New insights into the bifunctionality of vanadium phosphorous oxides: A chemical switch between oxidative scission and pinacol rearrangement of vicinal diols

Article abstract of DOI:10.1016/j.cattod.2012.10.021

New insights into the bifunctionality of vanadium phosphorus oxides (VPOs) were obtained. This study reports the precise tuning of experimental conditions, which act as the switch to direct the course of reaction selectively to obtain either CC oxidative scission or an acid-catalysed rearrangement of vicinal diols. The catalyst was synthesized and characterized to identify the active vanadyl pyrophosphate phase. The present method was extended further to develop a new environmentally benign and green catalytic protocol for oxidative cleavage of a variety of vicinal diols with 100% selectivity towards the corresponding aldehydes or ketones using H₂O₂ as the oxidizing agent.

Full text of DOI:10.1016/j.cattod.2012.10.021

Catalysis Today 208 (2013) 60–65
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New insights into the bifunctionality of vanadium phosphorous oxides:
A chemical switch between oxidative scission and pinacol rearrangement
of vicinal diols
Dharita J. Upadhyayaa,∗, Shriniwas D. Samantb
a School of Pharmacy, Queen’s University Belfast, 97 Lisburn Road, Belfast BT9 7BL, UK
b Organic Chemistry Research Laboratory, Institute of Chemical Technology, Matunga, Mumbai 400 019, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 25 July 2012
Received in revised form 9 October 2012
Accepted 12 October 2012
Available online 9 January 2013
New insights into the bifunctionality of vanadium phosphorus oxides (VPOs) were obtained. This study
reports the precise tuning of experimental conditions, which act as the switch to direct the course of
reaction selectively to obtain either C C oxidative scission or an acid-catalysed rearrangement of vicinal
diols. The catalyst was synthesized and characterized to identify the active vanadyl pyrophosphate phase.
The present method was extended further to develop a new environmentally benign and green catalytic
protocol for oxidative cleavage of a variety of vicinal diols with 100% selectivity towards the corresponding
aldehydes or ketones using H2O2 as the oxidizing agent.
Keywords:
Vanadium phosphorus oxides
Bifunctional catalysts
© 2012 Elsevier B.V. All rights reserved.
Oxidative cleavage of diols
Pinacol–pinacolone rearrangement
states of vanadium on catalyst surface (V+4/V+5). The surface acid-
ity plays a crucial role in rate-determining step in the activation
1. Introduction
Vanadium-phosphorus oxides (VPOs) represent a class of redox
catalysts of high commercial significance, due to their ability to
induce selective oxidation of alkanes; in particular, for the synthesis
of maleic anhydride from n-butane [1,2]. In fact VPOs are the only
industrialized catalyst system used for oxidation of light alkanes
[3–7]. The catalyst has been synthesized by numerous methods till
date and extensively characterized using X-ray diffraction (XRD),
N2 adsorption, atomic emission, NH3 Temperature-Programmed
Desorption, and IR (Infra red), Raman, 31P NMR, XPS, LEIS and
EPR spectroscopic techniques in order to understand the nature
of active species [8]. VPOs exhibit rich and complex chemistry.
Vanadyl pyrophosphate (VO)2P2O7 has been widely accepted as
the active and selective phase of this catalyst, however, a mixture
of different V+4/V+5 phosphate phases are also observed resulting
from a very easy redox cycle of V+4/V+5 depending on the experi-
mental conditions like reaction temperature, time-on-stream and
redox properties of reactants [9–11]. The high efficiency of this cat-
alyst has been attributed to its bifunctional nature containing redox
sites as well as surface acidity. The redox nature of this catalyst
has been ascribed to the availability and easy switching (reversible
sequential reduction and re-oxidation) between different oxidation
of paraffin and VPO catalysts are known to contain both Bronsted
and Lewis acid sites in their structure [8,12]. Although oxida-
tion catalysts based on VPOs are well established, their use as
bifunctional acidic/redox catalysts has not been reported previ-
ously. Herein, we show that VPO based catalysts can be utilized as
remarkably selective bifunctional catalysts with high activity using
two exemplar reactions, the C C oxidative scission of vicinal diols
and acid-catalysed pinacol–pinacolone rearrangement of vicinal
diols.
The oxidative scission of C C bond in vicinal diols (vic-diols)
to form corresponding aldehydes/ketones and carboxylic acids
is a significant and most frequently employed tool in organic
synthesis [13]. This reaction is very similar to many biochem-
ical oxidative processes performed by redox enzymes, which
possess a mononuclear iron-heme prosthetic group with addi-
tional cysteine or histidine residues, for instance the side chain
cleavage of cholesterol catalysed by cytochrome P450 monooxy-
genase, and cleavage of arylglycerol-␤-arylether linkages catalysed
by the heme peroxidase ligninase [14,15]. Also the oxidative
cleavage of vic-diol moieties in sugars has been used for prepa-
ration of chiral synthones [16]. Conventionally, this reaction has
been carried out with high-valent inorganic oxidants; periodic
acid and lead tetra acetate being the most common reagents
[17,18]. In addition, the cleavage has also been studied with ceric
ammonium nitrate, manganese dioxide, chromium trioxide,
∗
Corresponding author.
E-mail address: dharitaupadhyaya@hotmail.com (D.J. Upadhyaya).
0920-5861/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.cattod.2012.10.021

D.J. Upadhyaya, S.D. Samant / Catalysis Today 208 (2013) 60–65
61
sodium bismuthate, NaIO4–SiO2, copper (II) bromide-lithium-t-
butoxide [19–23]. However, such inorganic oxidants are required
in high stoichiometric ratios and generate large volumes of inor-
ganic by-products. In the past few years, in order to develop
environment friendly methods, this reaction has been studied
with heteropoly acid catalysts like Anderson-type polyoxometalate
micro syringe. All reactions were monitored on TLC for complete
consumption of starting compound. After completion, the reaction
mixture was filtered to separate catalyst, and solvent was evapo-
rated by distillation under reduced pressure to yield crude product.
The purification was done by column chromatography using silica
gel and using petroleum ether (60–80 ◦C)-ethyl acetate (6:4, v/v) as
an eluent. The products were characterized by IR, 1H NMR spectra,
physical constants; and the data are in consonance with reported
ones. The reaction was studied with respect to various parameters
such as amount of catalyst, reaction time, nature of solvent, and
use of co-oxidant.
[Imo6O24]
5−, ammonium molybdate, HPA with cetylpyridinium
chloride as a phase transfer agent [24–27]. The reaction has
also been studied with many organometallic complexes such as
(dihydrogentellurato)-cuprate (III) and bis-(dihydrogentellurato-
argentate (III) [28], iron (II) trisphenanthroline complex [29],
cobalt acetyl acetone [30], Ru(PPh3)3Cl2 on active carbon [31],
tin porphyrin [32] and octahedral mononuclear manganese (III)
complexes with tridentate equatorial ligand o-phenylenebis (oxa-
mate) (opba)–dioxygen [33]. However, it is noteworthy that tedious
experimental procedures are often employed for the preparation of
such organometallic complexes.
3. Results and discussion
3.1. Synthesis and characterization of vanadyl pyrophosphate
catalyst
In the course of our studies on pinacol rearrangement [34], we
realized the versatility of diols, which can selectively form acid-
catalysed rearrangement products under one set of conditions and
oxidative cleavage products in presence of a powerful oxidant. This
prompted us to investigate bifunctionality of VPO catalyst for this
purpose. We report here for the first time the precise tuning of
experimental conditions using VPO as selective bifunctional cat-
alyst, which acts as a chemical switch to direct the course of
reaction selectively to obtain either C C oxidative scission or an
acid-catalysed rearrangement of vic-diols. The present method was
extended further to develop a new environmentally benign and
green catalytic protocol for oxidative cleavage of a variety of vic-
diols with 100% selectivity towards corresponding aldehydes or
ketones using H2O2 as an oxidizing agent.
The preparation procedure of this catalyst has a marked effect
on its activity. Vanadyl pyrophosphate catalyst was prepared in
organic medium using the reported procedure [37]. The method
of catalyst preparation consisted of two steps; synthesis of the
precursor, vanadyl hydrogen phosphate hemihydrate (VOHPO4·0.5
H2O), followed by subsequent dehydration to give active cata-
lyst, vanadyl pyrophosphate (VO)2P2O7. The active VPO catalyst,
(VO2)P2O7 with a P/V ratio of 1.2:1 was prepared by refluxing V2O5
and H3PO4 in isobutyl alcohol for 3 h. After filtration and washing,
vanadium phosphate hemihydrate, VOHPO4·0.5 H2O was obtained,
which was dried in vacuum at 120 ◦C to yield a catalyst precursor,
VOHPO4·0.5H2O. The active catalyst phase, (VO)2P2O7 was pre-
pared by dehydration of VOHPO4·0.5H2O, by calcination in nitrogen
ꢁ
at 550◦C for 4 h.2VOHPO4 · 0.5H2O−→ (VO)2P2O7 + 2H2O
The formation of catalyst precursor, VOHPO4·0.5H2O was veri-
fied by XRD and IR analyses. The XRD pattern of VOHPO4·0.5.H2O
(Fig. 1a) showed very sharp characteristic peaks at 2ꢀ values of
15.533 (0 0 1), 19.624 (1 0 1), 24.232 (0 2 1), 27.081 (1 2 1) and
30.484 (2 2 0); indicating a well-crystallized single phase [38]. It
did not show presence of V2O5 phase indicating that all V2O5 has
been consumed in the formation of new phase. It also confirmed
the purity of VOHPO4·0.5H2O formed without any trace of impurity
peaks. The IR-spectrum of VOHPO4·0.5H2O showed characteristic
bands at 3375 and 1637 cm−1 due to co-ordinated water (Fig. 2a).
Further, the band at 1132 cm−1 is due to the (P OH) stretching
of hydrogen phosphate group present in VOHPO4·0.5H2O. Bands
2. Materials and methods
2.1. Materials
All the chemicals were of reagent grade quality, and used as
received. V2O5 and 85% H3PO4 were procured from s.d. fine chem-
icals Ltd., Mumbai. All the vic-diols were prepared according to
literature procedures [35].
2.2. Preparation of catalyst precursor
VPO catalyst (with P/V ratio of 1.2:1) was synthesized by mix-
ing an appropriate quantity of V2O5 and H3PO4 in isobutanol
and refluxing at 105 ◦C for 3 h under N2 atmosphere. The sus-
pension obtained was cooled and the precipitate was filtered,
washed with isobutanol and dried in a vacuum oven at 120 ◦C to
yield VOHPO4·0.5H2O precursor [36]. Activation of VOHPO4·0.5H2O
precursor was carried out to transform it into active catalyst.
The precursor was calcined at 550 ◦C in nitrogen for 4 h to yield
(VO)2P2O7 which was used for the reactions.
2.3. Catalyst characterization
XRD (X-Ray diffraction analysis) of catalyst was carried out using
Siemens D5000 diffractometer with Cu K␣ radiation, running at
40 kV/30 mA in the 2ꢀ range 10–80◦ with a step size of 0.05◦. The IR
spectra were recorded on a Perkin-Elmer 882 spectrophotometer
as KBr pellets.
2.4. Oxidative cleavage of vic-diols (reactivity studies)
The reactions were carried out by mixing vic-diol (1 mmol)
and required amount of catalyst in methanol as solvent at 60 ◦C
at atmospheric pressure in air. H2O2 was added dropwise using a
Fig. 1. X-Ray diffraction pattern of (a) VOHPO4·0.5 H2O and (b) (VO)2P2O7.

62
D.J. Upadhyaya, S.D. Samant / Catalysis Today 208 (2013) 60–65
Fig. 2. IR-spectrum of synthesized (a) VOHPO4·0.5 H2O and (b) (VO)2P2O7.
present in region 900–1200 cm−1 can be attributed to P O stretch-
ing [39].
3.2.2. Effect of co-oxidizing agent
In order to improve the catalytic efficiency and decrease the
catalyst loading, it is necessary to restore the V+5/V+4 balance, by
oxidizing the V+4 species to V+5 by using a co-oxidizing agent such
as H2O2. We selected H2O2 as an oxidant because of its easy avail-
ability, relative cheapness, and ability to act as a mild and selective
oxidant. In addition, it produces water as by-product, and hence
behaves as a green oxidizing agent. Benzopinacol cleavage was car-
ried out with decreasing quantity of VPO in conjunction with H2O2.
Thus, 1 mmol of benzopinacol in presence of 30% H2O2 (0.45 ml)
effectively yielded 95% of product using as low as 0.1 mmol of
VPO catalyst (Table 1, entries 4 and 5), increasing the VPO catalyst
efficiency, decreasing reaction time and increasing the turnover fre-
quency by 32 times than the use of equivalent or higher amount of
VPO catalyst in the absence of H2O2, this indicates that the redox
cycle of V+5/V+4 species is driving the reaction forward catalytically
and stoichiometric amount of VPO catalyst is not required as was
observed in the absence of H2O2 (Table 1, entries 1, 2, 3). All further
studies were carried out in presence of H2O2 as an oxidant.
The formation of active catalyst, vanadyl pyrophosphate,
(VO)2P2O7 was also verified similarly by XRD and IR analyses. The
powder XRD pattern of catalyst showed characteristic peaks of
(VO)2P2O7 phase at 2ꢀ values of 23.022 (2 0 0), 28.397 (0 1 3) and
29.867 (0 2 3) (Fig. 1b) [40]. The catalyst formed is fairly crystalline
in nature. This is in accordance with the data reported in litera-
ture [39–41]. The IR-spectrum of synthesized (VO)2P2O7 is shown
in Fig. 2b. The band at 966 cm−1 is assigned to V O stretching in
pyrophosphate structure. The bands at 1248, 1142, 1100 cm−1 are
4−
attributed to stretching mode of P2O7
agreement with reported values [39].
group. All values are in
3.2. Catalytic activity of VPO catalyst in oxidative cleavage of
vic-diols and acid catalysed rearrangement of vic-diols
The catalytic activity of VPO catalyst was tested in oxidative
scission and acid catalysed rearrangement reaction of vic-diols.
Preliminary studies were carried out using benzopinacol as model
substrate in either methanol or benzene as solvent, under reflux
conditions using VPO as catalyst. The reaction conditions were
optimized with respect to various parameters like reaction temper-
ature, amount of catalyst, choice of solvent and use of co-oxidant
and a good control over the course of reaction was obtained. It
was observed that choice of solvent played an important role in
directing the course of reaction towards oxidative scission or acid
catalysed rearrangement.
3.2.3. Effect of solvent
Solvents play an important role in the course of reaction as
they can directly affect the product selectivity by interaction with
the catalyst surface, blocking surface acidic sides as a function of
increasing Lewis basicity, enhancing the reactant mobility or by
changing the adsorption geometry of reactants on the surface of
catalyst. Therefore, we decided to study effect of different solvents.
Interesting trend was observed with various polar protic as well as
aprotic solvents, and non-polar solvents, as shown in Table 2. It was
seen that in case of polar protic solvents like, methanol, VPO cata-
lyst efficiently catalysed the reaction showing maximum activity. In
case of solvents, like acetonitrile and dioxane also, it showed good
activity (Table 2, entries 5 and 6). Higher yields and cleaner reaction
were obtained when methanol was used as a solvent as compared
to 1,4-dioxane and acetonitrile. However, in case of non-polar sol-
vents, like benzene and toluene in presence of H2O2 as oxidant at
60 ◦C, yield of benzophenone was as low as <20% (Table 2, entries
7 and 8). As observed above, solvent played a predominant role
in VPO catalysed oxidative cleavage reaction. Amongst all the sol-
vents studied, methanol was found to be most effective solvent
to make VPO function as oxidation reagent. However, methanol
being a protic polar solvent, its own involvement in reaction may
be visualized, e.g. methanol may block the acidic centres of VPO
catalyst, as methanol is a weak Lewis base. In order to throw more
3.2.1. Effect of catalyst loading
The effect of catalyst loading was studied over a range of
0.1–2 mmol for 1 mmol of benzopinacol. Table 1 represents the
effect of increasing catalyst loading on isolated yield of benzophe-
none in oxidative cleavage of benzopinacol (Table 1, entries 1, 2, 3).
With an increase in the amount of VPO catalyst loaded, the yield of
benzophenone increased linearly, along with the linear decrease in
reaction time. However, a substantial loading of VPO catalyst was
required to achieve high yield (>95%) of benzophenone. During the
reaction, benzopinacol reduces the part of V+5 species to V+4 which
cannot oxidize back to V+5 easily in the absence of oxidizing agent.
As a result of this, although the reaction proceeds but it does not
regenerate the V+5 species on the catalyst to close the redox cycle
of V+5/V+4 species.

D.J. Upadhyaya, S.D. Samant / Catalysis Today 208 (2013) 60–65
63
Table 1
Effect of catalyst loading.
Entry
VPO catalyst (mmol)
Time (h)
30% H2O2 (ml)
Yield of benzophenone* (%)
TOF (h−1
)
1
2
3
4
5
6
0.65
1
2
0.1
0.32
No catalyst
5.5
4
1
1.5
1.5
4
–
–
–
0.45
0.45
5
71
80
96
95
96
–
0.198
0.2
0.48
6.4
2
–
Reaction conditions: benzopinacol = 0.4 g (1 mmol), MeOH = 20 ml, temperature 60 ◦C.
*
Yields are of the isolated products.
Table 2
Table 3
Oxidative cleavage of benzopinacol using VPO catalyst in presence of different sol-
A
switch over from oxidative cleavage of benzopinacol to acid catalysed
vents at 60 ◦C.
rearrangement.
Entry
Solvent
Time (h)
30% H2O2 (ml)
Yield of
Entry
1
Solvent
Product
*Yield of product (%)
benzophenone* (%)
O
1
2
3
4
5
6
7
8
MeOH
EtOH
IPA
t-BuOH
1,4-Dioxane
Acetonitrile
Benzene
Toluene
1.5
1.5
2
4
1.5
1
0.45
0.6
0.77
1.0
0.45
0.45
0.45
0.45
95
90
87
88
71
80
19
17
MeOH
PhH
96
Ph
Ph
Ph
O
Ph
5
5
Ph
Ph
2
3
82
83
Reaction conditions: benzopinacol = 0.4 g (1 mmol), VPO catalyst = 0.032 g
(0.1 mmol), solvent = 20 ml.
Ph
Ph
*
Reaction temperature = 60 ◦C, yields are of the isolated products.
light on role of methanol, we decided to carry out the reaction in a
series of different primary, secondary and tertiary alcohols (Table 2,
entries 2, 3 and 4). The general trend observed was that in case of
secondary and tertiary alcohols, longer reaction time was required
and comparatively more amount of H2O2 was consumed to bring
about oxidative cleavage. The increase in the Lewis basicity of alco-
hols from 1◦ to 3◦ alcohol did not have any remarkable difference
on the course of the reaction.
Ph
PhCH3
Ph
O
Reaction conditions: benzopinacol = 1 mmol, VPO catalyst = 2 mmol, reaction
time = 1 h, solvent = 20 ml, reflux conditons.
*
Yields are of the isolated products.
substituted vic-diols. Most of the diols selected for the purpose
were di-tertiary; however the reaction was also applied to sec-
ondary diols (Scheme 2). The results are shown in Table 4.
3.2.4. Switch over from oxidative cleavage to acid catalysed
rearrangement
Oxidative cleavage of benzopinacol carried out in methanol as
a solvent gave very high yield of benzophenone but as it can be
seen in Table 2, entries 7 and 8, in benzene and toluene as a solvent
the yield of benzophenone was very low ∼20%. In order to know
whether methanol plays any significant role in directing the course
of reaction, we carried out the reaction in non-polar aprotic sol-
vent benzene at its reflux temperature (Table 3). We observed that
when a switch over was made from methanol to benzene, a change
in the course of reaction was observed. In presence of benzene
as a solvent, pinacol–pinacolone rearrangement of benzopinacol
occurred selectively to form rearrangement product with 82% yield
(Table 3, entry 2). In order to avoid carcinogenicity of benzene, we
also attempted the reaction using toluene as a solvent at 80 ◦C and
similar effect of acid-catalysed rearrangement was observed with
similar yield as that in benzene (Table 3, entry 3). In benzene and
toluene as reaction solvent, VPO functioned selectively as an acidic
catalyst. Thus, we arrived at two sets of conditions – under one
set, selective oxidative cleavage of benzopinacol occurred yielding
benzophenone (in presence or absence of H2O2 as a co-oxidant);
whereas, under second set, selective acid catalysed rearrangement
occurred resulting in formation of benzopinacolone in high selec-
tivity (Scheme 1).
3.2.6. Reusability of recovered VPO catalyst
The reusability of this recovered VPO catalyst was established
by carrying out repeated cycles of oxidative cleavage of benzopina-
col with recovered catalyst (Table 5). At the end of a reaction, the
catalyst was filtered, dried in air at 80 ◦C for 1 h, and reused in sub-
sequent cycle. No make up quantity of catalyst was added during
subsequent experiment. It was found that VPO catalyst could be
effectively recycled without significant loss in catalytic activity (4
runs). ICP analysis of filtrate confirmed no leaching of V species
Table 4
Oxidative cleavage of different vic-diols using VPO-catalyst in MeOH at 60 ◦C.
Entry
Substrate
Time (h)
30% H2O2 (ml)
Product
Product
yield* (%)
1
2
3
4
5
6
7
8
9
1a
1b
1c
1d
1e
3a
3b
3c
5a
5b
1.5
1
1
2
3
1
1
1
0.5
0.5
0.45
0.5
0.5
0.7
0.7
0.5
0.5
0.5
0.5
0.5
2a
2b
2c
2c
2d
4a
4b
4c
6a
6b
91
96
97
88
86
90
85
93
73
82
3.2.5. C C bond cleavage of vic-diols using VPO as bifunctional
catalyst
Once the method was set, the conditions were successfully
applied to bring about scission of C C bond of a many different
10
Reaction conditions: vic-diol = 1 mmol, VPO catalyst = 0.032 g (0.1 mmol),
MeOH = 20 ml, temperature 60 ◦C.
*
Yields are of the isolated products.

64
D.J. Upadhyaya, S.D. Samant / Catalysis Today 208 (2013) 60–65
Scheme 1. Effect of change in reaction conditions on the course of the reaction.
O
Ph
Ph
R=
1a= H
VPO catalyst- H2O2
MeOH ; 60oC
1b=CH3
1c = OCH3
1c= Cl
1d= Br
R-C6H4
R-C6H4
R-C6H4
C6H4-R
C6H4-R
C6H4-R
2
Ph
C6H4-R
OH OH
1
2
O
CH3
CH3
R=
3a= H
3b=CH3
3c= OCH3
VPO catalyst- H2O2
MeOH ; 60oC
H3C
C6H4-R
OH OH
3
4
O
H
H
R=
5a= H
5b=CH3
VPO catalyst- H2O2
MeOH ; 60oC
H
C6H4-R
OH OH
5
6
Scheme 2. Products obtained in the oxidative cleavage of different vicinal diols.
occurred. It was also seen that when catalyst was hot filtered at
∼50% conversion of benzopinacol, and reaction continued in the
absence of solid catalyst, no further reaction occurred. Thus, VPO
catalyst is re-usable for the oxidative cleavage of benzopinacol.
Table 5
Reusability of VPO catalyst in oxidative cleavage of benzopinacol.
3.2.7. Mechanistic aspects
Entry
Run
Time (h)
30% H2O2 (ml)
Yield of
The plausible reaction mechanism of oxidative cleavage of
vic-diols is shown in Scheme 3. On the basis of the observed exper-
imental results, in the absence of co-oxidizing agent H2O2, the
reaction required substantial amount of VPO catalyst which is due
to consumption of V+5 species being reduced to V+4 species. To
establish the reaction mechanism, a control experiment with H2O2
alone, in the absence of VPO catalyst was performed, but no oxida-
tive cleavage occurred, and almost 99% of starting compound was
benzophenone* (%)
1
2
3
4
1
2
3
4
1.5
1.5
1.5
1.5
0.45
0.45
0.45
0.45
95
94
93
94
Reaction conditions: benzopinacol = 0.4 g (1 mmol), (VO)2P2O7 = 0.032 g (0.1 mmol),
MeOH = 20 ml.
*
Yields are of the isolated products.

D.J. Upadhyaya, S.D. Samant / Catalysis Today 208 (2013) 60–65
65
O
Acknowledgements
[V+4
]
H2O2
DJU acknowledges G. D. Gokhale trust for a research fellowship.
The authors are grateful to Mr. Nilesh Kulkarni, Prof. Pushan Ayub,
TIFR for XRD-analysis.
R1
R2
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Scheme 3. Proposed reaction mechanism for oxidative cleavage of vic-diols and
regeneration of vanadium species in reaction mixture.
recovered back at the end of 4 h, after consumption of substan-
tial excess (5 ml) of H2O2 (Table 1, entry 6). Thus, a combination
of VPO–H2O2 was essential for an efficient oxidative cleavage of
vic-diols. The presence of co-oxidizing agent H2O2 was essential
to restore the balance of V+5/V+4 species. The oxidative cleavage in
presence of VPO catalyst is thought to involve a reversible V+5/V+4
redox cycle. Similar redox mechanism was also observed in the
allylic oxidation of cyclohexane using VPO catalyst in presence
of tertiary butyl hydroperoxide as co-oxidant [42]. Based on our
experimental results and the mechanism reported in literature, the
reaction pathway for oxidative scission of benzopinacol is shown in
Scheme 3. In this scheme, when benzopinacol undergoes oxidative
scission, a part of V+5 species was reduced to V+4 species, which was
oxidized back to V+5 species in presence of H2O2 to complete the
redox cycle. The (P:V) ratio in synthesized (VO)2P2O7 catalyst was
(1.2:1), the excess of phosphorus present on catalyst surface sta-
bilizes pyrophosphate framework and regenerates V+5 sites, which
were essential to V+4/V+5 redox mechanism.
In benzene and toluene as reaction solvent, VPO selectively func-
tioned as an acidic catalyst with the formation of benzopinacolone
in high selectivity. The pinacol–pinacolone rearrangement is pro-
posed to proceed through acid catalysed elimination mechanism.
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dinated onto the V5+ site of VPO through oxygen atoms, while the
H atoms on the hydroxyl groups may be stabilized by hydrogen
bonding with lattice oxygens of VPO to form acidic protons which
can easily transfer to the vicinal OH group in the pinacol molecule.
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pinacol. The back-donation of electron pair from V O bond to C
O
bond facilitates the formation of a pinacolone through the migra-
tion of methyl group. The pinacolone molecule is easily removed
by the solvent to regenerate the catalytic active site.
4. Conclusions
In conclusion, oxidative cleavage of vicinal diols is reported for
the first time in presence of VPO catalyst, using H2O2 as an oxidant.
The catalyst displayed excellent activity. Bifunctionality of vanadyl
pyrophosphate catalyst is very prominently exhibited in selectively
bringing out the oxidative cleavage of vic-diols giving correspond-
ing aldehydes/ketones or an acid-catalysed rearrangement product
depending on the choice of solvent. The protocol was successfully
extended to variety of ditertiary as well as secondary diols to yield
aldehydes and ketones selectively. The carbonyl compounds were
obtained in high isolated yields without over oxidation to acids.
The amount of catalyst used to induce the cleavage is catalytic
with respect to substrate molecule. The VPO catalyst can be re-
cycled efficiently without any loss in catalytic activity. The catalyst
regeneration mechanism has been proposed.
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