Catalytic homogeneous oxidation of monoterpenes and cyclooctene with hydrogen peroxide in the presence of sandwich-type tungstophosphates [M4(H2O)2(PW9O34)2]n?, M = CoII, MnII and FeIII

Article abstract of DOI:10.1016/j.molcata.2016.10.021

Catalytic efficiency of tetrabutylammonium salts of sandwich tungstophosphates B‐α‐[M₄(H₂O)₂(PW₉O₃₄)₂]^n?, M = Co^II, Mn^II, Fe^III, was studied in the oxidation of (R)-(+)-limonene, geraniol, linalool, linalyl acetate, carveol, and cis-cyclooctene with hydrogen peroxide, in acetonitrile. Oxidation of (R)-(+)-limonene gave limonene-1,2-diol as main product. Epoxidation of linalool takes place preferentially at the more substituted 6,7-double bond, the corresponding 6,7-epoxide reacting further, yielding furano- and pyrano-oxides, via intramolecular cyclization. Oxidation of linalyl acetate occurred preferentially at the more substituted 6,7-double bond for Mn₄(PW₉)₂, affording 6,7-epoxide at 82% selectivity. Linalyl acetate 1,2-epoxide was the major product with 51% and 77% selectivity for Co₄(PW₉)₂ and Fe₄(PW₉)₂, respectively. Oxidation of carveol occurred with very good conversions in the presence of Mn₄(PW₉)₂, Co₄(PW₉)₂ and Fe₄(PW₉)₂, yielding carvone and carveol 1,2-epoxide in similar amounts. Oxidation of cis-cyclooctene gave only the epoxide, while oxidation of geraniol at room temperature afforded 2,3-epoxygeraniol as the major product.

Full text of DOI:10.1016/j.molcata.2016.10.021

Accepted Manuscript
Title: Catalytic homogeneous oxidation of monoterpenes and
cyclooctene with hydrogen peroxide in the presence of
sandwich-type tungstophosphates [M4(H2O)2(PW9O34)2]n−,
M = CoII, MnII and FeIII
´
Author: Isabel C.M.S. Santos Jose A.F. Gamelas Tiago A.G.
´
˜
´
Duarte Mario M.Q. Simo es M. Grac¸a P.M.S. Neves Jose A.S.
Cavaleiro Ana M.V. Cavaleiro
PII:
S1381-1169(16)30443-5
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.molcata.2016.10.021
MOLCAA 10083
To appear in:
Journal of Molecular Catalysis A: Chemical
Received date:
Revised date:
Accepted date:
13-6-2016
7-10-2016
18-10-2016
´
Please cite this article as: Isabel C.M.S.Santos, Jose A.F.Gamelas, Tiago
´
˜
´
A.G.Duarte, Mario M.Q.Simo es, M.Grac¸a P.M.S.Neves, Jose A.S.Cavaleiro, Ana
M.V.Cavaleiro, Catalytic homogeneous oxidation of monoterpenes and cyclooctene
with hydrogen peroxide in the presence of sandwich-type tungstophosphates
[M4(H2O)2(PW9O34)2]n−, M=CoII, MnII and FeIII, Journal of Molecular Catalysis
A: Chemical http://dx.doi.org/10.1016/j.molcata.2016.10.021
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Catalytic homogeneous oxidation of monoterpenes and cyclooctene with hydrogen
peroxide in the presence of sandwich-type tungstophosphates
[M4(H2O)2(PW9O34)2]n-, M = CoII, MnII and FeIII
Isabel C. M. S. Santosa,*, José A. F. Gamelasb, Tiago A. G. Duartea, Mário M. Q.
Simõesa,*, M. Graça P. M. S. Nevesa, José A. S. Cavaleiroa, Ana M. V. Cavaleiroc
Dedicated to Professor Georgiy B. Shul’pin on the occasion of his 70th birthday
aChemistry Department & QOPNA, University of Aveiro, 3810-193 Aveiro, Portugal.
bDepartment of Chemical Engineering, CIEPQPF, University of Coimbra, Pólo II. R.
Sílvio Lima, 3030-790 Coimbra, Portugal.
cChemistry Department & CICECO, University of Aveiro, 3810-193 Aveiro, Portugal.
*Corresponding authors:
Mário M. Q. Simões
Tel.: +351 234 370 713
Fax: +351 234 370 084
e-mail address: msimoes@ua.pt
Isabel C. M. S. Santos
Tel.: +351 234 370 200
Fax: +351 234 370 084
e-mail address: ivieira@ua.pt
1

Graphical abstract
Highlights:

Sandwich-type
B--[M4(H2O)2(PW9O34)2]n-
(M = CoII,
MnII,
FeIII)
polyoxometalates



Oxidation of limonene, geraniol, linalool, carveol, and cyclooctene
Oxidation with 30% (w/w) aqueous hydrogen peroxide, in acetonitrile
The main/unique oxidation pathway for terpenoids and cyclooctene is epoxidation
Abstract
Catalytic efficiency of tetrabutylammonium salts of sandwich tungstophosphates
B--[M4(H2O)2(PW9O34)2]n-, M = CoII, MnII, FeIII, was studied in the oxidation of (R)-
(+)-limonene, geraniol, linalool, linalyl acetate, carveol, and cis-cyclooctene with
hydrogen peroxide, in acetonitrile. Oxidation of (R)-(+)-limonene gave limonene-1,2-
diol as main product. Epoxidation of linalool takes place preferentially at the more
substituted 6,7-double bond, the corresponding 6,7-epoxide reacting further, yielding
furano- and pyrano-oxides, via intramolecular cyclization. Oxidation of linalyl acetate
occurred preferentially at the more substituted 6,7-double bond for Mn4(PW9)2,
2

affording 6,7-epoxide at 82% selectivity. Linalyl acetate 1,2-epoxide was the major
product with 51% and 77% selectivity for Co4(PW9)2 and Fe4(PW9)2, respectively.
Oxidation of carveol occurred with very good conversions in the presence of
Mn4(PW9)2, Co4(PW9)2 and Fe4(PW9)2, yielding carvone and carveol 1,2-epoxide in
similar amounts. Oxidation of cis-cyclooctene gave only the epoxide, while oxidation of
geraniol at room temperature afforded 2,3-epoxygeraniol as the major product.
Keywords: sandwich-type polyoxometalates, monoterpenes, cyclooctene,
oxidation
1. Introduction
Sandwich-type
tungstophosphates
having
the
general
formula
B--[M4(H2O)2(PW9O34)2]n-, where M = 1st row transition metal, such as Mn, Fe, Co,
Ni, Cu or Zn, have a molecular structure based on the linkage of two tri-lacunary
Keggin sub-units B--[PW9O34]9- by a belt of four divalent metal ions (Fig. 1) [1-4].
The same structural unit is found for B--[Fe4(H2O)2(PW9O34)2]6- [5] or
B--[Mn4(H2O)2(PW9O34)2]9- and B--[Mn4(OH)2(PW9O34)2]9- [6], with trivalent Fe or
mixed MnIIMnIII belts, respectively. The presence of four 1st row transition metal
centres in the belt of these anions [abbreviated as M4(PW9)2] can be viewed as an
advantage for their use in oxidative catalysis of organic compounds, when compared to
other polyoxometalates (POMs) such as the transition metal monosubstituted Keggin-
type anions. Hence, a number of reports have been published describing the use of some
of these polyoxometalates as catalysts, namely in the oxidation with hydrogen peroxide
of alkenes, alkanes and terpenes [5,7-17]. Examples of studies with other oxidants [18-
19] or substrates [20-22] are also found.
Cyclooctene and cyclohexene are often used as model compounds to evaluate the
possible use of a catalyst in alkene oxidation. Several polyoxometalates have already
been used in the oxidation of cis-cyclooctene with H2O2, under homogeneous
conditions, namely the sandwich [M(PW11O39)2]10− (M = Zr, Hf) [23], manganese
containing POMs [24-26], sandwich-type tungstozincates [WZnM2(ZnW9O34)2]12–,
M = Mn, Zn [27,28] and others [29,30]. A very recent paper describes the oxidation of
these cycloalkenes with H2O2 in acetonitrile in the presence of the M4(PW9)2 anions,
3

with M = MnII, FeIII, CoII, NiII, and ZnII [17]. Previous studies on the oxidation of cis-
cyclooctene and/or cyclohexene by H2O2 in CH3CN used the iron sandwich complexes,
[FeII (H2O)2(PW9O34)2]10- [7] and [FeIII (H2O)2(PW9O34)2]6- [5], as well as the anion
4
4
with M = CuII [11]. Also, [MnII (H2O)2(PW9O34)2]10- and [CoII (H2O)2(PW9O34)2]10-
4
4
were used in oxidations with iodosylbenzene [18] and tert-butylhydroperoxide + O2
[19], respectively.
Limonene, geraniol, linalool and carveol are important natural compounds, which
can be found in the essential oils of several plants [31,32]. These compounds, along
with other monoterpenes, can be used as a source of epoxides, often starting materials
for the synthesis of fragrances, flavours and therapeutically active substances [33,34]. A
few recent reports are available concerning the use of M4(PW9)2 polyoxometalates for
the oxidation of geraniol or limonene with hydrogen peroxide in homogeneous and
heterogeneous conditions, using acetonitrile as solvent [12,14,15]. In these studies, the
heteropolyanions with M = FeIII or CoII were used. The oxidation of limonene with
H2O2/1,2-dichloroethane in a biphasic media was also reported, for anions with
MII = Co, Zn, Cu, Mn [9,13]. Contrastingly, we could not find reports on the use of the
M4(PW9)2 anions in the oxidation of linalool or carveol.
The M4(PW9)2 heteropolyanions have an M/W molar ratio higher than many metal
substituted polyoxotungstates used as catalysts in oxidative catalysis, like the
monosubstituted Keggin or Dawson anions or other sandwich-type polyoxometalates,
such as the tungstozincates [WZnM2(ZnW9O34)2]12-, M = MnII, ZnII, and others
[8,27-29,35,36], what makes them interesting candidates for their application as
catalysts in oxidative reactions. Following our previous experience in the use of the
monosubstituted Keggin anions in oxidative catalysis with H2O2 in acetonitrile
[15,24-26,36-39] and a few reported works with some M4(PW9)2 anions [10-12,14,15],
in the present work are presented the results of the use of the tetrabutylammonium
(TBA) salts of sandwich-type tungstophosphate anions, [M4(H2O)2(PW9O34)2]n-,
M = CoII, MnII (n = 10) and FeIII (n = 6), in the oxidation with hydrogen peroxide of
some monoterpenes [(R)-(+)-limonene, geraniol, linalool, linalyl acetate, carveol] and
cyclooctene.
2. Experimental section
2.1 Catalysts synthesis and reagents
II
II
[(C4H9)4N]7H3[Co4 (H2O)2(PW9O34)2],
[(C4H9)4N]7H3[Mn4 (H2O)2(PW9O34)2]
4

III
and [(C4H9)4N]6[Fe4 (H2O)2(PW9O34)2]·4H2O were synthesised as previously
described [10,12,40]. Analytical and spectroscopic data were in agreement with those
previously published [5,10,40]. All the reagents were of analytical grade and used as
purchased.
2.2 Oxidation reactions with hydrogen peroxide
The reactions were typically carried out in air at 80ºC (except when indicated), in
a stirred solution of the substrate (sub), catalyst (cat) and 30% (w/w) aqueous hydrogen
peroxide (ox) in acetonitrile. The oxidation of geraniol was always carried out in the
dark at room temperature. The amounts of the several components used in the reactions
were dependent on the substrate and are presented in the results and discussion section.
The conversion and percentages of each compound in the reaction mixtures were
determined by gas chromatography. Aliquots were withdrawn from the reaction mixture
and injected directly into the GC-FID or GC-MS apparatus (both equipped with a fused
silica capillary 5% phenyl 95% dimethylpolysiloxane column with 30 m  0.25 mm i.d.
and 0.25 m film thickness) using helium as the carrier gas (35 cm/s). The
chromatographic conditions for geraniol, limonene, linalool, carveol, and linalyl acetate
were: initial temperature: 100ºC; temperature rate: 5ºC/min; final temperature: 220ºC (2
min); injector temperature: 250ºC; detector temperature: 250ºC; for cyclooctene: initial
temperature: 80ºC (2 min); temperature rate: 20ºC/min; final temperature: 200ºC
(2 min); injector temperature: 220ºC; detector temperature: 220ºC.
Blank reactions: a) with H2O2 but without catalyst; b) with catalyst but without
H2O2 were performed for the conditions studied. Conversion values attained in the
blank reactions were in the range of 0 - 7%.
The assessment of the stability of catalysts was performed as follows: a drop of
the reaction mixture was dried on a KBr pellet and the infrared spectrum measured.
These experiments were performed in the oxidation of cyclooctene (ox/cat = 600), and
of geraniol (ox/cat = 1500).
2.3 Instrumentation and methods
Electronic absorption and FTIR spectra, C, H, N elemental analysis and
thermogravimetric analyses were obtained as previously described [26,41]. The reaction
products were identified by GC-MS, based on the mass spectra interpretation and by
comparison with the library of the equipment.
5

3. Results and discussion
The oxidation of the monoterpenes (R)-(+)-limonene, geraniol, linalool, linalyl
acetate, and carveol, and of cyclooctene in the presence of the Mn-, Co- or
Fe-containing sandwich polyoxotungstates was studied with 30% (w/w) aqueous
hydrogen peroxide as oxidant.
3.1. Oxidation of (R)-(+)-limonene
(R)-(+)-Limonene (1) is a monoterpene with a exocyclic and an endocyclic double
bond. The oxidation occurred at these double bonds and at the allylic position of the six
member ring, although with different selectivities, as summarized in Scheme 1 and
Table 1.
The (R)-(+)-limonene oxidation with H2O2 in the presence of sandwich-type
anions was performed at 80ºC using sub/cat = 333 and ox/sub = 2. After 2 h of reaction,
the best conversions were obtained with Fe4(PW9)2 (89%) and Co4(PW9)2 (86%),
followed by Mn4(PW9)2 with 68% conversion. Conversion increased slowly up to 7
hours of reaction, reaching 98% and 94% with Co4(PW9)2 and Fe4(PW9)2, respectively.
Typically, five products were observed (Scheme 1). The reactions seem to indicate that
epoxidation at the cyclohexene double bond is the main pathway, since the selectivity
for limonene 1,2-epoxide (2) and limonene glycol (limonene-1,2-diol, 3) together varies
between 56% and 73%. Moreover, diol 3 was obtained as the main product for all the
catalysts, with selectivity between 50-62%. The limonene diepoxide (4) was the second
most abundant product formed for Fe4(PW9)2 (30% selectivity) and Co4(PW9)2 (20%
selectivity), while for Mn4(PW9)2 the amount of 4 paralleled the amount of limonene
1,2-epoxide (2). The opening of the epoxide ring of 2 is possibly the reason for the
predominance of diol 3. Carveol (5) and carvone (6), resulting from allylic oxidation at
the six member ring, were obtained as minor products. In comparison with results
previously published, obtained in comparable conditions, but with a higher ox/sub
molar ratio [15], it was found that lowering the ox/sub molar ratio to 2 did not affect the
conversion obtained.
3.2. Oxidation of monoterpenoid allylic alcohols (geraniol, linalool, carveol)
6

Geraniol (7) and linalool (9) are isomeric acyclic monoterpenoid allylic alcohols
with two C-C double bonds with different reactivity. Geraniol was oxidized with H2O2
at room temperature, whereas linalool needed higher temperatures. The results obtained
for the catalytic oxidation of these compounds are summarized in Table 2.
The results presented here for the oxidation of geraniol (7) in the presence of the
Co and Fe sandwich-type catalysts are comparable to those already found in the
literature [12,15]; however, to our knowledge, no studies using the Mn4(PW9)2 anion
had been published so far. The Mn, Co, and Fe heteropolyanions, under the conditions
used here, showed analogous performance; moreover, the 97% conversion for the cobalt
heteropolyanion and the 94% selectivity for 2,3-epoxide (8) are very interesting, since
were obtained with less amount of oxidant than those previously published [15]. In fact,
the main product was always 2,3-epoxygeraniol (8), obtained with more than 99% of
selectivity for Fe4(PW9)2 or Mn4(PW9)2 (Table 2). It should be noted that a very small
decrease in 2,3-epoxide (8) selectivity was observed for all the catalysts, as the reaction
proceeds. Actually, after 3 hours of reaction the selectivity for 8 was always 100% [at
44% of conversion for Mn4(PW9)2, 58% for Co4(PW9)2, and 65% for Fe4(PW9)2]
although lowering into 94% selectivity after 5 hours with Co4(PW9)2 only. The selective
formation of 2,3-epoxygeraniol (8) as the major product is commonly observed in
studies with related sandwich-type POMs [11,28,42] or transition metal substituted
Keggin-type anions [12,15,36,38,43].
The epoxidation of linalool (9) takes place preferentially at the more substituted
6,7-double bond, but the corresponding non-allylic 6,7-epoxide (9a) reacts further,
yielding the furano- and pyrano-oxides (10) and (11) (Schemes 3 and 4), in a relative
molar ratio furano/pyrano ring of 1.8-2.5 after 5 h of reaction, via an intramolecular
cyclization [44,45].
According to Hutchings [45], the furano-oxide is the kinetically favoured product.
Moreover, the formation of 5-membered heterocycles is known to be preferred over 6-
membered heterocycles, although the 6-membered ring has the lower strain energy [45].
No preferential allylic-hydroxyl assisted epoxidation of linalool (9) seems to occur
under the present reaction conditions. This may be due to the steric situation around the
hydroxyl group, hindering the formation of (9b). Moreover, linalool diepoxide (12) was
7

formed in minor amounts, probably by over-epoxidation of (9a). Reactions performed at
80ºC lead to higher conversions [91%, 81%, and 60% for Fe4(PW9)2, Mn4(PW9)2, and
Co4(PW9)2, respectively] than those at 60ºC (43%, 48%, and 23% for Fe4(PW9)2,
Mn4(PW9)2, and Co4(PW9)2, respectively). So far, to our knowledge, no studies on the
oxidation of linalool with H2O2, using POMs as catalysts, were published. Yet, there are
reports on the oxidation of linalool catalysed by peroxotungstophosphates [44,46] or a
peroxotungstoborate [46].
The oxidation of linalyl acetate (13) occurred preferentially at the more
substituted 6,7-double bond only for Mn4(PW9)2, thus affording the 6,7-epoxide (14) at
82% selectivity. The other products were 1,2-epoxide (15) and aldehyde (16) resulting
from the cleavage of the 6,7-double bond. Interestingly, the selectivity changed
enormously when Co4(PW9)2 and Fe4(PW9)2 were used instead. Actually, 15 was the
major product with 51% and 77% selectivity with Co4(PW9)2 and Fe4(PW9)2,
respectively. Aldehyde (16) was formed as the second most abundant product for
Fe4(PW9)2, since only 1% of 14 was registered. Since the rearrangement of the 6,7-
epoxide (14) cannot occur here, due to the protection of the hydroxyl group of linalool
by the acetate group, maybe 6,7-epoxide (14) is an intermediate for the formation of 16,
as observed earlier for other olefins [39,47].
The oxidation of carveol (5) with hydrogen peroxide occurred with excellent
conversions in the presence of the Mn, Co, and Fe heteropolyanions (Table 2), yielding
carvone (6) and 1,2-epoxide (17) in similar amounts after 3 hours of reaction at 80ºC.
The 8,9-epoxide (18) was always a minor product. Carveol (5) and carvone (6) were
observed as minor products in the allylic oxidation of limonene (1) (Table 1), the ketone
6 probably resulting from the oxidation of 5, whereas epoxides 17 and 18, resulting
from the epoxidation of 5, were not detected in the oxidation of limonene (1).
Moreover, the epoxide ring opening of 17 was not observed (contrarily to what happens
with limonene, where 2 gives rise to large amounts of 3, consequently the putative triol
was not formed under the present conditions. The less substituted C8-C9 double bond
seems to be less reactive for epoxidation than the endocyclic C1-C2 double bond, and
the presence of the hydroxyl group in the allylic position seems to favour the
epoxidation at C1-C2, as happened for geraniol.
8

3.3. Oxidation of cis-cyclooctene
Since limonene (1) gave rise to somewhat high percentages of allylic oxidation
resulting products, 5 and 6, and in order to assess the selectivity of the present system
for epoxidation, relatively to allylic oxidation, the oxidation of cis-cyclooctene with
hydrogen peroxide was also studied in the presence of Mn4(PW9)2, Co4(PW9)2, and
Fe4(PW9)2. 1,2-Epoxycyclooctane was always obtained with 100% selectivity, so the
allylic oxidation did not occur under the present conditions. The conversion profile
obtained for cis-cyclooctene oxidation at 80ºC is illustrated in Figure 2.
In a recent study, Farsani et al. described the oxidation of cis-cyclooctene with
H2O2 in acetonitrile in the presence of similar Mn, Fe and Co sandwich
tungstophosphates [17]. Nevertheless, their tetrabutylammonium salts of the Co and Mn
sandwich heteropolyanions had different cation/anion ratios and, thus, different
chemical formula. Additionally, the reaction conditions involved higher catalyst amount
(lower sub/cat ratios) and lower substrate concentration [17]. The results presented here
correspond to higher conversion and selectivity than those already published [17],
independently of the used temperature, for the cobalt and manganese heteropolyanions,
and at similar temperature in the case of the iron heteropolyanion.
3.4. Catalysts stability
Previous reports on the stability of these sandwich-type heteropolyanions in the
presence of H2O2, under catalysis conditions, showed that Fe4(PW9)2 is stable under
conditions of deficiency of H2O2, at room temperature [5]. However, for the reactions of
cyclooctane oxidation performed at 80 ºC, with ox/cat molar ratio above 1330, all the
three catalysts, Co4(PW9)2, Mn4(PW9)2, and Fe4(PW9)2, were found to decompose into
the monosubstituted Keggin heteropolyanions (XW11M), being the degradation of the
manganese sandwich heteropolyanion more extensive than that of the analogous cobalt
anion [10]. A different situation was observed here, where catalysts’ recovery was
carried out for the reactions of cis-cyclooctene performed at 80ºC with ox/cat = 600 and
for those of geraniol oxidation performed at room temperature with ox/cat = 1500. The
infrared spectra of the solids recovered at the end of the reactions, after evaporation of
the solvent, suggested that the catalysts are stable under the reaction conditions studied,
as exemplified for Mn4(PW9)2 in the oxidation of cis-cyclooctene (Figure 3).
9

Oxidations of organic substrates with hydrogen peroxide in the presence of metal-
substituted Keggin-type polyoxometalates may occur by different pathways [37,42,48].
Studies performed in our laboratory with metal substituted Keggin anions in conditions
similar to those used in this work have shown that the reactions with H2O2 could occur
via radical or non-radical processes, possibly depending on the reactivity of the
substrates and reaction conditions. Overall, the oxidation of cycloalkanes, alkylbenzenes
and similar compounds catalysed by XW11M anions occurs probably through a radical
process [10,37,49-54], being completely inhibited in the presence of a radical
scavenger, whereas the oxidation of cis-cyclooctene [26,55], styrene [39], anthracene
[37] and geraniol [12] do not seem to involve radicals. Also, more than one route may
occur simultaneously, as found for the oxidation of cyclohexene with H2O2 in the
presence of [PMo11MnII(H2O)O39]5- [26]. This reaction yielded cyclohexene-1,2-diol
(resulting from the epoxide ring opening) as the major product together with some
products of allylic oxidation. The latter could be eliminated using a radical scavenger,
but this did not inhibit completely the reaction, and the former was always the main
product [26].
Observing the oxidation products obtained for the diverse substrates, it can be
proposed that the reactions described in 3.2 and 3.3 are possibly of a non-radical nature,
due to the predominance of the formation of epoxides and/or their derivatives and to the
absence of allylic oxidation. The occurrence of allylic oxidation observed for limonene
is possibly an indicative of a radical process, which, as mentioned above for
cyclohexene and referred previously by Casuscelli et al [56], may not be the only
mechanism.
The activation of H2O2 in the non-radical processes catalysed by the sandwich-
type M4(PW9)2 occurs possibly through the formation of a peroxo or hydroperoxo
complex centred at the metal M, which reacts with the double bonds, originating an
epoxide through the heterolytic oxygen transfer from the oxidant to the substrate, as
commonly proposed in the literature [24,42,48]. It should be noted that only two of the
four metal atoms M of the anions belt (those initially bound to water, see Fig. 1) are
possibly available for coordination with hydrogen peroxide. As mentioned for other
cases [43,57], the activation of the H2O2 may occur both at the tungsten and at the
substituting-metal, but this does not alter significantly the possible mechanism. The
preferential epoxidation of geraniol at the C2–C3 position has been explained by the
formation of a complex involving the polyoxometalate, the oxidant and the substrate,
10

which may coordinate to the metal center or interact with the polyoxometalate through
the OH group [24,42,43].
Radical species resulting from the homolytic breakage of the O–O bond of
hydrogen peroxide are expected to occur in the presence of transition metals with
consecutive oxidation states, like MnII/III, CoII/III, FeII/III [58,59]. Fenton and Fenton-like
mechanisms may occur through the formation of HO• radicals, as proposed for the
reactions of cyclooctane with H2O2 catalyzed by iron or manganese polyotungstates
[52], but other species have been proposed in other studies, like, for example, a POM-
TiIV–O• species [48], and it is not possible to advance a suggestion.
4. Conclusions
Sandwich-type tungstophosphates of general formula B--[M4(H2O)2(PW9O34)2]n-
(M = CoII, MnII, FeIII) were evaluated as catalysts for the oxidation of (R)-(+)-limonene,
geraniol, linalool, linalyl acetate, carveol, and cis-cyclooctene with 30% aqueous H2O2,
in acetonitrile. The oxidation of (R)-(+)-limonene gave limonene-1,2-diol as the
dominant product. Thus, for limonene the main pathway was the epoxidation of the
endocyclic double bond, followed by the epoxide ring opening affording limonene-1,2-
diol. The epoxidation of linalool takes place preferentially at the more substituted 6,7-
double bond, but the corresponding 6,7-epoxide reacts further, yielding the furano- and
pyrano-oxides, in a relative molar ratio furano/pyrano ring of 1.8-2.5 after 5 h of
reaction, via an intramolecular cyclization. The oxidation of linalyl acetate occurred
preferentially at the more substituted 6,7-double bond only for Mn4(PW9)2, thus
affording the 6,7-epoxide at 82% selectivity. Interestingly, the selectivity changed
enormously when Co4(PW9)2 and Fe4(PW9)2 were used instead. Actually, 1,2-epoxide
was the major product with 51% and 77% selectivity with Co4(PW9)2 and Fe4(PW9)2,
respectively. The oxidation of carveol occurred with very good conversions in the
presence of Mn4(PW9)2, Co4(PW9)2 and Fe4(PW9)2, yielding carvone and carveol 1,2-
epoxide at similar amounts. Carveol 8,9-epoxide was always a minor product. The
oxidation of cis-cyclooctene gave only the epoxide, thus without allylic oxidation, while
the oxidation of geraniol at room temperature afforded the 2,3-epoxygeraniol as the
major product.
11

Acknowledgments
Thanks are due to FCT/MEC for the financial support to QOPNA (FCT
UID/QUI/00062/2013) and CICECO (FCT UID/CTM/50011/2013), through national
founds and where applicable co-financed by the FEDER, within the PT2020 Partnership
Agreement.
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15

(A)
(B)
Fig. 1 (A) Structure of the complex B--[M4(H2O)2(PW9O34)2]n-. Each dashed octahedron represents a
MO6 group and the open circles correspond to oxygen atoms of co-ordinating water molecules. (B) Belt
of metal ions (grey) bound to oxygen atoms of the [PW9O34]9- ligands (red). Coordinated water not
shown.
100
80
60
40
Co4(PW9)2
Mn4(PW9)2
20
Fe4(PW9)2
0
0
1
2
3
4
5
6
Time (h)
Fig. 2. Cyclooctene (3 mmol) oxidation catalysed by sandwich-type POMs (5 mol), with 1.5 mL of
CH3CN at 80ºC and in the presence of H2O2 30% (3 mmol).
16

initial Mn4[PW9]2
Mn4[PW9]2 after reaction
1600
1400
1200
1000
800
600
400
Wavelength(cm-1)
Fig. 3. The infrared spectra of the initial Mn4(PW9)2 catalyst and the recovered solid after catalysis.
Scheme 1. Oxidation of limonene by hydrogen peroxide catalysed by M4(PW9)2 heteropolyanions
O
CH2OH
CH2OH
(7)
Scheme 2. Oxidation of geraniol by hydrogen peroxide catalysed by M4(PW9)2 heteropolyanions
(8)
Scheme 3. Oxidation of linalool by hydrogen peroxide catalysed by M4(PW9)2 heteropolyanions
17

Scheme 4. Linalool 6,7-epoxide (9a) intramolecular cyclization into five and six member heterocyclic
rings
Scheme 5. Oxidation of linalyl acetate by hydrogen peroxide catalysed by M4(PW9)2 heteropolyanions
Scheme 6. Oxidation of carveol by hydrogen peroxide catalysed by M4(PW9)2 heteropolyanions
18

Table 1. Oxidation of (R)-(+)-limonene with hydrogen peroxide catalysed by sandwich-type
tungstophosphates(a).
Catalyst
Time (h)
Conversion (%) TON(b)
Selectivity(c) (%)
(2)
23
5
(3)
50
62
56
53
62
54
(4)
20
20
30
19
20
26
(5)
3
(6)
4
Mn4(PW9)2
Co4(PW9)2
Fe4(PW9)2
Mn4(PW9)2
Co4(PW9)2
Fe4(PW9)2
2
2
2
7
7
7
68
86
89
80
98
94
227
287
297
267
327
313
4
9
0
5
9
21
5
2
5
4
9
3
4
13
(a) Reaction conditions: sub = 1 mmol; cat = 3 mol; CH3CN = 1.5 mL; 2 mmol of H2O2 30%; 80ºC; (b)
mol of converted substrate per mol of catalyst; (c) product numbering indicated on Scheme 1.
19

Table 2. Oxidation of geraniol, linalool, linalyl acetate, and carveol with hydrogen peroxide catalysed by
sandwich-type tungstophosphates(a).
Catalyst
Substrate
Temp
(ºC)
Time
(h)
Conversion
(%)
TON
Selectivity(c)
(%)
(8)
Mn4(PW9)2
Co4(PW9)2
Fe4(PW9)2
geraniol
geraniol
geraniol
RT(b)
RT(b)
RT(b)
5
5
5
90
97
87
300
323
290
>99
94
>99
(11)
30
(10)
64
65
70
62
66
61
(14)
82
39
1
(12)
6
Mn4(PW9)2
Co4(PW9)2
Fe4(PW9)2
Mn4(PW9)2
Co4(PW9)2
Fe4(PW9)2
linalool
linalool
linalool
linalool
linalool
linalool
60
60
60
80
80
80
5
5
5
5
5
5
48
23
43
81
60
91
160
77
30
5
143
270
200
300
28
2
36
2
26
8
36
3
(15)
15
(16)
3
Mn4(PW9)2
Co4(PW9)2
Fe4(PW9)2
linalyl acetate
linalyl acetate
linalyl acetate
80
80
80
6
6
6
40
44
48
133
147
160
51
10
22
(18)
6
77
(6)
45
49
51
(17)
45
Mn4(PW9)2
Co4(PW9)2
Fe4(PW9)2
carveol
carveol
carveol
80
80
80
3
3
3
99
98
99
330
327
330
44
2
46
2
(a) Reaction conditions: sub = 1 mmol; cat = 3 mol; CH3CN = 1.5 mL; 3 mmol of H2O2 30%; (b) RT =
room temperature, protected from light; (c) product numbering indicated on Schemes 2-6.
20