5].
Meanwhile, the cobalt-catalyzed oxidation of hydrocarbons by molecular oxygen
represents one of the largest scale applications of homogeneous catalysis that is
widely used in industry [7]. For instance, Co(salen) (Fig. 1) [8, 9] is a tetradentate,
planar Schiff’s base complex which is classified as an oxygen carrier [10]. It has
been used as a catalyst for the preparative oxygenation of phenols [11–13], indoles
[14, 15], flavonols [16], amines [17, 18], anilines [19, 20] and other substrates [11,
21] with both dioxygen or hydroperoxides as the oxygen source. It is most likely
that the ROOH formed initially reacts with the cobalt catalyst to form the
intermediate of cobalt alkylperoxy [22].
This paper reports on the role of Co(salen)-type complexes as oxygen activators
for the oxidation of cumene by molecular oxyen as well as on the reaction
mechanism. The effect of salen substitution on the selectivity of the reaction is
discussed.
R1
R1
R2
O
R2
O
Co
N
N
R3
R3
Number
R1
R2
R3
H
complex
Co(salen)
1
2
3
4
5
H
H
H
Br
H
H
Co(Brsalen)
Co(saltmen)
H
CH3
H
OMe
Br
Br
Br
Co(3-OMe,5-Brsalen)
Co(3,5-Brsalen)
H
Fig. 1 The Schiff base complexes applied in this work
123

Solvent-free oxidation of cumene
873
Experimental section
Materials and general considerations
All experiments were performed under N2 atmosphere using standard techniques
unless otherwise noted. Solvents (including deuterated solvents from Aldrich) were
degassed and dried according to the standard procedures. Ethylenediamine (98%),
salicylaldehyde, and Co(CH3COO)2ꢀ4H2O were purchased from Merck. Cumene
(Merck) was boiled under reflux over Na in a N2 atmosphere for 2 h. Then, its
distillation under N2 gave a fraction at b.p. of 152 °C.
The concentration of Cobalt salen-type complexes as catalyst was 0.01 M in
5 mL of neat cumene (as both reagent and solvent). All catalytic reactions as well as
oxygen absorption measurements were conducted in a thermo-bath-controlled
reactor at 60 °C.
The IR spectra were recorded on a Brucker IFS-88 in KBr pellets over the range
of 500–4,000 cm-1. The UV–visible spectra were recorded on a Rey Leigh UV-
2601. EI–MS spectra for LCo(III)-OOcumyl complex were measured by an Agilent
Technology (HP) quadrupole mass spectrometer with electron impact (70 eV) ion
1
source. H and 13C NMR spectra were recorded in CDCl3 on a Bruker-ARX 500
instrument at room temperature. Mass spectra was recorded by a Hewlett-Packard
(HP, Palo Alta, USA) HP 6890 series GC equipped with a split/splitless injector,
and a HP 5973 mass selective detector system was also used. The mass spectrometer
was operated in the EI mode (70 eV). Helium (99.999%) was employed as carrier
gas and its flow rate was adjusted to 1 mL/min. The separations were performed on
a 28.5 m 9 250 lm i.d. fused-silica capillary column coated with a 0.25-lm
bonded film of HP-1 MS. The GC column temperature was programmed at 40 °C
for 2 min and then raised to 260 °C at 10 °C/min, 10 min hold at 260 °C. The
injector temperature was set at 280 °C, and all injections were carried out in the
splitless mode. The ion source and quadrupole temperatures were set at 230 and
150 °C, respectively. The GC–MS interface was maintained at 280 °C. The MS was
operated using total ion current (TIC) mode, scanning from m/z 40 to 300. For
quantitative determination, the MS was operated in time scheduled selective ion
monitoring (SIM) mode. Quantitation was performed by calculating peak areas.
Oxygen consumption measurements and catalytic reaction experiments
In the former experiments, the reaction progress was followed by monitoring
dioxygen consumption applying an isobutyl phethalat manometer connected to a
buret as described previously [23]. Typically, a 250-mL flask equipped with
magnetic stir bar was charged with 5 mL neat cumene as both reagent and solvent.
The reactions were performed in a temperature-controlled water bath, which
maintained the reaction within 60 0.1 °C.
In the catalytic reaction experiments, the cumene in the flask was purged several
times with dioxygen. The reaction begins by dispersion of a sufficient amount of the
solid cobalt catalyst (0.01 M) in the mixture. The solubility of the catalysts in neat
cumene was spectrophotometrically determined as 0.1–1.2 mM. Then, the mixture
123

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B. Ghanbari et al.
Table 1 Important infrared bands of compound 1–5 and 1H NMR assignments of their ligands in CDCl3
Compound
mC=N Infrared band
Chemical shifts of the ligands in ppm relative
to Tetramethylsilane (TMS)
of complex (cm-1
)
–OH
CH = N
Aromatic
CH2–CH2–
CH3
1a
2
1,648
1,644
1,598
1,644
1,622
13.31
13.44
13.91
13.54
14.50
8.58
8.59
8.62
8.57
8.59
6.80–7.19
6.85–7.66
6.81–7.79
6.78–7.03
7.62–7.83
4.10
3.94
4.03
3.93
4.00
–
–
3
2.43
3.77
–
4a
5a
a
The complexes were prepared by in situ method according to the reported methods [9, 24–26] as well as
by direct reaction of the metal and the free ligand
was allowed to equilibrate at appropriate temperature and pressure under rigorous
stirring for 30 min. After isolation of the used catalyst by flash chromatography on
silica, the reaction mixture was chemically characterized by 1H NMR spectroscopy
in which such products as cumyl alcohol, acetophenone, and a-methylstyrene were
found. The concentration of each product was followed by GC–MS and confirmed
by comparison with (and addition of) authentic samples.
Preparation of Co(II) Schiff base complexes (1–5)
The compounds were prepared and characterized according to the procedures
previously reported [24–27]. The spectroscopic data (Table 1) confirmed on the
proposed structures for the ligands and their respective complexes.
Synthesis of LCo(III)-OOcumyl complex (Fig. 2)
Five mmol of 5 was added to 7.5 mmol of cumene hydroperoxide and the mixture
was transferred to a 250-mL flask which contained 200 mL dichloromethane.
The suspension was stirred for 10 min at ambient temperature until it became
transparent. Then, the solution dried over Na2SO4 prior to evaporation of solvent
and separation of the solid product. The remnant was filtered and identified. Anal.
Fig. 2 The proposed structure
for cumylperoxy complex
(LCo(III)-OOcumyl) by 1H
NMR spectroscopy
O
O
O
O
Co
N
N
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Solvent-free oxidation of cumene
875
Calcd for C25H25N2O4Co: C, 63.03; H, 5.29; N, 5.88. Found: C, 62.7; H, 5.4; N, 5.5.
EI–MS (70 eV, m/z?): calcd for Co(salen)OOcumyl, 476.4; found, 476.3.
IR bands (cm-1): C=N, 1,602; O–O, 957–860 .
1H NMR d (ppm): 8.37 N=CH, 7.15–7.50 ArH, 4.55, NCH2, 1.47 CCH3,
Results and discussion
Spectroscopic characterization of Cobalt salen-type 1–5 (Fig. 1)
and LCo(III)-OOcumyl complex (Fig. 2)
After preparation of the complexes according to the previous reports [ 24–27] from
their ligands in 2 and 3 and/or by in situ method in 1, 4, and 5, their C=N infrared
1
bands for the products were confirmed on Schiff base reaction (see Table 1). H
NMR spectra for the ligands of 2 and 3 showed their CH=N bands at 8.59 and
8.64 ppm, respectively.
1
Furthermore, the H NMR of LCo(III)-OOcumyl complex exhibited expected
resonances. A broad doublet at 1.47 ppm was observed which pertains to methyl
groups of cumyl substitution in which the coordination of cumyl hydroperoxy has
transferred them to higher chemical shifts. Two singlets (one broad, another sharp)
centered at 4.55 and 8.37 ppm were attributed to NCH2 and iminic N=CH,
respectively. The chemical shifts for the aromatic hydrogens were also observed at
7.15–7.50 ppm. Analysis by ESI mass spectral data of the complexes indicated ions
at m/z 716.4 (%4) [M]?.
Stoichiometric oxidation of cumene
In order to investigate the mechanism of the catalytic reaction, the oxygen
absorption of the prepared complexes (Fig. 1) was measured during the oxidation
process of cumene and they were compared with each other under identical
conditions. Basically, Co(salen)-type complexes are recognized as good oxygen
carriers [28] under reversible coordination of dioxygen and their partial oxidization
yields superoxo compounds [10]. In a series of experiments, their difference in
dioxygen absorption was measured and the results are shown in Fig. 3.
The catalytic reaction in the presence complex 1 was also conducted at different
temperatures, typically at 30, 40, 44, 47, and 60 °C; however, only in the latter case
a measurable amount of oxygen adsorption was observed (see Fig. 4). Furthermore,
in our next catalytic experiments conducted at different temperatures, it was found
that detection of the reaction products would give the best results only at 60 °C.
As it has been mentioned by Evgeny et al. [5], the kinetic curve for cumyl
hydroperoxide formation in emulsion oxidation of cumene by O2 at 358 K was so
slow that it did not produce a measureable amount of any product by GC-mass
analysis within 15 min. Our complementary experiments under these conditions
also confirmed that no measureable oxidation occurred in the absence of the
catalysis. Furthermore, the addition of a free radical inhibitor, e.g. benzoquinone,
precluded the reaction. These observations indicated a free radical chain reaction.
123

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B. Ghanbari et al.
Fig. 3 Oxygen absorption by the complexes 1–5 in neat cumene
Fig. 4 The effect of different temperatures: a 40, b 44, c 47, and c 60 °C on oxygen absorption of 1 in
cumene
In order to compare the dioxygen-adduct formation for these complexes [see Eq. 1,
LCo(II) denotes on Co(salen)], the rate of oxygen uptake by the complexes after
5 min were arranged arbitrarily and compared as shown in Fig. 5.
LCoðIIÞ þ O2 ! LCoðIIÞOO
ð1Þ
It can be seen that introducing electron withdrawing substitutions on the ligands
decreases oxygen absorption of the complexes remarkably. Furthermore, it has been
reported [10] that the rate constants for oxidations of C–H bonds by 1, including
cumene and a range of allylic and benzylic substrates, correlate with the strength of
the C–H bond being cleaved. This supports an H-atom transfer from cumene in a
rate-limiting step (Eq. 2) to these dioxygen Co(II) complexes (Eq. 1), LCo(II)OO,
which was responsible for the production of a stable cumyl radical (Eq. 2):
123

Solvent-free oxidation of cumene
877
Fig. 5 O2 absorption by the complexes after 5 min
Cumene þ LCoðIIÞ ! OO cumemeꢀ þ LCoðIIIÞOOH
ð2Þ
This was confirmed by the fact [29, 30] that, as the ligand (L) facilitated charge
transfer from Co(II) complex to dioxygen, O–O basicity increased. Such ligands are
capable of activating hydrogen atom abstraction from cumene.
Product distribution
After isolation of the used catalyst, the reaction mixture was first characterized by
1H NMR in CDCl3. Figure 6 shows the respective chemical shifts of the following
products in this mixture. Cumyl alcohol (2), a-methylstyrene (3), acetophenone (4)
and trace amount of dicumyl were confirmed by GC–MS in comparison with
authentic samples. The yields of these products are shown in Table 2.
Figure 7 shows the yield percent of each product in the cumene oxidation
reaction catalyzed by different substituted Co(salen)-type complexes in which the
complexes are arranged in the same order as in Fig. 5.
As can be seen in Fig. 7, the selectivity of the reaction (here defined as the
difference between the cumyl alcohol and acetophenone yield curves) increases
along the observed order for decreasing of oxygen absorption. It was observed that,
with this oxygen-absorption order for the complexes, a-methylstyrene and cumyl
alcohol productions decreased while the production of acetophenone decreased at
first then showed an increasing trend.
On the other hand, the best TON for these catalysts was measured for complex 3
while complex 4 rendered the least TON (see Table 3).
Generally, no relationship was found between TONs and oxygen absorption
properties or their product distribution of the complexes.
Autoxidation mechanism
In a separate experiment, it was found that oxygen absorption was accelerated upon
addition of AIBN to neat cumene. This should occur as a result of more production
of cumyl free radicals (Rꢀ in Eq. 3):
123

878
B. Ghanbari et al.
OH
O
Co(salen)
O2
+
+
3
1
2
4
+
trace
Fig. 6 1H NMR spectrum of the reaction mixture after filtration of the catalyst. The numbers refer to
chemical shifts (in ppm, versus TMS as reference) for: 1 cumene, 2 cumyl alcohol, 3 a-methylstyrene,
and 4 acetophenone
Table 2 The product yields
(%) of the cumene oxidation by
cobalt salen complexes
Catalyst
a-methylstyrene
Acetophenone
Cumyl
alcohole
1
2
3
4
5
30
23
20
20
13
30
25
24
27
34
24
25
25
18
15
123

Solvent-free oxidation of cumene
879
Fig. 7 The effect of substitution on the cobalt salen complexes on different product yields of cumene
oxidation; the yields were calculated with respect to cumene
Table 3 The comparison of
TONs for different substituted
cobalt salen complexes
Catalyst
TON
1
2
3
4
5
315
304
333
162
180
TON turnover number, moles of
product per one mole of catalyst
Rꢁ þ O2 ! ROOꢁ
ð3Þ
Then, the presence of cumylperoxy radicals in the solution leads to the following
step:
ROOꢁ þ RH ! ROOH þ Rꢁ
ð4Þ
The same situation was observed in cumene oxidation by LCo(II). After completion
of the reactions in Eqs. 1 and 2, subsequent cumylperoxide radical (Eq. 3) would be
capable of oxidizing LCo(II) to LCo(III) (Eq. 5). The resulting LCo(III)OH has
been shown to react with cumene hydroperoxide which results in the production of
the cobalt(III)cumylperoxy (LCo(III)OOR) complex (Eq. 6) [31].
ROOH þ LCoðIIÞ ! ROꢁ þ LCoðIIIÞOH
LCoðIIIÞOH þ ROOH ! LCoðIIIÞOOR þ H2O
ð5Þ
ð6Þ
Although LCo(III)OH was not identified in our experiments, we observed water
absorption by Na2SO4 from the reaction mixture. The mechanism was similar to that
123

880
B. Ghanbari et al.
Table 4 The yield percents for the cumene oxidation products catalyzed by the cobalt salen complex
Product
a-methylstyrene
Acetophenone
20
Cumyl alcohol
35
Dicumyl
Trace
Yield (%)
6
proposed by Drago et al. for cyclohexane and adamentan aerobic oxidation by
Co(NCMe)4(PF6)2 catalyst in 1991 [32].
In order to verify formation of the peroxy complex intermediate, kinetically inert
low-spin d6 Co(III) center, LCo(III)-OOR (L = salen and R = cumyl) was
synthesized and isolated to study its reactivity in the present catalytic reaction.
Discrete oxidation of LCo–OOR (in Fig. 2) was performed and the catalytic effect
of the complex during the cumene oxidation process was monitored in the same
condition applied for 1–5 but under anaerobic condition (N2 atmosphere). After
reaction completion by LCo(III)-OOR, the main reaction products were identified
by GC–MS as a-methylstyrene, acetophenone and cumyl alcohol. Table 4 shows the
separated products for this reaction.
The results showed strong evidence for formation of LCo(III)OOcumyl as
reaction intermediate because of production of the same products as the cumene
oxidation by the 1–5 complexes. Coordination of the alkyl hydroperoxide to cobalt
was assumed to weaken the O–O bond of the ROO group [2]. The process of
oxidation by LCo(III)OOR was closely related to the modes of peroxide complex
decomposition, resulting in homolysis of Co–O and/or O–O bonds [33]. There
might be a more probable homolytic scission for the O–O bond in LCo(III)OOR
than Co–O [7]. For instance, it has been reported in [Co(salen)-(L1)(OOR)]
(L1 = py, 4-Mepy) complexes that homolysis and radical formation can occur both
on O–O and Co–O bonds [33].
Fragmentation of LCo(III)OOcumyl generates oxygen-based radicals that initiate
a radical-based autoxidation process in this reaction. This fact allows oxidation of
hydrocarbons by these complexes under mild conditions [2]. These observations
lead us to propose the mechanism shown in Scheme 1.
In Path 1 of this mechanism, labile benzylic C–H bond in cumene produces
cumyl alcohol and carbonyl compound [7]. In a parallel reaction in this Path, the
ROꢀ cumyloxyl radical takes part in a b-scission reaction and releases CH3 radical
while it converts to acetophenone as the second reaction product [34]. Basically, it
has been found that, when relatively strong-field ligands are bonded to the cobalt
center of [LCo(III)-OOR] complexes, O–O homolysis is the primary mode of
decomposition [2].
In another reaction path, LCo(III)OOcumyl undergoes hemolytic O–O bond
breaking through Path 2 in which a-methylstyrene as well as acetophenone were
produced as the major products. We studied products of the oxidation reaction for
some of the intermediate compounds proposed in different paths of the mechanism
(Scheme 1). The role of 2-phenylpropion aldehyde was examined by determining
the products of the oxidation reactions of: (1) a-methylstyrene (Reaction 1), and (2)
2-phenylpropion aldehyde (Reaction 2) catalyzed by 5 in the presence of dioxygen
123

Solvent-free oxidation of cumene
881
Scheme 1 The product distribution of oxidation reaction of: (1) a-methylstyrene, and (2)
2-phenylpropion aldehyde catalyzed by 5 [Co(3,5-Brsalen)] in the presence of O2
in two separate experiments. Scheme 2 shows the product distribution and their
relative yields in each of these cases determined by GC–MS.
Furthermore, it was observed that the major reaction product of a-methylstyrene
was 2-phenylpropion aldehyde (20%) which was accompanied by a minor amount
of acetophenone. The formation of 2-phenylpropion aldehyde could be attributed to
further oxidation of a-methylstyrene by the ROOꢀ radical that was more preferable
in the presence of 5. In conclusion, having 2-phenylpropion aldehyde as
intermediate in the oxidation reaction catalyzed by Cobalt-salen type complex,
the production of a-methylstyrene fits into the Path 2 (Scheme 1) during homolytic
cleavage of Co–O bond in LCo(II)OOR. It has been reported [2] that, when ligands
bonded to cobalt are weak field, Co–O bond homolysis with concomitant reduction
of Co(III) to Co(II) (and formation of LCo(II)) occurs in most cases. That is why, in
Fig. 6, the complex 5 should have produced a-methylstyrene more than the strong-
field-ligand complex 1. While increasing concentration of cumyl peroxide (ROOꢀ)
radical, a-methylstyrene undergoes epoxidation prior to Lewis-acid-mediated
ring opening reaction [7] (see Scheme 1). Further hydrogen abstraction from the
aldehydic positions of 2-phenylpropanal promote the oxidation process by
producing acyl radical, which rapidly decarbonylates. The resulting 1-phenethyl
radical PhCꢀMe could also be another source of acetophenone.
However, Scheme 2 predicts why a-methylstyrene concentration does not follow
from the observed oxygen absorption trend of the complexes. This could be
described according to the weak ligand field feature of the complex and consequent
proposed Path 2 mechanism for such complexes. The products of the reaction in
these cases lead us to an unenviable change of a-methylstyrene to acetophenone
during the two cyclic reactions (Scheme 2). Finally, the non-linear dependence of
123

882
B. Ghanbari et al.
O
H2C
CH3
H3C
O
CH3
+
O
5 + O2
+
+
traces
H3C
CH3
Reaction 1
20%
3%
12%
O
H3C
H3C
O
H2C
CH3
+
O
5 + O2
+
traces
+
Reaction 2
H3C
CH3
3%
18%
6%
Scheme 2 The product distribution of oxidation reaction of: (upper) a-methylstyrene, and (lower)
2-phenylpropion aldehyde catalyzed by 5 [Co(3,5-Brsalen)] in the presence of O2
acetophenone production was attributed to its different source in both Paths in
Scheme 1.
Conclusions
Cobalt(II) Schiff-base complexes are efficient catalysts in the oxidation of cumene
by dioxygen. The products of cumene oxidation were determined as cumyl alcohol,
a-methylstyrene, and acetophenone. It was observed that the substitution on the
ligand had a prominent effect on the product distribution. LCo(III)-OOcumyl was
synthesized and shown to be the reaction intermediate. A reaction mechanism was
proposed in terms of the reaction products as well as the intermediate pathway.
Acknowledgment Financial support from Research Office of Sharif University of Technology is
acknowledged.
References
1. F.A. Chavez, J.M. Rowland, M.M. Olmstead, P.K. Mascharak, J. Am. Chem. Soc. 120, 9015 (1998)
2. F.A. Chavez, P.K. Mascharak, Acc. Chem. Res. 33, 539 (2000)
3. A. Bhattacharya, Chem. Eng. J. 137, 308 (2008)
4. S. Vetrivel, A. Pandurangan, J. Mol Cat. A. 227, 269 (2005)
5. T. Evgeny, I. Denisov, B. Afanas’ev, Oxidation and Antioxidants in Organic Chemistry and Biology
(Taylor & Francis Group, New York, 2005), p. 437
6. G.P. Armstrong, R.H. Hall, D.C. Quinn, J. Chem. Soc. 666–672 (1950)
7. T.W. Swaddle, Inorganic Chemistry-An industrial and environmental perspective, 1st edn.
(Academic Press, San Diego, 1997), pp. 399–401
8. T. Tsumaki, Bull. Chem. Soc. Jpn. 13, 252 (1938)
123

Solvent-free oxidation of cumene
883
9. R.H. Bailes, M. Calvin, J. Am. Chem. Soc. 69, 1886 (1947)
10. E.C. Niederhoffer, J.H. Timmons, A.E. Martell, Chem. Rev. 84, 137 (1984)
11. T. Matsuura, Tetrahedron 33, 2869 (1977)
12. E. Bolzacchini, A.M. Brambilla, M. Orlandi, B. Rindone, Life Chem. Rep. 13, 71 (1995)
13. Y. Aimoto, W. Kanda, S. Meguro, Y. Miyahara, H. Ohkawa, S. Kida, Bull. Chem. Soc. Jpn. 58, 646
(1985)
14. A. Nishinaga, H. Ohara, H. Tomita, T. Matsuura, Tetrahedron Lett. 24, 213 (1983)
15. A. Nishinaga, T. Tsutsui, H. Moriyama, T. Wazaki, T. Mashino, Y. Fujii, J. Mol. Catal. 83, 117
(1993)
16. A. Nishinaga, T. Tojo, T. Matsuura, J. Chem. Soc. Chem. Commun. 896 (1974)
17. F. Benedini, G. Galliani, M. Nali, B. Rindone, S. Tollari, J. Chem. Soc. Perkin Trans. 1963 (1985)
18. A. Nishinaga, S. Yamazaki, T. Matsuura, Tetrahedron Lett. 29, 4115 (1988)
19. A. Nishinaga, S. Forster, E. Eichhorn, B. Speiser, A. Rieker, Tetrahedron Lett. 33, 4425 (1992)
20. S. Forster, A. Rieker, K. Maruyama, K. Murata, A. Nishinaga, J. Org. Chem. 61, 3320 (1996)
21. M. Nali, B. Rindone, S. Tollari, L. Valletta, J. Mol. Catal. 41, 349 (1987)
22. E. Brazi, A. Robine, H. Mimoun, J. Fischer, R. Weiss, J. Am. Chem. Soc. 107, 3534 (1985)
23. B. Ghanbari, B. Tangeysh, J. Coord. Chem. 62, 3384 (2009)
24. D. Chen, A.E. Martell, Inorg. Chem. 26, 1026 (1987)
25. J. Endo, Nippon Kagaku Zasshi (Japan.) 70, 77 (1949)
26. M.R. Bermejo, A. Castineiras, J.C. Garcia-Monteagudo, M. Rey, A. Sousa, J. Chem. Soc. Dalton. 14,
2935 (1996)
27. C.A. Reed, S.K. Cheung, Proc. Natl. Acad. Sci. USA 74, 1780 (1977)
28. R.D. Jones, D.A. Summerville, F. Basolo. Chem. Rev. 79, 139 (1979)
29. A. Zombeck, R.S. Drago, B.B. Corden, J.H. Gaul, J. Am. Chem. Soc. 103, 7581 (1981)
30. R.S. Drago, R.P. Perito, J. Am. Chem. Soc. 107, 2903 (1985)
31. L. Saussine, E. Brazi, A. Robine, H. Mimoun, J. Fischer, R. Weiss, J. Am. Chem. Soc. 107, 3534
(1985)
32. A.S. Goldstein, R.S. Drago, Inorg. Chem. 30, 4506 (1991)
33. A. Nishiniga, H. Tomita, H. Ohara, Chem. Lett. 1751 (1983)
34. A. Brown, I. Lusztyk, J. Am. Chem. Soc. 115, 466 (1993)
123