K. Pamin et al. / Catalysis Communications 39 (2013) 102–105
103
30
complexes bearing electron-withdrawing perfluoroalkyl substituents
and/or t-butyl electron-donating substituents in the oxidation of
cycloalkanes with molecular oxygen (Scheme 1).
ketone
alcohol
25
20
15
10
5
2. Experimental
2.1. Synthesis
Manganese, iron, cobalt, nickel, copper and zinc complexes of salen
ligand 1 are well-known compounds and they were readily obtained
following standard literature methods [23]. Functionalized salen deriv-
atives 2–6 and the corresponding manganese complexes were also syn-
thesized according to procedures described elsewhere [24–28]. Their
spectroscopic data acquired from FT-IR, UV–vis and NMR techniques
were in agreement with the reported literature data.
0
2.2. Catalyst characterization
2.2.1. UV–vis spectroscopy
Fig. 1. Oxidation of different cycloalkanes by simple manganese salen complex Mn-1.
UV–vis measurements were performed on a Perkin Elmer Lambda
35 double beam spectrophotometer, using quartz cells of 1 cm optical
path. Electronic spectra of the metallocomplexes and heterogenized
metallocomplexes were measured in water solution with the concen-
tration of 2 ⋅ 10−5 mol/L.
products of cycloalkanes oxidation with molecular oxygen were
cycloketone and cycloalcohol. No oxidation occurred at 393 K in the
absence of the catalyst. For cyclopentane, only 1.1% yield to ketone
and 0.5% yield to alcohol were obtained in the presence of Mn-1
after 6 h. Under analogous conditions, cyclohexane was converted
to ketone and alcohol with yield of 3.2% and 1.2%, respectively. Final-
ly, the oxidation of cyclooctane catalyzed by Mn-1 resulted in 23.9%
yield to ketone and 2.3% yield to alcohol. In this case, the amount of
cyclooctane hydroperoxide in the reaction mixture evaluated by
iodometric titration [29] after 6 h was only 0.08%, too low to have
an impact on the results of the subsequent GC analysis. It is
well-known that the selective catalytic oxidation of C\H bonds re-
sults in the formation of hydroperoxides [30–35]. These compounds
in the course of the reaction upon the action of the catalyst convert
to alcohol and ketone. In our reaction conditions cycloalkyl hydroper-
oxide in the presence of metallosalen catalysts is almost completely
decomposed. Once the following order of catalytic activity was
established: cyclooctane > cyclohexane > cyclopentane, the most
reactive among investigated cycloalkanes was selected for further
investigations.
In a first series of experiments, the activity of various transition
metal complexes of plain salen ligand 1 was evaluated. All these cat-
alysts were active in the oxidation of cyclooctane (1) with molecular
oxygen resulting in cyclooctanone (2) and cyclooctanol (3) as the
main products. However, the results summarized in Table 1 show a
clear influence of the complexed metal cation on catalytic activity.
As one can see metallosalen complexes can be divided into two
groups. Manganese, iron and cobalt salen complexes are the most ac-
tive catalysts, while nickel, copper and zinc salen complexes are far
less useful. This is in line with the results obtained in oxidation pro-
cesses catalyzed by tetraarylporphyrin metallocomplexes [36,37].
Among plain salen metallocomplexes, cobalt complex Co-1 afforded
the highest products yield while manganese catalyst Mn-1 showed
the highest ratio of cyclooctanone to cyclooctanol and therefore the
latter complex was selected for further studies.
2.2.2. Infrared spectroscopy
FT-IR spectra were recorded on a Nicolet 800 spectrometer in KBr
pellets over the range of 400–4000 cm−1 under the atmospheric
conditions.
2.2.3. NMR spectroscopy
1H NMR (300 MHz), 13C NMR (75.4 MHz) and 19F NMR (282 MHz)
spectra were recorded on a Brucker AC 300 spectrometer with
tetramethylsilane (δ = 0), CDCl3 (δ = 77) and CFCl3 (δ = 0) as
internal standards.
2.3. General procedure for the oxidation of cycloalkanes
The oxidation of cyclooctane (cyclopentane or cyclohexane) was
performed in a stainless steel batch reactor system at 393 K and
under the air pressure of 10 atm, with the substrate to oxygen molar
ratio set at 6.5. In a typical experiment, 60 mL of substrate containing
the amount of catalyst providing a concentration of 3.3 × 10−4 M was
introduced in the deaerated autoclave and the whole system was heat-
ed until a temperature of 393 K was reached. Air was then introduced
and after 6 h the oxidation products were analyzed by GC Agilent
6890 N equipped with an Innowax (30 m) column. The yield values
were verified by addition of an internal standard, chlorobenzene, at
the end of the reaction.
3. Results and discussion
Three members of homologous series, viz. cyclopentane, cyclohex-
ane or cyclooctane were applied as substrates and studied in oxida-
tion catalyzed by manganese salen complex Mn-1 (Fig. 1). The main
Following these preliminary assays, manganese complexes of
modified salen ligands, 2–6 (Fig. 2) bearing electron-withdrawing
n-C8F17 fluorous substituents and/or t-butyl electron-donating sub-
stituents on the 3,3′ and/or 5,5′ positions of the aryl ring, were also
studied in the oxidation of cyclooctane with molecular oxygen. The
data in Table 2 highlight the influence of the substitution pattern on
the catalytic activity of these complexes. Regardless of the substitu-
ents nature, Mn-2–Mn-6 showed a higher catalytic activity than
O
OH
metallosalen catalyst
O2(air),120°C,10 atm,6 h
+
1
2
3
Scheme 1. Oxidation of cyclooctane with molecular oxygen in the presence of
metallosalen complexes.