G Model
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A.P.C. Ribeiro et al. / C. R. Chimie xxx (2015) xxx–xxx
3
2.2. General procedure for the peroxidative oxidation of
cyclohexane
3. Results and discussion
The bis(
m
4-(ae)-cyclohexane-1,4-dicarboxylato-O,O0,
4-O,O0,O00,O0000
-
The cycloalkane oxidations were carried out under air, in
a biphasic based IL system, contained in a round bottom
flask, with vigorous stirring, and using [bmim][PF6] as a
solvent(uptoatotalvolumeof5.0 mL). Typically,thecopper
catalyst was added to the solventas a solid orin the form of a
stock solution in the IL. Cyclohexane (2.3 mmol) was then
introduced, and the reaction started when hydrogen
peroxide (50% in H2O, 0.68 mL, 11 mmol) was added in
one portion. The final concentrations of the reactants in the
reaction mixture were as follows: catalyst precursor
(2ꢀ10ꢁ4–2ꢀ10ꢁ2 molꢀLꢁ1), substrate (0.46 molꢀLꢁ1), H2O2
(2.2 molꢀLꢁ1) and pyridine (0.005 molꢀLꢁ1). The reaction was
stopped and 5 mL of diethylether were added for extraction
of the organic products.
The catalyst’s recyclability was investigated using the IL
medium, for up to three consecutive cycles. Each cycle was
initiated after the preceding one upon addition of new
typical portions of all other reagents. After completion of
each run, the products were analyzed and the IL with the
catalyst was recovered by drying in vacuo overnight at
70 8C.
O00,O000)-tetracopper(II) complex [(CuL)2(
m
CDC)]2ꢀ2H2O (1) [HL = 2-(2-pyridylmethyleneamino)ben-
zenesulfonic acid, CDC = cyclohexane-1,4-dicarboxylate]
(Fig. 2) was tested as a catalyst for the peroxidative (with
H2O2) oxidation of cyclohexane (CyH), using 1-butyl-3-
methylimidazolium hexafluorophosphate ([bmim][PF6])
as a solvent, at 50 8C (Scheme 1).
In view of the oxidative nature of the catalysis, the
redox stability of [bmim][PF6] was first investigated by
cyclic voltammetry (CV) at a platinum-working electrode
at room temperature. It did not exhibit any redox activity
in the wide potential range from–2.0 to 2.0 V vs. Fc/Fc+
(internal reference) as depicted in Fig. 3. However,
this wide electrochemical window, which is favorable to
the application of the IL, is significantly reduced in the
presence of water (Fig. 3) on account not only of the redox
properties of water but also, e.g., of the susceptibility of
[PF6]– to hydrolysis [4(b),4(m)]. Hence, the presence of
water cannot be neglected. In any case, the higher stability
of [PF6]– relatively to other common used anions in ILs [10],
together with the ability of [bmim][PF6] to easily dissolve
1, makes this IL a good choice for use as a solvent.
Cyclohexane is oxidized to cyclohexanol (CyOH) and
cyclohexanone (CyO), via cyclohexyl hydroperoxide
(CyOOH, primary product), according to Scheme 1, and
the results are shown in Table 1. Shul’pin‘s method [9] was
used, where the addition of PPh3 to the samples prior to the
GC analysis is applied to prove indirectly the presence of
CyOOH. PPh3 reduces CyOOH to CyOH, resulting in an
increase in the alcohol amount and a decrease in that of
CyO. This behavior has been observed in several perox-
idative oxidation systems [2(h),2(r),3(g),11,12].
All products formed were identified by GC and their
retention times confirmed using those of commercially
available products. Nitromethane (0.05 mL) was used as a
GC internal standard. Chromatographic measurements
were performed in a Fisons Instruments GC 8000 series gas
chromatograph with
a
BP20/SGE (30 m ꢂ 0.22 mm ꢂ
0.25 mm) capillary column (FID detector) and using
helium as a carrier gas, whereas the analyses of the
chromatographic peaks were done by the corresponding
Jasco-Borwin v.1.50 software.
A PerkinElmer Clarus
600 gas chromatograph, equipped with two capillary
columns (SGE BPX5; 30 m 0.32 mm 25 mm), one having an
EI–MS (electron impact) detector and the other one with a
FID detector, were used for analyzing the reaction
mixtures. Helium was used as the carrier gas. The reaction
mixtures were analyzed twice by GC: with and without
adding an excess of solid triphenylphosphine. The addition
of this phosphine to the final organic phase reduces
cyclohexyl hydroperoxide, if present, to the corresponding
alcohol, and hydrogen peroxide to water. Comparison of
the results of both analyses allows estimate the amount of
cyclohexyl hydroperoxide, following a method developed
by Shul’pin [9]. Blank experiments in ionic liquid were
performed and confirmed that no alkane oxidation
products (or only traces, below 1%) were obtained in the
absence of the metal catalyst. The catalytic activity of the
ligand was also tested and no products were detected.
The effects of a basic and an acid additive were also
tested and a yield-enhancing effect of pyridine was found
(Table 1, entries 11 and 16), whereas no promoting effect
was observed upon addition of trifluoroacetic acid (TFA)
(Table 1, entries 16 and 17). Hence, the reactions were
typically studied in the presence of pyridine. This base
conceivably promotes the proton-transfer steps involved
in the formation of the hydroxyl radical from H2O2 [13(a)].
A blank control experiment carried out in [bmim][PF6]
but in the absence of 1 resulted in a negligible product
yield (lower than 1%, Table 1, entry 18).
The effect of catalyst loading in the overall yield was
investigated, and the highest yield (36%) was obtained at
the highest concentration of catalyst used (2ꢀ10ꢁ2, Table 1,
entry 11 and Fig. 4). The highest yield is obtained after a
2-h reaction time, beyond which a minor decrease is
OOH
OH
O
aq. H2O2, Cu-Catalyst
+
Scheme 1. Peroxidative oxidation of cyclohexane catalyzed by the tetranuclear Cu(II) complex 1, in [bmim][PF6].
Please cite this article in press as: Ribeiro APC, et al. Catalytic oxidation of cyclohexane with hydrogen peroxide and a