Oxidovanadium(IV) complexes of 3-hydroxy-4-pyrone and 3-hydroxy-4- pyridinone ligands: A new generation of homogeneous catalysts for the epoxidation of geraniol

Article abstract of DOI:10.1007/s10562-010-0281-8

A novel generation of homogeneous catalysts for the epoxidation of geraniol, in the presence of tert-butyl hydroperoxide oxidant is reported. Oxidovanadium(IV) complexes of 3-hydroxy-4-pyrone and 3-hydroxy-4-pyridinone ligands exhibit excellent activity and selectivity towards the 2,3-epoxygeraniol product, only differing in the reaction times and turnover frequencies. The pyrone based complexes are the most efficient catalysts showing performances similar to that of the well known oxidovanadium(IV) acetylacetonate catalyst. Graphical abstract: [Figure not available: see fulltext.]

Full text of DOI:10.1007/s10562-010-0281-8

Catal Lett (2010) 135:98–104
DOI 10.1007/s10562-010-0281-8
Oxidovanadium(IV) Complexes of 3-Hydroxy-4-pyrone
and 3-Hydroxy-4-pyridinone Ligands: A New Generation
of Homogeneous Catalysts for the Epoxidation of Geraniol
•
Clara Pereira Andreia Leite Ana Nunes
•
•
•
Susana L. H. Rebelo Maria Rangel
•
Cristina Freire
Received: 23 December 2009 / Accepted: 17 January 2010 / Published online: 4 February 2010
ꢀ Springer Science+Business Media, LLC 2010
Abstract A novel generation of homogeneous catalysts
for the epoxidation of geraniol, in the presence of tert-butyl
hydroperoxide oxidant is reported. Oxidovanadium(IV)
complexes of 3-hydroxy-4-pyrone and 3-hydroxy-4-pyrid-
inone ligands exhibit excellent activity and selectivity
towards the 2,3-epoxygeraniol product, only differing in
the reaction times and turnover frequencies. The pyrone
based complexes are the most efficient catalysts showing
performances similar to that of the well known oxidova-
nadium(IV) acetylacetonate catalyst.
homogeneous catalysts for the epoxidation of non-func-
tionalized alkenes and allylic alcohols, hydroxylation of
alkanes and arenes, oxidation of sulfides and of primary
and secondary alcohols [4, 6–13]. Among all the oxidation
products, epoxides have a pivotal position, being valuable
building blocks in the synthesis of natural products and
biologically active substances [4, 11]. Several vana-
dium(IV and V) based complexes have been reported as
highly active, stereo- and/or regioselective catalysts in the
epoxidation of allylic alcohols, leading to moderate-to-
good epoxide yields. In particular, the oxidovanadium(IV)
acetylacetonate ([VO(acac)2]) catalyst is one of the pre-
vailing choices in the epoxidation of allylic alcohols using
tert-butyl hydroperoxide (t-BuOOH) as oxidant, presenting
high activity, stereo- and regioselectivity [10–16]. Due to
the importance of such reactions, several research groups
have been engaged in the design of novel and efficient
vanadium catalysts for the stereoselective epoxidation of
allylic and homoallylic alcohols. A major breakthrough in
this area has been the synthesis of several active and
enantioselective vanadium complexes with chiral and non-
chiral hydroxamic acid ligands containing diverse struc-
tural features [10, 11, 17–32]. Sharpless and co-workers
were the first researchers to report a promising chiral
vanadium catalyst for this reaction involving [VO(acac)2]
and a chiral hydroxamic acid, with t-BuOOH as oxidant
[17, 18]. More recently, in the past decade a remarkable
progress has been achieved in this area, with the devel-
opment of modified versions of this catalytic system, by
using chiral hydroxamic acids derived from 2,20-binaphthol
[19], a-amino acids [20, 21], [2.2]paracyclophane [11, 22],
(?)-ketopinic acid [23], sulfonamides [24–26], or chiral
C2-symmetric bis-hydroxamic acids [27–30]. The asym-
metric induction has also been achieved through the com-
bination of optically active hydroperoxide oxidants with
Keywords Geraniol epoxidation ꢀ
Oxidovanadium(IV) complexes ꢀ Homogeneous catalysis ꢀ
3-Hydroxy-4-pyrones ꢀ 3-Hydroxy-4-pyridinones ꢀ
Oxidovanadium(IV) acetylacetonate
1 Introduction
The oxidation of organic substrates catalyzed by transition
metal compounds is a subject of paramount importance in
Fine Chemistry [1–5]. Vanadium complexes are promising
Electronic supplementary material The online version of this
article (doi:10.1007/s10562-010-0281-8) contains supplementary
material, which is available to authorized users.
C. Pereira ꢀ A. Leite ꢀ A. Nunes ꢀ S. L. H. Rebelo ꢀ
C. Freire (&)
´
REQUIMTE, Departamento de Quımica e Bioquımica,
Faculdade de Ciencias, Universidade do Porto, Rua do Campo
´
ˆ
Alegre, 4169-007 Porto, Portugal
e-mail: acfreire@fc.up.pt
M. Rangel
REQUIMTE, Instituto de Ciencias Biomedicas de Abel Salazar,
ˆ
Universidade do Porto, 4099-003 Porto, Portugal
´
123

Oxidovanadium(IV) Complexes of 3-Hydroxy-4-pyrone and 3-Hydroxy-4-pyridinone Ligands
99
achiral hydroxamic acid ligands [31, 32]. In this context,
the quest for efficient and selective oxidovanadium cata-
lysts for epoxidations still continues to be a thriving topic
in Fine Chemistry.
detailed study of the catalytic performance of oxidovana-
dium(IV) complexes with 3-hydroxy-4-pyrone and 3-hydroxy-
4-pyridinone ligands (Fig. 1, pyrone complexes: 2a–2d;
pyridinone complexes: 3a–3j) is reported; the [VO(acac)2]
complex (Fig. 1, complex 1) is also included in this study since
it is considered as the reference catalyst. The influence of the
ligand structure (pyrone–pyridinone–acetylacetonate) on the
catalytic properties of the corresponding complexes is also
examined. The names of all the complexes examined in this
work are summarized in Table S1, Supplementary Material.
Oxidovanadium(IV) complexes of 3-hydroxy-4-pyrones
and 3-hydroxy-4-pyridinones have been the object of a
considerable number of studies due to their pharmaceutical
application in the oral treatment of Diabetes Mellitus. In
particular, the bis(maltolato)oxidovanadium(IV) complex
has been extensively studied from the chemical and phar-
maceutical point of view and the oxidovanadium(IV) com-
plex of ethylmaltol has passed Phase I clinical trials in 2000
and recently Phase II clinical trials [33–35]. A series of
oxidovanadium(IV) complexes of 3-hydroxy-4-pyridinones
has been synthesized and characterized, the evaluation of its
insulin-like effect in vitro showed promising results [36–39]
and their potential activity in an animal model is currently
under study. These complexes present a central vanadium
atom (V=O) coordinated to two O2-type bidentate ligands
and their chemical properties indicate that they may also find
application in other fields such as catalysis.
2 Experimental
2.1 Reagents, Solvents and Complexes
[VO(acac)2], chlorobenzene, geraniol and t-BuOOH solu-
tion 5.0–6.0 M in decane were purchased from Aldrich.
The solvent dichloromethane was supplied by Romil
(HPLC grade). Silica gel 60 and thin layer chromatography
(TLC) plastic sheets of silica gel 60 F254 were from Merck.
Detailed procedures concerning the preparation and
characterization of the pyrone and pyridinone ligands and
In this work, a novel application for these complexes is
envisaged in the catalytic epoxidation of geraniol, at room
temperature, using t-BuOOH as oxidant. In this context, a
Fig. 1 Molecular structures and
abbreviations of the
oxidovanadium(IV) complexes
used in this study
Acetylacetonate complex
R
R
O
V
O
O
O
O
1: [VO(acac)2]
R = CH3
R
R
Pyrone complexes
R3
R2
2a: [VO(maltol)2]
2b: [VO(bromomaltol)2] R1 = H;
R1 = H;
R2 = CH3
R2 = CH3
R2 = H
R3 = H
R3 = Br
R3 = H
R3 = H
R1
O
V
O
O
O
O
O
R1 = CH2OH;
O
2c: [VO(kojato)2]
R1
2d: [VO(chlorokojato)2] R1 = CH2Cl;
R2 = H
R2
R3
Pyridinone complexes
N-alkyl-pyridinone complexes
3a: [VO(mpp)2]
3b: [VO(etpp)2]
3c: [VO(dmpp)2]
3d: [VO(mepp)2]
3e: [VO(empp)2]
3f: [VO(depp)2]
R1 = H;
R1 = H;
R1 = H;
R1 = H;
R1 = H;
R1 = H;
R2 = CH3;
R2 = CH2CH3; R3 = H
R2 = CH3;
R2 = CH3;
R2 = CH2CH3; R3 = CH3
R3 = H
R2
R3
R1
R1
O
V
O
O
O
O
R3 = CH3
R3 = CH2CH3
N
N
R3
R2
R2 = CH2CH3; R3 = CH2CH3
N-alkyl-pyridinone complex with OH groups
3g: [VO(heepp)2]
R1 = H;
R2 = CH2CH3; R3 = CH2CH2OH
N-aryl-pyridinone complexes
3h: [VO(ppp)2]
3i: [VO(epp)2]
3j: [VO(pbp)2]
R1 = H;
R1 = H;
R1 = H;
R2 = CH3;
R2 = CH2CH3; R3 = C6H5
R2 = CH3; R3 = C6H5(CH2)3CH3
R3 = C6H5
123

100
C. Pereira et al.
of the corresponding oxidovanadium(IV) complexes have
been reported elsewhere [38, 40].
syringe, then filtered through 0.2 lm PTFE syringe filters
and directly analyzed by gas chromatography.
To provide a full characterization of the catalytic sys-
tems in terms of identification and quantification of all the
reagents and products, detailed studies of the reactions with
complexes 1 and 2a were firstly performed by GC-FID, 1H
NMR spectroscopy, column chromatography and GC–MS.
In this context, the reactions were monitored by GC–FID in
order to determine the reaction time, substrate conversion
and product selectivity and yield. At the end of the reac-
tions, the mixtures were fractionated by silica gel column
chromatography using a mixture of dichloromethane:ethyl
acetate (9:1) as eluent, affording 2,3-epoxygeraniol as the
main product. The chromatography was controlled by TLC
on silica sheets and the spots were visualized by treatment
with a mixture of sulphuric acid:methanol (1:1). The iso-
lated product was identified and characterized by 1H NMR
(Figure S1 in Supplementary Material) [41] and GC–MS.
The main product isolated from these reactions was 2,3-
epoxygeraniol, which was identified and characterized by
1H NMR (Figure S1 in Supplementary Material) [41] and
GC–MS. These techniques in association with GC-FID also
revealed the absence of the other possible products 6,7-
epoxygeraniol and 2,3-6,7-diepoxygeraniol. Nevertheless,
trace amounts (\5%) of unidentified products were detec-
ted by GC–FID.
2.2 Physico-chemical Characterization
The 1H NMR spectra of the isolated 2,3-epoxygeraniol
product and of the crude reaction mixtures catalyzed by
complexes 1 and 2a were recorded using a Bruker DRX
300 at 300.13 MHz and tetramethylsilane (TMS) as the
internal standard.
GC-FID chromatograms were obtained with a Varian
CP-3380 gas chromatograph equipped with a FID detector,
using helium as carrier gas and a fused silica Varian
Chrompack capillary column CP-Sil 8 CB Low Bleed/MS
(30 m 9 0.25 mm i.d.; 0.25 lm film thickness). The GC-
FID temperature program to monitor the substrate and the
products of geraniol epoxidations was: 40 ꢁC (1 min),
25 ꢁC min-1, 150 ꢁC, 5 ꢁC min-1, 200 ꢁC (1 min); injec-
tor temperature, 200 ꢁC; detector temperature, 250 ꢁC.
The reaction parameters—geraniol conversion (Conv.),
2,3-epoxygeraniol yield (2,3-EG yield) and turnover fre-
quency (TOF)—were calculated as follows, where A
stands for chromatographic peak area: Conv. (%) =
{[A(geraniol)/A(chlorobenzene)]t=0h - [A(geraniol)/A(chlo-
robenzene)]t=xh} 9 100/[A(geraniol)/A(chlorobenzene)]t=0h
;
2,3-EG yield (%) = n(2,3-EG) 9 100/n(geraniol)t=0h
where n(2,3-EG) was determined by GC-FID using the
expression n(2,3-EG) = [A(2,3-EG)/A(chlorobenzene)] 9
n(chlorobenzene) 9 response factor; TOF = TON/reac-
tion time, where TON = n(2,3-EG)/n(V in the catalyst)
and the reaction time corresponds to the time required to
reach 100% of geraniol conversion.
1
The H NMR spectra of the crude reaction mixtures
catalyzed by complexes 1 and 2a were also obtained in
CDCl3 (Figures S2 and S3, in Supplementary Material),
and the substrate conversion and product yield were cal-
culated based on the integration of chosen proton signals
(d(ppm) = 5.08 for 2,3-epoxygeraniol and d(ppm) = 5.10
and 5.40 for geraniol).
GC–MS analyses were performed with a Finnigan Trace
GC–MS (Thermo Quest CE Instruments) using helium as
the carrier gas (35 cm s-1) equipped with a SPB-5 capil-
lary column (30 m 9 0.25 mm i.d.; 0.25 lm film thick-
ness). The temperature program used for the epoxidation
1
2,3-Epoxygeraniol: H NMR (300 MHz, CDCl3, 22 ꢁC,
TMS): d(ppm) = 1.29 (s, 3H, H-10), 1.22–1.26 and 1.45–
1.49 (2 m, 2H, H-5), 1.61 (s, 3H, H-8), 1.68 (d,
J = 0.9 Hz, 3H, H-9), 2.04–2.12 (m, 2H, H-4), 2.98 (dd,
J = 4.1 and 6.8 Hz, 1H, H-2), 3.49 (s-broad, 1H, OH),
3.65 (dd, J = 6.8 and 12.2 Hz, 1H, H-1), 3.81 (dd, J = 4.1
and 12.2 Hz, 1H, H-1), 5.08 (tt, J = 1.3 and 7.1 Hz, 1H,
H-6). MS (70 eV, EI): m/z (%): 170 (1) [M?•], 152 (4), 139
(8), 137 (5), 121 (13), 109 (100).
reaction mixtures was: 80 ꢁC (1 min), 25 ꢁC min-1
,
155 ꢁC, 5 ꢁC min-1, 200 ꢁC; injector temperature, 220 ꢁC;
interface and oven temperatures, 280 ꢁC.
2.3 Catalytic Epoxidation of Geraniol
The GC–FID results were in accordance with those
1
obtained from H NMR spectroscopy and 2,3-epoxyger-
In a typical procedure, 1.00 mmol of geraniol (substrate),
0.50 mmol of chlorobenzene (GC internal standard) and
0.02 mmol of oxidovanadium(IV) complex (catalyst pre-
cursor) were mixed in 5.00 cm3 of dichloromethane, at room
temperature with continuous stirring. The oxidant,
1.50 mmol of t-BuOOH was progressively added to the
reaction medium using a Bioblock Scientific Syringe Pump
at a rate of 3.05 cm3 h-1. To monitor the reactions progress
during the catalytic experiments, 0.05 cm3 aliquots were
withdrawn from the reaction mixture with a hypodermic
aniol isolated yield and thus this technique could be used to
determine substrate conversion and 2,3-epoxygeraniol
yield in all the other reactions.
3 Results and Discussion
The catalytic performance of several oxidovanadium(IV)
complexes with different 3-hydroxy-4-pyrone and
123

Oxidovanadium(IV) Complexes of 3-Hydroxy-4-pyrone and 3-Hydroxy-4-pyridinone Ligands
101
100
3-hydroxy-4-pyridinone ligands (Fig. 1, complexes 2a–2d
(a)
and 3a–3j, respectively) was evaluated in the epoxidation of
geraniol, at room temperature, in dichloromethane using
t-BuOOH as oxidant, and compared to that of the well known
oxidovanadium(IV) acetylacetonate catalyst (Fig. 1, com-
plex 1). Geraniol was selected as the model substrate and the
reaction conditions were optimized to allow direct compar-
ison with the results previously reported for complex 1
[14–16, 42, 43]. [VO(acac)2] is one of the catalysts tradi-
tionally chosen for this particular reaction due to its high
activity, stereo- and regioselectivity [10–12, 14–16, 42–44].
The catalytic parameters, reaction time, substrate con-
version, 2,3-epoxygeraniol yield and TOF are reported in
Table 1. Figure 2 shows the kinetic profiles for the epox-
idations reactions catalyzed by the different families of
oxidovanadium(IV) complexes.
90
80
70
60
50
40
30
20
10
0
1
2a
2b
2c
2d
0.00
0.25
0.50
0.75
1.00
1.25
1.50
t (h)
100
90
80
70
60
50
40
30
20
10
0
(b)
All the complexes investigated present excellent activity
in the epoxidation of geraniol (100% geraniol conversion)
and very high selectivity towards the 2,3-epoxygeraniol
3a
3b
3c
3d
3e
3f
Table 1 Geraniol epoxidation, at room temperature, catalyzed by
oxidovanadium(IV) complexes in the homogeneous phase
3g
Complex t (h)a Conv.b (%) 2,3-EG yieldc (%) TOF (h-1
)
0
1
2
3
4
5
6
7
8
9
t (h)
1
0.25
0.5
0.5
0.5
1.5
9
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
99
200
192d
197d
170d
31
100
90
80
70
60
50
40
30
20
10
0
(c)
2a
2b
2c
2de
3ae
3be
3c
3d
3e
3f
99
89
92
3h
3i
3j
100
100
94
6
7
7
2
24
4
100
100
100
86
13
4
13
5
10
3g
3h
3i
3
14
5
92
9
0
1
2
3
4
5
6
7
8
9
5
95
10
t (h)
3j
5
95
10
Fig. 2 Kinetic profiles for geraniol epoxidation catalyzed by homo-
geneous oxidovanadium(IV) complexes: a 1, 2a, 2b, 2c and 2d, b 3a,
3b, 3c, 3d, 3e, 3f (N-alkyl-pyridinone complexes) and 3g (N-alkyl-
pyridinone complexes with OH groups) and c 3h, 3i and 3j (N-aryl-
pyridinone based complexes)
Reaction conditions: 1.00 mmol of geraniol, 0.50 mmol of chloro-
benzene (internal standard), 0.02 mmol of homogeneous catalyst, 1.50
mmol of t-BuOOH; solvent: dichloromethane
a
Reaction time corresponding to 100% of geraniol conversion
b
Based on geraniol consumption
c
Determined by GC-FID using chlorobenzene as internal standard
and isolated 2,3-EG product (see Sect. 2)
product, with yields of epoxide higher than 86%, thus being
successful homogeneous catalysts precursors for this reac-
tion (Table 1). The results indicate that the pyrone-based
complexes are more efficient catalysts than the pyridinone-
d
The TOF was calculated for t = 0.25 h for comparison with
complex 1
e
Insoluble in the reaction medium
123

102
C. Pereira et al.
based ones, presenting lower reaction times and conse-
quently higher TOFs. These differences are clearly reflected
in the corresponding kinetic profiles (Fig. 2). Complexes 2a,
2b and 2c, in particular, exhibit reaction times and kinetic
profiles comparable to those observed for complex 1 and led
to similar geraniol conversion and 2,3-epoxygeraniol yield
(only complexes 2a and 2b), being envisioned as very
promising homogeneous catalysts for this reaction, Fig. 2a).
Complex 2d presents higher reaction time and lower TOF
than its counterparts Fig. 2a), which can be due to its
insolubility in the solvent used in the reaction, although
complete solubilization occurs during the catalytic cycle.
The family of compounds with pyridinone-based ligands
(complexes 3a–3j) can be subdivided in different groups
considering the differences in the R2 and R3 substituents. In
the case of the complexes containing N-alkyl pyridinone
ligands (R2 and/or R3 = alkyl groups), complexes 3a–3f, the
highest activity is reached with complex 3c which only
contains methyl groups in those positions, Fig. 2b. The
increase in the carbon chain of R2 and/or R3 groups (com-
plexes 3d–3f) leads to an increase in the reaction time and
decrease in TOF. In this subgroup of N-alkyl pyridinone-
based complexes, the catalysts which present the lowest
reactivities and TOFs are complexes 3a and 3b, which only
have alkyl substituents in the R2 group of the pyridinone
ligands and a hydrogen atom directly bonded to the N atom
(R3 = H), suggesting that the absence of alkyl groups in R3
leads to a decrease in the catalytic activity of this family of
compounds. However, these complexes are insoluble in the
solvent used in the reaction, which is probably other reason
for their lower catalytic performance.
The N-aryl pyridinone complexes (with phenyl based R3
groups), complexes 3h, 3i and 3j, present similar catalytic
parameters indicating that, in this particular set of com-
pounds, the increase in the R2 carbon chain (complex 3i) or
the replacement of the phenyl R3 group by a bulkier aryl
substituent (complex 3j) does not affect their performance,
Fig. 2c.
To allow a direct comparison between the different
types of complexes, Fig. 3 summarizes the reaction times
achieved with all the oxidovanadium(IV) catalysts under
study.
As a general tendency, the catalytic performance of the
complexes of the three ligand families decreases in the
order,
acetylacetonate [ pyrone [ pyridinone,
thus
revealing that the type of ligand modulates the activity of
the VO(IV) metallic centre. In the particular case of the
pyridinone complexes, the introduction of hydroxyl func-
tionality in the ethyl chain of R3 increases the catalyst
reactivity for this particular reaction (complex 3f vs. com-
plex 3g). In contrast, a decrease in the reaction rate is
observed when comparing complexes 3e, 3f and 3i sug-
gesting that the replacement of the R3 methyl group
(complex 3e) by an ethyl or by a bulkier phenyl group
(complexes 3f and 3i, respectively) causes some steric
hindrance, reducing the catalytic reactivity. The same pat-
tern is observed for complexes 3c, 3d, 3h and 3j, where a
methyl R3 group (complex 3c) was replaced by an ethyl
(complex 3d) or by more hindered phenyl based groups
(complexes 3h and 3j) and R2 is a methyl substituent. The
complexes 3a and 3b were not compared with their coun-
terparts (complexes 3e, 3f and 3i, and complexes 3c, 3d, 3h
9
9
8
7
7
6
5
4
3
5
5
5
5
4
4
3
2
2
1
0
1.5
2d
0.5
2a
0.5
2b
0.5
2c
0.25
1
3a
3b
3c
3d
3e
3f
3g
3h
3i
3j
Complex
Fig. 3 Bar chart showing the reaction times for 100% conversion achieved with the different oxidovanadium(IV) complexes
123

Oxidovanadium(IV) Complexes of 3-Hydroxy-4-pyrone and 3-Hydroxy-4-pyridinone Ligands
103
and 3j, respectively) due to their insolubility in the reaction
medium which restrains any accurate conclusions.
and to high yields of 2,3-epoxygeraniol product (C86%).
The two series of complexes show different reaction times,
kinetic profiles and turnover frequencies, demonstrating the
influence of the ligand type on the catalytic properties of
the metallic centre.
Finally, when compared to other oxidovanadium/
hydroxamic acid catalytic systems reported in literature
[19, 21, 23–26, 29–31], these two families of pyrone and
pyridinone oxidovanadium(IV) complexes lead to similar
or higher 2,3-epoxygeraniol regioselectivity and yield in a
shorter reaction time. Furthermore, they provide several
advantages to the catalytic reactions, including the possi-
bility of using low catalyst loadings, mild reaction condi-
tions (at room temperature over a shorter reaction time),
aqueous oxidants and easy work-up procedures.
The pyrone-based complexes proved to be the most
efficient catalysts, exhibiting catalytic performances com-
parable to that of the well known [VO(acac)2]. In the case
of the pyridinone-based complexes, their catalytic activity
depends on the nature of the R2 and R3 ligands substituents,
with the highest activity being achieved for the complex
containing R2 and R3 methyl groups.
Although this new generation of oxidovanadium(IV)
complexes acts as slightly less efficient homogeneous
catalysts when compared to the commercially available
[VO(acac)2] complex, they open new perspectives in
asymmetric catalysis namely in the design of novel envi-
ronmentally friendly heterogeneous catalysts by immobi-
lization onto inorganic porous supports. In fact, the
covalent immobilization of [VO(acac)2] onto several types
of porous supports such as clays and pillared clays [42, 43],
activated carbon [45] and mesoporous silicas [42, 44] has
been performed by using 3-aminopropyltriethoxysilane as
spacer, through reaction of the amine groups of the func-
tionalized support with the acetylacetonate ligand (C=O
functionality) of the complex. However, these methods
lead to a change in the coordination sphere of the metal
centre, which invariably originates lower catalytic perfor-
mances for the heterogeneous [VO(acac)2] systems when
compared to the homogeneous counterpart.
In conclusion, this work provides a significant contri-
bution to the repertoire of homogeneous vanadium cata-
lysts for the epoxidation of allylic alcohols.
Acknowledgments This work was funded by Fundac¸a˜o para a Ci-
ˆ
encia e a Tecnologia (FCT) and FEDER, through project ref. PTDC/
CTM/65718/2006. C.P. and A.L. thank FCT for a PhD fellowship.
The NMR spectrometer used in this work is part of the National NMR
Network and was purchased in the framework of the National Pro-
gram for Scientific Re-equipment, contract REDE/1517/RMN/2005,
with funds from POCI 2010 (FEDER) and FCT.
References
1. Gates BC (1992) Catalytic chemistry. Wiley, New York
2. Parshall GW, Itel SD (1992) Homogeneous catalysis. Wiley,
New York
3. van Leeuwen PW (2005) Homogeneous catalysis: understanding
the art. Springer, London
The complexes reported in this work present different
reactive groups in their ligands (Br, Cl, OH), which can be
used to anchor them onto inorganic porous supports by dif-
ferent immobilization strategies and remarkably, without
changing the coordination sphere of the metal centre, thus
overcoming the drawback of the [VO(acac)2] heterogeneous
systems reported in literature [42–45]. Moreover, these types
of ligands are easily modified with other reactive function-
alities, which can be selectively chosen not only to fine tune
the catalytic performance of the metal centre (improving
their catalytic activity), but also to design novel immobili-
zation strategies for their heterogenization onto porous
supports. Therefore, the quest for novel alternatives to the
[VO(acac)2] catalyst is still a current area of research.
4. Xia Q-H, Ge H-Q, Ye C-P, Liu Z-M, Su K-X (2005) Chem Rev
105:1603–1662
5. Gupta KC, Sutar AK (2008) Coord Chem Rev 252:1420–1450
6. Ada˜o P, Pessoa JC, Henriques RT, Kuznetsov ML, Avecilla F,
Maurya MR, Kumar U, Correia I (2009) Inorg Chem 48:3542–
3561
7. Kuznetsov ML, Pessoa JC (2009) Dalton Trans 5460–5468
8. Ada˜o P, Maurya MR, Kumar U, Avecilla F, Henriques RT,
Kusnetsov ML, Pessoa JC, Correia I (2009) Pure Appl Chem 81:
1279–1296
9. Matsumoto K, Saito B, Katsuki T (2007) Chem Commun 3619–
3627
10. Ligtenbarg AGJ, Hage R, Feringa BL (2003) Coord Chem Rev
237:89–101
11. Bolm C (2003) Coord Chem Rev 237:245–256
12. Conte V, di Furia F, Licini G (1997) Appl Catal A General
157:335–361
13. Butler A, Clague MJ, Meister GE (1994) Chem Rev 94:625–638
14. Sharpless KB, Michaelson RC (1973) J Am Chem Soc 95:6136–
6137
15. Rossiter BE, Verhoeven TR, Sharpless KB (1979) Tetrahedron
Lett 20:4733–4736
4 Conclusions
A novel generation of highly active and selective oxido-
vanadium(IV) catalysts bearing 3-hydroxy-4-pyrone and
3-hydroxy-4-pyridinone ligands has been selected for the
epoxidation of geraniol, in the presence of t-BuOOH oxi-
dant. All the complexes led to 100% substrate conversion
16. Itoh T, Jitsukawa K, Kaneda K, Teranishi S (1979) J Am Chem
Soc 101:159–169
17. Sharpless KB, Verhoeven TR (1979) Aldrichimica Acta 12:
63–74
18. Michaelson RC, Palermo RE, Sharpless KB (1977) J Am Chem
Soc 99:1990–1992
123

104
C. Pereira et al.
19. Murase N, Hoshino Y, Oishi M, Yamamoto H (1999) J Org Chem
64:338–339
20. Makita N, Hoshino Y, Yamamoto H (2003) Angew Chem Int Ed
42:941–943
21. Hoshino Y, Yamamoto H (2000) J Am Chem Soc 122:10452–
10453
22. Bryliakov KP, Talsi EP, Ku¨hn T, Bolm C (2003) New J Chem
27:609–614
34. Thompson KH, Orvig C (2006) J Inorg Biochem 100:1925–1935
35. Thompson KH, Lichter JB, LeBel C, Caife MS, McNeil JH,
Orvig C (2009) J Inorg Biochem 103:554–558
36. Rangel M, Tamura A, Fukushima C, Sakurai H (2001) J Biol
Inorg Chem 6:128–132
37. Rehder D, Costa Pessoa J, Geraldes CFGC, Castro MMCA,
Kabanos T, Kiss T, Meier B, Micera G, Pettersson L, Rangel M,
Salifoglou A, Turel I, Wang D (2002) J Biol Inorg Chem 7:
384–396
38. Rangel M, Leite A, Amorim MJ, Garriba E, Micera G, Lodyga-
Chruscinska E (2006) Inorg Chem 45:8086–8097
23. Wu H-L, Uang B-J (2002) Tetrahedron Asymmetr 13:2625–2628
24. Malkov AV, Czemerys L, Malyshev DA (2009) J Org Chem
74:3350–3355
25. Bourhani Z, Malkov AV (2005) Chem Commun 4592–4594
ˇ
26. Malkov AV, Bourhani Z, Kocovsky P (2005) Org Biomol Chem
3:3194–3200
39. Rangel M, Amorim MJ, Nunes A, Leite A, Pereira E, de Castro
B, Sousa C, Yoshikawa Y, Sakurai H (2009) J Inorg Biochem
103:496–502
27. Li Z, Zhang W, Yamamoto H (2008) Angew Chem Int Ed
47:7520–7522
28. Zhang W, Yamamoto H (2007) J Am Chem Soc 129:286–287
40. Burgess J, Rangel M (2008) Adv Inorg Chem 60:167–243
41. Martins RRL, Neves MGPMS, Silvestre AJD, Simo˜es MMQ,
´
Silva AMS, Tome AC, Cavaleiro JAS, Tagliatesta P, Cresini C,
Mol J (2001) Catal A Chem 172:33–42
29. Barlan AU, Zhang W, Yamamoto
63:6075–6087
H
(2007) Tetrahedron
42. Pereira C, Biernacki K, Rebelo SLH, Magalha˜es AL, Carvalho
AP, Pires J, Freire C, Mol J (2009) Catal A Chem 312:53–64
43. Pereira C, Silva AR, Carvalho AP, Pires J, Freire C, Mol J (2008)
Catal A Chem 283:5–14
44. Jarrais B, Pereira C, Silva AR, Carvalho AP, Pires J, Freire C
(2009) Polyhedron 28:994–1000
30. Zhang W, Basak A, Kosugi Y, Hoshino Y, Yamamoto H (2005)
Angew Chem Int Ed 44:4389–4391
31. Lattanzi A, Piccirillo S, Scettri A (2005) Eur J Org Chem 1669–
1674
¨
32. Adam W, Beck AK, Pichota A, Saha-Moller CR, Seebach D,
Vogl N, Zhang R (2003) Tetrahedron Asymmetr 14:1355–1361
33. Thompson KH, Orvig C (2004) Met Ions Biol Syst 41:221–252
45. Jarrais B, Silva AR, Freire C (2005) Eur J Inorg Chem 4582–
4589
123