188
H. Zhan et al. / Applied Catalysis A: General 468 (2013) 184–189
Table 2 (Continued)
Entry
Benzoins
Product (Yield %)b
O
O
O
O
COOH
11
12
HO
1k
2k 55
COOH
O
OH
1l
2l 49
Cl
OH
2i, 2a
13
14
70, 71
O
1m
O
CO2H
CO2H
OH
2n 53
1n
a
Isolated yield.
b
GC yield.
the time (entries 14–20). And the results clearly indicated that the
yield of 2a increased to 72% and 3a would not be detected. Finally,
control experiments revealed that no product could be formed in
deprotonation of A took place to form hydroperoxide (OOH•), while
intermediate B could give product by two possible path. Path I:
intermediate B underwent nucleophilic attack by hydroperoxide
to gernate C. The intermediate D is formed through 1,2-insertion of
the negative oxygen to the neighboring carbonyl group and simul-
taneous elimination of hydroxide. Finally, the product is obtained
wih the help of H2O and AcOH. Path II: intermediate product 3a
was observed, which was possibly produced by electron transfer
of intermediate B. Consequently, intermediate E was formed from
3a with visible light. This radical would be recombined with the
superoxide radical and giving the perbenzoic acid F which would
give the final product with the help of formal liberation of O2.
With this optimized protocol in hand, we explored the scope
of mpg-catalyzed oxidation cleavage with differently substituted
␣-hydroxy ketones derivatives and the results are summarized in
Table 2. The reactions bearing electron-donating groups (entries
2 and 7) and electron-withdrawing substituents at the aromatic
ring of ␣-hydroxy ketones (entries 8 and 9) proceeded to give
the desired products in moderate to good yields. Notably, the
corresponding products were also isolated in good yields, when
the reactions were used 2-hydroxy-1,2-di(thiophen-2-yl)ethanone
(1j), 1,2-di(furan-2-yl)-2-hydroxyethanone(1k), 2-hydroxy-1,2-
di(naphthalen-1-yl)ethanone(1l) as substrates under optimal
conditions. Further, the asymmetric substrate 1-(4-chlorophenyl)-
2-hydroxy-2-phenylethanone (1m) was also tested and the desired
products 2h and 2i were obtained in 70% and 71% respectively.
All these results indicated that this photo catalysis process proved
extensively applicable and tolerated various valuable functional
groups on the aromatic ring such as CH3, OCH3, C2H5, Br, Cl.
We next turned our attention to catalyst recycling and reuse.
After completion of the reaction, the catalyst was recovered by fil-
tration and washed 2–3 times with ethanol. And then the catalyst
next reaction. It was found that the catalytic activity was still main-
tained even when the catalyst was reused for 3 times. The surface
chemical composition and chemical states of g-C3N4 catalysts were
analyzed by means of XPS (Fig. 4).
4. Conclusion
In conclusion, we have reported on the first mpg-catalazed oxi-
dation cleavages of carbon–carbon bond of ␣-hydroxy ketones
with visible light. Mesoporous carbon nitride appears to play a
vital role as a driving force in this transformation. This catalytic
system proved boardly applicable and tolerated various valuable
functional groups on the aromatic ring. In addition, this transfor-
mation represents a green and environmentally friendly process.
Further applications of this approach are currently underway in
our laboratories.
Acknowledgement
The work was financially Supported by Foundation for Distin-
guished Young Talents in Higher Education of Guangdong, China
(LYM11083).
A plausible reaction mechanism is shown in Scheme 1. A similar
mechanism has been mentioned in previous reports. The electron
(e−) was combined with the oxygen to generate the superox-
ide radical anion, which formed by the irradiation of mpg-C3N4,
Appendix A. Supplementary data
Supplementary data associated with this article can be found,
(O2•−). Abstraction of the hydrogen atom of 1a was occurred in
•−
the presence of O2
to give intermediate A. And then the same