Selective oxidations of allylic alcohols using vanadyl and cobalt(II) alkyl phosphonate modified silicas

Article abstract of DOI:10.1016/j.tetlet.2004.04.067

A range of allylic alcohols can be selectively oxidised to either the corresponding epoxide or the enone in good yields using catalytic quantities of vanadyl or cobalt(II) alkyl phosphonate modified silicas, CoEPS3 and VOEPS3 and tert-butyl hydroperoxide.

Full text of DOI:10.1016/j.tetlet.2004.04.067

Tetrahedron
Letters
Tetrahedron Letters 45 (2004) 4465–4468
Selective oxidations of allylic alcohols using vanadyl and cobalt(II)
alkyl phosphonate modified silicas
*
Magdalena Jurado-Gonzalez, Alice C. Sullivan and John R. H. Wilson
Centre for Materials Research, Department of Chemistry, Queen Mary, University of London, Mile End Road, London, E1 4NS UK
Received 23 February 2004; revised 7 April 2004; accepted 13 April 2004
Abstract—A range of allylic alcohols can be selectively oxidised to either the corresponding epoxide or the enone in good yields
using catalytic quantities of vanadyl or cobalt(II) alkyl phosphonate modified silicas, CoEPS3 and VOEPS3 and tert-butyl
hydroperoxide.
Ó 2004 Published by Elsevier Ltd.
The identification and implementation of effective cat-
alytic systems for liquid phase oxidation, particularly in
the manufacture of fine chemicals, is considered to be
Allylic alcohols are very important intermediates for the
preparation of a wide range of organic compounds
either via epoxidation of the olefin or oxidation of the
alcohol. Thus effective chemoselective heterogeneous
catalytic systems would be of particular interest to
conduct these transformations. Titanium containing
silicates with hydroperoxides have been extensively ex-
1
one of the chemical industry’s major targets. The need
for such systems is widely accepted, specifically to
replace inorganic oxidants that are used in greater than
stoichiometric quantities and which also possess signif-
icant handling and environmental issues. Several differ-
ent approaches are being investigated and include
homogeneous catalysis, microencapsulation and hetero-
geneous catalysis. In homogeneous catalysis significant
progress has been made utilising soluble metal salts and
oxidants such as hydrogen peroxide and organic per-
4
;5
plored leading to a number of commercial epoxidation
processes. Materials obtained via the isomorphous
substitution of vanadium in the framework of APOs and
6
;7
silicalites have been investigated
as heterogeneous
epoxidation catalysts. Under liquid phase reaction
conditions some of the vanadium is leached from the
lattice leading to significant homogeneous catalysed
oxidation. Similar problems were encountered with
2
oxides. Microencapsulation has also shown promise for
oxidation reactions involving a metal complex immo-
bilised within a polymer, such as polystyrene. Hetero-
geneous catalysis has many inherent advantages such as
ease of recovery and recycling of the catalyst as well as
being suitable for continuous processing. A number of
different strategies for the heterogenisation of redox
active elements are being explored and include frame-
work-substituted molecular sieves and encapsulation
and ion exchange in layered double hydroxides. These
8
vanadium supported on silica and vanadomolybdo-
9
phosphate polyoxometalate supported on mesoporous
ꢀ
MCM-41. [VO(acac)
2
] and [VO
3
]
have been immobi-
lised onto a variety of derivatised polystyrene resins as
well as ion-exchange resins. Although epoxidation pro-
ceeds well, difficulties in recycling and metal leaching
were encountered (see Ref. 2 and references cited
therein).
3
approaches have been recently reviewed.
A variety of heterogeneous catalytic systems have been
reported for the selective oxidation of allylic alcohols to
the corresponding carbonyl compounds. These include a
number of supported ruthenium based catalysts and
recent notable examples are ruthenium incorporated
Keywords: Allylic alcohols; Epoxidation; Oxidation; Vanadyl; Cobalt;
Phosphonate; Silica; Immobilised reagents.
10
11
hydroxy apatites and hydrotalcites, perruthenate
catalysts based on cross-linked polymer and meso-
12
*
Corresponding author. Tel.: +44-20-7882-3274; fax: +44-20-7882-
794; e-mail: a.c.sullivan@qmul.ac.uk
13
7
porous MCM-41 and ruthenium(III) complexes of
0
040-4039/$ - see front matter Ó 2004 Published by Elsevier Ltd.
doi:10.1016/j.tetlet.2004.04.067

4466
M. Jurado-Gonzalez et al. / Tetrahedron Letters 45 (2004) 4465–4468
1
4
1
,4,7-trimethyl-1,4,7-triazacyclononane on silica gel.
Table 1. Allylic alcohol oxidation activated by VOEPS3
15
a
Palladium(II) hydrotalcites have also been reported to
catalyse this selective oxidation. Immobilised substituted
TEMPO on polystyrene can also selectively oxidise
allylic alcohols to the unsaturated carbonyl compounds.
Substrate
Product
Time (h) Isolated
yield (%)
16
2
2
2
.5
.5
.5
80
81
b
c
0
OH
OH
17;18
O
d
We recently reported
acterisation of the first examples of phosphonate
and phosphonic acid modified silicas,
Si] PO(OR)
R ¼ H, alkyl; n ¼ 2; 3;
on the synthesis and char-
2.5
0.5
0
e
60
79
OH
OH
6
[
O
2
x
[O3=2Si(CH
2
)
n
2 y
]
O
O
x : y ¼ 1 : 1–10 : 1. The silica ethyl and propyl phos-
phonate materials (EPS) and (PPS) prepared to date
have Q:T ratios in the range 1:1–10:1. In all cases we
found the T components to have around 66% conden-
sation and Q components around 90%. We have
reported on a range of metal EPS derivatives including
7
7
71
73
OH
OH
OH
OH
O
19
20
21
cobalt(II), cerium(IV) and vanadyl materials and
some of their catalytic oxidation chemistry. In this paper
we wish to report our initial work on the chemoselective
catalytic oxidation of a variety of allylic alcohols util-
ising VOEPS3 and CoEPS3 (see Scheme 1). The EPS3,
VOEPS3 and CoEPS3, see Figure 1, used in this study
2
1
4
6
79
83
OH
OH
O
O
OH
OH
a
19;21
All reactions were conducted using 1 mmol organic substrate in
ꢀ
were prepared as we have previously described.
We
1
acetonitrile at 80 °C, 5.7 mol % vanadium (0.96 mmol V g catalyst)
and 6 mmol tert-butyl hydroperoxide.
chose first to investigate the effectiveness and selectivity
of the VOEPS3 material to catalyse the tert-butyl
hydroperoxide (TBHP) mediated epoxidation of allylic
alcohols.
b
Average yield from three runs using recycled catalyst. After each run
the catalyst was filtered off, washed well with water and with diethyl
ether and then dried at 100 °C at 0.01 Torr.
c
d
e
No catalyst was used.
The initial series of reactions was conducted on geraniol
and we found, as illustrated in Table 1, that epoxidation
proceeds quickly and in good yield. The VOEPS3-TBHP
catalyst system selectively epoxidises the 2,3 double
bond of geraniol and no traces of other epoxides or
carbonyl compounds were detected in the reaction
mixture. At the end of the reaction the VOEPS3 catalyst
was removed by filtration. The VOEPS3 catalyst can be
reused without any apparent loss of activity as illus-
trated by the recycle experiments involving geraniol as
the substrate. In the absence of the VOEPS3 catalyst
only the starting material, geraniol, was obtained. To
investigate whether the silica surface or the attached
phosphonate groups were possibly playing a role in the
epoxidation we conducted the reaction in the presence of
2
Na EPS3 (60 mg) was used in place of the catalyst.
The standard test for catalyst leaching was applied and no further
conversion was observed in hot filtrates of the reaction mixture after a
further 2 h.
reaction was observed and only starting material was
recovered. To check for leaching the catalyst was re-
moved by filtration, at the reaction temperature, after
half an hour and the filtrate was kept at this temperature
for a further 2 h. No further epoxidation occurred. This
suggests that epoxidation is occurring at the immobi-
2
2
lised vanadyl site. A typical procedure involves stirring
a gently refluxing mixture containing the substrate and
ꢀ
1
catalyst (0.96 mmol V g catalyst, 5.7 mol % V relative
to substrate) and excess tert-butyl hydroperoxide, as the
reoxidant, in acetonitrile. The VOEPS3 catalyst system
can be used to epoxidise selectively a range of other
allylic alcohols in good yield (Table 1). Epoxidation of
cyclohex-2-enol proceeds with complete cis stereoselec-
tivity. Although the reaction is noticeably slower,
homoallylic alcohols can also be selectively epoxidised in
good yield.
the precursor of the VOEPS3 catalyst, Na
2
EPS3. No
R
R
R
OH
O
O
OH
VOEPS3
TBHP
CoEPS3
TBHP
R1
R1
R1
R3
R3
R3
R2
R2
R2
Scheme 1.
O
O
O
O
O
Si
O
O
Si
O
Si
O
O
O
O
O
O
Co
O
P
Si
O
P
Si
O
Si
P
O
O
O
O
Si
O
Si
O
Si
O
Si
O
Si
O
OH
OH
O
O
O
O
O
Si
O
O
V
O
O
O
O
O
O
EPS3
CoEPS3
VOEPS3
Figure 1.

M. Jurado-Gonzalez et al. / Tetrahedron Letters 45 (2004) 4465–4468
ROOH
4467
Apart from our own investigations on CoEPS3 we are
Co(III) + RO + OH -
Co(II)
unaware of any other reports on the use of either
homogeneous or heterogeneous Co(II) phosphonates as
catalysts for liquid phase oxidation reactions. Thus we
decided to investigate the effectiveness and selectivity of
CoEPS3 to catalyse the TBHP mediated oxidation of
HO
ROOH
o
7
0 C
o
5
0 C
19
allylic alcohols. We recently reported the effective
allylic and benzylic oxidation of a wide range of sub-
strates using TBHP and CoEPS3, as catalyst. In the
examples we explored, there was no evidence to suggest
that any epoxidation had occurred. In addition no traces
of allylic alcohols were observed in any of the reaction
mixtures.
Co(III)
Co
O O R
O
O
ROH H O
2ROH
2
H O
2
Scheme 2.
Utilising identical reaction conditions for the allylic and
benzylic oxidation, treatment of 3,5,5-trimethylcyclo-
hex-2-en-1-ol with CoEPS3 and TBHP, at 50 °C, affor-
ded only trace amounts of the enone product and
starting material. The reaction proceeds at a higher
temperature, 80 °C, to give 3,5,5-trimethylcyclohex-2-
en-1-one, selectively. No trace of any epoxide could be
detected from spectral analysis of the reaction product.
No reaction occurred in the absence of the catalyst or in
tive points about the previously reported CoEPS3 ca-
19
talysed allylic oxidation and the current results with
the corresponding allylic alcohols. The outcome is the
same in both cases since both reactions proceed selec-
tively to the enones. However since the oxidation of the
alkene compounds proceeds at a lower temperature than
is required for the allylic alcohol oxidation, and given
that we do not observe the formation of any interme-
diate allylic alcohol in that case, it is clear that different
mechanisms pertain. A tentative proposal, based in part
on the Chong and Sharpless mechanisms for the
2
the presence of Na EPS3 in place of the catalyst.
CoEPS3 can be reused as catalyst without any apparent
loss of activity as illustrated by the recycle experiments.
To check for leaching the catalyst was filtered off, at the
reaction temperature after an hour and the filtrate was
kept for a further 2 h. No further oxidation occurred.
Table 2 provides some further examples of the selective
oxidation of allylic alcohols to the corresponding enones
using the CoEPS3-TBHP system.
23
homogeneous cases, is suggested (see Scheme 2).
The allylic alcohol oxidation is likely to involve an
alkoxide intermediate while the cyclic alkene is more
likely to involve a transition state where both alkene and
peroxide are simultaneously coordinated to the metal.
The alkoxide intermediate can proceed to the enone
product with reduction of cobalt. In the analogous
vanadyl case a transition state involving both coordi-
nated alkoxide and peroxide is believed to pertain from
which the syn-epoxide evolves. In conclusion we have
demonstrated that new catalytic systems based on van-
adyl or cobalt(II) alkyl phosphonate modified silica can
selectively oxidise allylic alcohols to the epoxide or
enone, respectively, in good yields utilising tert-butyl
hydroperoxide as the reoxidant.
Before we rationalise the difference between the cobalt
and vanadyl catalysts, it is pertinent to make compara-
Table 2. Allylic alcohol oxidation activated by CoEPS3
a
Substrate
Product
Time
h)
Isolated
yield (%)
(
14
14
14
14
1
89
88
OH
O
b
c
0
d
0
80
e
Acknowledgements
OH
OH
O
12
87
The authors thank the EPSRC for support.
OH
O
1
1
0
0
82
76
References and notes
O
1
. Lee, A. F.; Gee, J. J.; Theyers, H. J. Green Chem. 2000, 2,
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a
All reactions were conducted using 1 mmol organic substrate in
acetonitrile at 80 °C, 5.8 mol % Co (0.97 mmol Co(II) g catalyst)
and 6 mmol tert-butyl hydroperoxide.
ꢀ1
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b
Average yield from three runs using recycled catalyst. After each run
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ether and then dried at 100 °C at 0.01 Torr.
c
d
e
Reaction was run without catalyst.
2
Na EPS3 (60 mg) was used in place of the catalyst.
The standard test for catalyst leaching was applied and no further
conversion was observed in hot filtrates of the reaction mixture after a
further 2 h.

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