Metal-free oxidation of α-hydroxy ketones to 1,2-diketones catalyzed by mesoporous carbon nitride with visible light

Article abstract of DOI:10.1002/cjoc.201200284

As a photocatalyst, mesoporous carbon nitride (mpg-C₃N ₄) shows higher photocatalytic activities in organic synthesis. Herein we report an mpg-C₃N₄-catalyzed oxidation of α-hydroxy ketones to synthesize 1,2-diketones using visible light. This transformation represents a green and highly efficient synthetic route to synthesize 1,2-diketones for which catalytic approaches are scarce.

Full text of DOI:10.1002/cjoc.201200284

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DOI: 10.1002/cjoc.201200284
Metal-Free Oxidation of α-Hydroxy Ketones to 1,2-Diketones
Catalyzed by Mesoporous Carbon Nitride with Visible Light
Zheng, Zhishuo*,a(郑志硕)
Zhou, Xiaosong*,b(周小松)
a Department of Physics, Guangdong University of Education, Guangzhou, Guangdong 510310, China
b School of Chemistry Science & Technology, Zhanjiang Normal University, Zhanjiang,
Guangdong 524048, China
As a photocatalyst, mesoporous carbon nitride (mpg-C3N4) shows higher photocatalytic activities in organic
synthesis. Herein we report an mpg-C3N4-catalyzed oxidation of α-hydroxy ketones to synthesize 1,2-diketones us-
ing visible light. This transformation represents a green and highly efficient synthetic route to synthesize
1,2-diketones for which catalytic approaches are scarce.
Keywords mesoporous carbon nitride, photocatalysis, visible light, α-hydroxy ketones
Introduction
in numerous biological compounds such as trifenagrel
(Figure 1) and are broadly utilized as synthetic building
blocks for construction of complex structures in organic
synthesis.[11] Herein we report an mpg-C3N4-catalyzed
oxidation of α-hydroxy ketones to 1,2-diketones by
visible light without the aid of traditional catalysts such
as Cu, Pd, Ru, Rb.[12] This methodology represents a
green and highly efficient synthetic route to synthesize
1,2-dike-tones for which catalytic approaches are
scarce.
The development of green and convenient method to
construct valuable organic compounds is an important
goal of synthetic chemistry and also represents a con-
tinuing challenge.[1] With particular attention in envi-
ronmental problems, organic chemists expect not only
to design new chemical reactions but also to improve
the catalyst system to maximize resource utilization and
decrease waste, especially for environmental health and
safety.
In the past few years, many green and benign reac-
tions have been reported, including heterogeneous ca-
talysis,[2] ionic liquids,[3] aqueous phase[4] and super-
critical carbon dioxide.[5] Although several useful pro-
cedures have been developed to allow the preparation of
functionalized organic molecule, there is still an intrin-
sic need to design new and green reactions under much
milder conditions and simpler catalytic system.
N
O
N
N
H
Figure 1 Structure of trifenagrel.
Recently, as a metal-free photocatalyst, mesoporous
carbon nitride (mpg-C3N4) not only demonstrated higher
photocatalytic activities for hydrogen production[6] but
also promoted organic redox synthesis in a broader
fashion.[7] And mpg-C3N4 is one of the hottest topics in
photocatalysis at present. Blechert and Wang’s group
has reported mpg-C3N4-catalyzed oxidation of alcohols
process[8] and aerobic oxidative coupling of amines[9]
using visible light; and Antonietti has reported activa-
tion of CO2 by mesoporous graphitic carbon.[10] Thus,
the development of mpg-C3N4-catalyzed green organic
synthesis strategies and methodologies have attracted
great attention of chemists.
Experimental
General procedure for the mpg-C3N4-catalyzed
oxidation of 2-hydroxy-1,2-diphenylethanone (1a)
into benzil (2a)
In a typical procedure, mpg-C3N4 (60 mg) and
2-hydroxy-1,2-diphenylethanone (1a, 1.0 mmol ) were
added to CH3CN (10 mL) in a 100 mL stainless steel
autoclave. The O2 (1 bar) source was introduced and the
closed autoclave was heated to 100 ℃, followed by
visible light irradiation (＞420 nm) for 10 h. The reac-
tion mixture turned clear and the catalysts were depos-
ited on the button, recovered by filtration. And then the
solvent was removed under reduced pressure to afford
1,2-Diketones are very important structural moieties
*
E-mail: emmazhi@gmail.com; zxs801213@163.com; Tel.: 0086-020-34114295; Fax: 0086-020-34114295
Received March 29, 2012; accepted May 7, 2012.
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/cjoc.201200284 or from the author.
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Zheng & Zhou
Table 1 Optimization of reaction conditionsa
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an oily residue. The water (30 mL) was added into resi-
due. The aqueous solution was extracted with EtOAc
(10 mL×3) and the combined extract was dried with
anhydrous MgSO4. The solvent was removed and the
crude product was separated by column chromatography
(eluted with petroleum ether∶ethyl acetate＝20∶1) to
give a pure sample of 2a.
O
O
mpg-C3N4, hv (>420 nm)
Ph
Ph
Ph
solvent, O2 , r.t. - 100 oC
Ph
OH
O
2a
1a
Entry Solvent
Air/O2 (atm) Temp/℃ t/h Yieldb/%
1
Toluene
Benzene
CH3CN
Air
Air
Air
60
4
4
4
4
7
2
60
9
Results and Discussion
3
60
15
10
To begin our study, we examined various solvents,
O2 source, temperature and the reaction time for oxida-
tion of α-hydroxy ketones to 1,2-diketones with visible
light and the results were summarized in Table 1.
Among the various tested solvents, the polar solvent
CH3CN was the more effective solvent in the presence
of mpg-C3N4 for this transformation under atmospheric
pressure at 60 ℃ (Table 1, Entries 1—4). When the
reaction was carried out in the absence of mpg-C3N4,
the desired product was not detected even prolonging
reaction time to 24 h (Table 1, Entry 5). Subsequently,
we attempted to improve the yields by using oxygen
substituted air and even increasing oxygen pressure
(Table 1, Entries 6—8). It was found that the use of pure
oxygen had significant influence on the yields which
increased to 53%. But the yield was significantly
changed with the variation of oxygen pressure in the
reaction. Lower yields were obtained, when the reaction
was treated with oxygen at 3 and 5 atm. Next, the dif-
ferent temperature was scanned (Table 1, Entries 9—12).
Excitingly, 100 ℃ was found to be the most optimal
one for the transformation and other temperatures led to
lower yields of product 2a. Especially, the desired
product was not detected while the reaction was carried
out at room temperature in the prsence of mpg-C3N4
with visible light. And it was found that the reaction
time had a significant influence on the yields under fur-
ther investigations. The yield of product 2a was ob-
tained in 72%, when the reaction time was increased to
10 h (Table 1, Entry 14) or 15 h (Table 1, Entry 15).
Under the same conditions, the product was only ob-
tained in 45% yield, as the reaction time decreased to 3
h. Finally, a control experiment of dark reaction was
carried out (Table 1 Entry 17) under optimal condition
and the result indicated that the desired product was not
detected in all the process.
4
Chlorobenzene Air
60
5c
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
Air
60
20 —
6
O2 (1)
O2 (3)
O2 (5)
O2 (1)
O2 (1)
O2 (1)
O2 (1)
O2 (1)
O2 (1)
O2 (1)
O2 (1)
O2 (1)
60
4
4
4
4
4
4
53
41
20
61
63
17
7
60
8
60
9
80
10
11
12
13
14
15
16
100
40
r.t.
24 —
66
100
100
100
100
100
6
10 72
15 72
3
45
17d CH3CN
10 0
a In a typical reaction mpg-C3N4 (60 mg) was added to the desired
solvent (10 mL) in a 100 mL stainless steel autoclave. The O2 or
air source was introduced and the closed autoclave was heated to
r.t.—100 ℃, followed by visible light irradiation for 4—15 h.
b GC-yield. In the absence of mpg-C3N4. The reaction was
carried out in the presence of mpg-C3N4 in the dark.
c
d
the reaction. In addition, α-hydroxy ketones with ali-
phatic substituted was also examined, and the corre-
sponding product 2m was obtained in lower yield than
aromatic substituted (Entry 13).
A plausible mechanism of photocatalysis reaction
was described in Scheme 1. A mechanism similar to
those of previous studies on the oxidation of alcohols
was likely to be involved for the present reaction.[6]
Firstly, the photoelectrons (h＋) was combined with the
hydroxyl to generate the hydroxy radical anion, which
was formed by the irradiation of mpg-C3N4, (•OH). The
deprotonation of 2-hydroxy-1,2-diphenylethanone oc-
curred in the presence of •OH to form intermediate B
which had two possible way to generate the product.
Path I: intermediate B by deprotonation was to form
intermediate C, which lost an electron to produce the 2a,
while path II lost an electron to form D, which was de-
protonated to give product 2a.
As summarized in Table 2, this mpg-C3N4 catalyzed
oxidation of α-hydroxy ketones with visible light could
be extended to a wide variety of substrates. Firstly, we
carried out the reactions with various types of
α-hydroxy ketones with aromatic substituted (Table 2,
Entries 1—12). In all these cases we found that the cor-
responding products could be obtained in good yields
(Entries 1—3). Importantly, electron-withdrawing and
-donating substituents as well as ortho-substitution on
the aryl ring were all well tolerated. Notably, the prod-
uct 2k was formed in 63% yield, when 2-hydroxy-1,2-
di(thiophen-2-yl) ethanone was used as a substrate in
Conclusions
In conclusion, we have described a metal-free, pho-
tocatalytic oxidation of α-hydroxy ketones to
1,2-diketones by mpg-C3N4 catalyst using 1 atm of O2
as the oxidant and visible light. Photocatalyst,
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© 2012 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Chin. J. Chem. 2012, 30, 1683—1686

Metal-Free Oxidation of α-Hydroxy Ketones to 1,2-Diketones
Scheme 1 A possible reaction mechanism
O
light
H2O
C3H4
C3H4 (h e)
Ph
H2O
Ph
e
O
OH
C
Path I
O
OH
O
O
Ph
2a
Ph
Ph
Ph
OH
Ph
O
Ph
Path II
O
O
H2O
B
A
Ph
Ph
OH
e
O
D
Table 2 Mpg-C3N4 catalyzed synthesis of 1,2-diketones with
visible lighta
Continued
Product (Yield/%)b
Entry α-Hydroxy ketone
Entry α-Hydroxy ketone
Product (Yield/%)b
OMe
OMe
O
O
O
O
MeO
MeO
9
1
O
O
OH
OH
1a
2i (73)
2a (72)
1i
OMe
OMe
O
O
O
O
2
10
O
OH
O
OH
MeO
S
MeO
2b (67)
1b
1j
2j (71)
O
O
O
O
S
S
S
3
11
12
OH
O
O
HO
1k
2k (63)
2c (70)
1c
Cl
Cl
O
O
OH
O
4
OH
O
O
O
2d (69)
1d
2l (72)
1l
Cl
Br
Et
Cl
Br
O
O
O
O
5
13
O
2m (49)
OH
O
OH
Cl
Cl
Br
Et
1m
2e (73)
1e
a
Reaction condition: substrate (1 mmol), mpg-C3N4 catalyst (60
mg), CH3CN (10 mL), O2 (1 atm), 100 ℃. b Isolated yields.
O
O
6
OH
O
Br
2f (73)
mpg-C3N4 efficiently activated O2 to generate the super-
oxide radical anion (•O2-) which reacted with substrate
to form intermediate alkoxide anion. In particular, this
heterogeneous catalyst could be readily recovered and
reused, avoiding the employment of any metal deriva-
tive or organic oxidant, which was green and environ-
mentally friendly.
1f
Et
O
O
7
OH
O
Et
2g (70)
1g
OMe O
OMe O
Acknowledgement
8
OH OMe
O
OMe
We gratefully acknowledge financial support by Re-
search Fund of the Guangdong Provincial Laboratory of
2h (68)
1h
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Carlsson, J. M.; Domen, K.; Antonietti, M. Nat. Mater. 2009, 8, 76;
(b) Wang, X.; Maeda, K.; Chen, X.; Takanabe, K.; Domen, K.; Hou,
Y.; Fu, X.; Antonietti, M. J. Am. Chem. Soc. 2009, 131, 1680; (c)
Zhang, J.; Chen, X.; Takanabe, K.; Maeda, K.; Domen, K.; Epping, J.
D.; Fu, X.; Antonietti, M.; Wang, X. Angew. Chem., Int. Ed. 2010,
49, 441; (d) Zhang, Y.; Thomas, A.; Antonietti, M.; Wang, X. J. Am.
Chem. Soc. 2009, 131, 50.
Green Chemical Technology.
References
[1] (a) Polshettiwar, V.; Varma, R. S. Acc. Chem. Res. 2008, 41, 629; (b)
Li, C. J. Acc. Chem. Res. 2009, 42, 335; (c) Leitner, W. Acc. Chem.
Res. 2002, 35, 746; (d) Anastas, P. T.; Kirchhoff, M. M. Acc. Chem.
Res. 2002, 35, 686.
[2] (a) Yin, L.; Liebscher, J. Chem. Rev. 2007, 107, 133; (b) Phan, N. T.
S.; Sluys, M. V. D.; Jones, C. W. Adv. Synth. Catal. 2006, 348, 609;
(c) Narayanan, R.; El-Sayed, M. J. Am. Chem. Soc. 2003, 125, 8340;
(d) Kotani, M.; Koike, T.; Yamaguchi, K.; Mizuno, N. Green Chem.
2006, 8, 735; (e) Ellis, P. J.; Fairlamb, I. J. S.; Hackett, S. F. J.;
Wilson, K.; Lee, A. F. Angew. Chem., Int. Ed. 2010, 49, 1820; (f)
Mortko, C. J.; Garcia-Garibay, M. A. J. Am. Chem. Soc. 2005, 127,
7994; (g) Yan, N.; Zhao, C.; Luo, C.; Dyson, P. J.; Liu, H.; Kou, Y. J.
Am. Chem. Soc. 2006, 128, 8714.
[3] (a) Chakraborti, A. K.; Roy, S. R. J. Am. Chem. Soc. 2009, 131,
6902; (b) Martins, M. A. P.; Frizzo, C. P.; Moreira, D. N.; Zanatta,
N.; Bonacorso, H. G. Chem. Rev. 2008, 108, 2015; (c) Fan, X. S.; He,
Y.; Wang, Y. Y.; Zhang, X. Y.; Wang, J. J. Chin. J. Chem. 2011, 29,
773.
[4] (a) Vasylyev, M. V.; Maayan, G.; Hovav, Y.; Haimov, A.; Neumann,
R. Org. Lett. 2006, 8, 5445; (b) Shaughnessy, K. H. Chem. Rev. 2009,
109, 643.
[7] (a) Nicewicz, D. A.; MacMillan, D. W. C. Science 2008, 322, 77; (b)
Hoffmann, N. Chem. Rev. 2008, 108, 1052; (c) Wang, X. C.; Maeda,
K.; Thomas, A.; Takanabe, K.; Xin, G.; Domen, K.; Antonietti, M.
Nat. Mater. 2009, 8, 76; (d) Goettmann, F.; Fischer, A.; Antonietti,
M.; Thomas, A. Chem. Commun. 2006, 4530; (e) Goettmann, F.;
Fischer, A.; Antonietti, M.; Thomas, A. Angew. Chem., Int. Ed. 2006,
45, 4467; (f) Gawronski, J.; Wascinska, N.; Gajewy, J. Chem. Rev.
2008, 108, 5227; (g) Lang, X.; Ji, H.; Chen, C.; Ma, W.; Zhao, J.
Angew. Chem., Int. Ed. 2011, 50, 3934.
[8] Su, F. Z.; Mathew, S. C.; Lipner, G.; Fu, X. Z.; Antonietti, M.;
Blechert, S.; Wang, X. C. J. Am. Chem. Soc. 2010, 132, 16299.
[9] Su, F. Z.; Mathew, S. C.; Möhlmann, L.; Antonietti, M.; Wang, X. C.;
Blechert, S. Angew. Chem., Int. Ed. 2011, 50, 657.
[10] Goettmann, F.; Thomas, A.; Antonietti, M. Angew. Chem., Int. Ed.
2007, 46, 2717.
[11] (a) Adib, M.; Mohammadi, B.; Ansari, S.; Bijanzadeh, H. R.; Zhu, L.
G. Tetrahedron Lett. 2011, 52, 2299; (b) Deng, X.; Mani, N. S. Org.
Lett. 2006, 8, 269.
[5] (a) Nishiyama, Y.; Kaneda, M.; Saito, R.; Mori, T.; Wada, T.; Inoue,
Y. J. Am. Chem. Soc. 2004, 126, 6568; (b) Potluri, V. K.; Xu, J.;
Enick, R.; Beckman, E.; Hamilton, A. D. Org. Lett. 2002, 4 2333.
[6] (a) Wang, X.; Maeda, K.; Thomas, A.; Takanabe, K.; Xin, G.;
[12] (a) Kashiwabara, T.; Tanaka, M. J. Org. Chem. 2009, 74, 3958; (b)
Shimakawa, Y.; Morikawa, T.; Sakaguchi, S. Tetrahedron Lett. 2010,
51, 1786.
(Lu, Y.)
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