Visible-light-induced metal-free allylic oxidation utilizing a coupled photocatalytic system of g-C3N4 and N-hydroxy compounds

Article abstract of DOI:10.1002/adsc.201100175

Graphitic carbon nitride (g-C₃N₄) and N-hydroxy compounds can function as a non-metal photocatalytic system to activate O ₂ for the selective allylic oxidation under mild conditions, avoiding the employment of any metal derivative or organic oxidizing agents. Interestingly, the novel photocatalytic system affords a remarkably high selectivity towards the formation of aldehydes, especially in the oxidation of toluene. By combining the unique nature of g-C₃N₄ (surface basicity, semiconductor features, high stability) and the remarkable catalytic oxidation reactivity of nitroxyl radicals, this photocatalytic system opens up a mild and efficent access for C-H bond activation. Copyright

Full text of DOI:10.1002/adsc.201100175

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DOI: 10.1002/adsc.201100175
Visible-Light-Induced Metal-Free Allylic Oxidation Utilizing
a Coupled Photocatalytic System of g-C3N4 and N-Hydroxy
Compounds
Pengfei Zhang,a Yong Wang,a,b Jia Yao,a Congmin Wang,a Chao Yan,a
Markus Antonietti,b and Haoran Lia,*
a
Department of Chemistry, Zhejiang University, Hangzhou 310027, Peopleꢀs Republic of China
Fax : (+86)-571-8795-1895; phone: (+86)-571-8795-2424; e-mail: lihr@zju.edu.cn
Department of Colloid Chemistry, Max-Planck Institute of Colloids and Interfaces, Research Campus Golm, 14424
b
Potsdam, Germany
Fax : (+49)-331-567-9502
Received: March 11, 2011; Revised: March 28, 2011; Published online: May 23, 2011
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/adsc.201100175.
Abstract: Graphitic carbon nitride (g-C3N4) and N-
hydroxy compounds can function as a non-metal
photocatalytic system to activate O2 for the selec-
tive allylic oxidation under mild conditions, avoid-
ing the employment of any metal derivative or or-
ganic oxidizing agents. Interestingly, the novel pho-
tocatalytic system affords a remarkably high selec-
tivity towards the formation of aldehydes, especially
in the oxidation of toluene. By combining the
unique nature of g-C3N4 (surface basicity, semicon-
ductor features, high stability) and the remarkable
catalytic oxidation reactivity of nitroxyl radicals,
this photocatalytic system opens up a mild and effi-
ed conversion and selectivity.[2] From the standpoint
of green and sustainable chemistry, metal-containing
catalysts with high toxicity and expense should be
avoided.
Metal-free catalysts have been becoming increas-
ingly attractive in recent years, and considerable
effort has been made to examine the effect of metal-
free catalytic systems on the aerobic oxidation.[3] Re-
cently, carbon nitrides have attracted considerable at-
tention partly because of their potential applications
in sustainable chemistry as metal-free heterogeneous
catalysts.[4] For example, a recent break-through was
achieved with the use of these materials as photocata-
lysts for the oxidation of alcohols and amines by O2
under visible light irradiation.[5] These metal-free oxi-
dation processes are good for the proof of principle,
the activity of bare carbon nitride was, however, gen-
erally very low which makes the practical application
difficult. Some modifications of carbon nitride were
found to show improved activities in the oxidation re-
À
cent access for C H bond activation.
À
Keywords: carbon nitride; C H activation; N-hy-
droxyphthalimide; metal-free oxidation; NHPI;
photooxidation
actions.[6] However, it is still difficult to drive the oxi-
3
À
dation of sp C H bonds (for example: toluene and
Selective allylic oxidation with molecular oxygen as cyclohexane) with carbon nitrides and O2.
the terminal oxidant is a primary and essential tool
In the past decade, some N-hydroxy compounds,
for organic synthesis and industrial chemistry, as it is acting as a new kind of organocatalyst, such as 1-hy-
À
believed that introduction of oxygen atoms into C H droxybenzotriazole (HBT), N-hydroxysuccinimide
bonds through aerobic oxidation is the cheapest way (NHSI), and N-hydroxyphthalimide (NHPI), have at-
to synthesize more complex molecules from natural tracted interest in a broader way.[7] For example, N-
petroleum.[1] However, due to the triplet ground state hydroxy compounds (>N-OH) can act as a hydrogen
structure, oxygen is a relatively unreactive molecule donor and be oxidized to the N-oxyl radical (>N-OC),
À
under mild conditions, especially towards strong C H which is a neat and selective oxidant for the transfor-
bonds. Traditional allylic oxidation is usually accom- mation of hydrocarbons, like alkanes and alkylaro-
panied by the use of transition metal-containing cata- matics. However, co-catalysts (particularly metal
lysts under relatively harsh conditions (high tempera- salts) are necessary for the N-hydroxy compound
ture and high pressure), and it also suffers from limit- (>N-OH) to generate the N-oxyl radical (>N-OC).[7g,8]
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Can carbon nitrides bridge the gap between visible
To our delight, when the g-C3N4/HBT system was
light energy and N-hydroxy compound (>N-OH) ir- employed as catalyst under visible light, 23% of CA
radiation? Based on the previous reports, graphitic was transformed to 7-KOCA with 61% selectivity
carbon nitride (g-C3N4) can activate O2 to O2CÀ under (entry 1, Table 1), while g-C3N4 or HBT alone was not
the illumination of visible light.[5,9] This suggested to efficient for the transformation of CA (entries 2 and
us that if O2CÀ can abstract a hydrogen from >N-OH, 3, Table 1). These results illustrated that g-C3N4 and
the formed >N-OC radical could smoothly activate HBT were coupled as a catalytic system for the oxy-
À
the C H bond. To test this interesting hypothesis, a genation of CA, as expected.
combination of g-C3N4 and HBT was used for the se-
When NHSI was used in place of HBT, a relatively
lective allylic oxidation of cholesteryl acetate (CA) to good selectivity (96%) was obtained with similar con-
7-ketocholesteryl acetate (7-KOCA), which is a key version (19%) (Entry 4, Table 1). Encouraged by
step in the synthesis of vitamin D3 (Table 1).[10]
those results, the combination of g-C3N4 and NHPI
was examined, in which a good result (48% conver-
sion and >99% selectivity) was obtained (entry 5,
Table 1). This result is consistent with previous obser-
vations that NHSI is less reactive than NHPI in the
oxidation of fluorene.[7f] Needless to say, it was diffi-
cult to promote this oxidation with NHSI or NHPI
alone (entries 6 and 7, Table 1). The simple and cheap
N-hydroxy compound, dimethylglyoxime (DMG) in
combination with NHPI, showed no photocatalytic ac-
tivity in the transformation of CA (entry 8, Table 1).
Table 1. The results of the oxidation of cholesteryl acetate
with g-C3N4 and N-hydroxy compounds.[a]
À
The H-abstraction reaction from R H by the
Entry Catalyst
hn Conv.
Sel.
[%]
>N-OC radical is an important step in the oxidation
[%]
of hydrocarbons catalyzed by N-hydroxy compounds.
1
g-C3N4 (100 mg)/HBT
+
23
61
À
The bond dissociation energy (BDE) of the >NO H
(1 mmol)
bond is closely related to H-abstraction process by
the >N-OC radical.[11] In the oxidation of CA, HBT
exhibited a lower reactivity than NHPI, which can be
rationalized on the basis of the lower energy of the
2
3
4
g-C3N4 (100 mg)
HBT (1 mmol)
g-C3N4 (100 mg)/NHSI
(1 mmol)
+
+
+
<1
<1
19
–
–
96
À
5
g-C3N4 (100 mg)/NHPI
(1 mmol)
+
48
>99
>NO H bond of HBT, if compared with the corre-
sponding >NO H bond energy of NHPI (85 vs.
À
88 kcalmolÀ1, respectively).[7b] These results provide
an argument that the lower BDE of the >NO H
6
7
8
NHSI (1 mmol)
NHPI (1 mmol)
g-C3N4 (100 mg)/DMG
(1 mmol)
+
+
+
<1
3
<1
–
67
–
À
bond is, the easier is the corresponding>N-OC radical
formed with H-abstraction by O2CÀ. However, the
easily formed >N-OC radical shows weak or even no
ability for abstracting hydrogen from hydrocar-
bons[7f,g]. Since the g-C3N4/NHPI system presented ex-
cellent catalytic activity in oxygenationof CA, this
system was selected for further studies.
9
g-C3N4 (100 mg)/NHPI
(1 mmol)
À
À
À
+
+
4
73
10[b] g-C3N4 (100 mg)/NHPI
(1 mmol)
44
38
22
9
>99
62
11[c]
g-C3N4 (100 mg)/NHPI
(1 mmol)
g-C3N4 (50 mg)/NHPI
(1 mmol)
g-C3N4 (100 mg)/NHPI
(0.5 mmol)
12
>99
>99
To clarify the role of g-C3N4 and N-hydroxy com-
pounds, detailed investigations have been carried out
on the oxidation of CA. When the process was carried
out in the dark, only 4% conversion of CA was ob-
served under the same system (entry 9, Table 1). It
was surprising that almost the same resulting (44%
conversion and >99% selectivity) was obtained when
the oxidation was performed under visible light irradi-
ation for 2 h and then in the dark (entry 10, Table 1).
This result suggested that g-C3N4 may act as initiator
for the generation of phthalimide N-oxyl radical
(PINO) to some extent. To further make clear the
role of g-C3N4, we separated g-C3N4 out by filtration
after the oxidation had been carried out under visible
light irradiation for 2 h and a slightly lower conver-
sion 38% with 62% selectivity was achieved
13
14[d] BPO/NHPI (1 mmol)
15[e] AQ/NHPI (1 mmol)
À
À
À
20
<1
45
54
–
78
16[f]
Co
N
[a]
Reaction conditions: cholesteryl acetate (10 mmol), ace-
tone solvent (80 mL), O2 1 atm, 358C, 8 h, under visible
light irradiation.
The reaction was carried out with visible light irradiation
for 2 h and then in the dark for 6 h.
g-C3N4 was separated out after the reaction had been car-
ried out for 2 h.
BPO: benzoyl peroxide, 0.05 mmol.
[b]
[c]
[d]
[e]
[f]
AQ: anthraquinone, 0.1 mmol.
CoACHTUNGTRENNUNG(acac)2: cobalt(II) acetylacetonate, 0.05 mmol.
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Visible-Light-Induced Metal-Free Allylic Oxidation Utilizing a Coupled Photocatalytic System
(Entry 11, Table 1). The lost selectivity of about 38% subsequently abstracts
a
hydrogen atom from
is believed to be due to 7-hydroperoxide-cholesteryl
acetate, which was observed as the only side product decomposition of intermediate ROOH to the oxygen-
in this oxidation. It is evident that g-C3N4 demon- ated products.
>N-OH to form >N-OC. Finally, g-C3N4 catalyzes the
strates some Lewis base character which can promote
The superoxide radical anion (O2CÀ) formed by the
the decomposition of 7-hydroperoxide-cholesteryl reduction of O2 (by the excited electrons from the
acetate.[12] When amount of NHPI or g-C3N4 was conduction band of the g-C3N4) was confirmed by
halved under these conditions, lower conversions electron spin resonance (ESR) and the DMPO (5,5-
(22%, 9%) were achieved, while keeping the selectivi- dimethyl-1-pyrroline N-oxide) spin trap technique in
ty at >99% (entries 12 and 13, Table 1). Early re- methanol (Figure 1).[13] When NHPI was added to this
search indicated that NHPI can efficiently oxidize system, the ESR signals for DMPO-O2CÀ adducts de-
substrates with molecular oxygen upon adding a small creased under the same conditions. The change in the
amount of co-catalysts. Compared with these catalytic ESR signal corresponds to the oxidation of NHPI to
systems, g-C3N4/NHPI system exhibits an excellent PINO by O2CÀ. On the basis of that result, we believe
catalytic effect and especially a high selectivity for ke- that some N-hydroxy compounds with suitable BDE
tones (entries 14–16, Table 1).
values all can be oxidized to their corresponding ni-
A plausible reaction pathway for the allylic oxida- troxyl radicals by O2CÀ and then catalyze a wide varie-
tion by g-C3N4/>N-OH under visible light irradiation ty of free radical oxidations.
is illustrated in Scheme 1. The visible-light-induced
To further probe the potential of this metal-free
catalytic cycle is assumed to involve two coupling photocatalytic system, various substrates were oxy-
steps of electron transfer: (i) O2 captures the light-ex- genated by g-C3N4/NHPI system with O2 under visible
cited electron from the conduction band (CB) of g- light (Table 2). Cyclohexene was oxygenated smooth-
C3N4 to generate superoxide radical anion (O2CÀ); (ii) ly to afford 2-cyclohexen-1-one as the main product
after abstracting a hydrogen from >N-OH, O2CÀ is re- (entry 1, Table 2). To test the reusability of g-C3N4
duced to HOOÀ. Then HOOÀ can react with the posi- which can be easily recovered by simple filtration and
tive hole (h+) by injecting a electron into the valence washing with 0.2M NaOH, three recycling runs were
band (VB) of g-C3N4, at the same time oxidizing an- carried out on the oxidation of cyclohexene. g-C3N4
other >N-OH to >N-OC. The nitroxyl radicals, nota- can be reused at least three times without losing activ-
bly phthalimide N-oxyl (PINO) follow a free radical ity (entries 2–4, Table 2). Phenylethylene, a-isophor-
route, initiating by abstracting a hydrogen atom from one, a-pinene, and ethylbenzene were oxidized with a
RH. Under an O2 atmosphere the newly formed RC is good selectivity (66–84%) for ketones, although the
rapidly trapped by O2 to give a ROOC radical, which conversion (26–71%) was only moderate (entries 5–8,
Table 2). The oxidation of b-isophorone was quick,
Figure 1. Changes in the DMPO spin-trapping ESR spectra
for g-C3N4 (10 mg) and g-C3N4 (10 mg)/NHPI (50 mg) for
superoxide radical (O2CÀ) in methanol (1 mL). The mixture
was stirred with bubbling O2 and visible light irradiation for
10 min and then put into a quartz tube.
Scheme 1. Proposed potential oxidation mechanism for the
g-C3N4/>N-OH system.
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Table 2. The results of the oxidation of various substrates
only benzaldehyde and cyclohexanone were detected.
The g-C3N4/NHPI system is therefore rather selective,
with respect to the aldehyde and ketone product it
creates.
with g-C3N4 and NHPI.[a]
Entry Substrate Product
Time [h] Conv. [%] Sel. [%]
1
8
8
8
8
93
92
90
95
77
71
66
67
In summary, g-C3N4 asisted by N-hydroxy com-
pounds has been successfully applied for selective al-
lylic oxidations with molecular oxygen under mild
conditions. Four N-hydroxy compounds have been
tested in combination with g-C3N4, and the g-C3N4/
NHPI system exhibits the best catalytic performance,
affording high selectivities towards the formation of
aldehydes and ketones, especially in the oxidation of
toluene and cyclohexane. This visible-light-induced
metal-free catalyst system opens up an efficient and
mild access for selective oxidations with molecular
oxygen. Moreover, we believe that various visible
light-responsive materials can couple with N-hydroxy
compounds, especially NHPI, and provide more
metal-free catalyst systems for the oxidation of allylic
2[b]
3[c]
4[d]
5
24
33
70
6
20
26
79
7[e]
18
23
71
27
66
84
8
À
C H bonds.
Experimental Section
9[f]
10
4.5
100
>99
All the chemicals were of AR grade. They were commer-
cially purchased and used without further treatment. A w-
filament bulb (250 W) together with a 420 nm cut-off filter
was used as a visible light source for the irradiation of the
reaction system. The NMR spectra were recorded by
Bruker ARX 500 NMR spectrometer, and TMS as the inter-
nal standard. Electron spin resonance (ESR) signals were
recorded at room temperature (298 K) with a Bruker ESR
A300 spectrometer. All GC experiments were carried out
and recorded using a Shimadzu GC-2010. The structure of
products and by-products was identified using an HP6890
GC/MS spectrometer by comparing retention times and
fragmentation patterns with those of authentic samples.
20
24
9
6
>99
>99
11[g]
[a]
Reaction conditions: substrates (10 mmol), g-C3N4
(100 mg), NHPI (1 mmol), acetonitrile (20 mL), O2
1 atm, 608C, under visible light irradiation.
Second run to test the reusability of catalyst.
Third run to test the reusability of the catalyst.
Fourth run to test the reusability of the catalyst.
Benzonitrile (20 mL) as solvent.
[b]
[c]
[d]
[e]
[f]
g-C3N4 (50 mg) and NHPI (0.5 mmol) were used.
g-C3N4 (50 mg) and NHPI (0.1 mmol) were used.
[g]
General Procedure for the Oxidation of Cholesteryl
Acetate
The oxidation of cholesteryl acetate (CA) was carried out in
a three-necked glass-reactor (100 mL), which was fitted with
an O2 inlet tube and a reflux condenser. Cholesteryl acetate
(10 mmol), catalysts as described in the Table and acetone
(80 mL) were added to the reactor. A w-filament bulb
giving 100% conversion and remarkably high selectiv-
ity (>99%) towards the formation of keto-isophorone
in 4.5 h (entry 9, Table 2). The oxidation of toluene
gave benzaldehyde as the only product (>99% selec-
tivity) (entry 10, Table 2). Catalyzed by 50 mg of g- (250 W) together with a 420 nm cut-off filter was placed at a
distance of ~10 cm from the reactor as a visible light source
for the irradiation of this reaction system. Then the reaction
was performed at 358C in a water bath with O2 flowing at a
constant flow rate (20 mLminÀ1) for 8 h. To carry out the re-
action in the dark, the glass-reactor was covered with alumi-
nium foil. Reactions were monitored by TLC. After comple-
tion of the reaction, the solvent was removed under reduced
pressure at 508C. The residue was purified by silica gel
column chromatography (ethyl acetate/petroleum ether=1/
11) to afford 7-ketocholesteryl acetate (7-KOCA) as the de-
C3N4 with 0.1 mmol of NHPI, a 6% conversion of cy-
clohexane with cyclohexanone as the sole product
(>99% selectivity) was obtained (entry 11, Table 2).
As far as we know, in previous reports, no matter
what co-catalyst was coupled, NHPI-catalyzed oxida-
tions of toluene and cyclohexane all gave alcohols or
acids as part of the product palette.[1a] It should be
noted that when catalyzed by g-C3N4/NHPI (en-
tries 10 and 11, Table 2), the oxidation products of tol-
uene and cyclohexane were free of alcohols and acids, sired product.
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Visible-Light-Induced Metal-Free Allylic Oxidation Utilizing a Coupled Photocatalytic System
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General Procedure for the Oxidation of the
Substrates in Table 2
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In a typical oxidation of the substrates in Table 2, 10 mmol
of substrate, 100 mg of g-C3N4, 1 mmol of NHPI and of
20 mL acetonitrile were added into a 50-mL three-neck,
round-bottom reactor, which was fitted with a magnetic stir-
rer and an O2 inlet tube. The reaction was performed at
608C in a oil bath with fast stirring. The oxygen was flowing
into the reactor at a constant flow rate (20 mLminÀ1). After
completion of the reaction, g-C3N4 was filtered and then the
reaction mixture was injected into the GC for analysis with
benzyl ethanoate as the internal standard. The products
were confirmed by comparison with standard chemicals and
GC-MS.
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Reuse of g-C3N4
When the first oxidation had finished, after cooling down
the reaction mixture to room temperature, g-C3N4 was sepa-
rated by filtration. The yellow solid was washed with 0.2M
NaOH and then dried in a vacuum oven at 1008C for 12 h.
Finally, the recovered g-C3N4 was used in a subsequent reac-
tion.
Acknowledgements
This work was supported by the National Natural Science
Foundation of China (Nos. 20990221, 20803062 and
J0830413). The authors are grateful to Zhejiang NHU Com-
pany Ltd, China, for financial support.
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