Trace water-promoted oxidation of benzylic alcohols with molecular oxygen catalyzed by vanadyl sulfate and sodium nitrite under mild conditions

Article abstract of DOI:10.1002/adsc.200800700

An inexpensive catalytic system consisting of vanadyl sulfate and sodium nitrite was developed for the oxidation of benzylic alcohols with molecular oxygen under mild conditions. Benzyl alcohols with various substitutions were efficiently converted to the

Full text of DOI:10.1002/adsc.200800700

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DOI: 10.1002/adsc.200800700
Trace Water-Promoted Oxidation of Benzylic Alcohols with
Molecular Oxygen Catalyzed by Vanadyl Sulfate and Sodium
Nitrite under Mild Conditions
Zhongtian Du,a,b Hong Miao,a Hong Ma,a Zhiqiang Sun,a Jiping Ma,a,b
and Jie Xua,*
a
State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023,
Peopleꢀs Republic of China
Fax : (+86)-(0)411-8437-9245; phone: (+86)-(0)411-8437-9245; e-mail: xujie@dicp.ac.cn
Graduate School of the Chinese Academy of Sciences, Beijing 100039, Peopleꢀs Republic of China
b
Received: November 13, 2008; Revised: February 15, 2009; Published online: March 5, 2009
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/adsc.200800700.
Abstract: An inexpensive catalytic system consist-
ing of vanadyl sulfate and sodium nitrite was devel-
oped for the oxidation of benzylic alcohols with
molecular oxygen under mild conditions. Benzyl al-
cohols with various substitutions were efficiently
converted to their corresponding aldehydes with
high conversion and selectivity under 808C (e.g., 4-
nitrobenzyl alcohol was smoothly oxidized to 4-ni-
trobenzyl aldehyde with 94% yield under 0.5 MPa
of molecular oxygen). Halogen, noble metals, extra
base or complicated ligands were avoided. Addition
of a trace of water to this system before the reac-
tion was crucial for the high efficiency.
base (e.g., NaOAc or K2CO3) or complicated ligands
are often used. The quest for efficient and inexpen-
sive catalytic systems for the oxidation of alcohols
with molecular oxygen remains an important chal-
lenge.
Because of their unique redox property, vanadium
compounds were widely used for oxidation.[15–17] For
example, vanadium phosphorus oxide (VPO) for the
oxidation of n-butane to maleic anhydride has been
employed in industry for decades.[15,16] In the liquid-
phase oxidation catalyzed by vanadium species, re-
searchers mainly focused on hydrogen peroxide or or-
ganic peroxides as terminal oxidants.[17] Only a few
vanadium-based systems for the oxidation of alcohols
with molecular oxygen are known. V2O5/K2CO3, VO-
Keywords: benzylic alcohols; molecular oxygen; ni-
trite; oxidation; vanadium
ACHTUNGRTEN(NUNG acac)2/MS 3ꢁ, V-Cu/DABCO, VOCl3 etc, were de-
scribed.[18] However, in these catalysts systems, extra
bases, drying agents or ligands were often indispensa-
ble; or the scope of alcohols was quite limited. Effi-
cient example using inexpensive vanadyl sulfate
Oxidation of alcohols to carbonyl compounds is a fun- under mild conditions remained to be explored. Here
damental transformation in organic chemistry both at we disclose a novel and efficient, vanadium-based cat-
the laboratory and industry levels.[1] Furthermore, the alytic system for the oxidation of benzylic alcohols
oxidative conversion of the hydroxy group has also at- with molecular oxygen.
tracted much attention in biorefinery field recently.[2]
During the synthesis of VPO, benzyl alcohol was
Traditionally, such a transformation is performed with usually used as the reductant of V2O5.[15] Therefore
stoichiometric oxidants such as CrO3 and permanga- stoichiometric [V5+] species could oxidize benzyl alco-
nate.[1] Much progress has been made to develop cata- hol. If a catalytic cycle could be established, the key-
lytic systems using molecular oxygen as the terminal stone is the facile reoxidation of low-valent vanadium
oxidant.[3] Representative examples are the catalytic species by O2 under mild conditions. We noticed that
systems using transition metal catalysts (Pd,[4] Ru,[5] in industry cyclohexanone was oxidized to adipic acid
Au,[6] Cu,[7] Pt,[8] Ni,[9] polyoxometalates,[10] Mo,[11] with stoichiometric HNO3 catalyzed by vanadium and
manganese oxide octahedral molecular sieves,[12] assumed that low-valent vanadium species could be
Fe[13], etc.), or stable nitroxyl free radicals (e.g., oxidized back to [V5+] by nitric compounds.[16,19] Then
2,2,6,6-tetramethylpiperidyl-1-oxyl,TEMPO).[14] How- NaNO2 as a nitric source was introduced to establish
ever, noble metals, halogen (e.g., bromine), extra the vanadium redox cycle. Initial experiments were
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Adv. Synth. Catal. 2009, 351, 558 – 562

Trace Water-Promoted Oxidation of Benzylic Alcohols with Molecular Oxygen
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carried out using benzyl alcohol as the test substrate
(Table 1). Inexpensive VOSO4 was employed as vana-
dium source. The results clearly indicated that the in-
troduction of NaNO2 greatly improved the conversion
Table 1. Oxidation of benzyl alcohol with molecular oxygen
catalyzed by vanadyl sulfate/sodium nitrite.[a]
Entry Sodium nitrite [mol%] Time [h] Conversion [%]
1
2
3
0
4
2
2
2
2
10
trace
61.1
99.5
100
trace
99.8
2.5
5.0
7.5
5.0
5.0
4
5[b]
6[c]
Figure 1. Effect of water on VOSO4/NaNO2-catalyzed oxida-
tion of benzyl alcohol. Reaction conditions: 5 mmol benzyl
alcohol, 0.25 mmol VOSO4, 0.25 mmol NaNO2, 808C, 5 mL
CH3CN, 0.5 MPa O2, 2 h. Entry 5000: 100% H2O as solvent
and reaction time=10 h.
[a]
Reaction conditions: 5 mmol benzyl alcohol, 5 mol%
VOSO4, 808C, 5 mL CH3CN, 25 mL H2O, 0.5 MPa O2.
Conversion was based on GC analysis, selectivity for
benzaldehyde > 98% in all cases.
Without VOSO4; additional 50 mL CH3COOH were
added.
[b]
[c]
Reaction was performed at 508C.
of benzyl alcohol (Table 1, entries 1–4). Nearly com-
plete conversion was obtained with the addition of 5
mol% NaNO2 within only 2 h (Table 1, entry 3).
VOSO4 alone is a poor catalyst. Trace products were
detected within 4 h at 808C using 5 mol% VOSO4
(Table 1, entry 1). NaNO2 itself is not active as cata-
lyst either, though 50 mL acetic acid were added to
keep the acidic conditions (Table 1, entry 5). Vanadyl
sulfate/sodium nitrite was still effective even when the
temperature was decreased to 508C. High conversion
was also obtained at 508C when the time was pro-
longed to 10 h (Table 1, entry 6).
As water was formed as a by-product during the
oxidation of alcohols, the effect of water was often ig-
nored. Addition of a trace of water before the reac-
tion was crucial to the efficiency of this catalytic
system (Figure 1). Without additional water before re-
action, a long induction time was observed and only
Figure 2. Catalytic oxidation of benzyl alcohol with molecu-
lar oxygen using NaNO2 and different vanadium com-
pounds. Reaction conditions: 5 mmol benzyl alcohol,
0.25 mmol [V], 0.25 mmol NaNO2, 5 mL CH3CN, 25 mL
H2O, 80 8C, 0.5 MPa O2, 2 h.
11.3% conversion was obtained in 2 h. Addition of efficiency. It was interesting that VOACTHNURTGNENUG(acac)2, which
about 25 mL water was appropriate to keep the high was widely used in oxidation, exhibited a lower activi-
efficiency (5 mmol benzyl alcohol as substrate, 5 mL ty here.[17] V2O5 was reported to be effective for the
CH3CN as solvent, and 0.09 g H2O would be generat- catalytic oxidation of alcohols at 1008C in the pres-
ed gradually if 100% benzyl alcohol was converted). ence of K2CO3.[18e] However, here at 808C without
However, addition of too much water resulted in a bases, the conversion of benzyl alcohol was less than
decrease of activity. When the amount of water 1%. NaVO3 was completely inactive.
reached 250 mL, the conversion of benzyl alcohol re-
To further probe the potential of the VOSO4/
markably decreased to only 33.2% under the same NaNO2 catalytic system, the oxidation of other alco-
conditions. Furthermore, only 5.6% conversion was hols was studied (Table 2). All primary benzyl alco-
obtained even within 10 h when pure water was used hols were smoothly converted into their correspond-
as solvent. The phenomena will be discussed in the ing aldehydes with high conversions from 82.5% to
following section.
100% (Table 2, entries 1–7). To our delight, the selec-
Some other vanadium compounds were also tested tivities for the corresponding aldehydes were also
(Figure 2). Among them, VOSO4 showed the highest quite high under optimal conditions (>99%). No ad-
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559

Zhongtian Du et al.
Selectivity [%]
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Table 2. Oxidation of alcohols with molecular oxygen catalyzed by VOSO4/NaNO2.[a]
Entry
Substrate
Product
Time [h]
Conversion [%]
1
2
3
4
5
6
7
8
Benzyl alcohol
Benzaldehyde
2
2
2
2
99.5 (93)
100 (88)
96.8
99.1 (91)
82.5
95.2
99.3 (94)
4.8
18.2
7.6
98.6
13
99.0
99.5
99.7
99.5
99.6
~100
~100
~100
79.7
32.3
~100
96.1
~100
59.7
4-Methylbenzyl alcohol
2-Methylbenzyl alcohol
3-Chlorobenzyl alcohol
2-Chlorobenzyl alcohol
3-Methoxybenzyl alcohol
4-Nitrobenzyl alcohol
Cyclopentanol
Octan-2-ol
Hexanol
4-Nitrobenzyl alcohol
Octan-2-ol
4-Nitrobenzyl alcohol
Hexanol
4-Methylbenzaldehyde
2-Methylbenzaldehyde
3-Chlorobenzaldehyde
2-Chlorobenzaldehyde
3-Methoxybenzaldehyde
4-Nitrobenzaldhyde
Cyclopentanone
Octan-2-one
Hexanal
4-Nitrobenzaldhyde
Octan-2-one
4-Nitrobenzaldhyde
Hexanal
8
1.5
1.5
5
20
5
9
10
11[b]
4
12[b]
3
98.5
7.2
[a]
Reaction conditions: 5 mmol alcohol, 0.25 mmol VOSO4, 0.25 mmol NaNO2, 5 mL CH3CN, 25 mL H2O, 0.5 MPa O2. Con-
versions and selectivities were based on GC. Values in parenthesis are the isolated yields of the pure products.
4-Nitrobenzyl alcohol: 2.5 mmol, the other alcohol: 2.5 mmol.
[b]
ditive such as bases was required to keep the high se-
To the best of our knowledge, this is the first report
lectivity, and this is quite different from the V2O5 sys- of vanadyl sulfate and sodium nitrite for the oxidation
tem.[18e] Nitration products on the phenyl group were of benzylic alcohols with dioxygen. Noble metals, hal-
not detected. Electron-donating or electron-withdraw- ogen and additional base were not used. Side reac-
ing substitutions on the benzyl alcohols did not appa- tions like halogenation and the Cannizzaro reaction
rently affect the efficiency of the oxidation in terms were avoided. As the catalysts were simple inorganic
of the conversion (Table 2, entries 2 and 7). 4-Nitro- salts, they could be easily removed by filtration and
benzyl alcohol was often difficultly oxidized in some washing. This would be quite practical and useful. It
other systems; here it underwent facile oxidation with would seem to be economic and efficient oxidation
a high selectivity for 4-nitrobenzaldhyde (Table 2, process with promising chemoselectivity.
entry 7). Methoxy and halide substitutions on the
To explain these findings, preliminary investigations
phenyl group could survive the reaction conditions, of the mechanistic aspects were made. The oxidation
which showed good functional group tolerance of benzyl alcohol in the presence of a radical inhibitor
(Table 2, entries 4–6). Methyl substitutions on the o- (2,6-di-tert-butyl-p-cresol or p-hydroquinone) was ex-
or p-positions all served as good substrates, which in- amined. Addition of 5 mol% inhibitor affected the re-
dicated that a steric effect of the substitution is not action rate greatly. The conversion was reduced from
remarkable (Table 2, entries 2 and 3). However, ali- 99.5% to only about 1% within 2 h. This result
phatic alcohols were difficult to oxidize under the showed that a radical pathway was probably involved
same conditions (Table 2, entries 8, 9 and 10). But no in the main reaction course. It was quite different
by-products of ring cleavage were found for cyclopen- from Uemuraꢀs or Kiriharaꢀs system.[18a,c] The radical
tanol (Table 2, entry 8). Prolonging the reaction time inhibitor did not influence their systems at all. NOx is
resulted in the formation of ether as by-product for a free radical and was proposed to be responsible for
octan-2-ol (Table 2, entry 9).
the high selectivity of aromatic alcohols. In a con-
In competition experiments, an equimolar mixture trolled parallel experiment, 65% benzaldehyde was
of 4-nitrobenzyl alcohol and an aliphatic alcohol (pri- converted to benzoic acid using 2 mol% VOSO4
mary or secondary) was subjected to oxidation. When alone within 2 h, but when additional 5 mol% NaNO2
a mixture of 4-nitrobenzyl alcohol and octan-2-ol was was present, 87% benzaldehyde remained intact
used, 98.6% of the 4-nitrobenzyl alcohol was convert- under an oxygen atmosphere at 808C in 2 h.
ed within 4 h, whereas only 13% of the octan-2-ol was
oxidized (Table 2, entry 11). For a mixture of 4-nitro- taining O2 was oxidized primarily to NO2 anion with
benzyl alcohol and hexanol, 98.6% and 7.2% conver- little or no formation of NO3 .
It was reported that NO in aqueous sÀolution con-
À [20]
So the trace of
sions were obtained after 3 h respectively (Table 2, water was crucial to the redox cycle between NO and
entry 12). These results proved that this system was O2 and the formation of HNO2 in the beginning. But
quite useful for the chemoselective oxidation of VOSO4 reacted with hot water to form polyanions of
benzyl alcohols with strong electron-withdrawing sub- vanadium;[21] this might account for the decreasing
stitutions in the presence of aliphatic alcohols.
conversion when too much water was added. Stoichio-
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Trace Water-Promoted Oxidation of Benzylic Alcohols with Molecular Oxygen
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without any purification. (All products were determined on
GC-MS).
metric reactions under N2 were also studied. When
the oxidation of benzyl alcohol using a stoichiometric
amount of VOSO4 at 808C was performed, a trace of
benzaldehyde was detected within 3 h. On the other
hand, 54% benzaldehyde was obtained when VOCl3
was used instead of VOSO4 (24% benzyl chloride was
detected as by-product). Therefore, VOSO4 was not
Typical Separation Procedure for Aldehydes
Taking the oxidation of 4-nitrobenzyl alcohol (0.7656 g) as
an example, if GC showed that the conversion of 4-nitroben-
zyl alcohol was >99%, CH3CN was removed on a rotary
the active species for the direct oxidation of benzyl al- evaporator after filtration. Then solid were washed with sa-
turated NaHCO3 and extracted with CH2Cl2, The combined
CH2Cl2 layer was dried over anhydrous MgSO4, then evapo-
rated to afford a yellow solid; yield: 0.7063 g (94%). The
NMR spectrum was recorded using the isolated product di-
rectly.
cohol.
Based the experiments above, an overall redox
cycle was proposed (Scheme 1). The oxidation of
benzyl alcohols by oxygen was divided into several
Acknowledgements
We gratefully acknowledge the financial support from the
Natural Science Foundation of China (20672111 and
20736010) and the Knowledge Innovation Program of the
Chinese Academy of Sciences (DICP K2007D6).
Scheme 1. Proposed vanadium redox cycle.
steps. In our system, alcohols might be oxidized by
[V5+] species. NaNO2 resulted in the formation of
HNO2 in the presence of a trace of water under acidic
conditions. The transformation of [V4+] to [V5+] runs
smoothly with assistance of HNO2.[19] NOx played the
role of a mediator to bridge the gap between oxygen
and vanadium. The reoxidation of NO to NO2 could
be easily performed with O2.
In conclusion, an economical and efficient catalytic
system composed of vanadyl sulfate and sodium ni-
trite was developed for oxidation of benzylic alcohols
with dioxygen. A wide range of benzylic alcohols
were efficiently converted to their corresponding al-
dehydes with high conversion and selectivity at below
808C. Addition of a trace of water in the beginning
had great promotive effect. This work may be helpful
to the further development of vanadium-based oxida-
tion. Investigations of the detailed mechanism are
under way.
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