Aerobic Stereoselective Oxidation of Olefins on a Visible-Light-Irradiated Titanium Dioxide-Cobalt-Ascorbic Acid Nanohybrid

Article abstract of DOI:10.1055/s-0036-1588897

A visible-light-driven photocatalytically active nanocrystalline TiO₂ was prepared by surface modification with a cobalt-ascorbic acid complex. The photocatalyst exhibited high activity and excellent chemo-, diastereo-, and stereoselectivities

Full text of DOI:10.1055/s-0036-1588897

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© Georg Thieme Verlag Stuttgart · New York
2016, 27, A–D
letter
A
M. Jafarpour et al.
Letter
Synlett
Aerobic Stereoselective Oxidation of Olefins on a Visible-Light-
Irradiated Titanium Dioxide–Cobalt–Ascorbic Acid Nanohybrid
Maasoumeh Jafarpour*
Fahimeh Feizpour
Abdolreza Rezaeifard*
Catalysis Research Laboratory, Department of Chemistry,
Faculty of Science, University of Birjand, Birjand 97179-414,
Iran
mjafarpour@birjand.ac.ir
rrezaeifard@birjand.ac.ir
Received: 09.09.2016
Accepted after revision: 20.09.2016
Published online: 11.10.2016
catalysts in oxidation reactions of olefins has been limited
to a small range of substrates.4a–g Therefore, the design of a
novel heterogeneous cobalt(II)-based catalyst for aerobic
reactions that shows a high activity and ease of separation
remains a challenging task.
Organic–inorganic hybrid materials have attracted con-
siderable interest in the field of catalysis, especially in oxi-
dative transformations of organic compounds. However, to
the best of our knowledge, only a few reports have de-
scribed the catalytic activity of transition-metal-coordinat-
ed organic–inorganic hybrid materials for the aerobic epox-
idation of olefins.4k,l
DOI: 10.1055/s-0036-1588897; Art ID: st-2016-d0458-l
Abstract A visible-light-driven photocatalytically active nanocrystal-
line TiO2 was prepared by surface modification with a cobalt–ascorbic
acid complex. The photocatalyst exhibited high activity and excellent
chemo-, diastereo-, and stereoselectivities in the aerobic epoxidation
of olefins under visible-light irradiation in the absence of a reducing
agent. The catalyst proved efficient and could be recycled at least five
times.
Key words epoxidation, alkenes, titanium dioxide, nanoparticles,
ascorbic acid, nanohybrids
In continuation of our research into the development of
novel catalytic methods for aerobic oxidation of organic
compounds,5 we recently discovered that a TiO2–ascorbic
acid (AA)–Co nanohybrid is active in the selective aerobic
benzylic C–H oxidation of alcohols or hydrocarbons in ethyl
acetate.5g Here, we wish to describe the visible-light photo-
catalytic activity and selectivity of this combination for oxi-
dation of olefins in ethyl acetate under heterogeneous con-
ditions in the absence of a reducing agent (Scheme 1). Fur-
thermore, spectroscopic analysis and leaching experiments
demonstrated the preservation of the structural integrity of
the solid catalyst after several reaction cycles.
The oxidation activity of the TiO2–AA–Co nanohybrid,
was initially examined in the epoxidation of norbornene (1
mmol) with O2 (1 atm) in ethyl acetate (1 mL), as this reac-
tion does not proceed in the absence of the catalyst under
any conditions. The reaction conditions were optimized
with respect to the nature and quantity of solvent, tem-
perature, catalyst, quantity of N-hydroxyphthalimide
(NHPI), and nature of the oxidant [for the experimental de-
tails, see the Supporting Information (Figure S5)].
Aerobic epoxidation of olefins is of extreme interest
from an environmental as well as an economic viewpoint.
The plethora of heterogeneous or homogeneous catalytic
conditions available to accomplish this key reaction is testa-
ment to its value.1 Nevertheless, developing an efficient
heterogeneous catalyst for aerobic epoxidations in the ab-
sence of a sacrificial co-reductant is rather difficult.2
Among the various transition metals used to catalyze olefin
epoxidation, cobalt has been used extensively with various
oxidants,3 because it is much more abundant and cheaper
than other metals such as ruthenium or iridium.
Despite considerable progress, significant metal con-
tamination under homogeneous conditions has limited the
number of industrial applications of most transition-metal
catalysts. Development of heterogeneous cobalt catalysts
for aerobic epoxidation facilitates separation processes and
enhances the commercial viability of such catalysts.1a–e,4
Nevertheless, the use of heterogeneous catalysts typically
involves various problems, such as nontrivial preparation,
low chemical stability, nonuniform structure, low efficien-
cy, and the possibility of metal leaching during the transfor-
mation. In addition, the catalytic activity of most of these
An investigation of the effect of catalyst loading demon-
strated that our catalyst has a high atom efficiency. Under
visible-light irradiation, norbornene was converted com-
© Georg Thieme Verlag Stuttgart · New York — Synlett 2016, 27, A–D

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M. Jafarpour et al.
Letter
Synlett
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
TiO2
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
OH
OH
EtOAc, O2
Scheme 1 Aerobic olefin oxygenation catalyzed by a TiO2–ascorbic acid–Co nanohybrid
pletely within four hours by using a low loading of the cata-
lyst (0.2 mol%) to give exo-norbornene oxide exclusively in
94% yield. No improvement in the reaction rate was ob-
served (33% after 4 h) on adding 2-methylpropanal, a com-
monly used reducing agent, confirming the ineffectiveness
of a reducing agent in this aerobic system. Inspection of the
results in Figure S5(v) (Supporting Information) revealed
that the efficiency of oxidation was dependent on the
amount of NHPI, in that the reaction did not proceed in the
absence of NHPI under any conditions. NHPI has been re-
ported to act as a free-radical oxidation catalyst.2,6 There-
fore, a radical mechanism may be suggested for the title ox-
idation system using O2; this is similar to previous reports
on reactions conducted in the presence of cobalt complexes
(Figure S6 in the Supporting Information).2,7 Such a mecha-
nism was further supported by the retardation of the oxida-
tion of norbornene in the presence of radical scavengers
such as 2,6-di-tert-butyl-4-methylphenol under the same
conditions. Under the optimized conditions (0.2 mol% TiO2–
AA–Co and 15 mol% NHPI at 60 °C), with a continuous
stream of O2 in ethyl acetate, TiO2–AA–Co exhibited a high
efficiency and noteworthy selectivity toward aerobic oxida-
tion of a wide range of olefins (Table 1).
As summarized in Table 1, olefins generally behaved as
good substrates for this nanocatalyst. Besides norbornene,
1-methylcyclohexene, 1-phenylcyclohexene, and indene
selectively gave the corresponding epoxides (Table 1, en-
tries 1–4). The poor reactivities of cyclohex-1-en-1-ylben-
zene and indene (entries 3 and 4) possibly point to a degree
of steric hindrance around the active site of the catalyst.
When cyclooctene was subjected to the oxidation proce-
dure, cyclooctene oxide was formed as the major product
(85%) along with 15% of the related α,β-unsaturated ketone
formed through allylic oxidation (entry 5). As expected, cy-
clohexene, which is prone to allylic oxidation, gave cyclo-
hex-2-en-1-one as the only product (entry 6). Likewise, ox-
idation of cyclohex-2-en-1-ol occurred easily to give cyclo-
hex-2-en-1-one in high yield and with excellent selectivity
(entry 7). Nevertheless, linear allylic alcohols (entries 8 and 9)
were converted selectively into the corresponding epoxides
in moderate yield (40–54%) and with no product resulting
from oxidation of the hydroxy group. Oct-1-en-3-ol (entry
9) gave threo-1,2-epoxyoctan-3-ol exclusively in 40% yield,
1
according to H NMR analysis. In addition to norbornene,
which exclusively gave the corresponding exo-epoxide (en-
try 2), epoxidation of cis- and trans-stilbene proceeded
with total stereoselectivity (entries 10 and 11). Note that
no activity was observed for oct-1-ene, a less reactive linear
terminal olefin, or for electron-deficient trans-benzylidene-
acetophenone.8
Table 1 Aerobic Oxidation of Alkenes Catalyzed by the TiO2–AA–Co
Nanohybrid in EtOAca
Entry Alkene
Productb
Time Yieldc,d
(h) (%)
100
(95)
O
1
2
5
O
100
(94)
4
10
6
Ph
Ph
O
28
(21)
3
4
O
50
(43)
100
5
6
O
(85)e
© Georg Thieme Verlag Stuttgart · New York — Synlett 2016, 27, A–D

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M. Jafarpour et al.
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Table 1 (continued)
Entry Alkene
To provide evidence for the photocatalytic properties of
the TiO2 core, we examined the oxidation of norbornene in
darkness or under UV or visible-light irradiation in the
presence of NHPI (15 mol%) and a continuous stream of O2
at 60 °C. The accelerated rates observed under irradiation,
particularly with UV irradiation, confirmed that the TiO2
core has a photocatalytic effect on the oxidation efficiency
of the TiO2–AA–Co nanohybrid (Table 2, entry 3). The
smaller bandgap of the TiO2–AA–Co nanohybrid (2.8 eV),
compared with that of TiO2 nanoparticles (3.2 eV),5g might
account for this activity (Figure S8 in the Supporting Infor-
mation).
Productb
Time Yieldc,d
(h)
(%)
O
100
(96)
6
5
OH
O
100
(95)
7
4.3
O
OH
O
54
(48)
8
7
5
Table 2 Screening of the Photocatalytic Activity of TiO2–AA–Coa
OH
Entry
Catalyst
Conditions
Time (h)
Yieldb (%)
9
40
OH
1
2
3
4
5
6
7
8
9
TiO2
UV
4
4
2
4
4
4
4
4
4
26
71
quant
–
OH
TiO2–AA
TiO2–AA–Co
TiO2
UV
UV
68
(59)
10
5
visible light
visible light
visible light
darkness
darkness
darkness
TiO2–AA
TiO2–AA–Co
TiO2
42
94
–
O
O
56
(50)
TiO2–AA
TiO2–AA–Co
18
37
11
12
5
6
a The substrate/NHPI/catalyst molar ratio was 1000:150:2. The reactions
were carried out under a continuous stream of O2 (5–7 mL/min) in EtOAc (1
mL) at 60 °C.
O
b Yield by GC analysis.
30f
a The substrate/NHPI/catalyst molar ratio was 1000:150:2. Reactions were
carried out under a continuous stream of O2 (5–7 mL/min) in EtOAc (1 mL)
at 60 °C.
The heterogeneous nature of the catalyst was confirmed
by a filtration experiment. For this purpose, the catalyst
was recovered by centrifugation and decantation of the re-
action mixture. No catalytic activity was observed in the fil-
trate solution, and neither Co nor Ti was detected in the fil-
trate solution by ICP-AES analysis. Aerobic epoxidation of
norbornene proceeded well in the presence of the recov-
ered catalyst after it had been washed with ethyl acetate
and dried under vacuum.
The ease of recovery, combined with the intrinsic stabil-
ity of the TiO2–AA–Co nanocomposite, permitted the cata-
lyst to be used at least five times in aerobic epoxidation un-
der the standard conditions for this study11 (Figure S9 in the
Supporting Information). A comparison of the FTIR and UV-
vis spectra of used TiO2–AA–Co nanocatalyst with those of
the fresh catalyst showed that the structure of the catalyst
remained almost intact after five recovery cycles (Figure
S10 in the Supporting Information).
b The products were identified by 1H NMR or by comparison with the GC re-
tention times of authentic samples.
c GC yield; isolated yields are given in parentheses.
d The selectivity toward the products was >99%, based on GC analysis.
e The remainder was the related α,β-unsaturated ketone.
f The remainder was benzaldehyde (70%).
Replacement of TiO2–AA–Co nanoparticles by other
nanoparticulate metal oxides such as MoO3,9 monoclinic
ZrO2,10 γ-Fe2O3, or TiO2 or their nanocomposites, such as
TiO2–AA, AA–Co, γ-Fe2O3–AA–Co, and MoO3–AA–Co, ex-
tended the time for oxidation of norbornene under the
same conditions (Figure S7 in the Supporting Information).
No oxidation product was observed in the presence of AA or
AA/Co(OAc)2.
The results presented in the Supporting Information
(Figure S7) suggest that the catalytic performance of the
TiO2–AA–Co nanohybrid can be explained in terms of the
oxidation activity of the Co(II) centers2,7 in combination
with the photocatalytic activity of the TiO2 core, which also
acts as a heterogeneous support.
In conclusion, we have improved the visible-light pho-
tocatalytic properties of TiO2 for aerobic oxidation of vari-
ous olefins by incorporating Co(OAc)2 on ascorbic acid coat-
ed TiO2 nanoparticles. Oxidations with this catalyst showed
remarkable chemo-, diastereo- and stereoselectivities. Our
© Georg Thieme Verlag Stuttgart · New York — Synlett 2016, 27, A–D

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M. Jafarpour et al.
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results demonstrate a synergistic effect of the cobalt–ascor-
bic acid complex and the TiO2 nanoparticles on the visible-
light photocatalytic activity of the combination. The use of
oxygen as the simplest oxidant, the lack of a need for a co-
reductant, the use of visible light as an energy source, and
the reusability and recyclability of the photocatalyst are sa-
lient features of the present method from an environmental
point of view, and render the catalyst amenable to use in
industrial applications.
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Acknowledgment
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Support for this work by Research Council of University of Birjand is
highly appreciated.
Supporting Information
Supporting information for this article is available online at
http://dx.doi.org/10.1055/s-0036-1588897.
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(11) Typical Procedure and Reusability of Catalyst: To a mixture of
norbornene (1 mmol) and TiO2/AA/Co nanohybrid (0.2 mol%) in
EtOAc (1 mL) was added NHPI (15 mol%, 0.024 g) and the reac-
tion mixture was stirred under 1 atm O2 (5−7 mL/min) and
visible light at 60 °C for 4 h. After completion of the reaction,
TiO2/AA/Co nanohybrid was separated by centrifugation fol-
lowed by decantation (3 × 5 mL EtOAc). The isolated solid-phase
TiO2/AA/Co nanohybrid was washed with EtOH then dried
under reduced pressure and reused for the next run.
© Georg Thieme Verlag Stuttgart · New York — Synlett 2016, 27, A–D