Baeckvall J.-E. (ed.) Modern Oxidation Methods

Подождите немного. Документ загружается.

polyoxomolybdates, [PV

12x

]

(3 þ x)

(x ¼ 0, 2), with DMSO as solvent.

The oxidation of benzylic alcohols was quantitative within hours and selective to

the corresponding benzaldehydes, but the oxidation of allylic alcohols was less

selective. The oxidation of aliphatic alcohols w as slower but selective. In mecha-

nistic studies considering oxidation of benzylic alcohols, similar to the oxidation of

alkylarenes, a polyoxometalate–sulfoxide complex appears to be the active oxidant.

Further isotope-labeling experiments, kinetic isotope eff ects, a nd especia lly

Hammett plots showed that oxidation occurs by oxygen transfer from the activated

sulfoxide and elimination of water from the alcohol. Howe ver, the exact nature

of the reaction pathway is dependent on the identity of substituents on the

phenyl ring.

Summarizing the information disclosed in the section above, one notes that

polyoxometalates appear to be versatile oxidation catalysts capable of activating

various mono-oxygen donors such as iodosobenzene, periodate, ozone, nitrous

oxide, and sulfoxides. Some of these reactions are completely new from both

a synthetic and mechanistic point of view. The various reaction pathways expressed

are also rather unusual and point to the many options and reaction pathways available

for oxidation catalyzed by polyoxometalates.

9.4

Oxidation with Peroxygen Compounds

Before speciﬁcally discussing oxidation by peroxygen compounds using polyoxo-

metalates as catalysts, a few general comments concerning peroxygen compounds as

oxidants or oxygen donors are worth making. First, from a practical point of view,

hydrogen peroxide is certainly the most sustainable oxidant of this class, since it has

a high percentage (47%) of active oxygen, it is inexpensive, and the by-product of

oxidation is water. On the down side, its use as an aqueous solution presents

problems of compatibility and reactivity with hydrophobic organic substrates or

solvents, and some precautions must be taken such as working under reasonably low

concentrations (usually <20 wt% in polar organic solvent) to prevent safety hazards.

Various solid forms of hydrogen peroxide such as urea hydroperoxide, sodium

perborate, and sodium percarbonate, are also available. Alkyl hydroperoxides, notably

tert-butylhydroperoxide, have the advantage that they are freely soluble in organic

media and can thus be used in strictly nonaqueous solvents. The alcohol by-product

resulting from the use of alkyl hydroperoxides as oxygen donors can often be easily

recovered, for example, by distillation, and at least in principle the alkyl hydroper-

oxide can be re-synthesized from the alcohol. There are also other peroxygen oxidants

readily available; one notable inorganic compound is monoperoxosulfate, HSO



normally available as a triple salt, OxoneÔ.

From a mechanistic point of view it is important to realize that polyoxometalates

may interact with peroxygen oxidants in several different ways depending on the

composition, structure, and redox potential of the polyoxometalate compounds. On

the one hand, one may expect reaction pathways typical for any oxotungstate or

9.4 Oxidation with Peroxygen Compounds

323

oxomolybdate compounds with formation of peroxo or hydroperoxy (alkylperoxy)

intermediates capable of oxygen transfer reactions with nucleophilic substrates such

as alkenes to yield epoxides. On the other hand, depending on the redox potential

of the polyoxometalate, a varying degree of homolytic cleavage of oxygen-oxygen and

hydrogen-oxygen bonds will lead to hydroxy (alkoxy) and peroxy (peralkoxy) inter-

mediate radical species. The trend of increased formation of radical species as

a function of increasing oxidation potential is clearly evident in the series of Keggin

type heteropoly acids: H

> H

PMo

> H

. In particular,

hydroxy or alkoxy radicals will lead to further hydrogen abstraction from the substrate

molecules and formation of additional radical species. The rate of formation and fate

of these latter radical species will determine the conversion, selectivity, and identity of

the products formed in the reaction. This tendency for homolytic cleavage in the

peroxygen compounds can also be expected to be strongly inﬂuenced by the presence

of substituting transition metals in the polyoxometalate structure. A high oxidation

potential of the polyoxometalate and/or presence of redox-active transition metals

will also lead to dismutation reactions and thus nonproductive decomposition of the

peroxygen oxidant and low yields based on the oxidant. There is also a more remote

possibility that intermediate hydroperoxide species of a transition metal substituted

in polyoxometalate structure, for example a Fe(III)-OOH intermediate, will lead to an

Fe(V) ¼ O species or equivalent. To date, there has been no observation of such

a biomimetic transformation in polyoxometalate catalytic chemistry, although we

have recently isolated such an Fe(III)-OOH intermediate that, however, acts as

a reducing agent (benzoquinone to hydroquinone) and does yield higher-valent oxo

species [30].

Originally, tert-butylhydroperoxide was used together with transition metal-substi-

tuted Keggin type compounds and then later on more effectively with transition metal

substituted sandwich compounds for the oxidation of alkanes to alcohols and

ketones [31]. The oxidation of alkenes went with low selectivity. Although the

mechanism was not rigorously studied, it would seem quite certain that these

reactions proceed by a radical mechanism via hydrogen abstraction by alkoxy radicals

from the substrate. Oxone has been similarly used for the oxidation of benzylic and

aliphatic alcohols [32]. Interestingly, it has been observed that oxidation of alkanes

with tert-butylhydroperoxide catalyzed by a polyoxomolybdate, H

PMo

, may be

redirected from oxygenation to oxydehydrogenation yielding alkenes as the major

products [33]. Thus, both acyclic and cyclic alkanes were oxidized to alkenes by tert-

butylhydroperoxide in acetic acid with H

PMo

as catalyst with reaction selec-

tivity generally 90%. Some minor amounts of alcohols, ketones, and hydroperoxide

products formed via oxygenation with molecular oxygen were also obtained, as were

some acetate esters. The alkene product selectively tended toward the kinetically

favored product rather than the thermodynamically more stable alkenes. Therefore,

oxidation of 1-methylcyclohexane yielded mostly 3- and 4-methylcyclohexene rather

than 1-methylcyclohexene. Similarly, in the oxidation of 2,2,4-trimethylpentane, the

terminal alkene, 2,2,4-trimethyl-4-pentene, was formed in fourfold excess relative

to the internal alkene, 2,2,4-trimethyl-3-pentene. A reaction scheme to explain

the reaction selectivity is presented in Scheme 9.5.

324

9 Liquid Phase Oxidation Reactions Catalyzed by Polyoxometalates

Tert-butylhydroperoxide reacts with the H

PMo

catalyst to yield alkoxy and

alkylperoxy radicals (reactions a and b). The alkoxy radical, which can be trapped by

spin traps and observed by EPR, homolytically abstracts hydrogen from a reactive

carbon-hydrogen moiety (reaction c). Instead of the usual diffusion rate-controlled

oxygenation with molecular oxygen (reaction d), oxidative electron transfer occurs

yielding a carbocation that in turn is dehydrogenated to yield an alkene or is attacked

by acetic acid to give the acetate ester as by-product (reaction e). Tert-butylhydroper-

oxide has also been used for the highly selective oxidation of thioethers, for example,

tetrahydrothiophene, to the corresponding sulfoxides without further oxidation to

sulfones using H

as the catalyst [34]. Although one may automatically

assume that such an oxidation would take place by an oxygen transfer reaction from

a polyoxometalate-alkylperoxy intermediate to the sulﬁde, the evidence presented

indicates that in fact oxidation occurs via electron transfer from the thioether to the

polyoxometalate, where the role of the tert-butylhydroperoxide is to re-oxidize the

reduced polyoxometalate. This type of mechanism is in line with what is known about

oxidation catalyzed by H

with oxygen as terminal oxidant, as discussed

in Section 9.5 below (see Scheme 9.9).

Enantioselective oxidation catalysis to yield chiral products from prochiral

substrates had not until recently been observed using polyoxometalate catalysts.

However, in a combined effort of several research groups it has been shown that the

racemic vanadium-substituted sandwich type polyoxometalate, [(V

ZnW

(ZnW

)

]

12

, is an extremely effective catalyst (up to 40 000 turnovers) at near

ambient temperatures, for the enantioselective epoxidation of allylic alcohols to the

2R,3R-epoxyalcohol with the sterically crowded chiral hydroperoxide, TADOOH, as

oxygen donor [35], Scheme 9.6. The enantiomeric excesses, ee, attained using aryl-

substituted allylic alcohols was quite high, generally 70–90%, and at high conversions

>95%. However, less sterically hindered allylic alcohols such as geraniol gave a low

enantiomeric excess (20%) of chiral 2R,3R-epoxygeraniol. The chiral induction

observed in the reaction is thought to be due to the presence of a vanadium template

for the chiral hydroperoxide and the allylic alcohol. Thus, nonfunctionalized alkenes,

for example, 1-phenylcyclohexene, showed essentially negligible enantioselectivity.

Also, substitution of vanadium with other transition metals yielded signiﬁcantly

lower enantioselectivity and low conversion of allylic alcohols.

+ R-OOH

+ H + R-OO

Mo + R-OOH

+ OH + R-O

–

R-O

+ Mo

AcOH

OAc

+ H

Mo = H

PMo

(a)

(b)

(c)

+ H

(e)

Oxygenated

products

(d)

Scheme 9.5 Oxidation of alkanes with tert-butylhydroperoxide catalyzed by H

PMo

9.4 Oxidation with Peroxygen Compounds

325

[(VO)

ZnW(ZnW

)

]

12–

OOH

PhPh

TADDOLTADOOH

%ee

4-MeOPh

Scheme 9.6 Enantioselective epoxidation of allylic alcohols with a chiral hydroperoxide, TADOOH,

catalyzed by [(V

ZnW(ZnW

)

]

12

As noted above, the oxotungstate or oxomolybdate nature of polyoxometalate

compounds boded well for their activation of hydrogen peroxide. Thus, Ishii and his

coworkers described the ﬁrst use of polyoxometalates with 30–35% aqueous hydrogen

peroxide as the oxidant. They used the commercially available phosphotungstic acid,

, as catalyst. In order to utilize a biphasic reaction medium (organic

substrate/aqueous oxidant) they added a quaternary ammonium salt, hexadecylpyr-

idinium bromide, to dissolve the [PW

]

3

in the organic apolar solvent reaction

phase. Ishiis group and also others gave numerous examples of oxidation reactions

typical for the use of reactions with hydrogen peroxide in the presence of tungsten-

based catalysts. The ﬁrst examples dealt with the epoxidation of allylic alcohols [36] and

alkenes [37]. Generally, high epoxide yields, >90%, were obtained with only a relatively

smallexcess of hydrogenperoxide. An evaluationof the catalyticactivityfor epoxidation

reveals turnover frequencies of 5–15 h

1

per tungsten atom. Under more acidic

conditions and at higher temperatures the epoxides are sensitive to hydrolysis leading

to formation of vicinal diols, which are subsequently oxidized to keto-alcohols,

a,b-diketones [38] or, at longer reaction times, undergo oxidative carbon-carbon bond

cleavage to yield carboxylic acids and ketones. The phosphotungstate polyoxometalate

was also effective for oxidationof secondary alcoholsto ketones, whileprimary alcohols

were not reactiveallowingfor the high-yield regioselectiveoxidationof nonvicinal diols

to the corresponding keto-alcohols; a,v-diols did, however, react to give lactones (e.g.,

c-butyrolactone from 1,4-butanediol) in high yields [39]. Other research showed that

alkynes [40], amines [41], and sulﬁdes [37], could be oxidized efﬁciently to ketones,

N-oxides, and sulfoxides and sulfones, respectively. Various quinones were also

synthesized from active arene precursors [42].

While the synthetic applications involving oxidation of the various substrate types

was being investigated mostly by Ishiis group [36–42] using [PW

]

3

as catalyst,

other researchers have actively pursued studies aiming at an understanding of the

identity of the true catalyst in these reactions [44–47]. In this context it should be

noted that the isostructural compound [SiW

]

4

showed almost no catalytic

activity compared to [PW

]

3

under identical conditions [36]. At practically the

same time, Csanyi and Jaky [43], and the groups of Br



egault [44], Grifﬁth [45], and

Hill [46] suggested, and convincingly proved, using various spectroscopic and

kinetic probes, that the [PW

]

3

and even more so the lacunary [PW

]

7

polyoxometalate formed at pH 3–4 was unstable in the presence of aqueous

326

9 Liquid Phase Oxidation Reactions Catalyzed by Polyoxometalates

hydrogen peroxide, leading mainly to the formation of the peroxophosphotungstate,

{PO

[WO(O

)

]}

3

. This compound had been previously synthesized and charac-

terized by Venturello et al. and had been shown to have very similar catalytic activity in

various oxidation reactions with hydrogen peroxide [47]. A general conclusion

resulting from these studies of the groups of Ishii, Venturello, Csanyi and Jaky,



egeault, Grifﬁth, and Hill is that the {PO

[WO(O

)

]}

3

peroxophosphotungstate

compound is one of the best catalysts, especially from the point of view of synthetic

versatility of all of the many peroxotungstates that have been studied. A more

extensive review of the complex phosphotungstate solution chemistry in the presence

of hydrogen peroxide is beyond the scope of this present chapter. In more recent

years, it has been shown by Xi and coworkers that by the careful choice of the

quaternary ammonium counter cation and reaction solvent a possibly technologically

practical process for the epoxidation of propene to propene oxide could be envisioned

using {PO

[WO(O

)

]}

3

as catalyst [48]. For example, in the presence of hydrogen

peroxide using a combination of toluene and tributylphosphate as solvent, a soluble

{PO

[WO(O

)

]}

3

compound was obtained. Once the hydrogen peroxide is used up,

a {PO

[WO

]}

3

compound is formed that is insoluble in the reaction medium,

allowing simple recovery for recycling of the phosphotungstate species. Importantly,

it was claimed that the system could be coupled with the synthesis of hydrogen

peroxide from hydrogen and oxygen using the classic ethylanthraquinone process for

hydrogen peroxide preparation.

The hydrolytic instability of the simple and lacunary Keggin-type polyoxometa-

lates, [PW

]

3

and [PW

]

7

, in the presence of aqueous hydrogen peroxide,

leading to formation in solution of various peroxotungstate species of varying

catalytic activity, led to two intertwined issues. The ﬁrst issue that came up was the

necessity to carefully analyze the stability of polyoxometalates under hydrogen

peroxide/hydrolytic conditions. For example, it had been claimed that lanthanide-

containing polyoxometalates, [LnW

]

9

, were active catalysts for alcohol oxida-

tion [49]; however, subsequent research showed that they in fact decomposed to

smaller and known peroxotungstate species that were the catalytically active

species [50]. On the other hand, other Keggin compounds appeared to be stable in

the presence of aqueous hydrogen peroxide. For example, a stable peroxo species

based on the Keggin structure, [SiW

(NbO

)

]

7

, was synthesized, characterized,

and used for epoxidation of reactive allylic alcohols but not alkenes [51]. The

[PZnMo

]

5

polyoxometalate was used to oxidize sulﬁdes to sulfoxides [52].

The Q

[PV

](Q¼ quaternary ammonium cation) in aqueous hydrogen

peroxide/acetic acid was stable and catalyzed the oxidation of alkylaromatic substrates

in the benzylic position [53], while Q

[PV

] used for the oxidation of benzene

to phenol also remained intact during the reaction [54]. Likewise, titanium-substi-

tuted Keggin-type phosphotungstates are apparently stable in the presence of

hydrogen peroxide [55]. Kholdeeva and her coworkers have viewed such titanium-

substituted compounds as models for hydrogen peroxide-based oxidation on titani-

um centers [56]. The research kinetically followed the formation of the titanium

peroxo species. 2,3,6-Trimethyl phenol is oxidized to yield the oxygenated product,

2,3,5-trimethylbenzoquinone and the oxidatively coupled product, 2,2

,3,3

,5,5

hexmethyl-4,4

-biphenol, presumably obtained by electron transfer oxidation. The

9.4 Oxidation with Peroxygen Compounds

327

presence of a second proton on the peroxo moiety is crucial for determining the ability

of the titanium Keggin polyoxometalate to oxidize an alkene such as cyclohexene by a

heterolytic oxygen transfer, although, because of the acidic condition, the 1,2-trans-

cyclohexanediol is the major product obtained. More recently, another group has

carried out similar research [57]. It would also appear that various iron-substituted

Keggin compounds reported by Mizuno and coworkers for alkene and alkane

oxidation are also stable in the presence of hydrogen peroxide, although the study

was not completely deﬁnitive [58]. From these examples and others not noted, it is

clear that certain Keggin-type polyoxometalates can be stable under certain reaction

conditions. Parameters to consider in this context are pH and the relative stability of

the speciﬁc polyoxometalate at such a pH, also the solvent and temperature.

In recent years, a c-[SiW

]

4

polyoxometalate with a defect site,

originally prepared by Mizuno and coworkers, has been shown to have similar

activity (i.e. highly effective epoxidation of primary alkenes) to {PO

[WO(O

)

]}

3

(normalized per tungsten atom) [59]. The catalyst was formulated to have two aqua

and two oxo ligands at the active site. The fact that there was a signiﬁcant induction

period prior to epoxidation using c-[SiW

]

4

with H

and the disap-

pearance of this induction period by pretreatment with the oxidant indicated that

c-[SiW

]

4

was in fact the catalyst precursor. Hammett correlations

indicate the formation of a very electrophilic oxidant, and the trans selectivity

observed in the epoxidation of 3-methyl-1-cyclohexene was explained by strong steric

control at the active site. NMR and MS spectroscopic measurements showed that

addition of H

to c-[SiW

]

4

yielded an inactive compound that only

later formed the active species.

Others have been also been intrigued by this research. In a computational study,

it was concluded that the catalyst precursor is better formulated as having four

hydroxy terminal ligands rather than two aqua and two oxo ligands [60].

Further computational studies by the same team suggested that the reactivity of

Figure 9.4 Ortep representation of the c-[SiW

]

4

polyoxometalate.

328

9 Liquid Phase Oxidation Reactions Catalyzed by Polyoxometalates

c-[SiW

]

4

, formulated by them as c-[SiW

]

4

, could be explained

by an active WOOH end-on hydroperoxo species rather than a side-on s ¼ active

common usually invoked in tungsten-based catalysts for the activation of H

[61].

Others based on titration/

183

W NMR experiments together with DFT calculations

also demonstrate the importance of acidity for reactivity but conclude that the active

precursor should be deﬁned as originally presented by Mizuno and coworkers [62]. It

should ﬁnally be noted that it has been reported that substitution of phenyl

phosphonate moieties at the defect site of c-[SiW

]

4

leads to a catalyst

of similar reactivity but greater stability [63].

There have been some other notable developments related to the oxidative catalytic

behavior of c-[SiW

]

4

-based catalysts. One observation has been that

partial protonation of c-[SiW

]

4

in an organic solvent leads to dehydra-

tion of c-[SiW

]

4

and formation of an S-shaped disilicoicostungstate that

showed good activity for the Baeyer-Villeger oxidation of cycloalkanes to lactones,

although it appears quite unclear what the actual catalyst is although it appears to

be quite acidic in nature [64]. Substitution of vanadium into the defect sites of

c-[SiW

]

4

to yield a c-[H

SiV

]

4

compound has also led to

interesting results [65]. This compound also led to a highly electrophilic oxidant

and in addition catalyzed chemoselective epoxidation with H

. Thus, cis-2-octene

was 32 times more reactive than its geometric isomer, trans-2-octene. Terminal

alkenes were much more reactive than the substituted ones, for example, 1,4-

hexadiene yielded mostly 1,2-epoxy-4-hexene. Furthermore, high diastereoselectivity

was also observed; epoxidation of 3-methyl-1-cyclohexene and 2-cyclohexen-1-ol led

to the preferred formation of the trans oxirane, showing selective anti addition to

the double bond. The chemoselectivity and diasteroselectivity were attributed

to steric constraints at the active site, and reactivity is explained via formation of

a hydroperoxo and perhaps a m-g

-g

peroxo moiety. Similar substitution of titanium

into c-[SiW

]

4

yielded a m-oxo-bridged dimeric [{c-H

SiTi

}

(m-O)

]

8

compound that showed similar but much diminished reactivity to that

observed with c-[H

SiV

]

4

[66].

Another issue is whether there are polyoxometalate structures that are intrinsically

stable toward the hydrolytic conditions of aqueous hydrogen peroxide. We observed

that, in general, larger polyoxometalates, speciﬁcally polyoxotungstates of various

sandwich-type structures were solvolytically stable toward hydrogen peroxide.

Unfortunately, often the substituting transition metal catalyzes the fast decompo-

sition of hydrogen peroxide leading to low reaction yields and nonselective reactions

of little synthetic value. However, there is now a considerable body of research into

several types of transition metal-substituted polyoxometalates that are synthetically

useful. Various iron-containing polyoxometalates of sandwich-type structures

have been investigated by the Hill group and found to have good activity for

alkene oxidation with only moderate nonproductive decomposition of hydrogen

peroxide [67]. A relatively new class of transition metal-substituted compounds,

polyﬂuorooxometalates, [TM(H

O)H

]

q

which have a quasi Wells-

Dawson structure (see Figure 9.3c) and where there is partial replacement of oxygen

by ﬂuorine proved to be very active and stable oxidation catalysts, which can be

monitored by

F NMR, for epoxidation of alkenes and allylic alcohols with hydrogen

9.4 Oxidation with Peroxygen Compounds

329

peroxide [68]. The nickel-substituted compound was the most active of the series

studied. Previously, our group observed that a far more catalytically active class of

compounds that were also stable in oxidation reactions using aqueous hydrogen

peroxides were the {[(WZnTM

][(ZnW

)

]}

q

sandwich-type polyoxome-

talates. Originally we observed that, among this class of compounds, the manganese

and analogous rhodium derivatives dissolved in the organic phase were uniquely

active when reactions were carried out in biphasic systems, preferably 1,2-dichloro-

ethane–water [69]. At lower temperatures, highly selective epoxidation could be

carried out even with cyclohexene, which is normally highly susceptible to allylic

oxidation. Nonproductive decomposition of hydrogen peroxide at low temperatures

was minimal but increased with temperature and was also dependent on the reactivity

of the substrate. The rhodium compound was preferable in terms of minimization of

hydrogen peroxide decomposition, but of course it is more expensive. Up to tens of

thousands of turnovers could be attained for reactive hydrocarbon substrates [70]. The

synthetic utility of the {[(WZnMn(II)

][(ZnW

)

]}

12

polyoxometalate as

catalyst for hydrogen peroxide activation was then extended to additional substrate

classes having various functional units [71]. Thus, allylic primary alcohols were

oxidized selectively to the corresponding epoxides in high yields and >90% selectivity.

Allylic secondary alcohols were oxidized to a mixture of a- and b-unsaturated ketones

(the major product) and epoxides (the minor product). Secondary alcohols were

oxidized to ketones, and sulﬁdes to a mixture of sulfoxides and sulfones. The reactivity

of simple alkenes is inordinately affected by the steric bulk of the substrate. For

example, the general reactivity scale for the epoxidation of alkenes indicates a strong

correlation between the rate of the epoxidation and the nucleophilicity of the alkene,

which is in turn correlated with the degree of substitution at the double bond. Thus, it

was expected and observed that 2,3-dimethyl-2-butene would be more reactive than

2-methyl-2-heptene; however, other more bulky substrates such as 1-methylcyclohex-

enewerefound to be less reactive than cyclohexene,incontrasttowhatwould normally

be expected. Furthermore, a-pinene did not react at all. This led, for example, to

unusual reaction selectivity in limonene epoxidation, where both epoxides were

formed in equal amounts in contrast to the usual situation where epoxidation at the

endo double bond is highly preferred [72]. In these catalytic systems high turnover

conditions can be easily achieved and high conversions are attained for reactive

substrates, but sometimes for less reactive substrates such as terminal alkenes con-

versions and yields are low. The conversion can be increased by continuous or semi-

continuous addition of hydrogen peroxide and removal of spent aqueous p hases.

After the original studies on the activity of the {[(WZnMn(II)

]

[(ZnW

)

]}

12

polyoxometalates in the mid 1990s, recent industrial interest

revived research in this area. Originally, the large size of the sandwich-type structure

was thought to be disadvantageous for the large scale and practical applications

because, even at low molar percent loads of catalyst, relatively large amounts

of polyoxometalate would be required. However, the large molecular size

(high molecular weight) have an under-appreciated advantage in that they signi ﬁ-

cantly simplify catalyst recovery from homogeneous solutions via easily applied

nano-ﬁltration techniques [73]. This reverses some of the previous thinking in this

330

9 Liquid Phase Oxidation Reactions Catalyzed by Polyoxometalates

area. The newly initiated reinvestigation of the use of sandwich-type polyoxome-

talates, {[(WZnTM

][(ZnW

)

]}

q

, showed that for a signiﬁcant series of

transition metals, most notably the zinc analog, these were exceptionally active

catalysts for epoxidation of allylic alcohols using toluene or ethyl acetate as sol-

vent [74]. The identity of the transition metal did not affect the reactivity, chemos-

electivity, or stereoselectivity of the allylic alcohol epoxidation by hydrogen peroxide.

These selectivity features support a conclusion that a tungsten peroxo complex rather

than a high-valent transition-metal-oxo species operates as the key intermediate in

the sandwich-type POM-catalyzed epoxidations. The marked enhancement of reac-

tivity and selectivity of allylic alcohols versus simple alkenes was explained by a

template formation in which the allylic alcohol is coordinated through metal-

alcoholate bonding, and the hydrogen-peroxide oxygen source is activated in the

form of a peroxo tungsten complex. 1,3-Allylic strain expresses a high preference for

the formation of the threo epoxy alcohol, whereas in substrates with 1,2-allylic

strain the erythro diastereomer was favored. In contrast to acyclic allylic alcohols

the {[(WZnTM

][(ZnW

)

]}

q

-catalyzed oxidation of the cyclic allylic

alcohols by hydrogen peroxide yielded signiﬁcant amounts of enone rather than

epoxides. A comprehensive comparison of {[(WZn

][(ZnW

)

]}

12

with

other tungsten-based catalysts taking into account activity per tungsten atom,

reaction selectivity, time for formation of the active species, and recycle showed

appreciable advantages of {[(WZn

][(ZnW

)

]}

12

[75].

In the present section we have highlighted research that has been carried out using

polyoxometalates as catalysts for oxidation with peroxygen compounds. Not all of the

synthetic applications have been noted, but those missing have been previously

reviewed [2]. It is important to stress that from a synthetic point of view various

substrates with varying functional groups can be effectively transformed to desired

products. In addition, interesting reaction selectivity can be obtained in certain cases.

In this sense polyoxometalates are one class of compounds among others that may be

considered for such transformations. In general, the often-simple preparations of

catalytically signiﬁcant polyoxometalates along with conceivable recovery from

solution by nano-ﬁltration present a conceptual advantage in the use of polyoxome-

talates. From a mechanistic point of view, the wide range of properties available in the

various classes of polyoxometalate compounds allows one to express reactivity in a

number of ways ranging from nucleophilic–electrophilic reactions between peroxo

or hydroperoxy intermediates and organic substrates to radical and radical chain

reactions via alkoxy or hydroxy radicals formed by homolytic cleavage of peroxygen

compounds by polyoxometalates.

9.5

Oxidation with Molecular Oxygen

The basic ecological and economic advantage and impetus for the use of oxygen from

air as primary oxidant for catalytic oxidative transformations are eminently clear. Yet,

the chemical properties of ground state molecular oxygen limit its usefulness as an

9.5 Oxidation with Molecular Oxygen

331

oxidant for wide-ranging synthetic applications. The limiting properties are the

radical nature of molecular oxygen, the strong oxygen-oxygen bond, and the fact that

one-electron reduction of oxygen is generally not thermodynamically favored (DG

> 0). The ground state properties of molecular oxygen lead to the situation that under

typical liquid phase conditions, reactions proceed by the well-known autooxidation

pathways, Scheme 9.7.

Metal-based catalysts may affect such pathways in various, but most notably have

an inﬂuence in initiating the radical chain propagation and decomposing interme-

diate alkylhydroperoxide species to alkoxy and peraalkoxy radicals as discussed in

Section 9.4 above. It is also very instructive to note that in nature common

monooxygenase enzymes such as cytochrome P-450 and methane monooxygenase

use reducing agents to activate molecular oxygen (Scheme 9.8).

The scheme depicted is not presented as an exact mechanistic representation,

but rather to illustrate several basic points. First, one may observe that oxygen is a

unique oxidant compared to other oxygen donors – the oxygen donors being in

principle reduced relative to molecular oxygen. In fact, even the active oxidizing

intermediate in metal-catalyzed autooxidation pathways is the reduced peroxo

intermediate (Scheme 9.7, reaction d). In addition, only one oxygen atom of

M+RH(a)

M+RH

(n-1)+

M+

(n-1)+

H+

M+RH(b)

M+

(n-1)+

H+

electron and proton transfer

hydrogen abstraction

O+R(c)

ROO

(d)

ROOH + RRH+ROO

(e)

ROOH + M

(n-1)+

RO + M

+ OH

ROOH + M(f)

ROO + M

(n-1)+

+ H

propagation

hydroperoxide decomposition

Scheme 9.7 Metal-catalyzed autooxidation pathways.

(n-1)+

–

(n-1)+

—O

(n-1)+

—OOH

(n+2)+

–

+ H

ROH

Scheme 9.8 Oxidation under reducing conditions – monooxygenase-type reactions.

332

9 Liquid Phase Oxidation Reactions Catalyzed by Polyoxometalates