A mild catalytic oxidation system: FePcOTf/H2O2 applied for cyclohexene dihydroxylation

Article abstract of DOI:10.3390/molecules20058429

Iron (III) phthalocyanine complexes were employed for the first time as a mild and efficient Lewis acid catalyst in the selective oxidation of cyclohexene to cyclohexane-1,2-diol. It was found that the catalyst FePcOTf shown excellent conversion and moderate selectivity relative to other iron (III) phthalocyanine complexes. The optimum conditions of the oxidation reaction catalyzed by FePcOTf/H₂O₂ have been researched in this paper. Iron (III) phthalocyanine triflate (1 mol %) as catalyst, hydrogen peroxide as oxidant, methanol as solvent, and a mole ratio of substrate and oxidant (H₂O₂) of 1:1 were used for achieving moderate yields of 1,2-diols under reflux conditions after eight hours.

Full text of DOI:10.3390/molecules20058429

Molecules 2015, 20, 8429-8439; doi:10.3390/molecules20058429
OPEN ACCESS
molecules
ISSN 1420-3049
www.mdpi.com/journal/molecules
Communication
A Mild Catalytic Oxidation System: FePcOTf/H2O2 Applied for
Cyclohexene Dihydroxylation
Baocheng Zhou 1,2 and Wenxing Chen 2,*
1
Department of Chemistry, Zhejiang Sci-Tech University, Hangzhou 310018, China;
E-Mail: zhoubc1982@163.com
2
Key Laboratory of Advanced Textile Materials and Manufacturing Technology,
Ministry of Education of China, Zhejiang Sci-Tech University, Hangzhou 310018, China
* Author to whom correspondence should be addressed; E-Mail: wxchen@zstu.edu.cn;
Tel.: +86-571-86843611.
Academic Editor: Giuseppe Mele
Received: 27 March 2015 / Accepted: 24 April 2015 / Published: 11 May 2015
Abstract: Iron (III) phthalocyanine complexes were employed for the first time as a mild and
efficient Lewis acid catalyst in the selective oxidation of cyclohexene to cyclohexane-1,2-diol.
It was found that the catalyst FePcOTf shown excellent conversion and moderate
selectivity relative to other iron (III) phthalocyanine complexes. The optimum conditions
of the oxidation reaction catalyzed by FePcOTf/H2O2 have been researched in this paper.
Iron (III) phthalocyanine triflate (1 mol %) as catalyst, hydrogen peroxide as oxidant,
methanol as solvent, and a mole ratio of substrate and oxidant (H2O2) of 1:1 were used for
achieving moderate yields of 1,2-diols under reflux conditions after eight hours.
Keywords: iron (III) phthalocyanine; cyclohexene; triflate; cyclohexane-1,2-diols
1. Introduction
In 1988, Sharpless and Jacobsen developed the OsO4/N-methylmorpholine N-oxide (NMO)
oxidation system using cinchona alkaloid derivatives as chiral ligands for the asymmetric
dihydroxylation of alkenes to obtain 1,2-diols [1]. 1,2-Diols are useful building blocks in the design of
pharmaceuticals [2] and natural product synthesis [3]. These compounds can be synthesized from
alkenes through alkene dihydroxylation [4]. As many alkenes can be obtained directly from

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petrochemicals, the conversion of alkene to 1,2-diols by oxidation is of great interest in both academia
and industry. Although OsO4 is the most commonly used catalyst, several group VIII transition metal
oxides, such as Fe [5–7], Ru [8–10], have been employed to catalyst these dihydroxylations too. The
industrial use of OsO4 is restricted on account of its expensive price and toxic properties. Therefore,
several catalysts such as polyoxometalates [11,12], sodium tungstate [13], MgxFe3-xO4 complex
oxide [14], peroxovanadium [15] and oxorhenium [16] have been developed. However, most of them
have limitations such as the usage of stoichiometric oxidants as well as toxic solvents and they suffer
from low selectivity, harsh reaction conditions, and cumbersome work-up procedures and thus are not
suitable for modern industrial practice. Therefore, it would still be highly desirable to develop new
efficient methods for the dihydroxylation reaction, and clean dihydroxylations using hydrogen
peroxide under mild conditions continue to be a challenge in catalysis. The selective dihydroxylation
of olefins with high efficiency in terms of the amount of H2O2 is particularly difficult to achieve.
Metalloporphyrins in oxidation of substrates with various single-oxygen atom donors have played a
major role in the understanding of the biologically related reactions of cytochrome P-450 [17], where
oxo-metalloporphyrins are the accepted reactive intermediates. There is a stark contrast between the
state of the art in porphyrin and phthalocyanine oxidation chemistry. Although phthalocyanine
complexes with a similar planar structure to the porphyrins have been actively investigated as
oxidation catalysts, their catalytic chemistry is practically undeveloped in terms of mechanisms and
identification of the active species involved in these catalytic oxidations [18,19]. Phthalocyanines have
demonstrated a wide range of applications in catalytic oxidation reactions, including alkene
cyclopropanation [20], C-S bond formation [21], saturated C-H bond amination [22,23], and alkene
epoxidation [24]. Our research group has been involved in the development of the catalytic chemistry
of metal phthalocyanine complexes [25–28]. As for the phthalocyanine core, two methods were
employed to modify the phthalocyanine. The first approach was to modify the parent phthalocyanine
ring [22,29,30]. Another possible approach was to modify the parent ring system of the axial ligands,
thus obtaining new complexes.
The anionic axial ligands effects of iron (III) porphyrin complexes were researched by Nam [31]
and Bell [32–35]. They found that the electronegativity of anionic axial ligands affected the heterolysis
or homolysis of oxoiron (IV) porphyrin intermediates, and thus decided the oxidation products.
−
Recently Che and co-workers found that iron (III) porphyrins with triflate (CF3SO3 ) as a counter
anion show higher efficiency in the selective oxidation of terminal aryl and aliphatic alkenes to
−
−
aldehydes than other counter anions, such as Cl−, ClO4 , and SbF6 [36]. As for phthalocyanines, only
a few cases have focused on the catalysis of iron (III) phthalocyanines [18,23]. Sorokin [37–39] and
McGaff [40,41] reported recently that iron (III) “helmet” phthalocyanines generated by reacting iron
(II) phthalocyanines with 14,28-[1, 3-diiminoisoindolinato] species can efficiently epoxidize of olefins
using H2O2. It was found that the “helmet” plays an important role in the catalytic efficiency of these
iron (III) phthalocyanines. White and co-workers recently developed iron (III) phthalocyanines with
hexafluoroantimonate as a counter anion [FePc·SbF6] as catalysts showing highly selectivity in C-H
amination reactions [23]. As far as we know, there is no report about the selective oxidation of
cyclohexenes to 1,2-diols catalyzed by iron (III) phthalocyanines with triflate as a counter anion and
using H2O2 as an oxidant.

Molecules 2015, 20
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2. Results and Discussion
At the outset, we examined the oxidation of cyclohexene with H2O2 using FePcX [23] (generated
in situ by reacting commercially available FePcCl with AgX; Pc = phthalocyanine; X = NO3, CF3SO3,
BF4, SbF6) as the catalyst. Treatment of cyclohexene with H2O2 (1.0 equiv.) in the presence of a
catalytic amount of FePcX (1 mol %) in the DMF at 80 °C (Scheme 1, Table 1) did not produced
detectable amounts of 1,2-diol when FePc was employed as a catalyst under these conditions (Table 1,
entry 1).
O
OH
FePcCl
AgX
OH
OH
+
+
O
+
H2O2
5
3
4
2
1
Scheme 1. The iron (III) phthalocyanine catalyzed the oxidation of cyclohexene with H2O2.
Table 1. The selective oxidation of cyclohexene to 1,2-diols catalyzed by iron (III)
phthalocyanines with different counter anions by using H2O2 as an oxidant.
Selectivity (%)
Conversion
Entry a Catalyst
Cyclohexene 2-Cyclohexen-1- 2-Cyclohexen- Cyclohexane-
(%)
oxide (2)
13.7
ol (3)
32.4
16.5
14.7
18.6
23.1
45.1
ND
1-one (4)
53.9
1,2-diol (5)
1
2
3
4
5
6
FePc
94.0
85.0
92.3
97.1
96.4
94.9
trace
/
FePcCl
59.1
21.4
3.0
1.9
45.9
29.2
/
FePcNO3
FePcOTf
FePcBF4
FePcSbF6
AgCl
69.6
13.8
20.2
15.3
32.8
14.9
33.1
ND c
21.8
7 b
ND
ND
a
Conditions: the amount of catalyst was 1 mol %; the mole ratio of substrate and oxidant (H2O2) was 1:1;
b
reaction time was 8 h; temperature was 80 °C; solvent was DMF; AgCl was employed in the oxidation
reaction under the same conditions; c ND means not detected.
As an iron (III) porphyin with a weakly coordinating anion as the axial ligand would have the
strong Lewis acidity that is needed for promoting the isomerization of epoxides to aldehydes [42], we
tested the activity of five iron (III) phthalocyanines with different coordinating anions is this oxidation
reaction. The conversion of cyclohexene was good to excellent in this system. Interestingly, 1,2-diol
was detected in this system for the first time, which was totally different from other reports [29,30].
−
−
−
−
Replacing Cl− by other anions such as NO3 , CF3SO3 , BF4 , SbF6 afforded similar results (Table 1,
entries 2–6). According to [42], the Lewis acidity of the metallporphyrins in the solvents used seems to
affect the reactivity and selectivity of the oxidation reactions. Phthalocyanines are structurally related
to porphyrins, but have a distinct framework from that of porphyrins in that all the meso positions
of its tetrapyrrolic macrocycle system are substituted with nitrogen atoms [43]. Owing to the
electron-withdrawing nature of its meso nitrogen atoms, the iron (III) phthalocyanine with the weaker

Molecules 2015, 20
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−
−
−
electron donors (NO3 , OTf−, BF4 , SbF6 ) would be a stronger Lewis acid catalyst than iron (III)
phthalocyanine with the stronger electron donors (Cl−), and thus would be expected to exhibit
enhanced reactivity and selectivity toward the oxidation reaction. Indeed, as expected, the amount of
1,2-diol increased obviously when the weaker electron donors were employed (Table 1, entries 2–6).
−
Surprisingly, FePcOTf displayed excellent selectively to obtain 1,2-diol although NO3 was the weakest
electron donor. To explore the universality of the FePcOTf catalyst, several oxidants were tested as
stoichiometric oxidants. Besides H2O2, tert-butylhydroperoxide (TBHP), m-chloroperoxybenzoic acid
(m-CPBA) and O2 were also tested (Table 2). It was found that the conversion of cyclohexene was
good to excellent, however the 1,2-diol was not produced using other oxidants (Table 2, entries 1,2,4).
We have also optimized the reaction temperature (Table 3). As for the conversion aspect, the optimum
temperature was 50 °C, but the 1,2-diol selectively rose from 17.4% to 45.9% when the reaction
temperature was increased to 80 °C (Table 3, entry 3).
Table 2. The selective oxidation of cyclohexene to 1,2-diols catalyzed by iron (III)
phthalocyanines with triflate as a counter anion and different oxidants.
Selectivity (%)
Conversion
Entry a
Oxidant
Cyclohexene 2-Cyclohexen- 2-Cyclohexene- Cyclohexane-
(%)
oxide (2)
/
1-ol (3)
70.7
1-one (4)
29.3
1,2-diol (5)
1
2
3
TBHP
m-CPBA
30% H2O2
O2
86.0
84.0
97.1
46.0
/
30.0
20.2
13.7
42.1
27.9
/
45.9
/
18.6
15.3
4 b
32.4
53.9
a
Conditions: the amount of catalyst was 1 mol %; the mole ratio of substrate and oxidant was 1:1; reaction
time was 8 h; temperature was 80 °C; solvent was DMF; b The reaction was carried out in an autoclave under
1.5 MPa pressure.
Table 3. The selective oxidation of cyclohexene to 1,2-diols catalyzed by iron (III)
phthalocyanines with triflate as a counter anion by using H2O2 as an oxidant at
different temperatures.
Reaction
Selectivity (%)
Conversion
(%)
Entry a
temperature
Cyclohexene 2-Cyclohexen- 2-Cyclohexene- Cyclohexane-
(°C)
25
oxide (2)
58.2
1-ol (3)
14.4
1-one (4)
13.4
1,2-diol (5)
14.0
1
2
91.0
97.0
97.1
50
45.3
20.3
18.0
17.4
3
80
20.2
18.6
15.3
45.9
a
Condition: the amount of catalyst was 1 mol %; the mole ratio of substrate and oxidant (H2O2) was 1:1;
reaction time was 8 h; solvent was DMF.
In order to achieve optimum conditions, three different cyclohexene to aqueous 30% H2O2 molar
ratios, viz. 1:1, 1:2, 1:5 were considered for a fixed amount of cyclohexene (10 mmol) and catalyst
(1 mol %) in the DMF at 80 °C (Table 4). A maximum of 45.9% selectivity for the 1,2-diol was
obtained for a cyclohexene to H2O2 molar ratio of 1:1 in 8 h of reaction time. The conversion was
98.0% when the ratio was 1:2 and decreased slightly at 1:5. However, cyclohexene oxide was absent
when the ratio to 1:5 (Table 4, entry 3).

Molecules 2015, 20
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Table 4. The selective oxidation of cyclohexene to 1,2-diols catalyzed by iron (III)
phthalocyanines with triflate as a counter anion using H2O2 as an oxidant at different condition.
Selectivity (%)
Substrate: Conversion
Entry a
Cyclohexene 2-Cyclohexen- 2-Cyclohexene- Cyclohexane-
oxidant
(%)
oxide (2)
1-ol (3)
18.6
1-one (4)
15.3
1,2-diol (5)
45.9
1
2
3
1:1
1:2
1:5
97.1
98.0
97.2
20.2
18
/
30.3
12.5
39.2
30.5
37.5
32.0
a
Conditions: the amount of catalyst was 1 mol %; reaction time was 8 h; temperature was 80 °C; solvent
was DMF.
Under the operating conditions as fixed above, the effect of catalyst considering three different
amount viz. 0.1 mol %, 0.2 mol %, 1 mol %, 2 mol % as a function of time was studied and results are
listed in Table 5. It is clear that 1 mol % catalyst was the best one to obtain a maximum of 97.1%
conversion of cyclohexene and highest selectively for 1,2-diol (Table 5, entry 3).
Table 5. The selective oxidation of cyclohexene to 1,2-diols catalyzed by iron (III)
phthalocyanines with triflate as a counter anion by using H2O2 as an oxidant at
different condition.
Selectivity (%)
Amount of
Conversion
(%)
Entry a
catalyst
Cyclohexene 2-Cyclohexen- 2-Cyclohexene- Cyclohexane-
(mol %)
oxide (2)
20.0
1-ol (3)
27.4
1-one (4)
52.6
1,2-diol (5)
1
2
3
4
0.1
0.5
1
94.2
95.0
97.1
95.5
/
15.2
23.1
23.2
37.5
45.9
37.4
20.2
18.6
15.3
2
20.0
23.2
19.4
a Conditions: the mole ratio of substrate and oxidant (H2O2) was 1:1; reaction time was 8 h; temperature was
80 °C; solvent was DMF.
The conversion of cyclohexene and selectivity of the different reaction products under the
optimized reaction conditions have been analyzed as a function of time and are presented in Figure 1.
The time that the injection of H2O2 was finished was defined as the zero time (see the procedure in the
Experimental Section). It was clear from the plot that with a conversion of 68% of cyclohexene at the
zero time, no 1,2-diol was been detected. After 4 h, 8.4% selectivity of 1,2-diol has been obtained,
while the conversion of cyclohexene was 89%. The selectivity of cyclohexene oxide, 2-cyclohexene-1-
one, and 2-cyclohexene-1-ol obviously decreased as time goes on. After 8 h, the selectivity for 1,2-diol
was the highest and reached 45.9%. At the end of 10 h, a small decrease was detected both in the
conversion of cyclohexene and the selectivity of 1,2-diol.

Molecules 2015, 20
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Conversion
Cyclohexeneoxide
2-Cyclohexene-1-ol
2-Cyclohexene-1-one
Cyclohexane-1,2-diol
100
90
80
70
60
50
40
30
20
10
-1
0
1
2
3
4
5
6
7
8
9
10 11
Time / h
Figure 1. Plots showing percentage selectivity of cyclohexene oxide, 2-cyclohexene-1-one,
2-cyclohexene-1-ol and cyclohexane-1,2-diol formation and percentage conversion of
cyclohexene as a function of time. Conditions: the amount of catalyst was 1 mol %; the mole
ratio of substrate and oxidant (H2O2) was 1:1; temperature was 80 °C; solvent was DMF.
Finally, the influence of solvent (DMF, acetonitrile, methanol) was also been researched (Table 6).
5.0% cyclohexene oxide and 52.0% 1,2-diol have been obtained when using CH3OH as a solvent, and
the conversion of cyclohexene was a little higher than with other solvents. CH3OH was found to be
most suitable solvent in which the iron (III) phthalocyanine complexes showed a lesser protonation
tendency and sufficient stability of the formed oxo species [39].
Table 6. The selective oxidation of cyclohexene to 1,2-diols catalyzed by iron (III)
phthalocyanines with triflate as a counter anion using H2O2 as an oxidant in different solvents.
Selectivity (%)
Conversion
Entry a
Solvent
Cyclohexene 2-Cyclohexen- 2-Cyclohexene- Cyclohexane-
(%)
oxide (2)
5.0
1-ol (3)
24.5
1-one (4)
18.5
1,2-diol (5)
52.0
1
2
3
CH3OH
CH3CN
DMF
97.5
97.2
97.1
18.2
26.4
14.0
41.2
20.2
18.6
15.3
45.9
a Conditions: the mole ratio of substrate and oxidant (H2O2) was 1:1; reaction time was 8 h; the reaction was
researched under reflux condition ; the amount of solvent was 3 mL.
High-valent iron oxocomplexes are well-documented and characterized by Sorokin and co-workers.
High-valent iron oxocomplexes were considered be the intermediates or active species in the oxidation
of olefins [39]. According to the reports, a possible mechanism for the formation of this species is
proposed in Scheme 2. It should be noted that all iron (III) phthalocyanine complexes exhibit excellent
conversions. However, the iron (III) phthalocyanine complexes with weaker electron donors (i.e.,
−
−
−
−
CF3SO3 , NO3 , BF4 and SbF6 ) as axial ligands gave high selectivity for 1,2-diol in the H2O2
reaction, whereas the iron (III) phthalocyanine complexes with the axial ligands of stronger electron

Molecules 2015, 20
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donors (i.e., Cl−) gave no 1,2-diol in the H2O2 reaction [31]. Although we propose at this time that the
electron-donating ability of the anionic axial ligands is an important factor in controlling the catalytic
activity of the iron (III) phthalocyanine complexes in the H2O2 reactions, more detailed studies are in
progress to gain a better understanding of the exact roles of the anionic axial ligands by density
functional theory (DFT), low temperature UV-Vis and electrospray ionization mass spectrometry
(ESI-MS) [37–39].
OH
O
O
O
N
N
N
N
N
H2O2
N
N
N
Fe
N
N
FeIV
N
N
N
Fe
N
N
N
N
N
FeV
N
N
N
N
N
X
X
N
N
N
X
N
N
N
N
N
X
N
X: Cl, OTf, NO3, BF4, SbF6
Scheme 2. Proposed mechanism for the formation of high-valent iron oxo-phthalocyanine species.
In summary, a mild oxidation system has been developed to convert cyclohexene into 1,2-diol, with
excellent conversion. To the best of our knowledge, this is the first efficient method to prepare
1,2-diols catalyzed by the FePcOTf/H2O2 system. Such an interesting reactivity and selectivity of
FePcOTf/H2O2 may provide a new strategy for preparing 1,2-diols, even regio- and stereoselective
1,2-diols. Besides, the results provide a rationale for the application of the accessible iron
phthalocyanine complexes in catalytic oxidation.
3. Experimental Section
3.1. Materials
Iron (III) phthalocyanine chloride (FePcCl), AgNO3, AgOTf, AgBF4, AgSbF6 cyclohexene,
cyclohexene oxide, 2-cyclohexen-1-ol, 2-cyclohexen-1-one, and cyclohexane-1,2-diol were purchased
from Aldrich (Sigma-Aldrich Co., Ltd., Shanghai, China) and used as received. Dimethylformamide
(DMF), acetonitrile, dichloromethane and methanol of gas chromatography (GC) grade were purchased
from Hangzhou Jiachen Chemical Co., Ltd. (Hangzhou, China), as were tert-butylhydroperoxide
(TBHP, 70% in water), and m-chloroperoxybenzoic acid (m-CPBA), H2O2 (9.7 M) were purchased
from Hangzhou Jiachen Chemical Co., Ltd. which were used without further purification. O2 was
provided by Hangzhou Minxing Gas Co., Ltd. (Hangzhou, China).
3.2. Physical Measurements
The GC traces were recorder with a GC-2014C Gas Chromatograph (Shimadzu, Suzhou, China) fitted
with an FID detector, using a “Crossbond” 5% diphenyl/95% dimethylpolysiloxane capillary column
(30 m length, 0.25 mm ID, 0.25 μm df) . The parameters for analysis were: injector temperature = 250 °C,
detector temperature = 260 °C. Temperature program: initial temperature = 40 °C with a 2 min hold,

Molecules 2015, 20
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then warmed up to 105 °C at 5 °C·min−1 and held for 0 min, before finally heating up to 230 °C at
20 °C·min−1. Mass spectra were recorded with a 6890N/5973I GC-MS system (Agilent Technologies
Co., Ltd., Shanghai, China) coupled with an Agilent Technologies column of 30 m length, 0.25 mm
internal diameter and 0.25 μm df.
3.3. General Procedure for the Preparation of 1,2-Diols
FePcCl (60.3 mg, 0.1 mmol) and AgX (0.12 mmol) were added to a two-necked 25 mL
round-bottomed flask equipped with a condenser. DMF (3 mL) and cyclohexene (10 mmol) were
added. The mixture was refluxed, then H2O2 (10 mmol) was added slowly within 30 min. The reaction
mixture was allowed to cool to room temperature. The solution was diluted with CH2Cl2, and the
reaction monitored over time using gas chromatography (GC). The oxidation products were identified
by spiking using standards and by measurements of GC retention times in. The identity of products
was also determined by a gas chromatograph connected to a mass spectrometer (GC-MS).
4. Conclusions
Several iron (III) phthalocyanines with different counter anions have been researched in the selective
oxidation of cyclohexene using H2O2 as an oxidant. It was found iron (III) phthalocyanines with triflate
as a counter anion was the best catalyst, and cyclohexane-1,2-diol was first detected in this system. A
proposed mechanism for the formation of high-valent iron oxo-phthalocyanine species has been given.
Besides, many effects were studied to find optimum reaction conditions. Such an amazing reactivity
and selectivity of FePcOTf/H2O2 system may further develop the applied of metal phthalocyanines in
catalytic oxidized organic compounds.
Acknowledgments
This work was financially supported by the NSF of China (NO. 51133006), Zhejiang Provincial
Natural Science Foundation of China (NO. LQ13B020006), Educational Commission of Zhejiang
Province of China (Y201225426) and Zhejiang Provincial Top Key Academic Discipline of Applied
Chemistry and Eco-Dyeing & Finishing Engineering (NO. YR2011014).
Author Contributions
Wenxing Chen has made important contribution to the design and write of the paper. Baocheng Zhou
was contribution to the experiment research.
Conflicts of Interest
The authors declare no conflict of interest.
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Sample Availability: Samples of the compounds (FePcCl, AgNO3, AgOTf, AgBF4, AgSbF6
cyclohexene, cyclohexene oxide, 2-cyclohexen-1-ol, 2-cyclohexen-1-one, and cyclohexane-1,2-diol)
are available from the authors.
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