Chemistry and catalysis advances in organometallic chemistry and catalysis

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(2)

c

660

587

640

594

SiW

527

SiW

594

(PW

)

567

(PW

)

452

(PW

)

645

Fe

(3)

407

467

380

487

SiW

167

SiW

273

Reaction conditions: 1 mmol of substrate, 1.5

μmol of catalyst, 2 mmol of H

, 1.5 ml of CH

CN; after 12 h at reﬂux.

After 6 h of reaction.

After 9 h of reaction.

3 ml of CH

CN.

6 mmol of H

O

2

.

(1.1)

(2.1)

(3.1)

(1.2)

(2.2)

(3.2)

(1.3)

(2.3)

(3.3)

O

n

n = 1

n = 3

n = 7

OH

n

OOH

n

n

n = 1

n = 2

(4.1)

(5.1)

OOH

n

OH

(6.1)

COOH

(6.2)

O

(8.1)

(9.1)

(10.1)

OOH

(9.2)

OOH

OOH

(5.5)

CHO

n = 1

n = 3

n = 7

n = 1

n = 3

n = 7

n = 1

n = 2

(4.2)

(5.2)

OH

n = 1

n = 2

(4.3)

(5.3)

n = 1

n = 2

(4.4)

(5.4)

(7.1)

OOH

(7.2)

(8.2)

OH

Figure 5.4

Oxidation products obtained in the catalytic oxidation studies of compounds 1 to 10.

FUNCTIONALIZATION OF SP

AND SP

CARBON CENTERS CATALYZED BY POLYOXOMETALATES AND METALLOPORPHYRINS

the meso-tetra-2,6-dichlorophenylporphyrin manganese complex containing an electron-withdrawing group at the

β-pyrrolic

position, Mn(

β-NO

TDCPP)Cl. In the case of cumene (8), the higher conversion was obtained with Mn

(TF

5

PP

)Cl, and

2-phenyl-2-propanol (8.2) was always the main product obtained, independent of the catalyst used [56].

The oxidation of 1-ethylnaphthalene (11) and 2-ethylnaphthalene (12), which are known polycyclic aromatic hydrocarbons

(PAHs), was studied in the presence of TBA salts of Mn

III

or Fe

III

POMs [51]. Only the iron-substituted POMs were able

to catalyze the oxidation of these two substrates, in moderate conversions. The oxidation of 11 and 12 occurs mainly at the

alkyl substituent, along with the formation of phthalic anhydrides resulting from the aromatic ring oxidation. The higher

conversions of 11 and 12 were obtained in the presence of BW

Fe or PW

Fe and when a molar ratio of H

O

2

/sub = 9.8

was used [51].

5.3

FUNCTIONALIZATION OF sp

2

CARBON CENTERS UNDER HOMOGENEOUS CONDITIONS

Studies on the functionalization of sp

carbon centers (Fig. 5.3) carried out in the presence of either TMSPs or

metalloporphyrins, using hydrogen peroxide as oxidant, gave rise to the products exempliﬁed in Fig. 5.5.

The oxidation of some monoterpenes, such as

(+)-3-carene (13), nerol (14), and geraniol (15), were studied in the presence

of Mn(III) porphyrins [55] and Mn(III)-substituted POMs [41]. The oxidation of 13 showed high conversions with all Mn(III)

porphyrins tested, and four major products were identiﬁed, namely

α-3,4-epoxycarane (13.1), β-3,4-epoxycarane (13.2), 3-

caren-5-one (13.3), and 3-carene-2,5-dione (13.4) [55]. However, in the presence of BW

Mn at moderate conversions

and under appropriate reaction conditions,

(+)-3-carene (13) afforded only the α-epoxide 13.1 and no allylic oxidation

was observed. Higher conversions of 13 were accompanied by a decrease in selectivity [41]. Nerol (14) and geraniol (15)

oxidations gave rise to 2,3-epoxides (14.1, 15.1), 6,7-epoxides (14.2, 15.2), and 2,3,6,7-diepoxides (14.3, 15.3). Allylic

oxidation was never found with the systems studied. Using Mn(III) porphyrins, the terminal 6,7-double bond of nerol and

(14.1/15.1)

(13.1)

(13.3)

(13.4)

O

(13.2)

O

O

O

(14.2/15.2)

(14.3/15.3)

CHO

O

(16.1) n = 1

(17.1) n = 2

CHO

R

n

n

n

n

(16.2) n = 1

(17.2) n = 2

(16.3) n = 1

(17.3) n = 2

(16.4) n = 1

(17.4) n = 2

O

n

(18.1) n = 1

(19.1) n = 3

(20.1)

(21.1)

(22.1)

(23.1)

(24.1)

R = H

R = NO

R = CH

R = F

R = Cl

(20.2)

(21.2)

(22.2)

(23.2)

(24.2)

(20.3)

(21.3)

(22.3)

(23.3)

(24.3)

OH

(25.1)

(26.1)

(27.1)

(27.2)

(26.2)

Figure 5.5

Oxidation products obtained in the catalytic oxidation studies of compounds 13 to 27.

FUNCTIONALIZATION OF SP

CARBON CENTERS UNDER HOMOGENEOUS CONDITIONS

geraniol was preferentially epoxidized [55]. On the other hand, in the presence of the TBA salts of BW

Mn, 14 and 15 were

preferentially epoxidized at the C

− C

double bond with conversions that can reach 96% with 86% selectivity, depending

on reaction conditions, representing a possible hydroxyl-directed epoxidation of oleﬁns [41]. Other studies on the oxidation

of monoterpenes using PW

Fe and M

(PW

)

, M

= Fe

III

, Co

were also published [52, 78].

The oxidation of 1H-indene (16) and 1,2-dihydronaphthalene (17) usually do not proceed much further than the formation

of the corresponding epoxides 16.1 and 17.1, respectively [79, 80]. However, in the presence of the TBA salts of XW

M,

where X

= P, Si, B and M = Mn, Fe, these reactions lead to the formation of several oxygenated products, including those

obtained through C

− C or C=C bond cleavage [47]. The conversion and selectivity for these two substrates were found

to be dependent on the POM used, the amount of oxidant added, and the reaction time. For instance, the oxidation of 16

and 17 after 5–7 h of reaction, and when using a ﬁvefold excess of H

, yielded two or three main products, namely the

hydroxyketones 16.3 and 17.3 and the dialdehydes 16.2 and 17.2. Under these conditions, naphthalene (25) was also obtained

by oxidative dehydrogenation of 17 [47]. The formation of 16.2 and 17.2, through the oxidative cleavage of carbon– carbon

bonds of 1,2-diols and/or C

= C bonds, was achieved under mild and environmentally friendly conditions, using hydrogen

peroxide as oxidant. In synthetic organic chemistry, this important type of ring-opening cleavage usually requires stronger

oxidants such as potassium permanganate, periodic acid, sodium or potassium periodate, and lead tetraacetate [2, 81].

The catalytic behavior of Mn(TDMImP)Cl was described, for the ﬁrst time, in 2008 for the homogeneous epoxidation

of alkenes, such as 1H-indene (16), cyclohexene (18), cis-cyclooctene (19), and styrene (20) [64]. It is known that allylic

oxidation and epoxidation are two competing processes both in vivo and in vitro. With this catalyst, only the epoxides from

cis-cyclooctene, cyclohexene, and 1H-indene were obtained. For styrene, the corresponding epoxide 20.1 was obtained as

the main product (95%), together with phenylacetaldehyde (20.2) (5%) [64]. These studies were extended to the oxidation of

other styrene derivatives (21–24) with H

, catalyzed by Mn(TDMImP)Cl and other cationic Mn(III) imidazolium-based

porphyrins, and the results were compared with those obtained for the robust and well-studied Mn(TDCPP)Cl. It is known

that the conversion of styrene (20) into the epoxide 20.1 is not always a clean reaction since undesired side products, such

as phenylacetaldehyde (20.2), can be formed. In some cases, benzaldehyde (20.3) is also observed as a by-product and its

formation is usually associated with radical processes taking place [82, 83]. The tested catalysts have shown to be efﬁcient in

the oxidation of styrene and its derivatives with hydrogen peroxide. For

p-nitrostyrene (21) oxidation, only the epoxide 21.1

was produced, whereas for

p-methylstyrene (22) a considerable yield of the corresponding phenylacetaldehyde (22.1) was

obtained.

p-Fluorostyrene (23) and p-chlorostyrene (24) showed similar epoxide/phenylacetaldehyde ratio when compared

to the unsubstituted styrene [68].

An efﬁcient system for the epoxidation of aromatic hydrocarbons with hydrogen peroxide in the presence of

Mn(TDCPP)Cl, Mn(

β-NO

TDCPP)Cl, and Mn(TPFPP)Cl as catalysts was also described [57]. In particular, if using

Mn(TDCPP)Cl and Mn(

β-NO

TDCPP)Cl, naphthalene (25) and anthracene (26) afforded the anti-1,2 : 3,4-arene diepoxides

with very good conversion and selectivity. For example, 25 and 26 were oxidized in the presence of Mn(TDCPP)Cl

with high selectivity to the corresponding anti-1,2 : 3,4-arene diepoxides (25.1, 81%) and (26.1, 74%) at 91% and 100%

of conversion, respectively. These results demonstrate the development of a very useful procedure for the synthesis of

epoxides 25.1 and 26.1, in just one step, under mild conditions. Mn(

β-NO

2

TDCPP)Cl gave rise to similar results on the

oxidation of 25 and 26. The oxidation of phenanthrene (27) by Mn(TDCPP)Cl showed high selectivity for the epoxidation

of the 9,10-bond affording 27.1. With both catalysts Mn(TDCPP)Cl and Mn(

β-NO

2

TDCPP)Cl, complete phenanthrene

conversions were obtained. Mn(

β-NO

TDCPP)Cl afforded compound 27.2 as the major product. With Mn(TPFPP)Cl, the

aromatic hydroxylation of substrates (25–27) and the transformation of the phenols to the corresponding quinones were

always the main transformations observed [57]. TMSPs had a completely different behavior in the oxidation of aromatic

hydrocarbons. Anthracene (26) was selectively oxidized to 9,10-anthraquinone (26.2) in the presence of XW

M, X

= P, or

B and M

= Mn

III

or Fe

III

. In particular, BW

Mn gave rise to 100% of conversion. Contrarily, naphthalene (25) could not

be oxidized under the conditions tested [51].

Novel bifunctional catalysts, speciﬁcally salts with a porphyrin/POM (Porph/POM) stoichiometric ratio of 0.75 or 1, were

prepared by the reaction of Keggin-type POMs with Mn(III) porphyrins bearing appropriate positively charged substituents

at the meso positions, namely Mn(TPyP)Cl, Mn(TMePyP)Cl, Mn(TrisDCPPyP)Cl, and Mn(TrisDCPMePyP)Cl [62]. The

oxidation of

(+)-3-carene (13), geraniol (15), and cis-cyclooctene (19) with hydrogen peroxide was examined in the presence

of these new Porph/POM compounds. In the conditions studied, the Porph/POM associations, POM

= (PW

)

−

, afforded

conversions signiﬁcantly higher than those obtained with the corresponding Mn(III) porphyrin alone, without signiﬁcant

change in the regio, chemo, and stereoselectivity of the reactions: 19 afforded selectively epoxycyclooctane (19.1), 15 gave

rise to 6,7-epoxygeraniol (15.2) as the major product, and 13 was preferentially oxidized to the

α-3,4-epoxycarane (13.1).

The POMs seemed to contribute to the stabilization of the metalloporphyrins (used as counter cations) against deactivation

during the catalytic cycles, but their effect as catalysts was not clearly observed, with the exception of the oxidation of

FUNCTIONALIZATION OF SP

AND SP

CARBON CENTERS CATALYZED BY POLYOXOMETALATES AND METALLOPORPHYRINS

geraniol, in which conversions trebled in the presence of the POM. In this work, the Porph/POM compounds could be used

at higher temperatures than those usually described, and the Porph/POM combinations were more efﬁcient catalysts than

the corresponding metalloporphyrins alone [62].

5.4

FUNCTIONALIZATION OF sp

2

AND sp

3

CARBON CENTERS UNDER HETEROGENEOUS CONDITIONS

A common undesirable feature associated with homogeneous systems is the problematic catalyst/products separation and the

poor or nonexistent catalyst reusability. To overcome these problems, many approaches have been adopted to immobilize

known active homogeneous catalysts onto appropriate supports without loss of their intrinsic activity and selectivity. The

TMSPs, XW

M, where X

= P, Si, or B and M

III

= Fe or Mn, were successfully immobilized on triethylpropylammonium-

functionalized silica (silicaNEt

) by an electrostatic methodology (Fig. 5.6). Through different techniques, such as Fourier

transform (FT) Raman and infrared (IR) spectroscopy, as well as elemental analysis, it was possible to provide enough

evidence of the presence of the TMSPs supported on the modiﬁed silica. [50]. The heterogeneous oxidation of cyclooctane

(2) gave conversions as high as 71–74%, with moderate selectivity for cyclooctanone (2.1), cyclooctanol (2.2), and cyclooctyl

hydroperoxide (2.3), together with minor amounts of 1,2-epoxycyclooctane (19.1). Considering the recovery and reuse of

the catalysts, the best performance was observed for silicaNEt

/PW

Fe, with conversions still around 65% in the third

cycle. The advantages of the new materials (recovery and reuse) can compensate the lower conversion values obtained, when

compared to those under homogeneous conditions. The product selectivity is not affected by the reuse of the catalyst, giving

2.1 and 2.3 as the main products for silicaNEt

/PW

Fe or silicaNEt

/SiW

Fe and 2.1 for silicaNEt

/BW

Fe. However, the

selectivity obtained under heterogeneous conditions is different from that achieved under homogeneous conditions. Actually,

in the heterogeneous system, higher amounts of 2.2 are observed, and in addition minor amounts of 1,2-epoxycyclooctane

(19.1) are also detected. This different selectivity may be a consequence of cyclooctyl hydroperoxide (2.3) decomposition

due to silica acidity [50].

The epoxidation reaction of cis-cyclooctene (19) was also studied in the presence of the immobilized POM

silicaNEt

/PW

Mn [50]. Under heterogeneous conditions, 19 was epoxidized with an appreciably higher conversion (65%)

than that obtained in the homogeneous system (22%), always with 100% selectivity for the epoxide 19.1. The unproductive

dismutation of hydrogen peroxide is faster under homogeneous conditions, when compared to the reactions using the

heterogeneous silicaNEt

/PW

Mn, which can justify this remarkable behavior. In fact, the efﬁciency of utilization of H

O

2

is higher in the heterogeneous system. It was possible to maintain good performance under heterogeneous conditions during

four cycles; the ﬁfth run gave still 26% of conversion after 6 h of reaction. Moreover, the heterogeneous catalysis results

suggest that PW

Mn does not leach into the reaction mixture, thus proving the heterogeneous nature of this catalytic

system [50].

Heterogeneous Mn(III) porphyrin [63] and Mn(III) chlorin [65] covalently immobilized on silica (Fig. 5.6) were used in

the oxidation of cis-cyclooctene (19). The efﬁciency of the immobilized Mn(

β-NO

TDCPP)Cl was evaluated under several

conditions, and the best performance was achieved when a lower volume of solvent and a lower rate of oxidant addition

were used. In this case, 89% of conversion was obtained after 21 h at 20

◦

C. The recovered catalyst was reused, but affording

only 5% of substrate conversion. Nevertheless, by increasing the catalyst amount to a ratio

S/C of 100 instead of 600, the

N

n

silicaNEt

3

/XW

11

M

XW

11

M

n-

Ar =

Silica

silica-Mn(III) porphyrin

silica-Mn(III) chlorin

Silica

(a)

(b)

(c)

Figure 5.6

Heterogeneous catalysts. (a) Silicanet

/XW

M. (b) Silica–Mn(III) porphyrin. (c) Silica–Mn(III) chlorin.

FINAL REMARKS

67

second and third catalyst reuse afforded good conversions of 19 (88% and 51%, respectively). No Mn(III) porphyrin was

observed by UV–vis analyses of the reaction supernatant, and the analyses of the silica recovered after reaction showed

that only minor leaching had occurred [63]. The catalytic activity evaluation of the supported Mn(III) chlorin (a macrocycle

related to porphyrins) showed that this heterogeneous catalyst is efﬁcient, selective, and, under appropriate conditions, can

also be reused in the epoxidation of 19. One factor that seems to inﬂuence the catalyst activity is the frequency of addition of

2

O

. For example, when this addition is made every 60 min, the ﬁrst and second runs give 100% and 88% of conversion,

respectively, but this was inactive at the third cycle. Better results were obtained using t-butyl hydroperoxide (TBHP) as

oxidant, which allowed a good recyclability of the immobilized chlorin with no signiﬁcant loss of activity. Depending on

the conditions used, it is possible to reach a ﬁfth cycle with good conversion of 19 using TBHP as an oxidant [65].

Recently, the preparation and characterization of silica nanoparticles supporting TMSPs, particularly iron(III) containing

POMs, were reported [52]. These new POM

/SiO

composites, enclosing Keggin-type POMs (PW

Fe

/SiO

) and sandwich-

type POMs Fe

(PW

)

/SiO

, were synthesized by a reverse micelle system, giving rise to nanoscale spherical particles with

diameters between 25 and 35 nm. It was found that core/shell structures could be formed, with the POM being encapsulated

by the silica, while in other cases the POM was dispersed on the nanoparticles surface. The nanocomposites prepared were

tested as heterogeneous catalysts in the oxidation of geraniol (15) with H

. The best catalytic activity for this oxidation

reaction was achieved with the PW

/SiO

composite, reaching 96% of conversion of 15 after 3 h of reaction, with

88%–91% of selectivity for 2,3-epoxygeraniol. Globally, the new POM-nanostructured catalysts seem to be more selective

than the corresponding TBA salts used under homogeneous conditions, adding the advantage of possible separation and

catalyst reuse [52].

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