Binuclear iron(III) phthalocyanine(μ-oxodimer)/tetrabutylammonium oxone: A powerful catalytic system for oxidation of hydrocarbons in organic solution

Article abstract of DOI:10.1016/j.tetlet.2010.10.023

Binuclear iron(III) phthalocyanine-(μ-oxodimer) complex was tested in catalytic oxygenation reactions of several hydrocarbons using tetrabutylammonium oxone as the oxidant in dichloromethane solution at room temperature. Results of the study demonstrate that this is an extremely powerful catalytic system for oxidative conversion of aromatic hydrocarbons (anthracene, 2-tert- butylanthracene, 2-methylnaphthalene, 9, 10-dihydroanthracene, 1,2,3,4-tetrahydronaphthalene, indane, ethylbenzene, toluene, and benzene) to the respective p-quinones in high yields in 5-30 min. Under these conditions, adamantane is oxidized with 71% conversion after 10 min affording a mixture of 1 -adamantanol, 2-adamantanone, 1-hydroxy-2-adamantanone, and 4-protoadamantanone.

Full text of DOI:10.1016/j.tetlet.2010.10.023

Tetrahedron Letters 51 (2010) 6545–6548
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Binuclear iron(III) phthalocyanine(
l-oxodimer)/tetrabutylammonium oxone:
a powerful catalytic system for oxidation of hydrocarbons in organic solution
⇑
⇑
Heather M. Neu, Viktor V. Zhdankin , Victor N. Nemykin
Department of Chemistry and Biochemistry, University of Minnesota Duluth, Duluth, MN 55812, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 29 July 2010
Revised 28 September 2010
Accepted 6 October 2010
Available online 15 October 2010
Binuclear iron(III) phthalocyanine-(l-oxodimer) complex was tested in catalytic oxygenation reactions of
several hydrocarbons using tetrabutylammonium oxone as the oxidant in dichloromethane solution at
room temperature. Results of the study demonstrate that this is an extremely powerful catalytic system
for oxidative conversion of aromatic hydrocarbons (anthracene, 2-tert-butylanthracene, 2-methylnaph-
thalene, 9,10-dihydroanthracene, 1,2,3,4-tetrahydronaphthalene, indane, ethylbenzene, toluene, and
benzene) to the respective p-quinones in high yields in 5–30 min. Under these conditions, adamantane
is oxidized with 71% conversion after 10 min affording a mixture of 1-adamantanol, 2-adamantanone,
1-hydroxy-2-adamantanone, and 4-protoadamantanone.
Keywords:
Iron phthalocyanine
Oxygenation
Tetrabutylammonium oxone
Catalytic oxidation
l-oxodimer
Ó 2010 Elsevier Ltd. All rights reserved.
Transition-metal porphyrins, phthalocyanines, and related com-
pounds are widely used as catalysts in the reactions mimicking
natural oxidations performed by the heme-containing cytochrome
P-450 class of enzymes.1 It was demonstrated recently that porphy-
But
But
But
N
Fe
N
N
N
N
N
N
l l
-oxo-,2a–c and -nitridodimers,2d–f
But
N
rin and phthalocyanine iron
But
which were earlier considered as catalytically inactive compounds,
in many cases have high catalytic activity in different oxidation reac-
tions. Most commonly, hydrogen peroxide, organic peroxides, and
iodosylbenzene are used as terminal oxidants in the transition-me-
tal porphyrin-type catalyzed reactions.1 In particular, iodosylben-
zene,3 2-iodylbenzoate ester,4a,4b and oligomeric iodosylbenzene
sulfate (PhIO)3SO3,3g,4b–d have been demonstrated to be efficient
sources of oxygen in these reactions. It was also shown that the
oxidative activity of a porphyrin (or phthalocyanine) based catalytic
system strongly depends on the nature of the terminal oxidant. In
our recent studies of hypervalent iodine reagents as terminal
O
N
But
N
N
Fe
N
N
N
N
N
But
But
1
Figure 1. Iron(III) phthalocyanine-(l-oxodimer) 1.
oxidants in the iron(III) phthalocyanine(l-oxodimer)-catalyzed
Complex 1 was synthesized using direct high-temperature
reaction between 4-tert-butylphthalonitrile and iron(II) acetate as
described previously,6 while tetrabutylammonium oxone was
prepared by simple treatment of commercial OxoneÒ (2KHSO5Á
KHSO4ÁK2SO4) with tetrabutylammonium bisulfate.5a The results
of catalytic oxidation of anthracene 2, 2-tert-butylanthracene 4,
2-methylnaphthalene 6, 9,10-dihydroanthracene 11, 1,2,3,4-tetra-
hydronaphthalene 12, indane 16, ethylbenzene 20, toluene 22,
benzene 24, and adamantane 26 using tetrabutylammonium oxone
and catalyst 1 are summarized in Table 1.
oxygenations,4c we have found that the catalytic systems utilizing
oligomeric iodosylbenzene sulfate are significantly more reactive
compared to the systems based on traditional two-electron transfer
oxidants, such as iodosylbenzene or hydrogen peroxide. In the
present work, we report preliminary results on the iron(III) phthalo-
cyanine(l-oxodimer) 1(Fig. 1) catalyzed oxygenations using one of
the most powerful oxidants, tetrabutylammonium oxone [approxi-
mate composition 2Bu4NHSO5ÁBu4NHSO4Á(Bu4N)2SO4],5 which has
excellent solubility in non-polar organic solvents and thus allowing
catalytic reactions in solution under homogeneous conditions.
All oxidation reactions were carried out in dry dichloromethane
using two times excess of tetrabutylammonium oxone (6 mol
equiv of active oxygen per one molecule of substrate) with
0.05 equiv of the catalyst 1. Samples of the reaction mixture
⇑
Corresponding authors. Tel.: +1 218 726 6902; fax: +1 218 726 7394.
E-mail address: vzhdanki@d.umn.edu (V.V. Zhdankin).
0040-4039/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2010.10.023

6546
H. M. Neu et al. / Tetrahedron Letters 51 (2010) 6545–6548
Table 1
Catalytic oxidation of organic substrates using tetrabutylammonium oxone as the stoichiometric oxidanta
Entry
Catalyst (mol %)
Substrate
Time (min)
Conversion (%)
Products distribution (%)
O
1
None
2880
10
2
O
3 (100%)
2
3
1 (5)
2
2
5
30
100
45
3 (100%)
3 (100%)
Fe(III)TPFPPb (5)
O
But
But
4
5
1 (5)
10
30
100
0
4
5 (97%)c
O
None
4
None
O
O
O
O
O
OH
OH
O
O
6
1 (5)
5
100
O
6
8 (47%)
10 (2%)
7 (47%)
None
9 (4%)
7
None
6
1440
10
0
8d
1 (5)
100
3 (100%)
11
9
None
11
60
10
14
2 (93%), 3 (4%)
O
O
10
1 (5)
100
14 (8%)
O
13 (91%)
12
OH
14 (89%) +
11
None
12
60
5
15 (11%)
O
O
12
13
1 (5)
10
60
100
14
16
O
17 (74%)
18 (26%)
OH
18 (23%) +
None
16
19 (77%)
O
14
15
1 (5)
30e
90
0
20
O
None
O
21 (100%)
None
20
1440
16
17
1 (5)
5e
43f
0
22
O 23 (100%)
None
None
22
1440
O
18
19
1 (5)
5e
38f
0
24
O
None
25 (100%)
None
24
1440

H. M. Neu et al. / Tetrahedron Letters 51 (2010) 6545–6548
6547
Table 1 (continued)
Entry
Catalyst (mol %)
Substrate
Time (min)
Conversion (%)
Products distribution (%)
O
O
O
20
21
1 (5)
10e
71
0
OH
OH
26
27 (57)
28 (23)
30 (8)
29 (12)
None
26
1440
None
a
Reactions were performed in dichloromethane at room temperature using 5 mol % of catalyst 1 and tetrabutylammonium oxone (6 equiv of active O) unless otherwise
noted.
b
Fe(III)TPFPP = 5,10,15,20-tetrakis(pentafluorophenyl)-21H,23H-porphyrin iron(III) chloride.
According to GC–MS data the reaction mixture also contains 3% of products of hydroxylation of the anthracene core.
4 mol equiv of tetrabutylammonium oxone (8 equiv of active O) used.
Longer reaction times lead to oxidative degradation of catalyst 1 and do not improve the conversion.
Conversion measured by NMR.
c
d
e
f
(100
l
L) were collected every 1 min, the catalyst and tetrabutyl-
35% conversion after 5 min at room temperature. Previously, the
ammonium salts were removed by filtration through silica gel
and the obtained solution was analyzed by GC–MS to determine
the conversion of the hydrocarbon. Upon completion of the reac-
tion, the reaction mixture was concentrated and separated using
TLC plate and ethyl acetate/hexane (2:3 v:v) mixture as the eluent
to afford analytically pure products. All reaction products were
identified by comparison with commercially available samples
and using EI-MS databases. Blank experiments in the absence of
catalyst 1 were performed similarly using 3 mol equiv of tetrabu-
tylammonium oxone in dichloromethane at room temperature.
The oxidation of anthracene 2 with tetrabutylammonium oxone
in the absence of catalyst 1 at room temperature in dichlorometh-
ane proceeds extremely slow showing only 10% conversion to
anthraquinone 3 after 48 h (entry 1). The addition of 0.05 mol
oxidation of benzene to phenol and traces of p-benzoquinone
was performed at 60 °C using l-nitrido bridged diiron phthalocya-
nine in combination with hydrogen peroxide by Sorokin and
co-workers.2e,2f In the absence of catalyst 1, ethylbenzene 20,
toluene 22, and benzene 24 cannot be oxidized with tetrabutylam-
monium oxone (entries 15, 17, and 19).
Furthermore, the iron(III) phthalocyanine(l-oxodimer)/tetra-
butylammonium oxone catalytic system is capable of efficient oxi-
dation of adamantane 26 in 71% conversion after 10 min affording
a mixture of 1-adamantanol 27, 2-adamantanone 28, 1-hydroxy-2-
adamantanone 28, and 4-protoadamantanone 29 (entry 20). The
latter compound is a product of oxidative rearrangement of
1-adamantanol.7
Since tetrabutylammonium oxone is a well-known two-electron
oxidant, based on the preliminary UV–vis, APCI, and MCD data, we
speculate that the catalytically active species should include Pc
2ÀFeIV–O–Pc2ÀFeIV = O or Pc2ÀFeIII–O–Pc1ÀFeIV = O species, detailed
characterization of which will be presented later.
equiv of Fe(III) phthalocyanine(l-oxodimer) 1 leads to a dramatic
increase in the reaction rate with a 100% conversion reached just
in 5 min (entry 2). This is a much higher reactivity, compared to
the previously reported metalloporphyrin (or phthalocyanine)-cat-
alyzed oxidations of anthracene 2 using other terminal oxidants.
For example, the oxidation of 2 to 3 in the presence of catalyst 1
using oligomeric iodosylbenzene sulfate (PhIO)3SO3 requires 2 h
for completion.4b,4c A common porphyrin catalyst, 5,10,15,20-tet-
rakis(pentafluorophenyl)-21H,23H-porphyrin iron(III) chloride,
shows a significantly lower catalytic activity in this oxidation
(entry 3). The reactions of less-reactive aromatic substrates 4 and
6 are extremely slow in the absence of catalyst 1 (entries 5 and
7) but occur almost instantaneously under catalytic conditions to
afford products 5, 7–10, same as previously reported for other oxi-
dants,4c but much faster. The oxidation of 9,10-dihydroanthracene
11 in the absence of catalyst 1 is very slow and affords mainly
anthracene 2 as the product of oxidative aromatization (entry 9);
under catalytic conditions this reaction affords exclusively anthra-
quinone 3 with a 100% conversion in 10 min (entry 8).
Typical experimental procedure for the catalytic oxidation of
substrates: A solution of anthracene 2 (89 mg, 0.5 mmol) in dichlo-
romethane (5 mL) was mixed with iron
l-oxo dimer phthalocya-
nine 16 (40 mg, 0.025 mmol, 5 mol %), and tetrabutylammonium
oxone (containing about 37% of Bu4NHSO5)5a (2.9 g, 3 mmol,
6 equiv of active O), with stirring, at room temperature. Samples
of the reaction mixture (100 lL) were collected every 1 min, fil-
tered through 2–3 cm of silica gel suspended in a Pasteur pipet,
washed with a mixture of ethyl acetate and hexane (2:3 v:v), and
then analyzed using GC–MS. Upon completion of the reaction,
the reaction mixture was concentrated and separated using TLC
plate and ethyl acetate/hexane (2:3 v:v) mixture as the eluent to
afford 100 mg (96%) of analytically pure anthraquinone 3. All reac-
tion products were identified by comparison with commercially
available samples.
The molecules of substrates 12 and 16 have two competing
reacting sites, C–H bonds in the aromatic ring and the benzylic
C–H bonds. The oxidation of these substrates in the absence of cat-
alyst 1 is very slow and occurs exclusively in the benzylic position
(products 14 and 15, entry 11 and products 18 and 19, entry 13). In
the presence of catalyst 1, these oxidations are complete in 10 min
to afford p-quinones 13 and 17 as major products.
Selective oxidation of benzene 24 and alkylbenzenes 20, 22 to
the respective p-quinones 25, 21, and 23 provides an impressive
proof of the exceptional reactivity of the iron(III) phthalocya-
Acknowledgments
This work was supported by a research grant from the National
Science Foundation (CHE-1009038) and Petroleum Research Fund,
administered by the American Chemical Society (grant PRF-45510-
GB-3).
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