154
S.-S. Wang et al. / Catalysis Communications 28 (2012) 152–154
Table 3
For secondary aliphalic alcohols, the oxidation of cyclohexanol and
a
The recycling of the different catalysts for the aerobic oxidation of benzyl alcohol.
hexan-2-ol were successful, with 100% selectivity to the corresponding
ketones at somewhat prolonged reaction time (entries 8 and 9). For
the activation of primary aliphatic alcohols, cyclopropylmethanol con-
verted to cyclopropanecarbaldehyde with 93% selectivity (entry 10).
Unfortunately, as for hexan-1-ol, the poor selectivity to hexanal was
found due to alkyl chain cleavage (entry 11).
Run
2
3
RuCl2(PPh3)3
Conv. (%)
Conv. (%)
Sel. (%)
Conv. (%)
Sel. (%)
Sel. (%)
1
2
3
4
5
99
96b
84
73
40
94
90
94
96
97
99
97
94
90
98
98
83
73
b5
–
99
95
–
100b
96
92
90
–
–
–
3. Conclusions
a
Cat. 1 mol%, benzyl alcohol 5 mmol, O2 1.0 MPa, [Bpy]BF4 2.0 mL, H2O 1.0 mL,
100 °C, 1.5 h.
The ionic compound of 3 was synthesized and applied as a catalyst
for aerobic oxidation of alcohols free of any base and nitroxyl radical.
b
1 h.
With the presence of H2O, 3 containing the Ru(III)-complex cation
3−
the potential hydrolysis of PF6−. The latter always happens under
specific conditions, leading to the formation of HF [24].
and α-Keggin-type [PW12O40
]
anion proved to be an efficient and
recyclable catalyst by using the RTIL of [Bpy]BF4 as a solvent. The
robustness of 3 against oxidative degradation and hydrolysis could
account for its available recyclability.
The generality of 3 as the catalyst for the aerobic oxidation of alcohols
was examined on a series of alcohols with different electronic and
steric effects. Since H2O was found to spur the catalytic activity of 3
dramatically, 1.0 mL H2O was added additionally in each case. As
shown in Table 4, 4-tolylmethanol and (4-methoxyphenyl)methanol
with electron-donating substituents converted to the corresponding
aldehydes even faster than benzyl alcohol (entries 2 and 3 vs 1).
(4-Nitrophenyl)methanol converted to 4-nitrobenzaldehyde relatively
slowly due to possessing the electron-withdrawing substituent of–NO2
(entry 4). For the oxidation of 1-phenylethanol as an activated sec-
ondary aryl alcohol, the decreased conversion was observed due to the
steric hindrance, and the dehydrated by-product of styrene was found
(entry 5). The oxidation of (E)-3-phenylprop-2-en-1-ol possessing
C_C double bond (entry 6) or (pyridin-2-yl)methanol possessing
pyridyl group (entry 7) yielded cinnamaldehyde or picolinaldehyde
with excellent chemselectivity. The oxidation-sensitive C_C double
bond or pyridyl group was not destroyed under the applied conditions.
Acknowledgements
This work was financially supported by Program Cai Yuanpei
(2011–2013), the National Natural Science Foundation of China
(nos. 20973063 and 21273077), and 973 Program from Ministry of
Science and Technology of China (2011CB201403).
Appendix A. Supplementary data
Supplementary data to this article can be found online at http://
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81
92
90
82
75
91
95
85
98
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76c
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OH
1.5
3
OH
OH
N
3
100
OH
9
3
2
4
50
74
85
100
93
OH
10
CH2O
11
10e
OH
a
Subs. 5 mmol, 3 1 mol%, O2 1.0 MPa, H2O 1.0 mL, [BPy]BF4 2.0 mL, 100 °C.
Determined by GC and GC–mass.
Styrene was yielded as the by-product.
Cinnamaldehyde was yielded as the main product.
Many by-products coming from alkyl chain cleavage were found.
b
c
d
e