Nanocatalysis Synthesis and Applications (Polshettiwar/Nanocatalysis) || Nanocatalysts for...

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NANOCATALYSTS FORREARRANGEMENT REACTIONS

Joaquın Garcıa-Alvarez, Sergio E. Garcıa-Garrido,and Victorio Cadierno

INTRODUCTION

The concept of atom economy, that is, syntheticmethods should be designed tomaximizethe incorporation of all materials used in the process into the final reaction products, hasemerged as a major goal for chemists in recent years.1–3 Atom economy is an importantconcept within the green chemistry philosophy,4–8 and one of the most widely used toolsto measure the “greenness” of a synthetic protocol since it allows the rapid evaluationof the amount of waste generated.9–11 The skeletal rearrangements of organic molecules(e.g., isomerizations, cycloisomerizations, and pericyclic reactions) are the most typicalexamples of atom-economical reactions since no by-products are generated. These trans-formations not only constitute fundamental processes in synthetic organic chemistry butalso play a pivotal role in different industrial fields of outstanding importance like fuelupgrading or food industry.

Homogeneous catalysis currently provides some of the most powerful tools toaddress the atom economy issue in chemical processes. In this sense, a huge number ofchemo-, regio-, and stereoselective metal-catalyzed transformations, proceeding cleanlywith a high level of atom efficiency, are presently available and play a burgeoning rolewithin the toolbox of synthetic organic chemists.12, 13 However, despite their wide use,many homogeneous catalytic systems cannot be commercialized because of the difficulty

Nanocatalysis: Synthesis and Applications, First Edition. Edited by Vivek Polshettiwar and Tewodros Asefa.© 2013 John Wiley & Sons, Inc. Published 2013 by John Wiley & Sons, Inc.

251

252 NANOCATALYSTS FOR REARRANGEMENT REACTIONS

encountered in separating the catalyst from the final reaction products. Removal of traceamounts of catalyst from the end product is essential since metal contamination is highlyregulated, especially by the pharmaceutical industry. To overcome these separationproblems, the use of heterogeneous systems appears to be the best solution.14–16 In thiscontext, remarkable efforts have been devoted to the heterogenization of homogeneouscatalysts (by covalent binding or by simple adsorption) onto solid supports, with the vastmajority of examples involving silicamaterials due to their advantageous properties (e.g.,excellent stability, good accessibility, and high porosity).17, 18 Attempts have been madeto make all active sites on these solid supports accessible for reaction, thus allowing therates and selectivities of the catalytic reactions to be comparable to those obtained underhomogeneous conditions. However, only the surface sites on heterogeneous catalysts areavailable for catalysis, which results in a decrease of the overall reactivity of the catalystsystem. Another problem usually encountered in heterogeneous systems is the leachingof active metal complexes from the supports, which again leads to contamination of thefinal reaction product with trace metals.

In very recent years, the field of nanocatalysis has emerged as the most promisingalternative to circumvent many of these problems.19–23 In particular, the use of metalnanoparticles (NPs) as catalysts (or as catalyst supports) allows the increase of theexposed surface area of the active component of the catalyst, thereby enhancing its con-tact with the substrates and mimicking the homogeneous systems (heterogeneous NPsprovide a quasihomogeneous phase). On the other hand, like in the case of heteroge-neous catalysts, the insolubility of the metal NPs allows their easy separation from thereaction mixture, thus minimizing the effort of the product isolation stage. In addition,the rapid advances made in the nanotechnology field have enabled the preparation of avariety of NPs with controlled size, shape, morphology, and composition—propertiesthat can be tuned to achieve optimal activities and selectivities.

This chapter is intended to cover the progress made in the use of this new generationof catalysts to promote atom-economical rearrangement reactions of organic molecules.

SYNTHESIS AND CHARACTERIZATION OF NANOCATALYSTS

Synthesis of the Nanocatalysts

Two main families of nanocatalysts have been employed to promote rearrangementreactions: (i) those based on metal(0) nanoparticles and (ii) those based on nano-sizedmetal oxides. Concerning the former, they are usually synthesized via two classicalmethods19–23: the reduction of metal salts and the decomposition of organometalliccomplexes. Regardless of the route employed, a key issue in their synthesis is theirstabilization to avoid the formation of catalytically inactive bulk metal. In this sense,polymeric materials, dendrimers, and surfactants are the most commonly employedstabilizing agents. We must note that, in addition to these classical methods, silvernanoparticles (Ag-NPs) active in the Wolff rearrangement of �-diazoketones have beengenerated from Ag2O electrochemically.24,25 Concerning the preparation of MOx-NPs,they have been usually prepared by hydrolysis of the corresponding metal salts in basic

SYNTHESIS AND CHARACTERIZATION OF NANOCATALYSTS 253

NN

N

P

I(MeO)3SiFe3O4 +

Fe3O4 O Si

O

ON

NN

P

n

CHCl3 / Δ

MeOH / r.t.

Fe3O4 O Si

O

O

1

NN

N

P

n

Ru Cl

Cl[{RuCl(μ-Cl)(η6-p-cymene)}2]

I

I

SiO2 layer

Scheme 8.1. Schematic representation of the synthesis of the SiO2-coated Fe3O4-RAPTA

nanocatalyst.

media, followed by dehydration of the resulting hydroxides by calcination. Immobiliza-tion of both M(0) and MOx-NPs on solid supports (e.g., mesoporous silicas, zeolites,alumina, ceria, and carbonaceous materials) has been widely employed to facilitate theirrecycling. This was generally achieved using the following sequential order: (i) impreg-nation of the corresponding metallic precursors on the support, (ii) generation of theNPs, and (iii) washing of the solid.

On the other hand, the use of nanoparticles by themselves as supports for the hetero-genization of catalytically active transition metal complexes has expanded considerablyduring the last years.19–23 In this context, silica-coated magnetic Fe3O4-NPs 1 decoratedon their surface with an organometallic Ru(II)–arene complex containing the water-soluble 1,3,5-triaza-7-phosphatricyclo[3.3.1.13,7]decane (PTA) ligand, usually abbre-viated as RAPTA complexes, have been designed (see Scheme 8.1) and successfullyemployed in different isomerization reactions.26

Techniques Employed for the Characterization of theNanocatalysts

Electron microscopy techniques—transmission electron microscopy (TEM), scanningelectron microscopy (SEM), and scanning tunneling microscopy (STM)—have beenroutinely employed to determine the shape, size, and morphologies of the nanocatalysts.X-ray diffraction techniques, X-ray photoelectron spectroscopy, and energy-dispersiveX-ray spectroscopy have been used to determine the elemental composition, the oxi-dation states, and the thermal stability of the materials. Alternatively, the elementalcomposition could also be determined by thermogravimetric analysis,27, 28 inductivelycoupled plasma mass spectrometry (ICP-MS),26 or atomic emission spectroscopy tech-niques.29


In some cases, ultraviolet-visible (UV-Vis) measurements have been performed inorder to determine the disposition of the nanoparticles along the surface (or the inter-nal cavities) of the supports.27, 28 Temperature-programmed gas adsorption/desorptiontechniques have also been applied to determine the active surface-adsorbed area of thematerials.30–32

Fourier transformation infrared spectroscopy (FT-IR),26 as well as reflection-absorption infrared spectroscopy,33–35 has been used to characterize the functional-ization, thermal stability, and the cleanliness and quality of the nanomaterials used ascatalysts. When it was possible to do this, the functionalization of the nanoparticles hasalso been determined employing nuclear magnetic resonance techniques. Changes inthe electronic and magnetic properties of the catalysts during the reactions were in somecases examined by Mossbauer spectroscopy,36–38 using measurements of macroscopicmagnetization (SQUID) to determine the static magnetic susceptibility.

CATALYTIC ACTIVITY, STABILITY, AND REUSABILITY

Olefin Isomerization Processes

The selective conversion of trans-olefins to their less thermodynamically favored cis-isomers constitutes a reaction of critical importance in fields such as food industry,39,40

the manufacturing of some vitamins,41,42 or fine chemical syntheses.43,44 In particular,the control of the selectivity in these reactions has significant practical implicationsbecause, for instance, the trans-fatty acids produced during the partial hydrogenation ofnatural oils to edible fats are believed to have serious negative health effects.39, 40 In thiscontext, Belkacemi and coworkers have employed palladium and sulfur-doped Pd-NPshighly dispersed on the mesoporous silica support SBA-15 to perform the concomitanthydrogenation and trans-to-cis-isomerization of vegetable oils, such as sunflower andcanola oils, yielding final products with very low trans-fatty acids levels.45, 46

Related cis-to-trans-isomerizations of olefins have been described with catalystsconsisting of tetrahedral or cubic Pt-NPs dispersed onto a high-surface-area silica xerogelsupport.30–32 Thus, it is shown that the platinum tetrahedral nanoparticles, which onlyexpose (111) facets, are particularly apt to promote the conversion of trans-olefins totheir more unstable cis-isomers (Scheme 8.2). Otherwise, platinum cubic nanocrystals,with high fractions of (100) facets, present the opposite behavior, where the cis-to-trans-conversion is always faster than the reverse trans-to-cis-reaction (Scheme 8.2). Pt-NPs stabilized by fourth-generation hydroxyl-terminated poly(amidoamine) (PAMAM)dendrimers,47 as well as supported Pd-NPs grown in situ on planar Fe3O4/Pt(111)or Al2O3/NiAl(110) thin films,33–35 were also used as catalysts in the cis-to-trans-isomerization of but-2-ene. As a general trend, the cis-to-trans conversion is favoredwhen the process is performed at low temperatures, while the reverse trans-to–cis-isomerization becomes more important as the temperature at which the reaction takesplace increases. In addition to these examples, Shukla and coworkers observed, duringthe synthesis of bimetallic Fe–Pt-NPs in the presence of a 1 : 1 mixture of oleic acid(cis-9-octadecenoic acid) and oleylamine (cis-9-octadecenoic amine) as a surfactant, the

CATALYTIC ACTIVITY, STABILITY, AND REUSABILITY 255

+

trans-but-2-ene cis-but-2-ene

+

trans-but-2-ene(major)

cis-but-2-ene(minor)

1:1 trans-to-cis ratio

trans-to-cis ratio

+

trans-but-2-ene cis-but-2-ene

+

trans-but-2-ene(minor)

cis-but-2-ene(major)

1:1

Tetrahedral Pt-NPs

SiO2 xerogel

H2 / 102 °C / 1 h

Cubic Pt-NPs

SiO2 xerogel

H2 / 102 °C / 1 h

Scheme 8.2. cis-to-trans-Isomerization of but-2-ene catalyzed by tetrahedral and cubic Pt-

NPs.

conversion of the alkyl chains of this surfactant from the oleyl form (cis-9-octadecenyl)to the elaidyl one (trans-9-octadecenyl) when excess of iron was present.48

The catalytic isomerization of terminal olefins into their corresponding internalcounterparts has also been performed employing nanocatalysts. It is worth mentioningthat a number of industrial processes, such as the synthesis of various fatty acids,terpenes, alkaloids, and steroids, involve these double-bond migration reactions.49 Inaddition, related olefin isomerization reactions represent key steps in the higher olefin-manufacturing processes and in the production of various fragrances.50–52 In this context,the isomerization of the model compound but-1-ene, giving to a mixture of cis-and-trans-but-2-ene, has been described using a porous borosilicate material coated with ananocomposite sol–gel-derivedTiO2/SiO2 membrane dopedwithRh-NPs as a catalyst.53

The same reaction was also promoted by TiO2-NPs.54 In particular, using pure TiO2 ormixed TiO2–SiO2 (Ti content from 70 to 80%), but-1-ene could be converted into amixture of cis-and-trans-but-2-ene in up to 74% yield without a marked preference forone of the two stereoisomers.

Meltzer and coworkers found that isomerization of the C=C bond of allylbenzeneswas possible employing the systemRhCl3·3H2O/Aliquat-336 (Aliquat-336= N-methyl-N,N-dioctyloctan-1-ammonium chloride) sol–gel entrapped within hydrophobic silica([RhCl4][(C8H17)3NMe]/sol–gel). When the reactions were performed in water, therhodium compound was converted into catalytically active Rh(0)-NPs.55 Remarkably,this system was quite stereoselective toward the formation of the corresponding (E)-1-aryl-1-propenes (25–80% yield vs. 2–9% yield of their Z counterparts), and could berecovered and reused at least six times without a decrease in its activity (Scheme 8.3).

R R R+Surfactant / H2O / n-PrOH

80–140 °C / 3 hR = H, 4-Cl, 4-Me, 4-OMe (2–9% yield)

[RhCl4][(C8H17)3NMe]/sol-gel

(25–80% yield)

Scheme 8.3. Isomerization of allylbenzenes catalyzed by [RhCl4][(C8H17)3NMe]/sol–gel.


SiO

SiO

O O

SiO

Si

O O

Si O Si Si SiO O

HN

N

NN

NH

NH2

N

H2N

N

HN

N

H2N

NH

WP-1

SiO

SiO

O O

SiO

Si

O O

Si O Si Si SiO O

N

NHO2CH2C

NN

NH

NHCH2CO2H

N

H2N

N

HN

N

HO2CH2CHN

N

WP-2

CH2CO2H

SiO

SiO

O O

SiO

Si

O O

Si O Si Si SiO O

HN NH HN NH

BP-1

NH2

NH2

H2N

NH2

SiO

SiO

O O

SiO

Si

O O

Si O Si Si SiO O

HN NH HN NH

BP-2

N

NH

HN

NHO2CH2CCH2CO2H

CH2CO2H

HO2CH2C CH2CO2H

CH2CO2H

Figure 8.1. Schematic representation of the silica polymeric composites WP-1, WP-2, BP-1,

and BP-2.

Nano-sized gold, nickel, and mixed gold–nickel particles immobilized on � -Al2O3,56,57 as well as Pd(II) and Pd(0) species supported onto multilayered titanatenanotubes (H2Ti3O7),58 also exhibited catalytic activity in the C=C bond isomerizationof allylbenzene. As observedwith the aforementioned RhCl3·3H2O/Aliquat-336 system,the (E)-1-phenyl-1-propene was again the major reaction product.

Related isomerizations of terminal olefins have been regularly described as sec-ondary reactions of other transformations. For example, Allen and coworkers observedthe isomerization of hex-1-ene, oct-1-ene, and dec-1-ene during their investigationson the hydrogenation of these substrates employing Pd(OAc)2, adsorbed onto thenanoporous silica polyamine composites WP-1, WP-2, BP-1, and BP-2 (Figure 8.1),as catalysts.59 X-ray photoelectron spectroscopy and TEM measurements, beforeand after the reaction, revealed the conversion of surface-ligand bound metal ions tocatalytically active metal NPs. Similarly, olefin isomerizations have been described as


Cl

Cl

ClCl

+ Cl ClClCH2CH2Cl / O2 / 60–150 °C

NP-based catalysts

3/4 ratios >98:22 3 4

Scheme 8.4. Isomerization of 3,4-dichlorobut-1-ene into 1,4-dichlorobut-2-ene.

secondary reactions of hydroformylation processes using Rh60 and Au-NPs61 supportedonto different materials, such as activated carbon, poly(N-vinyl-2-pyrrolidone) (PVP),Al2O3, TiO2, Fe2O3, ZnO, CeO2, or Co3O4.

Chloroolefins constitute a particular class of terminal olefins since C=C migrationsin these substrates are usually accompanied by changes in the location of the chloridesubstituents. In this sense, Rostovshchikova and coworkers have described a large seriesof nanocatalysts able to perform the selective isomerization of 3,4-dichlorobut-1-ene 2into trans-1,4-dichlorobut-2-ene 3 (Scheme 8.4). In particular, employing Fe(III) oxide-NPs (�-Fe2O3 or � -Fe2O3) supported on silica36–38 or stabilized on the surface ofpolymeric matrices,62,63 as well as copper, nickel, and palladium nanostructured metalfilms supported on the surface of thermally oxidized silicon,64–66 2 could be convertedinto 3with almost complete stereoselectivity (only 2% of the cis-isomer 4was observed)and high turnover frequencies (up to 4500 h−1).

Rearrangement of cyclohexene to afford 1-methylcyclopentene as the main reactionproduct has also been described by Castillo and coworkers using bimetallic Pt–Pd-NPsdispersed on � -Al2O3 or TiO2.67,68 Almost quantitative conversions were obtained withall the samples tested, the solids with major content on platinum showing the highestactivities in this cycloolefin isomerization process.

On the other hand, the isomerization of the C=C bond of readily accessible allylicalcohols represents a simple and atom-economic route for the preparation of saturatedcarbonyl compounds.69–72 The process involves the spontaneous tautomerization of theintermediate enol formed during the C=C bond migration. In this context, the magneticnanoparticles 1, decorated on the surface with a RAPTA-type complex (see Scheme 8.1),have proven to be an efficient catalytic system for the redox isomerization of allylic alco-hols (Scheme 8.5).26 Remarkably, the reactions were performed in pure aqueousmediumemploying microwave irradiation as the heating source, and the products could be iso-lated by Kugelrohr distillation without the use of a single drop of organic solvents. Inaddition, the NPs were recovered by the aid of an external magnet and reused up tofive runs in the isomerization of the model substrate oct-1-en-3-ol (cumulative turnovernumbers of 283). Related transformations were also studied using Pd-NPs supported on

R1 R2

R3OH

R1 R2

R3O

1 (1.58 mol% of Ru)

H2O / MW (150 °C) / 0.25–24 h

R1 and R2 = H, alkyl or aryl group; R3 = H, Me

(59–95% yield; six examples)

Scheme 8.5. Isomerization of allylic alcohols catalyzed by the nanoferrites 1 in water.


H2O / MW (150 °C) / 0.25–6.5 h HO

R

O R

1 (1.58 mol% of Ru)

(62–97% yield)

R = H, Et, Ph, CH2CH=CH2, CH2C(Me)=CH2, C CPh, C CSiMe3

5 6

Scheme 8.6. Catalytic cycloisomerization of (Z)-enynols in aqueous medium.

nanocrystalline ceria (Pd/CeO2)73 or stabilized by alkanethiolate monolayers.27, 28 How-ever, although selective in the redox isomerization process, the catalytic performancesof these Pd-based systems were much lower as compared to that of the functionalizednanoferrites 1.

Finally, we must also note that the isomerization of some allylic alcohols has beenobserved by Bruening and coworkers as a secondary process during their studies directedto the hydrogenation of this type of substrate employing Pd-NPs embedded in multilayerpolyelectrolyte films as catalysts.29

Cycloisomerizations and Related Cyclization Processes

Metal-catalyzed electrophilic activation of the C≡C bond of functionalized alkynestoward intramolecular nucleophilic attack has attracted considerable attention duringthe last decades as it allows the rapid construction of different 5-, 6-, or 7-memberedcarbocycles and heterocycles.74–78 In this context, the magnetic nanocatalyst 1 hasproved to be a general, efficient, and easily reusable catalyst (up to ten consecutive runs)for the heteroannulation of (Z)-enynols 5 into furans 6 (Scheme 8.6).26 The use of alow ruthenium concentration, microwaves as the energy source, and environmentallyfriendly water as the reaction medium, without a single drop of organic solvent beingused during or after the reactions (the nanocatalyst can be easily separated with anexternal magnet and a biphasic water/product system is formed), renders this syntheticprotocol of furans truly green and sustainable.

Associated with the hypervalent iodine species PhICl2, both Pt and Pd-NPs,synthesized using a fourth-generation PAMAM dendrimer as the capping agent andsupported on the high-surface-area mesoporous silica SBA-15, were also successfully

Pd or Pt (4 mol%) / PAMAM / SBA-15

PhICl2 (12 mol%)

Toluene / RT or 100 °C / 15 h

OH

Ph

O

Ph

(95–98% yield)

7 8

Scheme 8.7. Hydroalkoxylation of 2-phenylethynylphenol using encapsulated Pd and Pt-NPs.


Au / CeO2 (5 mol%)

CD3CN / 60 °C / 24 h O NTs

OH

NTs

(99% yield)

9 10

Scheme 8.8. Phenol synthesis using Au-NPs supported on CeO2

employed in the intramolecular hydroalkoxylation of 2-phenylethynylphenol 7(Scheme 8.7).79, 80 The Pd nanocatalyst was particularly active as the reaction could berun at room temperature, yielding the desired benzofuran 8 in a remarkably higher yieldwhen compared to classical homogeneous systems such as PdCl2, PdBr2, or Pd(PPh3)4.In the case of the Pt-NPs, a high-temperature regime (100 ◦C) was required to attain asimilar conversion. However, we must note that this Pt-based system remained activein other catalytic �-bond activation reactions, such as the hydroamination of allenes orthe oxidative cyclization of alkynyl ureas.

As shown in Scheme 8.8, nanoparticles of gold supported on CeO2 were ableto catalyze the isomerization of the �-alkynylfuran 9 to phenol 10.81 Cationic Au3+

species are the active sites for the reaction as determined by FT-IR using CO as a probemolecule. Initial leaching of gold was observed, which could be minimized by calciningthe catalyst before use. Subsequent runs show that once all soluble species have leached,the surface-bound cationic gold species are still active in the process and can reachturnover numbers (up to 391) similar to those reported with homogeneous systems. It isimportant to note that this nanocatalyst represents the first example of a heterogeneousgold-based system active in phenol synthesis from �-alkynylfurans.

Au-NPs supported on TiO2 also catalyzed the cycloisomerization of aryl-propargylethers 11 into the corresponding 2H-chromenes 12 (Scheme 8.9).82 The hydroarylationreactions, which were performed in 1,2-dichloroethane at 70 ◦C under air, deliveredthe desired heterocycles 12 in good yields along with minor amounts of bichromeneby-products resulting from the oxidative dimerization of 12.

Metal-catalyzed intramolecular cyclization of alkynoic acids provides a direct andsimple entry to enol-lactones, an important class of densely functionalized heterocyclesthat are useful as synthetic precursors and intermediates for many chemical processes.Recently, Genet, Michelet, Parvulescu, and coworkers have demonstrated that Au-NPssupported in zeolite �-NH4+ exhibit a superior catalytic activity for the intramolecular

Au / TiO2 (1.2 mol%)

ClCH2CH2Cl / 70 °C / 1–48 h

O

R2

R1 R1

O

R2

(70–91% yield)

11 12R1 = H, Me, OMe, F, Br; R2 = H, Me

Scheme 8.9. Cyclization of aryl propargyl ethers catalyzed by Au-NPs supported on TiO2.


OH

R2

R1

O O

R1

R2O

R1 = CO2Et; R2 = n-Bu, Bn

R1 = CO2Me; R2 = CH2CH=CH2, CH2CH=CHMe

R1 = Ph; R2 = H

Au / β-NH4+ (1.5 mol%)

CH3CN / RT or 40 °C / 8 h

(50–99% yield)

Scheme 8.10. Cyclization of alkynoic acids using an Au/β-NH4+ nanocatalyst.

addition of carboxylic acids to alkynes (Scheme 8.10).83–85 Analysis of the supportedgold species with in situ X-ray photoelectron spectroscopy suggested that cationic goldspecies (possibly Au3+) might be responsible of the catalytic activity observed. Thecatalytic efficiency of this system allowed the reaction to be performed under remarkablymild conditions. In addition, the catalyst could be recycled for eight consecutive runswithout any significant loss of activity.

On the other hand, transition metal-catalyzed [2+2+2] cyclotrimerization ofalkynes is one of the most powerful synthetic tools presently available for the con-struction of aromatic arene rings.86–90 In particular, the intramolecular version of thisreaction is an elegant route of access to highly substituted polycyclic arene frameworks,with complete atom economy, in a single operation. In this context, it has been reportedthat the intramolecular [2+2+2] cyclotrimerization of the triacetylenic azamacrocycle13 catalyzed by in situ generated Pd-NPs (obtained from PdCl2 in molten n-Bu4NBr at130 ◦C) results more advantageous as compared to conventional protocols employinghomogeneous catalysts in organic solvents, allowing to obtain the aromatic polycycle14 in higher yield and presenting recycling possibilities (Scheme 8.11).91 More recently,related cyclizations of aromatic enediynes have been described using mesoporous silicagrafted with a tertiary amine as a basic and recyclable nanocatalyst.92

In addition to all these selective transformations of alkynes, cycloisomerization pro-cesses involving functionalized olefins have also been described. In this regard, Pd-NPswere found to catalyze the cycloisomerization of N,N-diallylamides and sulfonamides15 into the corresponding N-acyl- and N-sulfonyl-3,4-dimethyl-2,3-dihydropyrroles 16

N

N

SO2Ar

SO2Ar

NArO2S N

N

N

SO2Ar

SO2Ar

ArO2S

Pd-NPs(from 10 mol% of PdCl2)

n-Bu4NBr / 130 °C / 24 h

14

(73% yield)

13

Ar = 2,4,6-C6H2(i-Pr)3

Scheme 8.11. Cyclotrimerization of an azamacrocycle catalyzed by Pd-NPs.


NR NR

Pd-NPs (1 mol%)

BTBAB (20 mol%)

R = PhSO2, 4-MeC6H4SO2, 4-ClC6H4SO2, PhCO

4-MeC6H4CO, 3-MeC6H4CO, 2-MeC6H4CO

4-ClC6H4CO, 2-ClC6H4CO, 4-NO2C6H4CO

4-FC6H4CO, CH3CO, CH3(CH2)2CO

DMF / 70 °C / 4–12 h

(90–99% yield)1615

Scheme 8.12. Catalytic cycloisomerization of dienes employing Pd-NPs.

with a very high regioselectivity (Scheme 8.12).93 Performing the catalytic reactionsin the presence of benzyltri-n-butylammonium bromide (BTBAB) as a surfactant, indimethylformamide (DMF) at 70 ◦C, only 1 mol% of palladium was required to obtainthe products in excellent yields. The surfactant stabilizes the nanoparticles, thus prevent-ing agglomeration processes (20 mol% of BTBAB to attain the highest conversions),and enables their reuse in five consecutive runs without reducing the catalytic activitymore than 8%.

Au-NPs stabilized by the organic polymer PVP are also effective catalysts in theintramolecular hydroalkoxylation of alkenes.94 Thus, performing the catalytic reactionsin the presence of DBU (DBU= 1,8-diazabicyclo[5.4.0]undec-7-ene), a wide variety ofhydroxyalkenes 17 could be selectively converted into the corresponding tetrahydrofu-rans 18 in moderate-to-good yields (Scheme 8.13). It is important to note that, althoughthe reactions were conducted in air, no Wacker-type or oxygenation derivatives wereobserved as by-products. Pd and Pt-NPs supported on different inorganic materials (e.g.,SiO2, Al2O3, TiO2, ZrO2, MgO, or ZnO) were applied in the related conversion of cis-2-butene-1,4-diol into 2-hydroxytetrahydrofuran under hydrogen atmosphere. However,these systemswere not selective for the desired cycloisomerization reaction, as they gaveas well hydrogenation, hydrogenolysis, and cis-to-trans-isomerization by-products.95–97

The aforementioned Au-NPs stabilized by PVP were also active in the intramolec-ular hydroamination of alkenes under aerobic conditions, thus leading to a new atom-economical route to five-membered nitrogen-containing heterocycles.98–100 Thus, forsubstrates 19, the toluenesulfonamide group could be intramolecularly added to the

R2

R1 OH

R3R3

O

R3

R2

R1

R3

Au-NPs / PVP (10 mol%)

DBU (200 mol%)

H2O-DMF (2:1) / 50 °C / 16–24 h

(38–93% yield; six examples)1817

R1, R2, R3 = H, alkyl or aryl group

Scheme 8.13. Intramolecular hydroalkoxylation of alkenes catalyzed by Au-NPs stabilized

by PVP.


Au-NPs / PVP (5 mol%)

Cs2CO3 (300 mol%)

N

R1

CH3

R3

R2

Ts

R1

R2

R3

NHTs

R1 = R2 = H, Me, Ph; R3 = H, Me

EtOH / 50 °C / 1–16 h

(41–99% yield)2019

Scheme 8.14. Intramolecular hydroamination reactions catalyzed by Au-NPs stabilized

by PVP.

unactivated alkene unit to afford the pyrrolidine derivatives 20 selectively using 5 mol%of the Au/PVP catalyst and excess of Cs2CO3 (Scheme 8.14).98 The process was alsooperative with substrates containing primary amine units (NH2 instead of NHTs), butmixtures of the desired pyrrolidines with significant amounts of the corresponding iso-meric imines were obtained in these cases.99 It was found that the reactionwas dependenton the acidity of the solvent, the yield of desired pyrrolidine being maximized in weaklyacidic media or under neutral conditions. The ability of this Au/PVP system to catalyzethe intramolecular addition of amines to alkynes, yielding exo-methylene pyrrolidines,was also demonstrated.100

Other Rearrangements

The skeletal rearrangement of n-alkanes into the corresponding branched isomers (iso-alkanes) is a fuel-upgrading technology of outstanding importance. In the search ofstable and environmentally friendly catalysts for these transformations, much attentionhas been paid in recent years to solid acid catalysts.101,102 Among them, sulfated zirconia(SO42−/ZrO2) has attracted considerable interest because of its remarkable activity atrelatively low temperatures.101–104 An important factor affecting the performance of thissolid acid catalyst is its degree of dispersion that affects the concentration and number ofactive sites. If sulfated zirconia is synthesized by conventional routes, it is difficult to havegood control over its textural properties. For example, the surface area of active sulfatedzirconia is less than 100 m2 g−1 in the tetragonal crystalline phase. Recently, Althuesand Kaskel have demonstrated that microemulsion-derived sulfated zirconia NPs, withsurface areas up to 175 m2 g−1, present a higher activity in n-butane isomerizationcompared to conventional sulfated zirconia (4.7 × 10−7 vs 3.1 × 10−7 mol s−1 g−1

at 300 ◦C).105 The same reaction was also explored using nanoparticles of sulfatedzirconia anchored to different inorganic supports such as TiO2 or MCM-41 mesoporoussilica.106–110 Interestingly, in the course of these studies, a remarkable promotional effectof Al2O3 on the catalytic behaviour of this solid acid catalyst was disclosed, allowing toimprove the n-butane conversion up to six times compared to that of the correspondingunpromoted catalyst (selectivities up to 94% toward iso-butane could be reached).109,110

The balanced distribution of Lewis and Brønsted acid sites and a higher dispersion ofzirconia were evoked to explain this beneficial effect. Related isomerization reactions ofn-pentane, n-hexane, and n-hexadecane have been described using as catalysts Ni-, Mo-,


andW-based nano-oxides supported on zeolites,111 ZrO2,112 or zirconium phosphates.113

However, only very low conversions and nonselective transformationswere attainedwiththese systems.

A series of Pt, Pd, Rh, and Au and mixed Pt–Au, Pd–Au, Pt–Sn, Pt–Rh, or Pt–Cs-NPs anchored to different zeolites and mesoporous silicas, as well as Pt–Au and Pt–Snnanoalloys, have been employed to promote the hydroisomerization of n-alkanes.114–126

However, although some of these systems presented a remarkable activity and highlevels of reusability, they systematically led to reaction mixtures containing the corre-sponding iso-alkanes along with cracking products resulting from the hydrogenolysisof the substrate. In this case, best results in terms of selectivity were obtained withPt-NPs dispersed on mesoporous Al–MCM-48 materials.123 These systems showedselectivities in the isomerization of n-octane up to 81%, with conversions typically of50% at 375 ◦C, favoring the formation of the 3-methylheptane isomer over the 2- and4-methylheptanes. Nanocomposite materials of type Pt/WO3/ZrO2, involving highlydispersed WO3 domains and Pt nanoclusters supported on ZrO2 nanocrystals, alsoshowed a superior activity and selectivity for C7+ paraffin hydroisomerization.127 Forexample, 80–90% conversions with 80–82% isomerization selectivities were reachedfor n-heptane and n-octane working at 150 ◦C.

On the other hand, the isomerization of oximes to amides (Beckmann rearrange-ment) is one of the most straightforward synthetic routes for the production of amides (orlactams if cyclic oximes are employed as the starting materials).128,129 The Beckmannrearrangement generally requires a very high reaction temperature and strong acidicconditions. However, Gai and coworkers have recently revealed that conversion of vari-ous ketoximes efficiently takes place under a moderate-temperature regime (130 ◦C) inthe presence of tungstated zirconia solid acid nanocatalysts (5–35 wt% of W).130,131 Bythe aid of one of these systems, composed of active WOx nanoclusters and nanoparticles(0.6–1 nm) located at grain boundaries on the surface of ZrO2 nanograins (∼15 wt% ofW), the nonsteroidal anti-inflammatory drug paracetamol 22 could be generated in 92%gas chromatography (GC)-yield from the readily accessible oxime 21 after only 6 h ofheating in benzonitrile (Scheme 8.15). The vapor-phase Beckmann rearrangement ofcyclohexanone oxime 23 into caprolactam 24, which is the feedstock in the industrialproduction of the synthetic polymer nylon-6, has also been described using ZrO2/TiO2(1 : 1) mixed nanoparticles (Scheme 8.15). Reactions, which were performed in a fixed-bed downflow reactor, led to amaximum oxime conversion of 40% and a good selectivitytoward the desired lactam (98%) using particles of 20 nm average size at 300 ◦C.132

Since its discovery in 1912,133 the Claisen rearrangement has become one of themost widely used synthetic tools for the selective formation of new carbon–carbonbonds.134–136 In this context, the Claisen rearrangement of allyl phenyl ether 25 wasstudied using Keggin-type 12-tungstophosphoric acid (TPA) nanocrystals encapsulatedover TiO2/SBA-15 nanocomposites.137 One of these nanocatalysts, composed of 15 wt%TPA deposited on TiO2 (22.4 wt%)/SBA-15, was found to be active and selective in theformation of 2,3-hydro-2-methylbenzofuran 27 (Scheme 8.16). The reaction proceededvia the expected generation of the allyl phenol intermediate 26, which undergoes aprototropic rearrangement to the more stable bicyclic compound 27. The two steps arecatalyzed by the acid sites of the nanocatalyst (see mechanistic details next). It is worthy


HO

N

OH

HO

HN

O

N

OHNH

O

Benzonitrile / 130 °C / 6 h

21

WOx / ZrO2

22

(92% yield)

EtOH / 300 °C / 1 atm / 1 h

Flow rate = 15 ml/min23

ZrO2 / TiO2 (1:1)

24(40% conversion; 98% selectivity)

Scheme 8.15. Examples of Beckmann rearrangements using nanocatalysts.

of note that the activity of TPA encapsulated over TiO2/SBA-15 is remarkably higherthan that of pure TPA, TiO2/SBA-15, or TPA supported on SBA-15. This fact clearlyindicates that the acidity and the structural control of the nanocomposite materials arecritical for obtaining an optimal catalytic activity. In addition, the catalyst proved to bereadily recyclable after separating it from the reaction mixture by filtration, washingwith 1,2-dichloroethane and subsequent reactivation at 500 ◦C in air, giving in a secondand third run the products in similar yields with negligible leaching of TPA.

The Wolff rearrangement of �-diazocarbonyl compounds has been of tremendoussynthetic utility for over 100 years, particularly for the homologation of carboxylicacids (Scheme 8.17).138–140 The rearrangement involves the initial generation of a short-lived �-keto-carbene, by extrusion of a nitrogen molecule, which readily undergoesa stereospecific 1,2-carbon shift leading to the formation of a highly reactive ketene.Further nucleophilic attack of water on this ketene intermediate leads to the carboxylicacid. Among other methods, the elimination of N2 can be initiated by the aid of metalcatalysts such as Ag(I) salts. In this context, recent studies have demonstrated that in situ

OO

OH

Tetrachloroethylene / 110 °C / 12 h

25 27

26

TPA / TiO2 / SBA-15 (10 mol%)

(81% conversion; 61% selectivity toward 27)

Scheme 8.16. Claisen rearrangement of 25 using a TPA/TiO2/SBA-15 nanocomposite as

a catalyst.


O

R1R2

N2

O

R1R2

O

R2R1

H2OO

HOR2

R1

hν, Δ or Ag+

- N2(g)

Scheme 8.17. The Wolff rearrangement for the homologation of carboxylic acids.

generated Ag(0)-NPs (0.5–1.5 nm) are the real active species in theWolff rearrangementof �-diazoketones catalyzed by Ag2O and other Ag(I) species, as clearly evidencedby UV-Vis, TEM, and SEM imaging measurements.141 Based on these observations,an efficient preparative route of carboxylic acids from �-diazoketones, in which theactive Ag-NPs are generated from Ag2O electrochemically, was developed by Sudrik,Vijayamohanan, and coworkers.24, 25

�-Pinene oxide 28 is known as a reactive molecule that rearranges easily under theinfluence of an acid catalyst. Thereby, many products, such as campholenic aldehyde29, trans-carveol 30, pinocarveol 31, or isopinocamphone 32, can be formed (Figure8.2). The most industrially desired compound among these is campholenic aldehyde29 since it is a key molecule for the synthesis of various highly intense sandalwood-like fragrance chemicals.142 In this context, a series of Lewis acids (FeCl3, ZnCl2, orH3BO3) supported on silica and titania nanoparticles have been tested in the selectiveisomerization of 28 into 29.143 In general, the Lewis acids supported on titania werefound to be more active than analogous silica-supported ones due to the higher surfacearea of the TiO2 support. The selectivity was instead found to be independent of thesupport employed. Among the investigated catalysts, the ZnCl2-based systems resultedin the most selective catalysis, leading to a maximum yield of 74% of campholenicaldehyde 29 under optimized reaction conditions (70 ◦C in cyclohexane). Catalystscomprising Fe2O3-NPs dispersed on mesoporous silicas were also shown to catalyze the

O

OH

O

OH

O

28

31

32

30

29

Figure 8.2. Product range of isomerization of α-pinene oxide.


Au / TiO2

ClCH2CH2Cl / 80 °C / 2–10 h

R1, R2, R3, R4 = H, alkyl, aryl or ether group

O

R2

R1R3

R4

OH

R2

R1R3

R4

(55–98% yield; 13 examples)

Scheme 8.18. Transformation of epoxides into allylic alcohols catalyzed by Au/TiO2.

isomerization of 28 toward the formation of aldehyde 29, albeit with lower selectivities(up to 53%).144,145

In contrast, the isomerization of 28 by means of Au-NPs supported on TiO2 resultedin the generation of pinocarveol 31 as the major reaction product (65% selectivity).146

In fact, using 1,2-dichloroethane as a solvent at 80 ◦C, this bifunctional acid–basesystem (see mechanistic details next) proved to be a general catalyst for the selectiveisomerization of epoxides into the corresponding allylic alcohols (Scheme 8.18).146 Itis important to note that, in these reactions, the new double bond is always formed withhydrogen atom abstraction from the least hindered position.

Photochemical rearrangement of phenyl esters into hydroxy aryl ketones via radicalmigration of the acyl group, known as the photo-Fries rearrangement, is a well-knownprocess in organic chemistry.147,148 Such a concerted rearrangement, coupled with non-concerted recombination pathways (oxidations), has been invoked by Sivalingam andMadras in the photocatalytic degradation of the industrially important and nonbiodegrad-able plastic poly(bisphenol-A-carbonate) (Figure 8.3) under UV or solar exposure.149

Dihydroxybenzophenones, phenyl salicylate, and phenolic species are generated as themajor decomposition products. Interestingly, in the presence of nano-sized high-surface-area TiO2, the degradation rates of poly(bisphenol-A-carbonate) were found to increaseby an order or magnitude compared to those observed under catalyst-free photochem-ical conditions, representing a nice example of the potential utility of nanocatalysis inenvironmental chemistry.

INSIGHT ON MECHANISTIC ASPECTS

Density functional theory (DFT) was applied to investigate the cis-to-trans-hydroisomerization of but-2-ene by Pd9 metal clusters (unsupported and anchored to

OO

O

n

Figure 8.3. Structure of the engineering plastic poly(bisphenol-A-carbonate).

INSIGHT ON MECHANISTIC ASPECTS 267

H3C CH3

HH

H2Pd9

CH2

H3C

CH3H

HPd9

H3C H

CH3H

H2Pd9

CH2

H3C CH3

H

HPd9

75–94 kJ mol–1 13 kJ mol–1

45–48 kJ mol–1

Scheme 8.19. cis-to-trans-Isomerization of but-2-ene on Pd9 clusters.

carbon nanotubes or zeolite supports).150 The first and rate-limiting step of the pro-cess involves the migration of a hydrogen atom from the hydrogenated cluster to theadsorbed cis-but-2-ene molecule, which requires energy barriers of 75–94 kJ mol−1

(Scheme 8.19). Then, an almost free rotation (13 kJ mol−1) around the –HC–CH2–bond takes place, followed by backtransfer of H to Pd9. This latter step, which givesthe product trans-but-2-ene adsorbed on the cluster, has a barrier of 45–48 kJ mol−1

remarkably lower than the initial H-transfer process from the substrate to the cluster.Related DFT calculations, along with H-D exchange experiments, were also performedfor the cis-to-trans-isomerization of buten-2-ene catalyzed by tetrahedral Pt(111)-NPs,31

and Pd-NPs grown in situ on planar Fe3O4/Pt(111) or Al2O3/NiAl(110) thin films.33–35

As commented previously, Au–NPs stabilized by the hydrophilic polymer PVPhave been described as an efficient catalytic system for both the hydroalkoxylationand the hydroamination of unactivated alkenes under aerobic conditions (Schemes 8.13and 8.14).94,98–100 Based on studies using deuterated solvents and labeled substrates, areaction mechanism was proposed for the hydroalkoxylation reactions94 (Scheme 8.20).The process is initiated by the formation of the key intermediate A, which possesses anelectron-deficient site generated by adsorption of O2 onto the surface of the Au-NPs. Aacts as a Lewis acid, activating both the alkoxide and alkene units of the deprotonatedsubstrate by adsorption onto the surface of the nanoparticles (B), and giving C byinsertion of the alkene moiety into the O–Au bond. From C, neither �-elimination orO2 insertion, nor hydrolysis takes place; only homolytic dissociation of the Au–C bondoccurs. By this way, the radical intermediateD is formed and readily evolves into the finaltetrahydrofuran by hydrogen abstraction from DMF, a process which is accompaniedby the regeneration of the active species A. For the related hydroamination processes, asimilar reaction mechanism was postulated.98–100

DFT calculations were also applied to investigate the structure, reactivity, andcatalytic activity in n-alkane hydroisomerization of model Pt6 particles encapsulated indifferent zeolites.124–126 The results show that interaction of the platinumparticlewith theacid sites of the support suppresses both its Lewis and Brønsted acidities. On this basis,a mechanism for n-alkane isomerization and cracking, avoiding the direct participationof these acid sites, was proposed, with the Pt-NPs stabilized in the zeolite channelsacting as the real active sites during catalysis (Scheme 8.21). Thus, the initial oxidative


R2

R1 O

R3R3

O

R3

R2

R1

R3

Au

O O

Au

A

Au

O O

B

OR1

R2

R3

R3

Au

O O

C

OR1

R2

R3

R3

X

X

O2

DMF

DMF

DMF

D

O

R3

R2

R1

R3OH

O

R3

R2

R1

R3

Au

O OOR1

R2

R3

R3

X

H2O

O

R3

R2

R1

R3

δ-

+ O2

δ+

β-Elimination

or

17

17

18

18

17

Scheme 8.20. Proposed mechanism for the hydroalkoxylation reactions catalyzed by Au-NPs.

addition of the n-alkane to the nanoparticle gives a surface metal–alkyl structure E. TheC–H bond cleavage in E then leads to the formation of a metal–cyclobutane species F,which evolves by C–C cleavage into the carbene intermediate G in which the resultingalkene remains adsorbed. Further transformation ofGmay result in either isomerizationinto the corresponding iso-alkane or cracking. Thus, while the isomerization proceeds

Pt Pt

* *

+

R

Pt Pt

* *

R

E

Pt Pt

* *

R

F

Pt Pt

* *G

R

Pt Pt

* *

+

CH4

R

R

Pt Pt

* *

R

Pt Pt

* *

R

Pt Pt

* *

R

Pt Pt

* *

+

- H - H Cracking

+4 H

Isomerization

+ H+ H

Scheme 8.21. n-Alkane transformations over Pt/zeolite catalysts.

INSIGHT ON MECHANISTIC ASPECTS 269

O

H

OOH

H

O

H OH

H

H

OH

HOH

H

O

25

26

26 27

Scheme 8.22. Schematic representation of the Claisen rearrangement of 25 into 27.

via rotation of the adsorbed alkene and recombination with the carbene, the crackingroute involves the direct hydrogenation of the hydrocarbon fragments of G.

The Claisen rearrangement of allyl phenyl ethers to 2-allylphenols is a well-studiedreaction.134–136 However, as previously commented, rearrangement of allyl phenyl ether25 in the presence of a TPA/TiO2/SBA-15 nanocomposite leads to the major forma-tion of 2,3-dihydro-2-methylbenzofuran 27 instead of the expected 2-allylphenol 26(Scheme 8.16).137 The highly acidic character of the nanocatalyst employed was evokedto explain this unexpected result. Thus, once formed, the adsorbed phenol 26 can bereadily protonated at the allylic double bond by the acid sites of the nanocomposite toproduce a secondary carbenium ion, which reacts intramolecularly with the phenolicoxygen to generate the dihydrobenzofuran 27 (Scheme 8.22).

Concerning the role of silver nanoparticles (Agn) during the Wolff rearrangementof �-diazoketones,24,25 two nonclassical electron-transfer pathways have been proposedon the basis of electrochemical data (Scheme 8.23). In the first one (path A), the key�-keto-carbene is generated through a silver-carbene intermediate and instantaneouslyrearranges into the ketene, while in the second one (path B) it is a carbene cation radical,which ultimately leads to the ketene. In both cycles, the Agn+/Agn0 redox couple allowsthe initial removal of an electron from the substrate and its backdonation after N2elimination.

Finally, on the basis of kinetic studies using deuterium-labeled substrates, a con-certed mechanism for the conversion of epoxides into allylic alcohols, catalyzed byAu-NPs supported on TiO2 (Scheme 8.18), was proposed.146 Apparently, the Au/TiO2system acts as a bifunctional catalyst where gold, acting as a Lewis acid, activates theepoxide, and the titania support, acting as a base, simultaneously abstracts an hydrogenatom from one of the geminal carbons of the epoxide (Scheme 8.24). This extrapo-lated site selectivity is in agreement with the remarkable regioselectivity experimentallyobserved, where hydrogen atom abstraction occurs from the less hindered carbon atom.


OR1

R2 NN

OR1

R2 NN

Agn+

OR1

R2 NN

Agn0

N2

OR1

R2

Agn0

OR1

R2 Agn+

OR1

R2O

R1 R2

H2O

R1 R2

CO2H

O

Agn0

R1 R2

O

Agn+

R1 R2

Agn+

Path A

n/2 Ag2O + n/2 H2O + ne-

Agn - (Agn0/Agn+)

n OH-

Path B

Agn+

Scheme 8.23. The role of silver nanoparticles (Agn) during the Wolff rearrangement of α-

diazoketones.

SUMMARY AND FUTURE OUTLOOK

In line with the current demand for a sustainable development, the search for new low-cost and recyclable catalytic systems, showing improved activities and selectivities, toreplace existing homogeneous/heterogeneous ones is an important issue. The use ofnanocatalysts has emerged as one of the most promising solutions to overcome the

Au / TiO2

O

R1 R2

R3

R4

- Au(I)

O

R1 R2

R3

R4

Au

O

R1 R2

R3

HO

Au

R4

OH

R1 R2

R3

R4from TiO2

‡

H+ transfer from

protonated titaniaδ+δ+

δ+

Scheme 8.24. Proposed mechanism for the isomerization of epoxides catalyzed by Au/TiO2.

REPRESENTATIVE EXPERIMENTAL PROCEDURES 271

problems associated with these classical systems as clearly indicated by the growingnumber of emerging publications in the field.

In this context, herein we have summarized the utility of metal and metal–oxidenanoparticles to promote the rearrangement of organic molecules through an up-to-dateoverview of the published literature. As commented previously, these atom-economictransformations play a pivotal role in organic synthesis, and present some implicationsin different industrial fields of outstanding importance. From the various examplesdiscussed in this chapter, one can conclude that the performance of any nanocatalystfor a particular reaction strongly depends on the particle shape, composition, size, andinteraction with the support. However, relationships between these key properties andreactivity are, in general, poorly understood and still need to be rationalized in order toextend the applicability of the new generation of catalysts. Stability, durability, and costare also issues that need to be fully addressed for a future usage in industry. Obviously,this fast-growing area remains open with many opportunities for new discoveries ahead.Therefore, an intense research in the field is expected to continue for the coming years.

REPRESENTATIVE EXPERIMENTAL PROCEDURES

cis-to-trans-Isomerization of But-2-Ene Catalyzed by Tetrahedraland Cubic Pt-NPs Dispersed onto a High-Surface-Area SilicaXerogel Support30–32

The synthesis of both tetrahedral and cubic platinum particles was performed by follow-ing essentially the same procedure. The platinum precursor (H2PtCl6 or K2PtCl6) and theappropriate capping agent (PVP or sodium polyacrylate) were added to distilled water,thoroughly mixed, treated with bubbling argon for 20 min, and reduced with bubblinghydrogen gas for 10 min. The flask was sealed, wrapped in aluminum foil, and storedin the dark for 12 h. At this point, the color of the solution turned from pale yellow tobright gold or brown, indicating nanoparticle formation. The silica xerogel support wassynthesized by combining 30% aqueous ammonia and ammonium fluoride solutionswith a 1 : 2 v/v ethanol/water mixture, stirring the resulting liquid into a separate ethanolsolution of tetraethylorthosilicate, and evaporating the solvent at 52 ◦C until a whitepowder was obtained. The silica xerogel was calcined to 402 ◦C before use. To preparethe supported catalyst, the silica xerogel was added to a water-based colloidal solutionof the appropriate Pt-NPs, and the mixture was sonicated and filtrated. The resulting wetpowder was dried and calcined in air and oxidized and reduced three times in a quartztube under flowing O2 and H2, respectively.

The catalytic activity of the Pt-supported catalysts was investigated in a smallbatch reactor consisting of a stainless loop with 150 ml total volume evacuated with amechanical pump to a base pressure of 5× 10−2 torr and equipped with a thermocouplegauge and a capacitancemanometer for pressure determinations. Samples of the catalysts(5 mg) were sandwiched between two quartz wool plugs and placed in one of the verticalarms of a U-shaped quartz tube cell (10 mm inner diameter) inserted in the reactorloop, which could be heated in a furnace controlled by a temperature controller. The


reactant gases (argon–2-butene–hydrogen= 590 torr : 10 torr : 0.2 torr) were introducedsequentially into the reactor loop andmixed using a recirculation bellows pump.Aliquots(10 ml) of the gas sample were taken every 20 min during the catalytic reactions byusing an eight-port valve and analyzed with a gas chromatograph.

Isomerization of Allylbenzenes Catalyzed byRhCl3·3H2O/Aliquat-336 Ion-Pair Encaged within Silica Sol–Gel55

Preparation of the Sol–Gel-Entrapped Rhodium Catalyst. A mixture ofphenyltrimethoxysilane (0.45 ml; 2.26 mmol), tetramethoxysilane (TMOS; 3.6 ml;24.2 mmol), RhCl3·3H2O (30 mg; 0.114 mmol) dissolved in triply distilledwater (2.0 ml), and Aliquat-336 (N-methyl-N,N-dioctyloctan-1-ammonium chloride,0.124 mmol) dissolved in MeOH (3.0 ml), was stirred at room temperature for 3 days.The gel was allowed to stand at 25 ◦C for 3 days and then dried at 80 ◦C and 0.5 torrfor 24 h. The dry sol–gel matrix was washed and sonicated with boiling CH2Cl2 (30 ml)and redried at 0.5 torr at 80 ◦C for 5 h before use.

General Procedure for the Catalytic Isomerizations. For the preparation ofthemicroemulsions, amixture of the allylbenzene (1.35mmol), triply distilledwater (17–20 ml), the appropriate surfactant sodium dodecyl sulfate or cetyltrimethylammoniumbromide (0.6–0.8 g), and n-PrOH (1.6–2.0 ml) was stirred magnetically at 25 ◦C until aclear transparentmicroemulsionwas formed.One half of the above immobilized rhodiumcatalyst (containing usually 0.057 mmol of Rh) was roughly ground and admixed withina mini autoclave equipped with a sampler together with one of the aforementionedfreshly prepared microemulsions. The autoclave was inserted in a preheated thermostat.After stirring the reaction mixture at the desired temperature, and for the necessarylength of time, the autoclave was cooled to 20 ◦C. The sol–gel material was filtered off,washed, and sonicated for first 30 min with triply distilled water (30 ml) and then withhexanes. The sol–gel-free filtrate was treated with NaCl (2 g), which caused breakageof the microemulsions and phase separation. The aqueous layer was extracted withhexanes (2 × 30 ml); the extract was dried over MgSO4, concentrated, and subjected togas chromatographic separation and analysis. The dried sol–gel was reused for a furthercatalytic run.

Isomerization of Cyclohexene into Methylcyclopentene Catalyzedby Bimetallic Pt–Pd-NPs Supported on TiO2

68

Chloroplatinic acid (H2PtCl6), palladium chloride (PdCl4), and TiO2 were dispersedand stirred in aqueous solution for 6 h at room temperature. Different atomic concen-trations of Pd and Pt were used to obtain samples with 1 wt% of total metal contentsupported. Then, solids were separated by evaporation. The solids were then dried at80 ◦C overnight, before thermal treatment under hydrogen flow at 400 ◦C for 4 h.

The catalytic tests in the cyclohexene isomerization reaction were performed underatmospheric pressure in a microflow fixed-bed reactor (6 mm inner diameter) using100 mg of catalysts (grain size 63–125 �m) at 250 ◦C. Prior to any measurement, the


catalysts were submitted to in situ heating in N2 at 200 ◦C for 2 h (ramp 0.02 ◦C/min).Cyclohexene was feed by bubbling N2 through a saturator at 25 ◦C. The reactant wasthen passed through the catalyst at 250 ◦C, and the effluent sampled at regular intervalsof 20 min for analysis by GC.

Redox Isomerization of Allylic Alcohols Catalyzed by theNanoferrite-Supported RAPTA Complex 126

Fe3O4/SiO2 Magnetic Nanoparticles. Fe(II) chloride tetrahydrate (10.0 g;50 mmol) and Fe(III) chloride hexahydrate (16.2 g; 60 mmol) were separately dis-solved in water (100 ml) by stirring at room temperature. The two solutions were thenmixed and stirred for 15 min. Ammonium hydroxide (25%) was added slowly to adjustthe pH of the solution to 9–10. The reaction mixture was continually stirred for 4 h atroom temperature. The precipitated Fe3O4-NPs were separated magnetically, washedwith water until the pH reached 7, and then dried under vacuum at 60 ◦C for 2 h. ThisFe3O4 ferrite (2.0 g; 8.64 mmol) was dispersed in isopropanol (500 ml) and the mixturewas sonicated for 30 min. To this solution, concentrated ammonium hydroxide (25%;50 ml) was then added, followed by the dropwise addition of a solution of tetraethy-lorthosilicate (1 ml; 4.5 mmol) in isopropanol (100 ml) for 1 h. The solution was stirredat room temperature for 6 h. The silica-coated magnetic nanoparticles were recoveredby centrifugation, washed thrice with water (50 ml) and thrice with ethanol (50 ml), andthen dried under vacuum at 60 ◦C for 2 h.

RAPTA Complex Supported on the Fe3O4/SiO2 Magnetic Nanoparticles.4 g (9 mmol) of 1-(3-(trimethoxysilyl)propyl)-3,5-diaza-1-azonia-7-phosphatricyclo[3.3.1.13,7]decane iodide and 4 g of the silica-coated nanoferrites were dispersed in100 ml of CHCl3 (sonicated), and the mixture was refluxed for 48 h. The solid was thenseparated magnetically, washed with CHCl3 (3× 50 ml), and dried under vacuum at 60◦C for 2 h. Then, synthesized PTA-functionalized nanoferrites (2 g) were dispersed inmethanol (20 ml) under N2 atmosphere. A solution of [{RuCl(�-Cl)(�6-p-cymene)}2](2.5 g; 4.1 mmol) in methanol (100 ml) was added to them and stirred at room tempera-ture for 24 h. The resulting product 1was separated magnetically, washed with methanol(3 × 30 ml) and dried under vacuum. The weight percentage of P and Ru in the solidwas found to be 1.2% and 1.6%, respectively, by the ICP-MS analysis.

Catalytic Isomerization Reactions. Under nitrogen atmosphere, nanoferrite1 (100 mg; 1.58 mol% of Ru), water (3 ml), and the corresponding allylic alcohol(1 mmol) were introduced into a crimp-sealed thick-walled glass tube equipped with apressure sensor and a magnetic stirrer. The reaction tube was placed inside the cavity of aCEM Discover R© focused microwave synthesis system, operated at 150 ◦C (temperaturemonitored by a built-in infrared sensor), 100 W, and 100–120 psi for 0.25–24 h. Thecourse of the reaction was monitored by regular sampling and analysis by GC. Aftercompletion of the reaction, the mixture turned clear and the catalyst was deposited onthe magnetic bar within 30–45 s. After magnetic separation of the catalyst, the mixture


was cooled and two phases appeared. Fractional Kugelrohr distillation of the organicphase allowed isolation of the final carbonyl compound.

Intramolecular Cyclization of Alkynoic Acids Catalyzed by Au-NPsSupported on Zeolite β-NH4

+83

�-Zeolite (1 g) was vigorously stirred for 3 h with an aqueous 1 M solution of NH4NO3(100 ml) at 80 ◦C. After this time, the slurry was filtered and carefully washed, dried for6 h at 60 ◦C, and calcined for 24 h at 500 ◦C. The Au-supported catalyst was preparedby the deposition–precipitation method, by adding the support (1 g) to an aqueoussolution of HAuCl4 (10−3 mol l−1) previously adjusted at pH 8.5 with a NaOH solution(0.2 mol l−1). The slurry was kept at 70 ◦C under vigorous stirring for 3 h. After stirring,the sample was filtered, washed with deionized water until complete elimination ofchloride, and then dried under vacuum at 60 ◦C for 24 h.

Amixture of the acetylenic acid (0.26mmol) and gold catalyst (20mg) in acetonitrile(1.2 mol l−1) was stirred under an air atmosphere at room temperature or 40 ◦C. Aftercompletion of the reaction (∼8 h), the mixture was centrifuged. By simple decantation,the catalyst was removed from the reaction mixture and the solvent was evaporatedunder reduced pressure to give the corresponding lactone.

Cycloisomerization of N,N-diallylamides andN,N-Diallylsulfonamides Catalyzed by Pd-NPs93

In a mixture of 1.0 ml of concentrated HCl and 2 ml of deionized water, 0.02 g(0.11 mmol) of PdCl2 was dissolved. The pH of the solution was then adjusted to7.0 using 0.1 M NaOH. Under magnetic stirring at 50 ◦C, 0.11 g of hexadecylpyri-dinium chloride (0.32 mmol) and 15 ml of acetone were added to this solution. Afterstirring for about 10 min, 6 ml of 80% hydrazine hydrate was added. The colour ofthe solution then changed from yellow-brown to black, indicating the formation of pal-ladium colloid. Finally, the mixture was separated by centrifugation. The deposit waswashed with deionized water and ethanol for several times. The Pd-NPs were obtainedafter vacuum drying.

In 2 ml of DMF, 1 mmol of the corresponding N,N-diallylamide or N,N-diallylsulfonamide 15 was added. Then, 0.01 mmol of the Pd-NPs (1 mol%) and0.2 mmol of BTBAB were added in turn. The mixture was heated at 70 ◦C withstirring under a nitrogen atmosphere for the desired reaction time (monitored by thinlayer chromatography (TLC)) till reaction was completed, and then centrifuged. Thesolution was separated and the precipitate was washed with diethyl ether (5 × 3 ml).The solutions were combined and washed with water, dried over anhydrous Na2SO4,and purified by column chromatography on silica gel with hexane–ethyl acetate (15 : 1)as eluent to yield the pure product 16.

The precipitate was further washed sufficiently with methanol and diethyl ether,and then dried. Pd-NPs were thus recovered and could be reused for five times.


Intramolecular Hydroalkoxylation of Alkenes Catalyzed by Au-NPsStabilized by PVP94

In an aqueous solution of HAuCl4 (1 mM, 50 ml), 555.5 mg of PVP was added so thatthe molar ratio of AuCl4− and monomer unit of PVP was kept at 1 : 100. The mixturewas further stirred for 30 min in an ice bath kept at 0 ◦C. Then, an aqueous solution ofNaBH4 (100 mM, 5 ml) was rapidly sprayed into the mixture under vigorous stirring.The colour of themixture immediately turned from pale yellow to dark brown, indicatingthe formation of small Au-NPs. The Au/PVP-NPs were subsequently dialyzed overnightto remove inorganic impurities such as Na+ and Cl−, which is a crucial treatment toenhance the stability of the Au/PVP-NPs against coalescence. The dialyzed hydrosol ofthe Au/PVP-NPs was diluted to 100 ml and stored at 2 ◦C.

The corresponding hydroxyalkene 17 (0.05 mmol) and DBU (15 �l, 0.10 mmol)were dissolved in 5 ml of DMF. Then, the aqueous solution of Au/PVP (10 ml; 10 mol%of Au) was added and the reaction mixture was stirred vigorously (1300 rpm) at 50 ◦Cfor 16–24 h. The reactionmixture was extracted with methyl tert-butyl ether (3× 10ml),and the combined organic layers were washed with water (100 ml) and brine, dried overNa2SO4, and concentrated in vacuo. Purification of the resulting tetrahydrofurans 18was carried out by preparative thin-layer chromatography.

Hydroisomerization of n-Octane Catalyzed by Pt-NPs Dispersed onMesoporous Al–MCM-48123

The appropriate amount of tetraamine Pt(II) nitrate for the chosen Pt loading in samples(from 0.5 to 5 wt%) was dissolved in ethanol and subsequently mixed with the Al–MCM-48 support,151 stirring the mixture in a rotary evaporator at 40 ◦C for a few hours.Ethanol acts as the reducing agent in the deposition of the Pt-NPs. The materials werethen calcinated in air (60 ml min−1) at 400 ◦C for 2 h and then reduced in hydrogen(100 ml min−1) at 400 ◦C for 3 h.

The catalytic reactions (typically performed with 0.1 g of the catalyst) were carriedout in a continuous-flow system with PC control at 375 ◦C, employing hydrogen as acarrier (25 ml min−1) at 5 bar. The n-octane (50 �l min−1) was fed into the system bymeans of a peristaltic pump. Reaction products (n-propane, n-butane, n-hexane, toluene,and 2-, 3-, and 4-methylheptane) were identified and characterized by GC and GC-MS.Typical conversions after 4 h were 44–54%, with selectivities of 70–81% toward thedesired methylheptanes.

Beckmann Rearrangement of Ketoximes Using TungstatedZirconia Solid Acid Nanocatalysts130,131

Zirconium oxohydroxide ZrOx(OH)4-2x was prepared by the controlled addition of aque-ous zirconium oxychloride ZrOCl2·8H2O solution to an NH4OH (2 M)/NH4Cl (2 M)buffer solution to keep the pH at a constant value of 10.5. The resulting solid wasfiltered and thoroughly washed with distilled water to remove the chlorides. Then,ZrOx(OH)4-2x was impregnated with an aqueous solution of ammonium metatungstate


(NH4)6H2W12O40 to obtain W loadings in the range 5–35 wt%. The impregnated mate-rials were dried at 60 ◦C overnight and calcined (700 ◦C) for 4 h.

The rearrangement of oximes (0.9 mmol) with these nanocatalysts (20 mg) wascarried out in benzonitrile (20 ml) at 80–150 ◦C in a 50 ml glass reactor equipped with acondenser and a magnetic stirrer. Tetradecane was added as a GC internal standard and,to monitor the reaction, 0.1 ml samples of the reaction mixture were taken periodicallyand analyzed by GC. Using a catalyst with a approximately 15 wt% content of W,77–92% yields in the amides were typically observed by GC after 6 h of heating at130 ◦C.

Claisen Rearrangement of Allyl Phenyl Ether Using TPANanocrystals Encapsulated over a TiO2/SBA-15 Nanocomposite137

Mesoporous silica SBA-15 (2 g) was first impregnated with an aqueous solution ofTiCl4 in a rotary evaporator under a vacuum of 10−7 torr attained by means of a turbopump. The TiCl4 solution was prepared in deionized water using high-speed stirring inan ice-water bath under air-tight conditions. An excess of this solution (10, 22.4, 30,50, and 70 wt%) was kept in contact with silica for 6 h under vacuum. The volatileswere then stripped off and the solid was dried at 100 ◦C in air. A similar procedure wassubsequently used for the wet-impregnation of TPA (10, 15, 30, 50, 70, and 90 wt%)on the prepared TiO2/SBA-15 nanocomposites. All the catalysts were dried overnight at100 ◦C in air in an oven and further calcined at 850 ◦C.

The Claisen rearrangement reactions were carried out in batch mode in a 50 mlthree-necked round-bottomed flask under nitrogen atmosphere, using 3 g of allyl phenylether 25 in 10 g of tetrachloroethylene with 0.3 g of freshly calcined catalysts at 110 ◦C.To monitor the reaction, aliquots of the reaction mixture were taken periodically andanalyzed byGC.Using a catalyst with an approximately 15wt%content of TPA encapsu-lated over TiO2(22.4 wt%)/SBA-15, 81% conversion of allyl phenyl ether was observedby GC after 12 h leading to the major formation of 2,3-dihydro-2-methylbenzofuran 27(61% selectivity).

ACKNOWLEDGMENTS

Financial support from the Spanish MICINN (Projects CTQ2010-14796/BQU,CTQ2009-08746/BQU, CTQ2008-00506/BQU, and CSD2007-00006) is acknowl-edged. S.E.G.-G. and J.G.-A. also thank MICINN and the European Social Fund for theaward of “Ramon y Cajal” contracts.

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