Biomimetic catalytic system driven by electron transfer for selective oxygenation of hydrocarbon

Article abstract of DOI:10.1021/ja047297b

Hydrocarbon oxyfunctionalization is a crucial industrial process. Most metallic catalysts require higher temperatures and often show lower selectivities. One of the intellectual approaches is the mimicry for bio-oxidation. We have established a biomimetic

Full text of DOI:10.1021/ja047297b

Published on Web 08/05/2004
Biomimetic Catalytic System Driven by Electron Transfer for Selective
Oxygenation of Hydrocarbon
†
‡
,†
Guanyu Yang, Yinfa Ma, and Jie Xu*
Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian, 116023, P. R. China, and
Department of Chemistry, UniVersity of Missouri-Rolla, Rolla, Missouri 65409
Received May 9, 2004; E-mail: xujie@ dicp.ac.cn
Table 1. Catalytic Oxygenation of 1 by O2a
Selective oxyfunctionalization of hydrocarbons is a crucial
industrial process and remains a challenge. Molecular oxygen,
1
selectivity (%)c
conversionc
d
yield of 3
which is environmentally benign and economic, would be an ideal
ultimate oxidant for oxygenation of hydrocarbons. However, most
current processes require elevated temperatures and often show
lower selectivity. Mimicry for biological oxygenation may be an
inspirational approach for selective oxyfunctionalization of hydro-
feed catalystsb
entry
3
4
2
(%)
(%)
1
2
3
4
5
6
7
8
9
AQ/NHPI/HY
EAQ/NHPI/HY
CAQ/NHPI/HY
92.8
91.8
92.9
3.8
3.1
5.8
4.2
3.4
5.0
1.3
0
37.3
36.9
41.7
66.2
41.2
67.9
2.7
10.9
0.7
1.0
33.2
32.5
36.5
61.1
12.5
48.4
e
e
e
e
DACAQ/NHPI/HY 95.8
AQ/NHPI
DACAQ/NHPI
DACAQ
NHPI
32.5 14.8 52.7
73.5 13.1 13.4
21.5 30.8 47.7
0
96.5
1.2
2
carbon with O , because biological oxygenation exhibits high
selectivity under mild conditions.
2
On the evolution of enzyme mimicry, the biomimetic oxygen-
ation model should encompass three elementary factors: a redox
center, a one-electron transfer chain, and multiple binding sites
similar to the surrounding protein environment of the enzyme.
A large number of oxidoreductases contain redox centers, which
can be organic radicals or metallic compounds.3 Biological
oxygenations of C-H bonds stem from the abstraction of hydrogen
12.5 87.5
2.5 1.0
3.4 95.4
HY
10
a
Reaction conditions: 1 (2 mL), Q (2.5 mol %), NHPI (10 mol %), HY
b
(
2.0 w %), acetonitrile (7 mL), 80 °C, O2 (0.3 MPa), 10 h. AQ, EAQ,
CAQ, and DACAQ represent anthraquinone, 2-ethylanthraquinone, 2-chlo-
roanthraquinone, and 1,4-diamino-2,3-dichloroanthraquinone, respectively.
c
GC results. d Isolated yields of 3. e Too little to be accurately isolated.
4
from organic compounds by the redox centers. It is known that
5
electrophilic radicals can readily abstract hydrogen atoms. The
HY (HY), 66.2% of 1 was oxygenated. In comparison, only 10.9,
.7, or 0.7% of 1 was oxygenated when NHPI, DACAQ, or HY
was used individually. Results (Table 1) clearly show that the
biomimetic system can significantly promote oxygenation of 1.
phthalimide N-oxyl radical (PINO) is highly electrophilic and can
2
2
efficiently promote hydrocarbon oxyfunctionalization with O , as
6
demonstrated by recent intensive studies. PINO can be generated
from N-hydroxyphthalimide (NHPI) by donating both an electron
and a proton and can convert to NHPI by abstracting a hydrogen
atom from hydrocarbons. Therefore, PINO can be a nonmetalic
redox center in catalytic hydrocarbon oxygenation.
1
-Phenylethyl hydroperoxide (2) is an initial product from 1
autoxidation, and considerable quantities always exist in the mixture
of products. Some of 2 may further convert into a variety of by-
products following a random radical mechanism.10 When mixtures
of these products were analyzed by GC, according to literature meth-
ods,10 it was surprising to find that 95.8% selectivity of acetophen-
one (3) could be obtained. GC-MS measurements showed that 3,
The action of the redox center in biologic oxygenation relies on
a chain of one-electron transfers, resulting in formation of radicals.7
A ubiquitous quinone derivative (i.e., coenzyme) is often one of
the units of the electron-transfer chain.8
1
-phenylethanol (4), and 2 were produced. Such high selectivity
Indeed, the essence of biosynthesis is the control of selectivity
by some noncovalent binding of the substrate to the enzymatic
protein domain. This suggests that porous zeolites could be used
as host materials for host-guest composites with sequestered or-
ganic molecules, analogous to enzyme-substrate complexes in bio-
oxidation, and enhance molecular orientation for chemical reac-
tions.9
It is tempting to suggest that the quinones can make NHPI
convert to PINO via one-electron transfer and subsequently facilitate
hydrocarbon autoxidation, in which zeolite is used to promote
reaction selectivity. Such a biomimetic system could be applied
of 3 without any appreciable amount of over-oxidation byproducts
and aromatic hydroxylated products clearly demonstrated that our
biomimetic oxygenation system had a high product orientation.
The action of Qs and NHPI was investigated in the absence of
HY. When anthraquinone (AQ) and NHPI were employed, conver-
sion of 1 reached 41.2%. In comparison to the results in the presence
of HY, the selectivity of 3 decreased to 32.5%; 2 and 4 increased
to 52.7 and 14.8%, respectively. When HY alone was employed,
the conversion was 0.7% with a selectivity of 3 up to 96.5%. This
showed that Qs and NHPI were coupled as a catalytic system for
efficient oxygenation of 1 mainly to 2, as expected. Even though
HY cannot catalyze oxygenation, it can selectively catalyze the
cleavage of 2 to 3.
2
for selective oxygenation of hydrocarbons by O under moderate
conditions. The present study examined this biomimetic system with
anthraquinone derivatives (Q) as electron-transfer mediators and
ethylbenzene (1) as a probe molecule.
Oxygenation of 1 was performed by employing different Qs in
this biomimetic catalytic system using a 70 mL Teflon-lined auto-
The interaction between NHPI and 2-ethylanthraquinone (EAQ)
was investigated by a special liquid in situ FTIR system, composed
of a flow-through liquid cell, a peristaltic pump, and a water bath
(see “Supporting Information”). Spectra are shown in Figure 1.
When the same molar amounts of NHPI and EAQ in acetonitrile
2
clave with O (0.30 MPa) at 80 °C in acetonitrile for 10 h. When
2.5 mol % 1,4-diamino-2,3-dichloro-anthraquinone (DACAQ) was
-
1
used, in combination with 10 mol % NHPI and 2.0 w % zeolite
were heated at 80 °C, broad peaks at 3632 and 3544 cm appeared
in the spectrum (Figure 1C) and were assigned to the -OH
stretching vibration of 2-ethylanthracenediol. Correspondingly, the
†
Dalian Institute of Chemical Physics.
University of Missouri-Rolla.
‡
10542
9
J. AM. CHEM. SOC. 2004, 126, 10542-10543
10.1021/ja047297b CCC: $27.50 © 2004 American Chemical Society

C O MMU N I C A T I O N S
Scheme 2. Suggested Mechanism of Ketonic Cleavage of 2
through an Adsorbed Transition State on HY
molecules. It is suggested that the weaker acidic 1-H of 2 can also
participate in proton exchange with HY.
The mechanism of ketonic cleavage of 2 in the presence of HY
is proposed (Scheme 2). During the reaction, 2 comes into contact
with HY. The adsorption complex, acting as a cleavage transition
state, was formed by O of the hydroxyl of 2 coordinating to the
acidic proton on the HY surface and 1-H coordinating to the
neighboring framework oxygen. Afterward, the complex syner-
getically decomposes, resulting in proton exchange on HY, dehy-
drolysis, and ketonization of 2. This nonradical mechanism can
suppress random radical cleavage of 2 and orient toward 3
generation, so that the selectivity of 3 increases greatly.
Figure 1. Liquid in situ FTIR study of NHPI and EAQ interactions. This
was performed with 0.063 M NHPI and 0.063 M EAQ in acetonitrile in a
special in situ system at 80 °C. The same concentrations of NHPI and EAQ
were measured by the same method. Spectra of EAQ (line A), NHPI (line
B), and a mixture of NHPI and EAQ (line C) were obtained by subtracting
the spectral background of acetonitrile.
Scheme 1. Redox Cycle for Oxidation of 1 by the Q/NHPI System
In summary, the Qs, NHPI, and HY three-component biomimetic
system can efficiently catalyze oxygenation of 1 by O under
2
moderate reaction conditions. NHPI acts as a nonmetallic redox
center and Qs as redox-active cofactors, and both contribute to a
redox catalytic cycle. HY catalyzes nonradical cleavage of 2 to 3.
The high selectivity of 3 without any appreciable over-oxidation
byproducts suggests the application of this catalytic system for
methylene ketonization of hydrocarbons.
-
1
carbonyl peak of EAQ in the region of 1677 cm decreased, and
-
1
the 3226 cm peak belonging to the stretching vibration of -OH
of NHPI (Figure 1B) disappeared (Figure 1C). A strong peak also
-
1
Acknowledgment. We thank The Natural Science Foundation
of China (Project 20233040) and The National High Technology
Research and Development Program of China (Project 2002AA-
321020) for financial support.
appeared at 1290 cm (Figure 1C), which was not the peak of
-1
EAQ at 1294 cm (Figure 1A). It may be assigned to the stretching
vibration of NO bond of PINO since its vibration often shows a
broad and strong peak at this frequency region.11 These changes
indicated that PINO was formed from NHPI by reacting with EAQ,
while EAQ was converted to 2-ethylanthracenediol.
Supporting Information Available: Detailed experimental pro-
cedures; GC measurement method; description of liquid in situ FTIR
system; and original IR spectra of EAQ, NHPI, and EAQ/NHPI
interaction in acetonitrile solution (PDF). This material is available free
of charge via the Internet at http://pubs.acs.org.
It can be concluded that there is a coupling redox transformation
between Q/anthracenediols (QH ) and NHPI /PINO via a one-
2
electron transfer. The actions of Qs resemble the redox activity of
quinones in biological processes. The formed PINO abstracts a
hydrogen atom from 1 to produce 1-phenylethyl radical (5) and
then reverts to NHPI. 5 interacts with O
-phenylethylperoxyl radical (6). After further H-abstraction from
QH or QH , 6 converts to 2; QH or QH transforms to Q at the
same time. The redox cycle of the oxidation of 1 is created (Scheme
2
and converts to
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13
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J. AM. CHEM. SOC.
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VOL. 126, NO. 34, 2004 10543