Multiple oxo-vanadium schiff base containing cyclotriphosphazene as a robust heterogeneous catalyst for regioselective oxidation of naphthols and phenols to quinones

Article abstract of DOI:10.1007/s10562-012-0852-y

Grafting of multiple oxo-vanadium Schiff base moieties to cyclotriphosphazene provided an efficient and recyclable heterogeneous catalyst for the regioselective oxidation of naphthols and phenols to quinones by using tbutylhydroperoxide as oxidant. One of the main advantages of the developed catalyst was the presence of multiple oxovanadium moieties in close proximity which made the developed catalyst more active as compared to its homogeneous oxo-vanadium Schiff base. After the reaction, the catalyst was easily recovered from the reaction mixture by simple filtration and reused for five runs without loss in activity.

Full text of DOI:10.1007/s10562-012-0852-y

Catal Lett (2012) 142:1020–1025
DOI 10.1007/s10562-012-0852-y
Multiple Oxo-Vanadium Schiff Base Containing
Cyclotriphosphazene as a Robust Heterogeneous Catalyst
for Regioselective Oxidation of Naphthols and Phenols
to Quinones
Praveen K. Khatri Suman L. Jain
•
Received: 20 February 2012 / Accepted: 2 June 2012 / Published online: 20 June 2012
Ó Springer Science+Business Media, LLC 2012
Abstract Grafting of multiple oxo-vanadium Schiff base
moieties to cyclotriphosphazene provided an efficient and
recyclable heterogeneous catalyst for the regioselective
oxidation of naphthols and phenols to quinones by using t-
butylhydroperoxide as oxidant. One of the main advantages
of the developed catalyst was the presence of multiple oxo-
vanadium moieties in close proximity which made the
developed catalyst more active as compared to its homo-
geneous oxo-vanadium Schiff base. After the reaction, the
catalyst was easily recovered from the reaction mixture by
simple filtration and reused for five runs without loss in
activity.
[3, 4]. The synthetic utility of cyclotriphosphazenes is
mainly due to the presence of reactive halogens and
their ease of nucleophilic substitution [5]. Hexachlorocyc-
lotriphosphazene (N P Cl ), a readily available starting
3
3
6
material, is used increasingly for the synthesis of func-
tionalized dendrimers and macrocyclic compounds by
nucleophilic substitution of chlorine with number of
potential donor ligands [6–8]. Nevertheless, few deriva-
tives containing silicon substituents have been reported
in the literature [9–11], metal containing derivatives of
cyclotriphosphazenes are scarcely reported [12]. We have
previously reported [13] the application of cyclotriphos-
phazene grafted silica for immobilizing the oxo-vanadium
Schiff base moieties for hydroxylation of benzene. The
developed catalyst was found to be highly active than its
corresponding oxo-vanadium Schiff base, probably due to
the presence of multiple organometallic anchored frag-
ments on close proximity. Inspired with our own results,
we targeted to explore the applications of cyclotriphosp-
hazenes for linking of the organometallic moieties.
Keywords Cyclotriphosphazene ꢀ Vanadium ꢀ
Oxidation ꢀ Naphthol ꢀ Quinone
1
Introduction
Recent trend in the evolution of environmentally benign
synthesis has evoked considerable interest in developing
reusable metal catalysts and reagents that maintain high
activity, selectivity and facile recovery of the catalyst [1,
Regioselective oxidation of naphthols and phenols to
quinones is an important reaction as quinones are versatile
and highly useful compounds as synthetic intermediates as
well as biologically active compounds. Some quinones
show bioactivities and are used as medicine intermediates.
For example, trimethyl-p-benzoquinone is a key compound
in the vitamin E synthesis, and 2-methyl-1,4-naphthoqui-
none is used as blood coagulating agent and as supplement
in animal food [14–16]. Although, oxidation of naphthols
and phenols to the corresponding quinones is well docu-
mented in the literature [17–19], the scope for developing
an efficient and recyclable catalyst for this valuable reac-
tion is still remained. In continuation to our work on
development of new synthetic methodologies [20–24], we
herein report an efficient catalytic approach for the
2
]. As transition metal complexes are often expensive to
purchase or prepare, making them recyclable offer several
advantages such as simplicity in handling, facile recovery
from reaction mixture and reusability. Cyclophosphazenes
are a remarkable class of inorganic compounds which have
found extensive uses in the development of polymeric
materials intended for a variety of commercial applications
P. K. Khatri ꢀ S. L. Jain (&)
Chemical Sciences Division, CSIR-Indian Institute
of Petroleum, Dehradun 248005, India
e-mail: suman@iip.res.in
1
23

Multiple Oxo-Vanadium Schiff Base Containing Cyclotriphosphazene
1021
regioselective oxidation of naphthols and phenols to cor-
responding quinones using tert-butylhydroperoxide as the
sole oxidant in the presence of multiple oxo-vanadium
schiff base conatining cyclotriphosphazene as catalyst.
separated by filtration and washed with dichloromethane
and dried under vacuum for further use. The filtrate was
concentrated under reduced pressure and residue so
obtained was dissolved in dichloromethane (10 ml). The
organic layer was washed with water (2 9 15 ml), dried
over anhydrous MgSO . The solvent was removed under
4
2
Experimental
vacuum to give corresponding oxidized product. The
conversion of the products was determined by GCMS and
1
selectivity was determined by H NMR.
2
.1 Material
All chemicals, solvents, and reagents were of reagent grade
quality and were used as purchased from commercial
sources. All solvents were dried and purified before use.
Oxo-vanadium Schiff base 1 was prepared by following the
literature procedure [25].
3 Result and Discussion
3.1 Synthesis and Characterization
The synthetic route of the multiple oxo-vanadium Schiff
base containing cyclotriphosphazene 3 is shown in Scheme 1.
The required oxo-vanadium Schiff base 1 was readily
prepared by the reaction of 4-hydroxysalicylaldehyde,
2-aminoethanol and vanadyl acetylacetonate by following
the literature procedure [25]. The reaction between oxo-
vanadium Schiff base 1 and hexachlorocyclotriphosphazene
(N P Cl ) 2 in a molar ratio (6:1) in dry DMF at 80 °C
resulted in the formation of dark brown colored solid, which
was precipitated out on cooling. The precipitated cyclotri-
phosphazene containing oxo-vanadium Schiff base 3 was
isolated by filtration. The solid thus obtained was washed
several times with DMF, hot methanol and dried under
vacuum.
2
.2 Measurements
FT-IR spectra were obtained on a Perkin Elmer FTIR X
760 spectrophotometer with the samples prepared as KBr
1
1 13
pellets. H and C NMR Spectra were recorded on a
Bruker Avance 300 Spectrometer in CDCl . Elemental
3
analysis was done by using ASTM D-3828 (Kjeldhal
method). Analysis for metal contents were carried out by
using inductively coupled plasma atomic emission spec-
trometer (ICP-AES, PS-3000UV) by Leeman Labs.
3
3
6
2
.3 Preparation of Multiple Oxo-Vanadium Schiff
Base Containing Cyclotriphosphazene 3
The synthesized catalyst was characterized by the IR,
elemental analysis and TGA. The catalyst 3 exhibits a
To a stirred solution of vanadium Schiff base 1 (1.84 g,
-
1
intense IR band at 982 cm characteristic to the V=O
4
1
.0 mmol) in dry DMF (15 ml) was added NaH (0.24 g,
-
1
0 mmol) at 0 °C in one portion and the mixture was
and strong bands in the region of 1,150–1,215 cm
slowly warmed to room temperature. After stirring for 30 min
corresponding to cyclotriphosphazene structure, confirm-
ing the covalent attachment of complex 1 to the cyclo-
triphosphazene support 2. In addition disappearance of
at room temperature, hexachlorocyclotriphosphazene 2 (0.34 g,
1
.0 mmol) was added and the resulting mixture was vig-
-
1
orously stirred and heated for 8 h at 80 °C. After being
cooled to room temperature, the black solid thus obtained
was separated by filtration and washed with DMF, water,
methanol and dried under vacuum. Yield 1.65 g (94 %). IR
phenolic band (3,200–3,400 cm ) and lower shift of a
-
1
band from 1,631 to 1,600 cm corresponding to C=N of
the Schiff base further revealing the formation of het-
erogenized complex 3. Since, the displacement of all the
chloro groups by oxo-vanadium Schiff base moieties to
form a condensed structure as 3 is difficult, thus we
assumed that the obtained heterogeneous material might
be containing structural units having different number
of oxo-vanadium Schiff base moieties attached to the
cyclotriphosphazene molecule. We performed the syn-
thesis of catalyst in DMF, thus attachment of solvent
molecules to metal centre is obvious, which is further
confirmed by the presence of an intense band at
-
1
(
cm ): 2924, 2656, 1654, 1600, 1383, 1292, 1207, 1146,
9
2
82; Analytical data: C 30.31 %; H 4.64 %; N 11.48 %; Cl
.58 %.
2
.4 General Experimental Procedure for the Oxidation
of Naphthols and Phenols
To the stirred mixture of naphthol or phenol (1 mmol),
anhydrous TBHP (5–6 M solution in decane; 2 mmol) in
acetonitrile (3 ml) was added catalyst 3 (1 mol%,
-
1
1749 cm in IR. Furthermore, we analyzed the prepared
material by elemental analysis and the observed per-
centage of chlorine (2.58 %) and nitrogen (11.48 %)
suggested the structure of the prepared material as
0.01 mmol). The resulting mixture was heated at 70 °C for
the time reported in the Table 1. Progress of the reaction
was monitored by TLC. After completion, the catalyst was
123

1
022
P. K. Khatri, S. L. Jain
Table 1 Oxidation of
naphthols and phenols with t-
butylhydroperoxide
Time
(h)
Conv.
(%)*
Yield
(%)
a
Entry
Substrate
Product
b
O
OH
O
85
78
15
1
4.0
3.5
c
7
0
O
OH
2
80
74
O
O
OH
OH
O
3
4
5
5.5
4.5
6.5
70
72
60
66
68
57
Br
Br
O
MeO
MeO
O
O
OH
COOMe
O
COOMe
O
OH
OH
6
7
3.0
2.5
2.5
8.5
3.0
92
94
92
40
90
88
90
87
30
86
Me
Me
Me
Me
O
O
t-Bu
t-Bu
O
t-Bu
t-Bu
*Conversion determined by
HPLC and GCMS and
selectivity of the products was
1
confirmed by H NMR
O
O
OH
CH
3
CH
3
8
a
Reaction conditions: substrate
CH
3
CH3
OH
(
(
(
1 mmol), TBHP in decane
2.0 mmol), catalyst 3
1 mol%), acetonitrile (3 ml) at
O
O
9
7
b
0 °C
Isolated yield
NO
OH
2
NO
2
c
O
Using homogeneous oxo-
vanadium complex 1 as catalyst,
0
resulting the mixture of 4,4 -
binaphthol and 1,2-
naphthoquinone
10
Me
Me
O
N P Clx[VO(Schiff base)dmf] 3. The percentage of
6-x
12.5 % percentage of vanadium in the supported catalyst
3. In the mass spectrum of 3, the presence of molecular
ion peak at m/z 1,493, suggested the proposed structure
as shown in Scheme 1.
3
3
vanadium in the supported catalyst was determined by
thermogravimetric (TGA) analysis under oxygen atmo-
sphere. The prepared catalyst was found to be stable
thermally up to ca 150 °C and started losing weight
between 150 and 200 °C due to the loss of coordinated
solvent molecules. Further the weight loss was observed
at different stages at different temperatures between 300
and 500 °C, probably due to the decomposition of the
part of ligand moiety and phosphorus structure. Finally
sublimation with 40 % residue at 750 °C suggested the
formation of V O at the end which corresponds to the
3.2 Catalytic Studies
3.2.1 Oxidation of Naphthols and Phenols with Catalyst 3
The prepared catalyst 3 has been used for the oxidation of
napthols and phenols using anhydrous t-butylhydroperox-
ide (solution in decane) as oxidant (Scheme 2). Simple
2
5
1
23

Multiple Oxo-Vanadium Schiff Base Containing Cyclotriphosphazene
1023
Cl
Cl
Cl
OR
RO OR
P
OH
P
P
N
N
P
N
P
N
P
N
P
N
P
N
P
Cl
Cl
Cl
Cl
RO
RO
Cl
OR
RO
Cl
+
+
O O
V
N
N
O
N
O
N
Cl
Cl
OR
V
2
O
O
RO OR
P
RO OR
P
NaH
N
P
N
P
OR
+
N
P
N
P
1
DMF, 80 0
OR
RO
OR
OR
C
OH
8
h
N
N
Cl
RO
9
4 %
3
O
O
V
DMF
O
R=
O
N
Scheme 1 Synthetic route of catalyst 3
mixing of the 2-naphthol (1 mmol) and catalyst 3
1 mol%) in presence of anhydrous TBHP (2 mmol) in
vanadium catalyst. Therefore, we have chosen t-butylhy-
droperoxide as an optimum oxidant for the present trans-
formation. In a controlled experiment, the oxidation of
2-naphthol with TBHP in the absence of catalyst was not
occurred even after prolonged reaction time (6 h). The
reaction was found to be slow at room temperature, how-
ever 70 °C was found to be optimum for this reaction.
Further increase in reaction temperature did not improve
the results to any significant extent.
(
acetonitrile at 70 °C allowed the convenient and high
yielding oxidation to o-naphthoquinone. After completion
of the reaction as monitored by TLC analysis, the catalyst
was separated by filtration and reused as such for sub-
sequent runs. The filtrate was subjected to the usual workup
to give the corresponding oxidized product. Importantly,
the leaching of metal or ligand was not detected in this
course, which was ascertained by subjecting the filtrate
sample and product to ICP-AES analysis.
Further we studied the oxidation of various naphthols
and phenols under similar reaction conditions. The results
of these experiments are presented in Table 1. All the
substrates were efficiently converted to corresponding
quinones selectively without any evidence for the forma-
tion of coupling product. The reaction times varied from
2.5 to 8.5 h, depending upon the functional groups
substituted on the starting substrate. The reaction was
failed in case of unsubstituted phenol, however, substituted
phenols and 1-naphthol, were selectively converted to p-
quinones in good to excellent yields (Table 1, entries 2,
6–7). However, in case of 2,4-dimethyl phenol, 4-nitro-
phenol and 2-naphthols, where p-position was blocked,
the reaction favoured the synthesis of the corresponding
o-quinones selectively (Table 1, entries 1, 3–5, 8, 9). As,
p-isomers are more stable and symmetric than o-isomers,
therefore, thus the reactions favours the regioselective
synthesis of 1,4-quinones in most of the phenolic
substrates.
The development of synthetic methodologies for the
oxidation using molecular oxygen, hydrogen peroxide as
oxidants is important both from environmental and eco-
nomical viewpoint. Keeping this in view, we also per-
formed the oxidation of 2-naphthol by using molecular
oxygen as oxidant under similar reaction conditions. The
reaction did not take place even after prolonged reaction
time. However, the use of hydrogen peroxide as oxidant
affected the selectivity of reaction adversely and yielded an
intricate mixture of the products. This is probably due to
the non-productive decomposition of hydrogen peroxide by
O
OH
R
R
O
O
O
OH
To evaluate the effect of solvent, we studied the oxi-
dation of 2-naphthol under described reaction conditions
by using different solvents such as acetonitrile, toluene,
dichloroethane. Among the various solvents studied, ace-
tonitrile was found to be solvent of choice from conversion
and reaction time points of view. In case of toluene and
dichloromethane, the reaction was found to be very slow
and provided very poor yield of the desired product.
We also compared the catalytic potential of the hetero-
geneous catalyst 3 with the corresponding homogeneous
cat 3 (1mol%)
TBHP
3
0
CH CN, 70 C
O
OH
R'
O
R''
R''
R'
R=Me, OMe, NO2
R''=H, Br, OMe
R'=H, COOMe
Scheme 2 Oxidation of naphthols and phenols to quinones
123

1
024
P. K. Khatri, S. L. Jain
Table 2 Comparison of catalyst 3 with other reported catalysts
100
O
9
0
OH
O
Catalyst
Oxidant
80
7
6
5
4
0
0
0
0
Entry Catalyst
Reaction
time (h)
Yield Ref.
(%)
a
85
1
Cyclotriphosphazene linked oxo- 4.0
vanadium Schiff base 3
–
2
3
4
5
6
7
8
IBX (o-iodoxybenzoic acid)
10.0
84
57
65
65
87
83
75
19
26
27
28
29
30
31
30
b
Rh (cap)
2
4
4.0
2
0
0
0
PhIO
2
in AcOH
12 0
1
Polymer supported vanadium salt 12
Martin’s reagent
12
1.0
–
b
b
1
2
3
4
5
Heteropolytungstate
Cycle
Vanadium-substituted Keggin
heteropolyacid
Fig. 1 Result of recycling experiments
a
GC yield
b
Isolated yields
oxidant and 2-naphthol. The reaction was continued for
h under described reaction conditions. No reaction was
6
oxo-vanadium Schiff base 1 by performing the oxidation of
-naphthol under similar reaction conditions. The cyclotri-
observed; confirming that there was no leaching occurred
during the reaction and therefore the reaction was truly
heterogeneous in nature.
2
phosphazene linked oxo-vanadium 3 was found to be more
efficient and selective catalyst, which yielded 1,2-naph-
thoquinoe exclusively. Whereas in case of catalyst 1, the
reaction was found to be slow and yielded poor yield of the
0
4 Conclusion
desired product along with the formation of 4,4 -binaphthol
as by-product (Table 1, entry 1). Enhanced catalytic activ-
ity of the catalyst 3 in comparison to catalyst 1, is probably
due to the presence of multiple oxo-vanadium moieties in
close proximity to the cyclotriphosphazene support.
In conclusion, we have demonstrated an efficient multiple
oxo-vanadium Schiff base containing cyclotriphosphazene
catalyst for the regioselective oxidation of the naphthols
and phenols to corresponding quinones by using t-butyl-
hydroperoxide as oxidant. The advantageous features of the
developed catalyst are its facile synthesis, higher catalytic
activity, regioselective nature and recycling ability without
significant loss in catalytic activity. Because of the unique
chemistry of cyclotriphosphazenes, we believe that
the present report will open a wide scope in develop-
ing new catalytic materials/methodologies for organic
transformations.
The comparison of cyclotriphosphazene grafted oxo-
vanadium Schiff base 3 with the reported catalysts by
considering the oxidation of 2-naphthol to o-naphthoqui-
none as model substrate is shown in Table 2. The higher
yield, enhanced selectivity and lower reaction time with the
added benefits of facile recovery and recycling ability of
catalyst noticeably reflects the superiority of the developed
catalyst 3 over existing methods.
Furthermore, recycling of the catalyst was tested by
performing the oxidation of 2-naphthol with TBHP under
described reaction condition. After completion of the
reaction, the catalyst was easily recovered from the
reaction mixture by simple filtration and reused as such
for subsequent runs. As shown in Fig. 1, the developed
catalyst showed consistent catalytic activity for five runs
and afforded similar yield of the desired product. Last
but not least, we also checked the leaching of the metal
or ligand from the catalyst by refluxing the catalyst 3 in
acetonitrile for 3–4 h. The catalyst was separated by
filtration and filtrate so obtained was charged with
Acknowledgments We thank Director, IIP for his kind permission
to publish these results. We thank Analytical Sciences Division, IIP
for IR, elemental analysis and Dr. JK Gupta for TGA analysis.
References
1
2
. Leadbeater NE, Marco M (2002) Preparation of polymer-sup-
ported ligands and metal complexes for use in catalysis. Chem
Rev 102:3217–3274
. Burgess K (2000) Solid phase organic synthesis. Wiley, New
York
1
23

Multiple Oxo-Vanadium Schiff Base Containing Cyclotriphosphazene
1025
3
4
5
6
. Allcock HR (2003) Chemistry and applications of polyphosp-
hazenes. Wiley-VCH, New York
oxidation of 2-methyl-1-naphthol in the presence of iron phtha-
locyanine supported catalyst. Comptes Rendus Chimie 10:598–
603
. Gleria M, De Jaeger R (2004) Applicative aspects of poly(or-
ganophosphazenes). Nova Science Publishers, New York
. Allen CW (1991) Regio- and stereochemical control in substi-
tution reactions of cyclophosphazenes. Chem Rev 9:119–135
. Gall M, Breza M (2009) QTAIM study of transition metal
complexes with cyclophosphazene-based multisite ligands I:
zinc(II) and nickel(II) complexes. Polyhedron 28:521–524
. Chandrasekhar V, Nagendran S (2001) Phosphazenes as scaffolds
for the construction of multi-site coordination ligands. Chem Soc
Rev 30:193–203
19. Magdziak D, Rodriguez AA, Van De Water RW, Pettus TRR
(2002) Regioselective oxidation of phenols to o-quinones with
o-iodoxybenzoic acid (IBX). Org Lett 4:285–288
20. Verma S, Jain SL, Sain B (2011) An efficient biomaterial sup-
-
?
ported bifunctional organocatalyst (ES-SO3 C5H5NH ) for the
synthesis of b-amino carbonyls. Org Bio Chem 9:2314–2318
21. Verma S, Jain SL, Sain B (2011) Starch immobilized Ru-con-
taining ionic liquid catalyzed oxidative cyanation of tertiary
amines with hydrogen peroxide. ChemCatChem 3:1329–1332
22. Kumar S, Verma S, Jain SL, Sain B (2011) Thiourea dioxide
(TUD): a robust organocatalyst for oxidation of sulfides to sulf-
oxides with TBHP under mild reaction conditions. Tetrahedron
Lett 52:3393–3396
7
8
9
. Uslu A, Guvenaltın S (2010) The investigation of structural and
thermosensitive properties of new phosphazene derivatives
bearing glycol and amino acid. Dalton Trans 39:10685–10691
. Allcock HR, Brennan DJ, Graaskamp JM, Parvez M (1986)
Reactions between phosphazenes and organosilicon compounds:
synthesis and molecular structure of methylsilane- and methyl-
siloxane-cyclotriphosphazenes. Organometallics 5:2434–2446
23. Verma S, Nandi M, Modak A, Jain SL, Bhaumik A (2011) Novel
organic–inorganic hybrid mesoporous silica supported oxo-
vanadium Schiff base for selective oxidation of alcohols. Adv
Synth Catal 353:1897–1902
24. Singhal S, Jain SL, Sain B (2009) An efficient aerobic oxidative
cyanation of tertiary amines with sodium cyanide using vanadium
based systems as catalysts. Chem Commun 17:2371–2372
25. Ando R, Yagyu T, Maeda M (2004) Characterization of oxo-
vanadium (IV) Schiff-base complexes and those bound on resin,
and their use in sulfide oxidation. Inorg Chim Acta 357:
2237–2244
26. Ratnikov MO, Farkas LE, McLaughlin EC, Chiou G, Choi H,
Khalafy SHE, Doyle MP (2011) Dirhodium-catalyzed phenol and
aniline oxidations with T-HYDRO. Substrate scope and mecha-
nism of oxidation. J Org Chem 76:2585–2593
27. Moriarty RM, Prakash O (2001) Oxidation of phenolic com-
pounds with organohypervalent iodine reagents. Org React 57:
327–416
1
1
1
1
0. Allcock HR, Connolly MS, Sisko JT, Al-Shali S (1988) Effects of
organic side group structures on the properties of poly(organo-
phosphazenes). Macromolecules 21:323–324
1. Allcock HR, Brennan DJ (1988) Organosilicon derivatives of
cyclic and high polymeric phosphazenes. J Organomet Chem
3
41:231–239
2. Rao MR, Gayatri G, Kumar A, Sastry GN (2009) Cyclotriphos-
phazene ring as a platform for multiporphyrin assemblies. Chem
Eur J 15:3488–3492
3. Khatri PK, Singh B, Jain SL, Sinha AK, Sain B (2011) Cyclo-
triphosphazene grafted silica: a novel support for immobilizing
the oxo-vanadium Schiff base moieties for hydroxylation of
benzene. Chem Commun 47:1610–1612
1
1
1
4. Shimizu M, Orita H, Hayakawa T, Takehira K (1989) A conve-
nient synthesis of alkyl-substituted p-benzoquinones from phe-
nols by a H
71–474
2
O
2
/heteropolyacid system. Tetrahedron Lett 30:
28. Suresh S, Skaria S, Ponrathnam S (1996) Polymer-supported
vanadium salt as a catalyst for the oxidation of phenols. Synth
Commun 26:2113–2117
29. Luo HB, Xie YY (2003) Regioselective oxidation of phenols to
o-quinones with Dess-Martin periodinane(DMP). Chin Chem
Lett 14:555–556
30. Egusquiza G, Romanelli G, Cabello C, Botto C, Thomas H
(2008) Phenol and naphthol oxidation to quinones with hydrogen
peroxide using vanadium- substituted Keggin heteropolyacid as
catalyst. Catal Commun 9:45–50
4
5. Saladino R, Neri V, Mincione E, Filippone P (2002) Selective
oxidation of phenol and anisole derivatives to quinones with
hydrogen peroxide and polymer-supported methylrhenium tri-
oxide systems. Tethaedron 58:8493–8500
6. Zalomaeva OV, Sorokin AB (2006) Access to functionalized
quinones via the aromatic oxidation of phenols bearing an alcohol
or olefinic function catalyzed by supported iron phthalocyanine.
New J Chem 30:1768–1773
1
1
7. Cooksey CJ, Land EJ, Riley PA (1996) A simple one-pot prep-
aration of 4-alkoxy and 4-alkyl thio-catechols and o-benzoqui-
nones. Org Prep Proced Int 28:463–467
8. Zalomaeva OV, Kholdeeva OA, Sorokin AB (2007) Preparation
of 2-methyl-1,4-naphthoquinone (vitamin K3) by catalytic
31. Romanelli G, Villabrille P, V a´ zquez P, C a´ ceres C, Tundo P
(2008) Phenol and naphthol oxidation to quinones with hydrogen
peroxide using vanadium-substituted Keggin heteropoly acid as
catalyst. Lett Org Chem 5:332–335
123