Metal-free allylic oxidation with molecular oxygen catalyzed by g-C 3N4 and N-hydroxyphthalimide

Article abstract of DOI:10.1007/s10562-014-1200-1

Polymeric graphitic carbon nitride (g-C₃N₄) is a layered graphite-like nitrogen-rich material, bearing the potential ability to reductively adsorb molecular oxygen for catalytic allylic oxidation. Furthermore, N-hydroxyphthalimide (NHPI) has been recognized as an efficient catalyst for aerobic oxidation of various organic compounds under mild conditions in the presence of various co-catalysts. We present here a promising strategy for employing such nitride-rich g-C₃N₄ combined with NHPI to form an all-organic metal-free composite and have examined its activity for allylic oxidation with molecular oxygen as the primary terminal oxidant. In the case of allylic oxidation α-isophorone catalyzed by g-C₃N₄/NHPI gave priority to its corresponding carbonyl compound and epoxide. The effects of various reaction conditions on the catalytic reaction were optimized, affording 74.8 % conversion with 44.4 % selectivity of ketoisophorone at 130 C in 5 h. Repeated runs demonstrated that the catalyst was stable for at least three cycles without noticeable loss of its catalytic activity. Graphical Abstract: [Figure not available: see fulltext.]

Full text of DOI:10.1007/s10562-014-1200-1

Catal Lett (2014) 144:717–722
DOI 10.1007/s10562-014-1200-1
Metal-Free Allylic Oxidation with Molecular Oxygen Catalyzed
by g-C N and N-Hydroxyphthalimide
3
4
Guiyin Liu
•
Ruiren Tang Zhen Wang
•
Received: 4 November 2013 / Accepted: 17 January 2014 / Published online: 4 February 2014
Ó The Author(s) 2014. This article is published with open access at Springerlink.com
Abstract Polymeric graphitic carbon nitride (g-C N ) is
3
variety oxidants employed so far, the noteworthy examples
include metal salts (SeO [3], CrO [4]), tert-butyl hydro-
4
a layered graphite-like nitrogen-rich material, bearing the
potential ability to reductively adsorb molecular oxygen for
catalytic allylic oxidation. Furthermore, N-hydroxyphthal-
imide (NHPI) has been recognized as an efficient catalyst
for aerobic oxidation of various organic compounds under
mild conditions in the presence of various co-catalysts. We
present here a promising strategy for employing such
nitride-rich g-C N combined with NHPI to form an all-
2
3
peroxide (TBHP) [5], hydrogen peroxide (H O ) [6] and
2
2
molecular oxygen (O ) [7, 8]. Among these oxidants,
2
although metal salts could obtain ideal conversion and
selectivity, they bore certain defects such as environmental
hazards, high expense and waste of metal resources. TBHP
and H O are obviously more environmentally benign than
2
2
metal salts, however, for safety, there is a need to remove
the excess oxidants, except that, TBHP is so expensive for
large-scale industrial application. Molecular oxygen is
cheap, forms water as the only by-product and obtains
3
4
organic metal-free composite and have examined its
activity for allylic oxidation with molecular oxygen as the
primary terminal oxidant. In the case of allylic oxidation a-
isophorone catalyzed by g-C N /NHPI gave priority to its
maximum atom efficiency [9], O is more ideal oxidant for
2
3
4
corresponding carbonyl compound and epoxide. The
effects of various reaction conditions on the catalytic
reaction were optimized, affording 74.8 % conversion with
both economical and environmental benefits. However,
molecular oxygen is a relatively unreactive oxidation
toward the strong bonds of C–H for its triplet ground state
structure [10]. Heretofore, many catalysts which can effi-
ciently catalyze allylic oxidation with molecular oxygen
have been reported, such as heteropoly acid [11], phtha-
locyanine [12], Schiff-base transition metals (Mn, Cu, Co,
Ni) [13–15]. Even so, rigorous solvent (DMSO) [11] and
combination with metal ions made them unacceptable for
the sustainability of the process. Bearing in mind these
concepts, efficient metal-free catalyst, which can active
molecular oxygen, is indispensable from the eco-friendly
point of view.
4
4.4 % selectivity of ketoisophorone at 130 °C in 5 h.
Repeated runs demonstrated that the catalyst was stable for
at least three cycles without noticeable loss of its catalytic
activity.
Keywords Allylic oxidation Á g-C N Á
3
4
N-Hydroxyphthalimide Á Molecular oxygen
1
Introduction
In our preceding papers, the organic catalyst, N-hy-
droxyphthalimide (NHPI) has been recognized as a
remarkable catalyst for aerobic oxidation of various
organic substrates via hydrogen abstraction by intermediate
phthalimide-N-oxyl (PINO) radical [16]. Although the
generation of the PINO radicals through the precursors
NHPI can be achieved efficiently by using transition metal
salts [10, 16, 17] as the mediators, the catalytic system is
detrimental to environment. Green chemistry teaches us
Selective allylic oxidation has been studied comprehen-
sively over the past decade, for its extensive application in
organic synthesis and industrial chemistry [1, 2]. Of the
G. Liu Á R. Tang (&) Á Z. Wang
School of Chemistry and Chemical Engineering, Central South
University, Changsha 410083, Hunan, China
e-mail: trr@mail.csu.edu.cn
123

7
18
G. Liu et al.
that there is an urgent need for eco-friendly methods to
carry out organic reactions. Then creating a system based
on NHPI and a recoverable co-catalyst is one feasible
access to obviate this disadvantage. Currently, a nitrogen-
rich graphitic carbon nitride (g-C N ) [18, 19], a solid-state
3
4
semiconductor, has attracted much attention. This material
resembles the polymeric melamine and is regarded as the
most stable allotrope of covalent carbon nitride solids at
ambient conditions, owing to the fact that it is formed by
2
the sp hybridization of carbon and nitrogen generating the
heptazine heterocyclic ring (C N ) unit, later that the rings
6
7
Q
are linked by edge N forming the
conjugated graphitic
layers [20]. Beyond that, g-C N exhibits good stability,
3
4
ease of reusability, electronic and catalytic properties [21],
which make it promising applications as heterogeneous
metal-free catalysts for selective alcohol oxidation [22],
coupling of amines [23], selective oxidation of primary
carbon hydrogen bonds [21], and hydrogen evolution by
splitting water [24]. Very recently, Li and co-workers [25]
have applied g-C N assisted by N-hydroxy compounds in
Fig. 1 XRD patterns of g-C N materials
3
4
3
4
selective allylic oxidation with molecular oxygen under
visible light irradiation. However, visible light spectrum
2.2 Preparation of Catalyst
The g-C was prepared by heating melamine directly
(
400–800 nm) only takes possession 46 % of the solar
N
3 4
spectrum. In addition, common glass apparatus is of inhi-
bition effect on the absorption of light, and the quartz glass
instrument is demanded. Employing stainless steel auto-
clave as reaction vessel under heating alone is more con-
venient for industrial production. Inspired by these
previous findings, we would like to build a catalytic system
of combining g-C N with NHPI as catalyst and molecular
according the previous reports [26, 27]. In a typical pro-
cedure, melamine was put into an alumina crucible with a
cover, heated at a rate of 3 °C/min reaching temperature of
550 °C in a muffle furnace, treated at this temperature for
further 4 h. When cooling to room temperature, grinding
the pale yellow polymer obtained the g-C
N powder.
3 4
3
4
oxygen as the primary terminal oxidant in the allylic oxi-
dation under heating alone within stainless steel autoclave,
which is unprecedented and seems to be a intriguing
method for converting hydrocarbons into their corre-
sponding carbonyl compounds.
2.3 Oxidation Reaction
Reactions were carried out in a 100 mL stainless steel
autoclave with a magnetic stirrer. a-Isophorone (a-IP,
10 mmol), solvent acetonitrile (10 mL) were placed in the
vessel together with g-C N (200 mg), NHPI (1 mmol,
3
4
10 mol%), and the reactor was sealed. Prior to heating to
2
Experimental
desired temperature (130 °C) under stirring, the atmo-
sphere over the autoclave reactor was replaced three times
2
.1 Materials and Methods
with O and pressurized with O (1.2 MPa). When the
2 2
reaction was finished, the autoclave reactor was cooled to
the ambient temperature and slowly depressurized. The
reaction mixture was analyzed by GC–MS and GC.
All the chemicals were of analytical grade apart from
industrial-grade a-isophorone. They were commercially
purchased and used without further treatment. Infrared
spectra were determined on a Nicolet NEXUS 670 FT-IR
-
1
spectrophotometer using KBr discs in the 400–4,000 cm
3 Results and Discussion
region. Powder X-ray diffraction (XRD) patterns of the
heterogeneous samples were measured with Rigaku D/Max
III VC diffractometer with Cu KR radiation at 40 kV and
3.1 Characterization of g-C N
3 4
4
0 mA in the range of 2h = 2–9°. The reaction products of
In Fig. 1, XRD patterns of the samples show the charac-
teristic (002) interplanar stacking peak around 27.4°, cor-
responding to the interlayer distance of aromatic systems of
d = 0.326 nm, suggesting the formation of well-built
oxidation were determined by AGILENT QP2010 GC/MS
instrument and analyzed by GC-7890II gas chromatograph
with a 50 m 9 0.25 mm 9 0.25 lm OV-1701 column.
1
23

Metal-Free Allylic Oxidation
719
Figure 3 depicts the scanning electron micrograph
SEM) images of fresh and recycled g-C N and shows the
(
3
4
typical stacked lamellar structure, and it has the basic
characteristic of g-C N of a graphite-like structure [30].
3
4
Beyond that, no obvious change of the catalyst g-C N is
3
4
viewed after the reaction.
3.2 Effect of Various Reaction Conditions
on the Catalytic Reaction
3.2.1 Effect of Reaction Temperature and Time
The effects of reaction temperature and time on the oxida-
tion were examined using 200 mg g-C N and 5 mol%
3
4
NHPI, and the results are shown in Table 1. As expected, the
conversion of a-isophorone went up with the concomitant
gradual increase of temperature and reached a maximum at
130 °C (entry 3) and then changed slightly even at higher
3 4
Fig. 2 The FT-IR spectra of fresh and used g-C N
temperature. However, no significant variation in the
selectivity of ketoisophorone (KIP) was observed during the
course of the reaction. This fact has mainly been caused by
the following accounts: under low temperature, the energy
was not sufficient for g-C N to activate oxygen molecule,
g-C N layer structure [28]. Another peak at 13.1° agrees
3
4
with (100) interplanar of tri-triazine units, which is indexed
as the 100 peak [29]. Anything else, no obvious difference
can be detected between the recycled and fresh g-C N ,
3
4
a-IP was hardly oxidized. As illustrated, with prolonging
reaction time from 3 to 5 h, the conversion of a-IP increased
from 33.9 to 60.7 % (entries 6 and 8). While as the reaction
continued, the selectivity of KIP and a-IP conversion
remained almost unchanged. Hence, 130 °C and 5 h should
be the suitable reaction conditions.
3
4
which indicates that the g-C N is of good stability.
3
4
The FT-IR spectra of fresh and recycled g-C N are
3
4
-
1
shown in Fig. 2. Several bands in the 1,200–1,600 cm are
ascribed to the stretching vibration of the heterocyclic aro-
matic ring units, while the sharp absorption peak at
-
1
8
00 cm is considered as the characteristic breathing mode
of the triazine cycle [30]. In addition, the broad absorption
-
3.2.2 Effect of Oxygen Pressure
1
peak located at 3,000–3,300 cm corresponding to the
N–H components (uncondensed amino) and the O–H bands
It is interesting to note that the pressure of molecular
oxygen has significant influence on the allylic oxidation of
a-IP (Fig. 4). Below 1.2 MPa a positive impact of O2
pressure on the reaction is observed clearly. The a-IP
(
the absorbed H O molecules) are observed [31]. Upon
2
comparison, the fresh and recycled g-C N bear similarity to
3
4
each other, which correspond to the literature report.
3 4 3 4
Fig. 3 SEM images of the samples. a fresh g-C N and b used g-C N
123

7
20
G. Liu et al.
Table 1 The effects of reaction temperature and time on a-IP
oxidation
Table 2 The effects of g-C
oxidation
N
3 4
and NHPI amount on the a-IP
Entry
T (°C)
t/h
Conversion (%)
Selectivity (%)
Entry
g-C
(
3
N
mg)
4
NHPI
(mol%)
Conversion
(%)
Selectivity (%)
KIP
OIP
KIP
OIP
1
2
3
4
5
6
7
8
9
110
120
130
140
150
130
130
130
130
130
130
130
130
5
5
35.1
49.1
60.7
57.6
56.0
33.9
46.3
60.7
56.8
55.1
52.0
50.8
54.0
47.7
34.9
42.4
39.9
34.4
42.6
43.6
42.4
46.2
43.5
42.1
44.2
41.9
15.4
18.9
21.7
20.7
21.7
27.8
24.2
21.7
22.5
22.9
24.6
25.7
26.3
1
2
100
200
300
400
200
0
5
5
53.7
60.7
65.1
52.8
8.7
37.6
42.4
34.3
31.8
24.4
41.9
39.7
42.9
44.4
43.8
18.4
21.7
14.1
20.1
26.5
7.0
5
3
5
5
4
5
5
5
0
3
6
5
23.5
24.6
66.9
74.8
67.1
4
7
200
200
200
200
2.5
7.5
10
1.25
24.1
19.6
12.0
14.0
5
8
6
9
1
1
1
1
0
1
2
3
7
10
8
Reaction conditions: 10 mmol a-IP, g-C
1.2 MPa O , 130 °C, 5 h. Analyzed by GC
3 4 3
N , NHPI, 10 mL CH CN,
9
2
10
Reaction conditions: 10 mmol a-IP, 200 mg g-C
N
3 4
and 5 mol%
NHPI, 10 mL CH CN, 1.2 MPa O . Analyzed by GC
3
2
3.2.3 Effect of Catalyst Amount
From Table 2, it is observed that the conversion of a-IP
improves with the increase of g-C N amount until 300 mg
3
4
(entries 1–3). After that, the conversion decreases slightly,
due to the solution can’t stir evenly. The selectivity
decreases obviously (42.4 and 34.3 %, respectively) with
increasing the amount of g-C N from 200 to 300 mg.
3
4
Taking both conversion and selectivity into consideration,
00 mg g-C N is the suitable amount. Needless to say, it
2
3
4
is difficult to promote this oxidation with g-C N or NHPI
3
4
alone (entries 5 and 6). To further make clear the effect of
NHPI, different amount of NHPI was added into the
reaction, the catalytic efficiency was achieved to the
maximum using 10 mol% of NHPI with 74.8 % conversion
of a-IP and 44.4 % KIP selectivity (entry 9). Even though
more quantity of NHPI was added, the a-IP conversion and
the KIP selectivity changed slightly (entry 10).
Fig. 4 The effect of O
2
pressure on a-IP oxidation
3.2.4 Oxidation of Different Hydrocarbons Catalyzed
by g-C N /NHPI
3
4
conversion improves from 29.5 to 60.7 % with increasing
To further probe the generality of this metal-free catalytic
system, further researches were studied to oxidize various
hydrocarbons by g-C N /NHPI with O under heating
O pressure from 0.6 to 1.2 MPa, and then changes very
2
slightly after that. The selectivity of KIP has always been
rather constant. It has been generally accepted that the
dioxygen solubility in the liquid phase increases with the
3
4
2
alone (Table 3). Tetralin afforded a remarkably high con-
version of 95.7 % towards the formation of tetralone with
60.1 % selectivity. In the case of oxidizing toluene, the
benzoic acid was obtained as the main product giving
89.4 % conversion and 69.7 % selectivity (entry 2). Fur-
thermore, the reaction system was found to be acceptable
for oxidizing indane and fluorene to indanone and fluore-
none affording the satisfactory selectivity of 72.2, 87.3 %
increase of dioxygen pressure. At a lower O pressure, the
2
O concentration in solution is so low that insufficient O
2
2
could be activated by catalysts, which contributes to the
lower conversion. When the oxygen solubility in the
solution reaches saturation, excessive oxygen pressure has
no much influence on the oxidation efficiency.
1
23

Metal-Free Allylic Oxidation
721
3 4
Table 3 Oxidation of various hydrocarbons by the g-C N /NHPI
Reaction conditions: substrates 10 mmol, g-C
5
3 4
N 200 mg, NHPI
Fig. 5 Recycling study of the catalyst
mol%, acetonitrile 10 mL, O
2
1.2 MPa, 130 °C, 5 h. Analyzed by GC
and the conversion of 54.3, 66.4 %, respectively (entries 3
and 4). 64.9 % cyclohexene was also converted into cor-
responding 2-cyclohexene-1-one with 82.1 % selectivity
oxidize another NHPI to PINO. Then, PINO abstracts a
hydrogen atom from a-IP forming a-IP radical, while
PINO itself converted into NHPI again [34]. Synchro-
nously, the newly formed a-IP radical is trapped by O2
producing peroxy radical, which finally results in oxygen-
ated products.
(
entry 5).
3
.2.5 Proposed Oxidation Mechanism
Based on the results, a possible catalytic mechanism of
3.2.6 Reused of g-C N
3
4
g-C N /NHPI in the allylic oxidation of a-isophorone is
3
4
proposed and is given in Scheme 1. The electrons in
After the first oxidation reaction, the g-C N can be easily
3 4
g-C N can be activated and transmit from valence band to
3
recovered simply by washing with 0.2 M NaOH [22] and
then dried in a vacuum oven at 65 °C overnight. Then, the
obtained g-C N was used directly in subsequent reaction
4
conduction band under heating [32, 33]. The superoxide
-
radical anion (O2 ) formed by molecular oxygen absorbing
3
4
the excited electrons from the conduction band of the
g-C N [22], captures a hydrogen from NHPI generating
cycles and no obvious loss of its catalytic activity and
selectivity were observed, as shown in Fig. 5, giving 74.8,
72.9, 71.8 % conversion for the first, second, third runs,
3
4
-
PINO. At the same time, the O2 forms HOO which can
-
Scheme 1 The proposed
mechanism for the oxidation
reaction
123

7
22
G. Liu et al.
respectively. That means the g-C N was stable in the
3
8. Zheng GX, Liu CH, Wang QF, Wang MY, Yang GY (2009) Adv
Synth Catal 351:2638
4
catalytic reaction media.
9
. Dhakshinamoorthy A, Alvaro M, Garcia H (2012) J Catal
89:259
2
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0. Yang XM, Zhou LP, Chen Y, Chen C, Su YL, Miao H, Xu J
(2009) Catal Commun 11:171
4
Conclusion
1
1
1. Liu CH, Li F, Tang RR (2010) Bull Korean Chem Soc 32:1723
2. Tonucci L, Nicastro M, d’Alessandro N, Bressan M, D’Ambrosio
P, Morvillo A (2009) Green Chem 11:816
In summary, the fangled metal-free, heterogeneous catal-
ysis protocol consisting of g-C N combined with NHPI
3
4
13. Li ZY, Tang RR, Liu GY (2013) Catal Lett 143:592
14. Mukherjee S, Samanta S, Roy BC, Bhaumik A (2006) Appl Catal
A 301:79
was applied to catalyze allylic oxidation of a-isophorone
into ketoisophorone using molecular oxygen as the primary
terminal oxidant. 74.8 % conversion of a-IP and 44.4 %
selectivity of KIP were obtained under optimized reaction
conditions (10 mmol a-IP, 200 mg g-C N , 10 mol%
1
5. Chang Y, Lv YR, Lu F, Zha F, Lei ZQ (2010) J Mol Catal A
20:56
3
16. Chen LH, Tang RR, Li ZY, Liang S (2012) Bull Korean Chem
Soc 33:459
1
1
3
4
7. Yao Z, Hu XB, Mao JY, Li HR (2009) Green Chem 11:2013
8. Wang Y, Wang XC, Antonietti M (2012) Angew Chem Int Ed
NHPI, 1.2 MPa O , 130 °C, 5 h). This catalytic process
could eliminate the employment of any metal derivative,
2
5
1:68
g-C N was found to be stable and reusable. The clean and
4
19. Dong GH, Zhao K, Zhang LZ (2012) Chem Commun 48:6178
20. Zheng HR, Zhang JS, Wang XC, Fu XZ (2012) Acta Phys Chim
Sin 28:2336
3
one pot reaction process described here provided a poten-
tial way to oxidize various compounds possessing these
analogous structures.
2
2
1. Li XH, Wang XC, Antonietti M (2012) ACS Catal 2:2082
2. Su FZ, Mathew SC, Lipner G, Fu XZ, Antonietti M, Blechert S,
Wang XC (2010) J Am Chem Soc 132:16299
Acknowledgments This work was financially supported by the
National Natural Science Foundation of China (No. 21071152).
23. Su FZ, Mathew SC, Mohlmann L, Antonietti M, Wang XC,
Blechert S (2011) Angew Chem Int Ed 50:657
2
4. Wang XC, Maeda K, Chen XF, Takanabe K, Domen K, Hou YD,
Fu XZ, Antonietti M (2009) J Am Chem Soc 131:1680
Open Access This article is distributed under the terms of the
Creative Commons Attribution License which permits any use, dis-
tribution, and reproduction in any medium, provided the original
author(s) and the source are credited.
25. Zhang PF, Wang Y, Yao J, Wang CM, Yan C, Antonietti M, Li
HR (2011) Adv Synth Catal 353:1447
26. Yang JH, Wu XT, Li XF, Liu Y, Gao M, Liu XY, Kong LN,
Yang SY (2011) Appl Phys A 105:161
2
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7. Zhai HS, Cao L, Xia XH (2013) Chin Chem Lett 24:103
8. Zhang JS, Chen XF, Takanabe K, Maeda K, Domen K, Epping
JD, Fu XZ, Antonietti M, Wang XC (2010) Angew Chem Int Ed
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