of equivalent amounts of VOPO
only 1.5% veratraldehyde in 4 h at room temperature in water.
In contrast, the conversion of veratryl alcohol was 35.9% when
4
and veratryl alcohol afforded
Acknowledgements
We acknowledge the financial support from the Natural Science
Foundation of China (Projects 20736010 & 20803074).
5
mol% TEMPO was added, and the color of mixture changed
from reddish-brown to green. These results indicated that the
oxidation of veratryl alcohol with VOPO was catalyzed by
4
Notes and references
V
IV
TEMPO, a process that probably involves V /V redox cycles.
This was further confirmed by electron paramagnetic resonance
IV
‡
The nuclear spin of the V is I = 7/2, and the EPR lines of
IV
mononuclear V are split into 8 lines (2I + 1). One of the eight lines
for V overlaps with the strong signal of TEMPO.
IV
(
EPR) in controlled experiments under N
pp. 8–10). There was no apparent EPR signal for VOPO
aqueous solution. After adding TEMPO and agitating at
2
atmosphere (ESI,
4
1
G. Tojo and M. Fernandez, Oxidation of Alcohols to Aldehydes and
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IV
room temperature under N atmosphere, a V signal appeared
2
19
V
IV
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(
Fig. 3).‡ This result suggested that V could be reduced to V
directly by TEMPO. When veratryl alcohol was added, the EPR
IV
signal of V was further strengthened, and veratraldehyde could
be detected by GC in the mixture. The color of the mixture also
gradually changed to dark green. Therefore, such redox cycles
are proposed to play a part in the catalytic route (Scheme 1):
3
C. J. Li and T. H. Chan, Comprehensive Organic Reactions in Aqueous
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+
V
V
TEMPO is oxidized to TEMPO by V , and V regenerated
4 D. T. Richens, Chem. Rev., 2005, 105, 1961–2002.
IV
from V in the presence of O
2
; then veratryl alcohol is oxidized
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system at a molecular level remains unknown.
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(
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IV
Fig. 3 EPR spectrum of V species.
1
1
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Scheme 1 Proposed redox cycles.
1
1
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In summary, we have developed VOPO in combination with
4
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2
5
92 | Green Chem., 2010, 12, 590–592
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