Selective Oxidation of Aromatic Olefins Catalyzed by Copper(II) Complex in Micellar Media
2
.2 Instruments and Methods
3 Results and Discussion
Ultraviolet–visible absorbance measurements were per-
formed with a spectrophotometer (Hitachi U-2910, Japan).
Elemental analyses were implemented with an elemental
analyzer (MOD 1106, Carlo Erba Company of Italy). The
3.1 Catalytic Oxidation of Olefins
In this work, the oxidation of aromatic olefins 1a and 2a by
H O to secondary alcohols and ketone in the presence of
2
2
−
1
typical reaction solution containing 0.01 mol L olefin,
CuL, TEMPO–CuL or TEMPO were investigated at 40 °C,
respectively (Scheme 1). The concentrations of alcohols,
ketones and olefins in reaction solution were quantitatively
detected by GC and HPLC (Fig. S2). The olefins 1a, 2a
were oxidized by H O to corresponding secondary alco-
−
1
−1
0
.004 mol L copper(II) complex and 0.2 mol L H O
2 2
was sealed and reacted at pH 2 and 40 °C. The concentra-
tions of reaction reactants and products in the reaction
solution were quantitatively detected by HPLC and GC.
The reaction solution was analyzed by GC (FULI 9790,
China), GC–MS (Agilent 5973 Network 6890 N, Japan)
and HPLC (waters 1525, USA) with a UV–vis detector
2
2
hols MVA, HMOPE catalyzed by CuL complex in CTAB,
SDBS, TX-100 and aqueous solution, respectively (Table 1).
From Table 1, it is noted that high selectivity (S>90%) of
secondary alcohols 1c and 2c could be achieved when the
oxidation reaction carried out in micellar media. Compared
with in micellar media, only 6.1 and 6.9% of selectivity of
1c and 2c could be found in aqueous solution, respectively,
which indicated that the CuL complex displayed excellent
catalytic ability for the oxidation of 1a, 2a to secondary
alcohols in the micellar media. 2,2,6,6-tetramethylpiperidine
1-oxyl (TEMPO), a typical N–O radical, was widely applied
in oxidation of organic compounds owing to the advantage
of reaction selectivity and mild conditions. In order to inves-
tigate the effect of TEMPO, we introduce TEMPO to this
reaction. It should be noted that the reaction rate was obvi-
ously accelerated and selectivity also changed when TEMPO
participated in the oxidation reaction (Table 2). As can be
seen from Table 2, the 100% conversion of substrates and
over 95% selectivity of ketones could be observed when both
CuL complex and TEMPO existed (Table 2, entries 5–8).
Especially, it can be seen that the olefins can be oxidized
into ketones with 100% selectivity in CuL-TEMPO associ-
ated system in aqueous solution (Table 2, entry 5). As seen
in Tables 1 and 2, micelle medium can change the reaction
rate as well as the selectivity of product. The aromatic sec-
ondary alcohols were the chief products (S> 90%) in the
CuL–H O system in micellar media (Table 1, entries 2–4).
(
detected at λ = 254 nm).
2
.3 Preparation of Copper(II) Complex
The ligand 6,8,15,17-tetramethyl-7,16-dihydrodibenzo-
1
,4,8,11-tetraazacyclotetrade-cine (L) and copper(II)
complex CuL was prepared, purified and characterized
as described in the literature [22]. Analysis: Anal. calcd.
for C H N Cu: C, 64.76; H, 5.93 N, 13.73%; found:
2
2
24
4
C, 64.81; H, 5.90; N, 13.75%; ICP-AES analysis for Cu:
1
5.56 wt%. The chemical structure of copper(II) complex
was outlined in Fig. 1S in supporting information.
2
.4 Synthesis of Substrates DEVB and MOVP
1
,2-dimethoxy-4-vinylbenzene (DEVB) was synthesized
as follows: DMOAP (1.76 g) was added to absolute metha-
nol (20 mL), then sodium borohydride (0.88 g) was added
slowly into the mixture and the solution was stirred for
4
h under ice water bath. Then the reaction solution was
refluxed for 10 h. The suspension gradually became a clear
solution, then it was adjusted to pH 3.0 with hydrochlo-
ric acid. The solvent was removed by vacuum distillation
then the reaction mixture was poured into iced water (20
mL) and extracted with 1,2-dichloroethane. The extract
was eluted with the mixture of ethyl acetate and n-hexane
in 1:1 (v/v) and then distilled to obtain the product DEVB.
It was determined by elemental analysis and GC/MS.
m/z: 164, 149, 121, 103, 91, 77, 65, 51, Anal. Calcd. for
C H O : C, 73.15; H, 7.37%; found: C, 73.17; H, 7.36%.
2
2
However, when TEMPO was used as oxidant, lots of aro-
matic ketone generated in micellar solution (Table 2, entries
2–4). Further, it can be observed that when this reaction
was carried out in aqueous solution, the main product was
aromatic ketone (Table 1, entry 1; Table 2 entries 1 and 5).
This implies that the oxo species could play important role
in the oxidation reaction. Catalyst CuL, as a cheap transition
metal complex, can activate hydrogen peroxide to generated
“associated radical CuL–·OOH”, which was described in
our previous studies [25, 26]. TEMPO can oxidize alcohol
to ketone. Further we found the complex CuL can catalyze
the reduction state TEMPOH to oxidation form TEMPO in
the presence of H O . Therefore in the presence of TEMPO,
1
0
12
2
The compound of 2-methoxy-4-vinylphenol (MOVP)
was similar to the method of DEVB synthesis. It was deter-
mined by elemental analyses. Anal. Calcd. for C H O : C,
9
10
2
7
1.98; H, 6.71%; found: C, 72.02; H, 6.70%.
2
2
CuL catalyzes the reduced TEMPOH back to TEMPO, this
greatly accelerated the conversion from alcohol to ketone,
1
3