13Copper Nanoparticle-Catalyzed Carbon-Carbon

DOI:10.1002/cssc.201100348

Copper Nanoparticle-Catalyzed CarbonàCarbon and CarbonàHeteroatom Bond Formation with a Greener Perspective

Brindaban C.Ranu,*Raju Dey,Tanmay Chatterjee,and Sabir Ahammed[a]

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1.Introduction

The division of larger particles into smaller ones with sizes be-tween1–100nm imparts unusual properties to these small species compared to those of bulk material.[1,2]These particles are called ultrafine particles[3]or nanoparticles(NPs),[4,5]where-as nanoclusters are defined as a material of at least one dimen-sion between1–10nm with a narrow size distribution.[6,7] Nanometer-sized single crystals are often referred to as nano-crystals.Nanoparticles,in general,are of great scientific interest as they effectively serve as a bridge between bulk materials and atomic or molecular structures.A bulk material should have constant physical properties regardless of its size;howev-er,at the nano-scale,size-dependent properties are often ob-served.The properties of materials change as their size ap-proaches the nano range,and the percentage of atoms at the surface of a nanomaterial becomes significant.For bulk materi-als larger than one micrometer(or micron),the percentage of atoms at the surface is insignificant with respect to the number of atoms in the bulk of the material.The interesting and sometimes unexpected properties of nanoparticles are, therefore,largely attributable to the large surface area of the material,which dominates the contributions made by the small bulk of the material.This characteristic change in the properties of nanoparticles compared to its counter bulk mate-rials may be attributed to the quantum size effect.With the decrease of particle size,the resultant surface-to-volume ratio increases considerably,which influences the surface-related properties of the materials.High surface area can provide better dispersion of the active sites and easy diffusion of the reactants and,hence,makes them suitable for catalytic activity. In general,the advantageous features of nanoparticles are: a)High surface-to-volume ratios of nanoparticles,which pro-vide a large number of active sites per unit area compared to their heterogeneous counterparts;[8]b)higher zeta potential for nanoparticles compared to bulk material,which prevent ag-gregation of nanoclusters;[9]c)low reduction potentials for nanoparticles,which facilitate oxidative addition in organome-tallic reactions;d)high reactivities of metal nanoparticles, which avoid the use of ligands;[10]and e)easy separation and recyclability of metal nanoparticles,which make them cost-ef-fective and minimize the chance of contamination of the cata-lyst with the product.[11]Thus,nanoparticles are endowed with several features of an efficient catalyst including green aspects useful for organic transformations.[12]

Although gold and silver received special attention mostly because of their ability to control economy,copper is also of considerable importance as an alternative to these metals. However,although copper occupies a position among transi-tion metals in the periodic table,metallic copper barely shows any catalytic activity like other members of this family.With the advance of nanotechnology and nanoscience,nanomateri-als received considerable interest because of their unique properties and applications in all branches of science.Similar to other nanomaterials,copper with nanometer-sized dimen-sions shows remarkable activity in applications from medicine to material to catalyst.

2.Copper Nanoparticles:Preparation and Characterization

Usually,two generic approaches are followed for the prepara-tion of nanoparticles:[13]top-down[14]and bottom-up[15] (Scheme1).The former is essentially the division of a massive solid into smaller portions.This approach involves the use of several methods,such as milling or attrition,chemical meth-ods,and volatilization of a solid followed by condensation of the volatilized components.In the bottom-up approach,the fabrication of nanoparticles is performed through condensa-tion of atoms or molecular entities in gas phase or in solution. This approach is gaining popularity in the preparation of nano-particles.

It is worth mentioning that dispersions of nanoparticles are intrinsically and thermodynamically metastable,[16]primarily be-cause of their high surface area.Uniform dispersion of the nanoparticles is dictated by the activation energy.When the activation energy is not sufficient to affect dispersion,an in-crease in nanoparticle size has been observed due to Oswald’s ripening.[17]Thus,highly dispersed nanoparticles are only kinet-ically stabilized[18]and cannot be prepared under

conditions [a]Prof.Dr.B.C.Ranu,R.Dey,T.Chatterjee,S.Ahammed

Department of Organic Chemistry

Indian Association for the Cultivation of Science

Jadavpur,Kolkata–700032(India)

Fax:(+91)33-2473-2805

E-mail:ocbcr@iacs.res.in G r e e n C h e m i s t r y b y N a n o c a t a l y s i s

that exceed some threshold.In addition,the use of surface-sta-bilizing agents has been used in synthesizing many nanomate-rials to hinder sintering,recrystallization,and aggregation.To prepare Cu,Cu2O,and CuO nanoparticles with well-defined morphology and chemical composition,both physical methods (chemical vapor deposition)and chemical methods(including sol–gel,hydrothermal,and electro-chemical deposition)have been followed.Some of them are discussed below.

2.1.Preparation of PEG-stabilized Cu Nanoparticles using microwave irradiation

Recently,Zhu et al.[19]reported the preparation of highly dis-persed copper nanoclusters through the reduction of a metal salt with suitable reducing agents(Scheme2)by using a solvo-thermal method.A basic aqueous solution of CuSO4was re-duced by N2H4·H2O in the presence of ethylene glycol as stabi-lizer,followed by microwave irradiation to give Cu nanoparti-cles.The energy-dispersive X-ray(EDX)spectrum(Figure1)and transmission electron microscopy image(TEM,Figure2)clearly indicate the formation of Cu nanoparticles with a size of4–6nm.These nanoparticles are highly reactive towards C(aryl)àS[20]and C(aryl)àSe[21]bond formation.

Henglein et al.observed that irradiation of a solution of copper perchlorate[Cu(ClO4)2]containing sodium formate led to the formation of colloidal copper[22]through reduction of Cu2+by both,solvated electrons and CO2generated during radiolysis.

2.2.Preparation of polyvinylpyrrolidone-coated Cu2O nano-particles

Chen and Kong et al.[23]developed a simple method for the synthesis of polyvinylpyrrolidone-coated Cu2O nanoparticles (Scheme3).The nanoparticles were capped inside the polymer matrix to restrict agglomerization.In a typical experimental procedure,an aqueous solution of copper acetate(Cu(OAc)2) was added to an aqueous mixture of sodium borohydride con-taining a polyvinyl polymer(40kDa)while stirring vigorously at room temperature.After2–3h,the solution turned dark brown.Then,the solution was centrifuged at10000rpm for 10min,and the precipitate was dried under vacuum.

The particle size distribution of PVP-coated Cu2O nanoparti-cles is20?10nm,and the catalyst is highly active towards azide–alkyne click reactions in water.

Academy of Sciences and

recipient of the J.C.Bose

N.S.Narasimhan Award

(2009)medal of the Chemical Presently,he is pursuing his Benign Solvents,Reagents,

for the Cultivation of Sci-

supervision of Prof. B.C.

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2.3.Solvothermal preparation of CuI nanoparticles

Xu and Feng et al.[24]reported a solvothermal method for the preparation of CuI nanoparticles by the reaction of [Cu(dmg)2](DMG =dimethylglyoxime)and KI in an autoclave (Scheme 4)

with ethanol as solvent.dmgH and Cu(OAc)2·H 2O were added into absolute ethanol in sequence,which was stirred at 08C for 30min to obtain a brown precipitate of Cu(dmg)2.The collect-ed precipitate was dispersed in absolute ethanol followed by addition of KI,and the mixture was stirred vigorously for 2h.Then,the mixture was transferred into a teflon-lined stainless steel autoclave.

The autoclave was sealed and heated at 1808C for 6h.The black precipitate thus obtained was centrifuged and washed with ethanol and deionized water,which was followed by vacuum drying for 12h at room temperature to give CuI nano-particles (30–80nm)of high purity.This catalyst is very efficient and selective for the synthesis of phenols,anilines,and thio-phenols from aryl halides in aqueous

solution.

Scheme 1.Conventional routes for the preparation of

nanoparticles.

Scheme 2.Preparation of PEG-stabilized Cu 0nanoparticles using microwave

irradiation.

Figure 1.EDX spectra of copper nanoparticles (adopted from Ref.

[20]).

Figure 2.TEM image of the Cu nanoparticles showing spherical particles with diameters in the range 4–6nm (adopted from Ref.

[20]).

Scheme 3.Preparation of PVP-coated Cu 2O nanoparticles by using a strong reducing agent at room

temperature.

Scheme 4.Preparation of CuI nanoparticles at very high temperatures in an autoclave.

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Copper-Catalyzed Bond Formation with a Greener Perspective

2.4.Core–shell nanoparticles consisting of Cu/Cu 2O

Hyeon and coworkers [25]demonstrated a straightforward and efficient method for the preparation of core–shell nanoparti-cles consisting of Cu/Cu 2O (Scheme 5)and the subsequent use of the nanocatalyst in the Ullmann-type amination of aryl

chlorides.In an experimental procedure,Cu(acac)2dissolved in oleyl amine was slowly heated to 2308C and held at this tem-perature for 6h,which produced a red colloidal solution.A TEM image showed that uniform 15nm-sized Cu nanoparti-cles were formed.When these Cu nanoparticles were exposed to air,the color of the nanoparticle solution turned blue.

The TEM image showed that the size and shape of the nano-particles remained practically unchanged after oxidation in air.The XRD pattern (Figure 3)suggested the formation of a Cu 2O shell from the copper core.The TEM and high resolution TEM

(HRTEM)image of an oxidized sample indicated the formation of polycrystalline Cu 2O on the surface.2.5.Supported nanoparticles

Because of the small size of metal nanoclusters and their solu-bility in reaction media,it is not always easy to separate them from the solution.Thus,nanoparticles are anchored onto a solid surface to achieve an easy separation,and a few proce-dures are described below.

2.5.1.Silica-supported Cu nanoparticles

Highly dispersed Cu nanoparticles supported on silica were prepared by using the straightforward and convenient precipi-tation–gelation technique (Scheme 6)reported by Chen and Xia et al.[26]An aqueous NaOH solution was added dropwise to a solution of Cu(NO 3)2at a constant rate while stirring vigo-rously to form a precipitate.Next,a calculated amount of col-loidal aqueous silica solution was added to the solution of the precipitate to form a gel,and the gel was allowed to age at 363–373K for 4h.Finally,the slurry of the gel was filtered,thoroughly washed with hot distilled water,dried at 1308C overnight,and calcined at 4008C under air for 3h.

To produce silica-supported Cu nanoparticles,the immobi-lized CuO/SiO 2was autoclaved in a stainless steel bomb at 3008C for 3h in an H 2stream.

X-ray photo electron spectra (XPS,Figure 4)clearly indicate the presence of Cu 2+in the catalyst after calcination,and it was completely converted to Cu 0after reduction.Cu/SiO 2cata-lysts prepared by using this method showed high selectivity (>98%)towards glycerol hydrogenolysis.

2.5.2.Mesoporous silica-supported Cu nanoparticles

Copper(II)sulfate was added to the basic aqueous solution of b -cyclodextrin.To this clear blue solution,b -methyl cyclodex-trin was added while stirring.After filtration,the filtrate was poured into a vigorously stirred prehydrolyzed silica gel (tetra-methyl orthosilicate)in an acidic medium.This mixture was kept in an open flask at room temperature for approximately one week.The resulting silica hybrid composites were subject-ed to calcination at 5008C to provide mesoporous silica con-taining Cu nanoparticles with an average pore size of 6–8nm.[27]

2.5.

3.Deposition of Cu on activated carbon

A solution of CuI in absolute ethanol (30mL)was refluxed for 4h in the presence of activated carbon under an inert atmo-sphere to produce Cu nanoparticles deposited on carbon.[28]The resulting materials were washed with ethanol and dried under vacuum.Inductively coupled plasma analysis

revealed

Scheme 5.Preparation of Cu 2O-coated Cu

nanoparticles.

Figure 3.XRD pattern of Cu 2O coated Cu

nanoparticles.

Scheme 6.Preparation of silica-supported Cu

nanoparticles.

Figure 4.XPS spectra (Cu 2P)of a)calcined sample and b)calcined sample after reduction.

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B.C.Ranu et al.

that the copper content of the catalyst was approximately 9.97%.

However,the CuI crystals tend to deposit on the activated carbon because of the low solubility of CuI in ethanol.The SEM image (Figure 5)showed the morphology of CuI crystals.

The poor solubility of CuI under the reaction conditions re-sults in a low concentration of Cu +in the solution.Thus,Cu +produces Cu 0nanoparticles (Scheme 7)and Cu 2+species through a disproportionation reaction,and the resultant Cu 2+is reduced by the iodide present in the reaction mixture to re-produce CuI.The above sequences of reactions are repeated continuously during the process to generate Cu/CuI,which plays an active role in the catalysis of organic reactions.

2.5.4.Magnetically separable Cu catalysts

Chi and Hur et al.developed a method for the preparation of Cu 3N nanoparticles supported on mesoporous superparamag-netic silica microspheres (Scheme 8).[29]In an experimental pro-cedure,Fe(NO 3)3·9H 2O,dioctyl ether,and a nonionic surfactant (Igepal CO-520)in ethanol were added to mesoporous silica,followed by stirring and sonication for 10min.The reaction

mixture was heated to 908C and centrifuged.The solid mass was dried in an oven at 808C for 24h.This was further an-nealed at 7008C in an NH 3atmosphere for 6h to give the Fe 3N-SiO 2matrix.This Fe 3N-SiO 2matrix was sonicated with Cu(OAc)2·H 2O,cetyl trimethylammonium bromide (CTAB),and octane in methanol for 10min followed by addition of 1-buta-nol to the solution.The reaction mixture was heated to 808C,and the resultant colloidal microspheres were separated by performing centrifugation and dried at 808C for 24h.This was further annealed at 2508C in an NH 3atmosphere for 6h to produce the Cu 3N-Fe 3N-SiO 2catalyst.

2.6.Characterization of nanoparticles

Several techniques are used for the characterization of nano-materials.Among them,TEM analysis is used to determine shape and size (diameter)of the nanoparticles (Figure 2).EDX of the selected area of nanoparticles gives an indication of the composition (Figure 1).The oxidation state of the metal pres-ent in the material can be easily determined by using XPS (Figure 4).The phase purity of the bulk material of the nano-particles can be determined by using XRD (Figure 3)with com-parison of those reported earlier.For nanomaterials with a comparatively large size,scanning electron microscopy images (SEM,Figure 5)gives information regarding its morphology.UV spectroscopy also gives useful information of the nature of nanoparticles.

3.Copper Nanoparticles as Catalysts

Because of growing environmental concerns,copper has re-ceived tremendous attention in organic synthesis owing to its environmentally benign character,easy availability,and low https://www.360docs.net/doc/1011790454.html,e of copper in catalysis has dramatically changed the scenario in organic transformations.A number of outstanding catalytic systems have been developed in recent years,which provide excellent stereo-,regio-,chemo-,and enantioselectivity in many organic reactions,such as formation of C(aryl)àC(aryl),C(aryl)àN,C(aryl)àO,C(aryl)àS,and C(aryl)àSe bonds as well as many other useful transformations.A brief account of all these reactions will be presented.

3.1.Carbon àcarbon bond formation 3.1.1.Ullmann coupling reaction

Ullmann coupling,named after Fritz Ullmann,between two aryl halides catalyzed by stoichiometric amounts of copper or copper bronze alloy producing biaryls is well-known in organic synthesis.[30]The traditional Ullmann reaction involves harsh re-action conditions and elevated reaction temperatures.More-over,lower yields of products observed with substituted aryl halides are another limitation for this reaction.Since its discov-ery a century ago,[31]many improvements and alternative pro-cedures have been introduced.Recently,Maitra et al.[32]report-ed a Cu nanoparticle-catalyzed Ullmann coupling (Scheme

9).

Figure 5.SEM image of Cu and CuI particles on activated

carbon.

Scheme 7.Formation of Cu nanoparticles on activated

carbon.

Scheme 8.Cu 3N nanoparticles supported on a mesoporous superparamag-netic silica.

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Copper-Catalyzed Bond Formation with a Greener Perspective

Uncapped and undefined macrosized copper powder showed only 43%conversion of iodobenzene to biphenyl in

5h under the experimental conditions.However,use of Cu nanoparticles (66nm)prepared by applying citrate capping showed 88%conversion,and it was further increased to 95%when finer Cu nanoparticles (8nm)were used.Further division of Cu nanoparticles did not further improve the performance.The reported reaction involves the formation of an aryl–copper intermediate (A)in the first step (Scheme 10),which then

reacts with another aryl halide molecule through oxidative ad-dition to form intermediate (B),which provides the biaryl on reductive elimination.

This procedure of Ullmann coupling using environmentally benign Cu nanoparticles is an attractive alternative to the ex-isting procedures.

3.1.2.Suzuki reaction

Cross-coupling of aryl halides with aryl boronic acids leading to biaryls was discovered by Suzuki and coworkers [33]and is one of the most powerful synthetic tools in organic synthesis.Because of its ease of operation and general applicability,Suzuki coupling has become the method of choice for labora-tory and industrial applications.Palladium-,nickel-,and iron-based catalysts are usually used for this reaction.Rothenberg et al.[34]reported a Cu nanoparticle-catalyzed Suzuki cross-cou-pling reaction (Scheme 11).Colloidal Cu nanoparticles showed significant activity to afford quantitative yields after 8h at 1108C.

However,this procedure is limited to aryl iodides only,and other halides are not reactive.

3.1.3.Stille reaction

Cross coupling between aryl halides and organotin derivatives is known as Stille coupling.[35]Li and Zhang et al.developed a versatile reaction (Table 1)catalyzed by Cu 2O nanoparticles using an ionic liquid,tetrabutylammonium bromide (TBAB),as reaction medium and KF as a promoter in the presence of P(o -tol)3as a ligand.[36]

The reaction is compatible with several substituents present on the aromatic rings.The reaction of other aryl halides includ-ing less reactive aryl chlorides proceeds well with the Cu 2O NPs/P(o -tol)3/TBAB reagent system.Moreover,the use of ionic liquid as solvent and inexpensive and readily available Cu 2O as reusable catalyst make this protocol an attractive green alter-native to the existing procedures for this reaction.However,the catalytic activity of the reusable system was reduced in the second run of the coupling of the deactivated aryl bromide.3.1.4.Heck reaction

The use of metal alloy as catalyst is well-known in organic syn-thesis.[37]However,the active catalytic species in the reaction were not precisely determined.Recently,Calo and

coworkers

Scheme 9.Ullmann coupling reactions of aryl iodide.Reagents:Iodoben-zene (0.5mmol),Cu NPs (1.6equiv.),DMSO (1

mL).

Scheme 10.Probable reaction mechanism for Ullmann coupling

reactions.

Scheme 11.Suzuki coupling reaction between aryl iodide and phenyl boronic acid.Reagents:Iodobenzene (0.50mmol),phenyl boronic acid (0.75mmol),K 2CO 3(1.5mmol),catalyst (2mol %),DMF (12.5mL).

Halide

Time [h]I 12Br

24Cl 60[a]Reagents and reaction (0.4mmol),Cu 2O NPs TBAB (1.5g);125-130Green Chemistry by Nanocatalysis

B.C.Ranu et al.

observed that when copper bronze alloy was treated with iodobenzene,Cu nanocolloids (%4nm)were formed,which played the active role in catalysis.[38]An efficient Heck reaction between activated alkenes and aryl iodides/bromides catalyzed by these colloidal NPs (Table 2)in TBAB was reported.[39]

After each cycle,the efficiency of the catalyst increased gradually and was maximum for the sixth cycle,as shown in the product conversion chart (Figure 6).Therefore,it was sug-gested that the tin metal present in the bronze was gradually consumed by aryl halide and small particles of metallic copper were generated.Agglomerization of these small particles ulti-mately led to the formation of Cu nanoparticles stabilized by TBAB.In addition,halide ions play a crucial role in the forma-tion of nanoparticles.[40]

3.1.5.Sonogashira cross-coupling

The coupling between aryl halides and terminal alkynes known as Sonogashira coupling [41]is usually performed by using a va-riety of palladium or palladium–copper mixed-metal catalysts.Rothenberg et al.[42]showed that Cu nanoparticles efficiently catalyze the reaction between phenyl acetylene and a variety of iodo-and bromo-arenes (Table 3).Significantly,these Cu

clusters can be recycled up to three cycles without any loss of activity,giving a final turnover number of 73.Surprisingly,the state of the catalyst after the third cycle and any data of leach-ing of the catalyst were not available.

A mechanism was proposed based on oxidative addition and reductive elimination.Phenylacetylene reacts with a Cu cluster,forming an [alkenyl–Cu]cluster (species 1),which reacts with the aryl halide to form 2,and this intermediate fi-nally leads to the product by reductive elimination as depicted in Scheme 12.

Significantly,the positive charge created during oxidative addition is shared among the copper atoms in the cluster,

Yield [%]Product

8190

[a]Reagents and reaction conditions:Aryl halide (1mmol),butyl (1.2mmol),TBAA (1.5mmol),TBAB (3g),copper bronze (3mol %);16h.[b]The corresponding aryl bromide was

used.

Figure 6.Recyclability chart of the coupling reaction of p -iodoanisole and butyl acrylate catalyzed by TBAB/Cu nanoparticles (adopted from ref.[38]).

Sonogashira reaction of terminal alkynes with aryl halides.Yield [%]Product

849999[b]

[a]Reagents and reaction conditions:Aryl halide (0.25mmol),acetylene (0.38mmol),TBAA (0.40mmol),Cu nanoclusters (1.2DMF (2.5mL);1108C,

24h.corresponding aryl bromide was Scheme 12.Probable mechanistic path for the Sonogashira reaction.

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Copper-Catalyzed Bond Formation with a Greener Perspective

which is contrast to monoatomic complexes.This facilitates product formation and reductive elimination steps.

3.1.6.Difunctionalization reaction

Park et al.[43]reported a PVP-stabilized Cu2O nanocubes cata-lyzed difunctionalization of vinyl arenes with cyclic ethers through CàH activation(Table4).

The catalytic oxyalkylation of various substituted styrenes was performed by refluxing a mixture of styrene,catalyst,and THF.The process is simple,and a wide range of oxyalkylated vinyl arenes are obtained in excellent yields with high regio-selectivity.The high chemical reactivity of the nanoparticles plays an important role in the improvement of the yield of the reaction.

3.1.7.Arylation of active methylene compounds

Kidwai et al.reported a CuO nanoparticle-catalyzed arylation of an active methylene compound(Table5).[44]C-Arylation of ace-tylacetone and dimethyl malonate with a variety of substituted iodobenzenes provided the corresponding products in high yields.

The CuO nanoparticles show superior reactivity over Cu(OAc)2,parent CuO,and even Cu nanoparticles with respect to the reaction yield and reaction time.After the completion of the reaction,CuO nanoparticles were recovered by centrifu-gation and reused four times.However,no data regarding leaching of the Cu catalyst was provided.3.1.8.Three-component condensation

Cu nanoparticles catalyzed the three component coupling re-action of aromatic aldehydes,amines,and alkynes through CàH activation to form propargyl amines(Table6),[45]which are

Difunctionalization reaction between alkene,air,and ether.

Yield

[%]

Product

98

100

95

[a]Reagents and reaction conditions:Styrene(13.9mmol), (1mol%),THF(10mL);0.3MPa,air.

Yield

[%]

Product

80

81

[a]Reagents Active methylene (3mmol),aryl(10mol%),Cs DMSO(2mL);

Table6.Coupling aldehyde,amine,and CuO nanoparticles.

Product Yield

[%]

Product

87

89

[a]Reagents and reaction conditions:Aldehyde(1.0mmol), amine(1mmol),phenyl(1.5mmol),Cu NPs(15mol (5mL);100–1108C,3.5–7

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very useful precursors to many biologically active compounds and natural products.The catalyst was recycled up to seven times.

Copper was the best among other metal nanoparticles,in-cluding nickel,silver,and gold.Nevertheless,copper is less ex-pensive and environmentally benign.

3.2.Carbonànitrogen bond formation

CàN bond formation is a very useful process,as it provides a powerful tool for the synthesis of numerous compounds with biological,material,and therapeutic importance.

3.2.1.Anilines from aryl halides in aqueous solutions Aromatic amines are of considerable importance,as they are used as building blocks for many structurally complex and bio-logically active molecules.The amines are also useful for the polymer,dye,and pharmaceutical industry.Recently,Xu and Feng[24]reported a CuI nanoparticle-catalyzed selective amina-tion(Table7)of aryl halides in water under mild conditions. The reactions proceeded in the presence of both,electron withdrawing(EWG)and electron donating(EDG)substituents present on the aromatic ring,providing high yields of products.

3.2.2.N-Arylation reaction

N-Arylation of amines through CàN cross-coupling reaction with aryl halides is a useful process for preparing aryl amines.[46]In conventional procedures,CàN bond forming re-actions usually require stoichiometric amounts of copper re-agent,[47]which makes waste disposal a serious issue for large scale reactions.To overcome this problem,methods utilizing catalytic amounts of metal salts have been developed.[48] Several ligands have been used in palladium and copper com-plexes for the amination of aryl halides.[49]However,Punniya-murthy et al.[50]have developed a CuO nanoparticle-catalyzed ligand-free CàN cross-coupling reaction of amines with iodo-benzene(Table8)in excellent yield.This catalytic system is also effective for the reactions of bromobenzenes and chloroben-zenes with substituted anilines.

A variety of alkyl amines,such as benzyl amine,furfuryl amine,n-butyl amine,and cyclohexyl amine,and N-heterocy-clic compounds,such as pyrrolidine,piperidine,and morpho-line,gave85–93%yields.A variety of substituted iodoben-zenes also underwent high yielding reaction with aniline.After completion of the reaction,the CuO nanoparticles were sepa-rated by performing centrifugation and reused for three suc-cessive runs without significant loss of activity.

The reaction was proposed to proceed through a catalytic cycle involving formation of CuàN bonds through insertion fol-lowed by oxidative addition of the aryl halide and reductive elimination as depicted in Scheme 13.

Amination of aryl halides.

Halide Yield

[%]

Product Halide

I97

Br87

Br91[b]

I89

[a]Reagents and reaction conditions:Aryl bromide(1.0mmol),

(3.0mol%,X=Br;1.5mol%,X=I),n Bu4NOH(3mmol),28%aqueous

(5equiv.,X=I;10equiv.,X=Br);(RT,X=I;808C,X=Br),N2atmosphere, 24–48h for X=I and Br,10equiv.NH

CuO NP-catalyzed N-arylation reactions.

Yield

[%]

Product

92

98

70

[a]Reagents and reaction conditions:Aniline(2.5mmol),iodobenzene

(2mmol),CuO NPs(1.2mol%),KOH(2mmol),

DMSO(4mL);

Scheme13.Proposed mechanism for N-arylation reactions.

G

r

e

e

n

C

h

e

m

i

s

t

r

y

b

y

N

a

n

o

c

a

t

a

l

y

s

i

s

Copper-Catalyzed Bond Formation with a Greener Perspective

3.2.3.Arylation of aromatic heterocycles

Li et al.[51]introduced a Cu 2O nanoparticle-catalyzed solvent-free procedure for the N -arylation of nitrogen-containing het-erocycles (Table 9),such as imidazole,triazole,and indoles,by

using aryl and heteroaryl halides.The cubic Cu 2O nanoparticle works best for the N -arylation reaction compared to other structures,such as bulky,octahedral,and spherical nanoparti-cles.Cu 2O nanoparticles show high efficiency for the N -aryla-tion of aryl halide and heteroaryl halide in neat conditions using tetra-n -butylammonium fluoride (TBAF)as a base at 1408C.

3.2.

4.Arylation of amides

The N -arylation of amides is of considerable significance,as the products are core units of many natural products and bio-logically active molecules (Figure 7).The Goldberg reaction [52]

is frequently used for preparing N -arylamides.However,these reactions are performed at elevated temperatures (>1508C).Several palladium-and copper-based catalysts with various types of ligands were also used for the reaction.[53]Cu 2O nano-particles in poleyethylene glycol (PEG)are efficient and recycla-ble catalysts for the amidation of aryl iodides (Table 10).[54]No ligand,additive,or cocatalyst is required for this reaction.

A disadvantage of this methodology is the use of expensive aryl iodides,which limits its use in industrial processes.To overcome this problem,Xianwen and coworkers [55]introduced a Cu 2O nanoparticle catalyst in combination with 10mol %di-methylethylenediamine (DMEDA),which facilitates amidation with less reactive aryl bromides and chlorides (Table 11)in high yields.

The steric effect was determined to be more significant than the electronic effect.The reaction is highly chemoselective,as benzamide is selectively arylated in the presence of free aro-matic amines.Secondary aliphatic amides afforded the desired products in high yields.The reaction of less reactive aryl chlor-ides was successfully performed in this catalytic system.The catalyst was reused in the next reaction without any loss of activity.

3.2.5.Intramolecular carbon ànitrogen bond formation Benzimidazole moieties play an important role in the recogni-tion of biological and therapeutic activities.Development of general methods for the synthesis of these compounds is highly appreciated in drug discovery and medicinal chemistry.Punniyamurthy et al.utilized an intramolecular C àN bond for-mation using CuO nanoparticles as the catalyst for the synthe-sis of substituted benzimidazoles (Table 12).[56]

A variety of functional groups are compatible with this pro-cedure.High yields of products are achieved irrespective of number and nature of R,R 1,or R https://www.360docs.net/doc/1011790454.html,e of

recyclable

Figure 7.Examples of some biologically active compound containing C àN bonds.

-arylation of nitrogen-containing aromatic heterocycle.Yield [%]

Product

77

63

[a]Reagents and conditions:Heterocyclic (0.5mmol),aryl halide Cu 2O NPs (10mol nanthroline (20mol 140–1458C,24–48h.

Arylation of amides using aryliodide.Yield [%]

Product

80

70

75

[a]Reagents and reaction Amide (1.2mmol),(1mmol),Cu 2O NPs PEG-4000(1g);1208C,3–16h.

Green Chemistry by Nanocatalysis

B.C.Ranu et al.

copper catalyst significantly reduces the E -factor (the ratio of the amount of waste to the amount of product),which makes this a greener procedure in comparison to other available methods.[57]

3.2.6.Aziridination reaction

In 1967,Kwart and Kahn described a copper–bronze alloy-cata-lyzed aziridination of cyclohexene (Scheme 14)with benzene sulfonyl azide.[58]Although the methodology has serious limita-tions with regard to yields of the products (15%),this concept led to the future development of efficient aziridine synthesis.

Recently,Kantam et al.reported an aziridination reaction (Table 13)catalyzed by alumina-supported Cu nanoparticles in moderate to high yields.[59]

Use of environmentally friendly and recyclable heterogene-ous copper nanocatalysts at room temperature is a significant advantage of this procedure.

3.2.7.Click reaction

Click chemistry is a newer approach to the synthesis of drug-like molecules that can [60]accelerate the drug discovery pro-cess by utilizing practical and reliable reactions.[60]Sharpless and coworkers [61]defined a click reaction as one that is wide in scope and easy to perform,uses readily available reagents,and is insensitive to oxygen and water.[62]In several cases,water is the ideal reaction solvent,providing the best yields and high-est rates.

11.Arylation of amide using aryl chloride,bromide and iodide.Halide Yield [%]Product

Halide Cl [b]82Br I 95I Br Cl [b]815267I Cl [b]

8078

Br [a]Reagents and reaction conditions:Amide (0.6mmol),aryl mide (0.5mmol),Cu mol %),DMEDA (10mol %),(0.75mmol.),toluene C,10h.[b]Neat conditions for 25h.

Table 12.CuO nanoparticle-catalyzed synthesis of substituted zoles.[a]

Product

Time [h]Yield [%]4

94

492

1888

Reagents and reaction conditions:Reactant (1mmol),

KOH (1.5mmol),DMSO (1mL);1108C,4–18h,air.

Scheme 14.Synthesis of aziridine through reaction of benzene sulfonyl azide with cyclohexene.

nanoparticle-catalyzed azidirination reactions.Product

Time [h]Yield [%]3

953

74

3

754

75

and reaction catalyst (4.5mol G r e e n C h e m i s t r y b y N a n o c a t a l y s i s

Copper-Catalyzed Bond Formation with a Greener Perspective

Sharghi et al.[28]developed a one-pot procedure for the syn-thesis of1,2,3-triazole derivatives through a three-component coupling reaction of terminal alkynes,benzyl or alkyl halides, and sodium azide in the presence of1mol%copper/carbon (Cu/C)nanoparticles.[63]Cu nanoparticles supported on activat-ed charcoal offers superior catalytic activity and1,4-regioselec-tivity towards the[3+2]Huisgen cycloaddition in water.[64]

The solvent plays a significant role in this reaction.The reac-tion of benzyl bromide,phenylacetylene,and sodium azide in the presence of1mol%of Cu/C in water provided the1,4-di-substituted triazole product in91%yield by stirring for45min at1008C in an open system(Table14).

Benzyl halides with an electron-donating group on the phenyl ring underwent facile reactions with high yields,where-as substitution of the electron-withdrawing group led to lower yields of the products.The reaction of an aliphatic acetylene, such as1-hexyne,requires longer reaction times.Internal al-kynes remained inert in this reaction.The reaction offers a good scope for the synthesis of new azacrown ethers and an-thraquinone derivatives of triazole.

Alanso and coworkers reported a similar reaction,which used activated carbon-supported Cu nanoparticles.[65]This method offers low catalyst loading,shorter reaction times,and higher yields.Water was the best solvent for this procedure. Other azide precursors,including epoxides,diazonium salts, anilines,or alkenes,successfully gave the corresponding1,2,3-triazoles,[66,67]while the catalyst was reusable.

Recently,Chi et al.[29]established that Cu3N is an efficient cat-alyst for the Huisgen cycloaddition reaction(Table15).Cu3N nanoparticles are supported on a magnetic microsphere and are recyclable.As the catalyst is not cytotoxic,it can be applied to living and environmentally benign systems.It is expected to have considerable potential for use in a wide range of toxic-free click-chemistry applications,such as imaging and labeling of bio-molecules on living cells.The essential element for the activity of the catalyst is believed to be the Cu I species in Cu3N.The reaction is usually completed within3h at room temperature.

3.3.Carbonàoxygen bond formation

Diaryl ether is of considerable importance,as several com-pounds containing this moiety show significant biological ac-tivity.[68]Although the classical Ullmann coupling reaction is limited by elevated reaction temperatures,lower yields,and use of stoichiometric quantities of copper,is has been widely used in diaryl ether synthesis.[69]In the last decade,significant improvements have been made by using Cu nanoparticles as a catalyst.

3.3.1.Oxygen arylation

Cu II has been widely used in cross-coupling reactions of aryl halides and phenol to form a C(aryl)àO bonds.[70]A simple, general,and efficient procedure for the cross-coupling reaction of oxygen nucleophiles with aryl halides has been developed

1,3-Dipolar Huisgen cycloaddition reaction.

Time

[h]

2.5

8 [a]Reagents and reaction conditions:

alkyne(1mmol),sodium azide(1.1

1008C.nanoparticles as catalyst.

Product Yield

[%]

79

84 Reagents and reaction conditions:Alkyne(1mmol), (20mg),Et3N(0.3equiv.),acetonitrile(4mL);

Green Chemistry by Nanocatalysis

B.C.Ranu et al.

by using CuO nanoparticles under ligand-free conditions.[71]A variety of substrates underwent this reaction efficiently pro-ducing the corresponding biaryl ethers in excellent yields (Table 16).Dimethyl sulfoxide (DMSO)was the best solvent and KOH the most suitable base for C àO cross-coupling reactions.The reaction did not proceed in the absence of the catalyst.

Iodobenzene is more reactive compared to phenyl boronic acid,bromobenzene,and chlorobenzene.The substrates with electron-donating groups showed greater reactivity compared to those with electron-withdrawing groups.By using this pro-cedure,polyhydroxy compounds,pentaerythritol,and trietha-nolamine react with iodobenzene to provide the correspond-ing polyaryl ethers in excellent yields (Scheme 15).

As aryl iodides with electron-withdrawing groups have shown better reactivity compared to those with electron-do-nating groups,it was assumed that the reactions occurred through oxidative addition followed by a reductive elimination process.

Recently,Kantam et al.developed an efficient procedure for C àO coupling reactions using inexpensive and environmentally benign CuI nanoparticles as catalyst under ligand-free and rela-tively mild conditions.[72]A mixture of aryl chlorides,phenol,and K 2CO 3in dimethylformamide (DMF)gave excellent product yields when treated at 1208C under air (Table 17).

Both electron-donating substituents on aryl chlorides and electron-withdrawing substituents on phenols afforded the corresponding coupling products with uniform yields.Sterically hindered phenols retarded the reaction.It is believed that the reaction may occur through oxidative addition followed by re-ductive elimination.

3.3.2.Magnetic nanoparticle-catalyzed carbon àoxygen bond formation

The use of magnetic nanoparticles for C àO cross-coupling re-actions of phenol with aryl halides is very useful.Sun et al.[73]reported a Cu-Fe 2O 4nanoparticle-catalyzed diaryl ether forma-tion by using aryl halides,phenols,Cs 2CO 3as a base,and ace-tylacetone as a ligand in DMF at 1358C under argon (Scheme 16).The use of acetylacetone as a ligand appeared to be essential for the reaction.

Iodobenzenes with electron-rich substituents as well as those with electron-withdrawing groups reacted with a wide variety of phenols to give the desired products in good to ex-cellent yields.Sterically hindered aryl iodides were disfavored for this reaction.Substituents on the phenol ring influenced the reaction significantly.The catalyst could be reused six times with only a slight loss of activity for the reaction in each cycle.The catalyst was recovered by using an external magnet,washed with water and EtOAc,and dried under reduced pres-sure for use in the next cycle.

Halide Yield [%]Product

Halide I Br

8935

I

I Br 8832I Br [a]Reagents and reaction conditions:CuO NPs (2.5equiv),ROH (1aryl iodide (1.2equiv.),KOH (1.5equiv.),DMSO (1mL);1108

C.

Scheme 15.CuO nanoparticle-catalyzed reaction of pentaerythritol with aryl iodides.

O -Arylation of phenols with chlorobenzenes.Yield [%]Product

99

8798

[a]Reagents and reaction conditions:Phenol (1.2mmol),aryl (1.0mmol),catalyst (1.25mol %),K 2CO 3(1.2mmol),DMF

(1mL);5h.

Scheme 16.CuFe 2O4-catalyzed diaryl ether formation.Reagents:Aryl iodide (0.5mmol),phenol (1.0mmol).

G r e e n C h e m i s t r y b y N a n o c a t a l y s i s

Copper-Catalyzed Bond Formation with a Greener Perspective

3.3.3.Copper nanoparticle-catalyzed oxidative cyclization of Schiffs’base

Benz-fused azoles are an important class of compounds be-cause of their medicinal and biological activity.Kidwai et al.de-veloped a method for the synthesis of 2-arylbenzoxazoles through the condensation of aldehydes with 2-aminophenol using Cu nanoparticles under ligand-free conditions.[74]This one-pot synthesis includes in situ preparation of Schiff’s bases,which undergo oxidative cyclization in the absence of metal oxides,organic oxidizing agents,or strong acids.In this reac-tion,the subsequent formation of C àO and C àN bonds leads to the product (Table 18).

Under optimum conditions,the best yield was obtained for the reaction of 2-aminophenol with benzaldehyde in the pres-ence of 10mol %catalyst using K 2CO 3as a base.Methanol was the solvent of choice.The mechanism for the catalytic activity of the nanoparticles was highly dependent on the size of the Cu nanoparticles.

3.3.

4.Carbon àoxygen bond formation through Ullmann cou-pling reaction

Kidwai et al.[75]developed an efficient,economic,and novel method for the synthesis of diaryl ethers through Ullmann-type coupling reactions using recyclable Cu nanoparticles.This method provides wide substrate applicability,avoids the use of a heavy metal cocatalyst,and gives diaryl ethers in good yields (Table 19).

The best product yield was obtained with 50mol %Cu nanoparticles in 1.5h.Cs 2CO 3played an important role in the reaction because of its enhanced basicity.Substituted phenols with electron-donating groups provided better yields than those with electron-withdrawing groups.

3.3.5.Formation of aryl ethers and oxygen heterocycles using Cu 2O nanocubes

Park et al.[76]described a simple and economical process for the coupling reaction of aryl halides with phenols catalyzed by thermally and air stable Cu 2O nanocubes under ligand-free conditions.The process provides an easy access to a diverse range of diaryl ethers in high yields with low catalyst loading and in short reaction times (Scheme 17).

The relative reactivity towards a halophilic attack of phenol was investigated for R àCl,R àBr,R àI,and the order was found to be C àI >C àBr >C àCl.Although the electronic effect of the substituents in phenol was not prominent,substituents in aryl iodides influenced the yields of the reaction.Steric crowding of phenol also affected the rate of the reaction.

3.4.Carbon àsulfur bond formation

The formation of C àS bonds through transition metal-cata-lyzed cross-coupling reactions of aryl halides with sulfur nucle-ophiles is a powerful tool in organic synthesis.[77]The aryl sul-fides are useful intermediates in several organic transforma-tions and many of these compounds are pharmaceutically active.[78]The traditional method for the formation of C àS bonds often requires harsh conditions.To overcome these diffi-culties,attention has been focused on the development of cat-alytic system for C àS cross-coupling reactions.[79]

3.4.1.Carbon àsulfur cross-coupling reactions of thiols with io-dobenzene

Recently,the attention was focused on the development of catalytic systems for the C àS cross-coupling reactions of thiols with aryl halides.Punniyamurthy et al.[80]reported a highly effi-cient C àS cross-coupling reaction of aryl and alkyl thiols with

18.Synthesis of 2-arylbenzoxazoles.Yield [%]Product

9586[a]Reagents and reaction conditions:2-Aminophenol (1equiv.),hyde (1equiv.),K 2CO 3(2equiv.),Cu NPs (10mol %),MeOH;80–1000.1MPa,3–5h.

nanoparticles as catalyst.Yield [%]Product 85

68[a]Reagents and reaction conditions:Aryl iodide (1equiv.),(1equiv.),Cs 2CO 3(1.5equiv.),Cu NPs (10mol %),CH 3CN;50–60

0.1MPa.

Scheme 17.C àO coupling reactions catalyzed by Cu 2O nanocubes.

Green Chemistry by Nanocatalysis

B.C.Ranu et al.

iodobenzene using readily available,inexpensive,and air stable CuO nanoparticles.The authors screened several copper catalysts and found CuO nanoparticles to be the best catalyst.The reactions are effective at 808C in DMSO in the presence of KOH under nitrogen.Iodobenzene was more reactive than chlorobenzene or bromobenzene (Table 20).Substrates with electron-donating groups were more reactive than those with electron-withdrawing groups.

Thiols with a longer alkyl chain (octane-,decane-,and do-decanethiol)and benzyl thiol required longer reaction times compared to aryl thiols.This coupling reaction is suggested to proceed through oxidative addition followed by reductive elimination (Scheme 18).

3.4.2.Microwave-assisted ligand-free copper nanoparticle-cat-alyzed aryl àsulfur bond formation

Ranu et al.[20]demonstrated that the cross-coupling reaction of aryl halides with thiols was efficiently performed when using Cu 0nanoparticles and K 2CO 3under ligand-free conditions.This was the first report of C àS cross-coupling reactions using Cu 0nanoparticles.A variety of functionalized aryl sulfides were pre-pared in excellent yields under microwave irradiation for 5–7min (Table 21).

The electronic nature of the substituents on the aromatic rings of thiol and aryl iodide did not have a significant influ-ence on the reaction.Moreover,substitution at the ortho-posi-tion did not affect the reaction.The reaction was highly che-moselective with aryl iodide while keeping chloro and bromo groups intact.The catalyst was not very active after the first re-action and was not recyclable.

The probable mechanism involves the free-radical intermedi-ate generated by a one-electron transfer from Cu nanoparticles to thiols to form the RS radical,which interacts with ArI and produces Ar-S-R (Scheme 19)and releases an iodo radical for the further propagation of the cycle.

3.4.3.One-pot synthesis of S -aryl-and S -vinyl dithiocarba-mates using copper nanoparticles

Related to this type of C àS cross coupling reactions,Ranu et al.[81]introduced a convenient,green,and efficient proce-dure for the synthesis of aryl and vinyl dithiocarbamates by performing a simple one-pot three-component condensation of an amine,carbon disulfide,and an aryl iodide or a styrenyl bromide catalyzed by Cu nanoparticles in water.Significantly,they obtained exclusively (E )-and (Z )-products from the (E )-and (Z )styrenyl bromides,respectively.The reactions were highly stereoselective,the Cu nanoparticles were recyclable,and the reaction did not require a base (Table 22).

with iodobenzene.Yield [%]Product

90

98[a]Reagents and reaction conditions:CuO NPs (1.26mol %),thiol (1mmol),iodobenzene (1.1mmol),KOH (1.5mmol),DMSO 4–15h,

N 2.

Scheme 18.CuO nanoparticle-catalyzed C àS cross-coupling reactions of butane-1,4-dithiol with iodobenzene.Reagents:Aryl halide (iodide,bromide,chloride:1equiv.),phenol (1equiv.),Cu 2O nanocubes (0.1mol %),Cs 2CO 3(2equiv.).

nanoparticles.Yield [%]Product

91

7295

[a]Reagents and reaction Aryl iodide (1mmol),thiophenol (1.1mmol),Cu NPs (30(0.8mL),K 2CO 3(2mmol);120

7min.

Scheme 19.Probable mechanism of Cu nanoparticle-catalyzed aryl àsulfur bond formation.

G r e e n C h e m i s t r y b y N a n o c a t a l y s i s

Copper-Catalyzed Bond Formation with a Greener Perspective

The open chain as well as the cyclic amines participated in this reaction,and a wide range of aryl iodides and styrenyl bro-mides underwent reaction by this procedure.The mechanism involves oxidative addition of aryl or styrenyl halide to Cu nanoparticles to form ArCuI,which combines with the dithio-carbamate anion,which is generated in situ by the reaction of the amine and carbon disulfide,to give an intermediate A ,which yields the product by subsequent reductive elimination

(Scheme 20).The liberated Cu 0initiates further reaction and propagates the cycle.Cu 0nanoparticles facilitated the oxida-tive coupling reaction with an aryl iodide by an easier transfer of an electron in comparison to metallic Cu.Water also played an important role in this reaction.

3.4.4.Stereoselective synthesis of vinyl sulfides catalyzed by using copper(II)oxide nanoparticles

Rao and coworkers [82]developed a method for the cross-cou-pling reaction of vinyl halides with thiols catalyzed by CuO nanoparticles under ligand-free conditions.During optimiza-tion,KOH was found to be the ideal base for the reaction.With increasing temperature,the yield increased and reached an optimum at 808C.Among the various solvents and catalysts tested,combination of nano-CuO with KOH in DMSO provided the best result (Table 23).

In the absence of metal oxide nanoparticles,the reaction did not produce any C àS cross-coupling products.The yields were highly dependent on the reaction temperature,base,solvent,and the nature of catalyst.This protocol works uniformly for

trans -b -iodostyrene with electron-rich,electron-neutral,and electron-deficient groups to produce the corresponding prod-ucts in excellent yields.For vinyl bromide,a longer reaction time was needed to obtain a good yield.The catalyst was easily recovered and reused for four cycles without significant loss of activity.

3.4.5.Synthesis of aryl sulfides through cascade reaction of aryl halides with thiourea

Nageswar et al.[83]reported a recyclable CuO nanoparticle-cata-lyzed synthesis of symmetrical diaryl sulfides under ligand-free conditions in the absence of any additive.The reaction used a simple catalyst system,which provided a broad functional group tolerance and good to excellent yields.The reaction was effective when using polar protic solvents,such as DMSO and DMF.Among the various bases (KOH,Cs 2CO 3,Na 2CO 3,NaOH)investigated,Cs 2CO 3was the most effective.The optimum temperature for the reaction was 1108C (Table 24).

with dithiocarbamate anion.Yield [%]

Product

8087[a]Reagents and reaction conditions:Aryl iodide/styrenyl bromide (1mmol),Cu NPs (3mol %),carbon disulfide (2.5mmol),(1.2

mmol),water;reflux,6h.

Scheme 20.Probable mechanism for the formation of aryl/vinyl thio-carbamate.

23.Cross-coupling reaction of thiol with trans -b -iodostyrene.Product

Yield [%]95

93

Reagents and reaction conditions:Trans -b -iodostyrene mmol),CuO NPs (1.5mmol),KOH Green Chemistry by Nanocatalysis

B.C.Ranu et al.

Iodobenzenes with electron-donating groups(e.g.,Me,Et, and OMe)produce the corresponding aryl sulfides in excellent yields,whereas those with electron-withdrawing groups(NO2)

show a decrease in the yield of the diaryl sulfide.A longer reaction time is needed for aryl bromides and hetroaromatic bromides.

3.5.Carbonàselenium bond formation

Diaryl selenides play an important role in organic chemistry, acting as versatile reagents in organic synthesis[84]and cataly-sis.[85]The biological and medicinal properties of selenium and organoselenium compounds are also increasingly appreciated as anticancer and antioxidant agents.[86]Cu-catalyzed reac-tions[87]of AràSeàAr have been of considerable interest,possi-bly because of the easy availability and low cost of copper derivatives and easy handling of the selenium reagent.

3.5.1.Coupling reaction of diaryl diselenide and aryl halides under ligand-free conditions

A new,efficient,and ligand-free cross-coupling reaction of aryl halides and diaryl diselenides using catalytic amounts of nano-crystalline CuO as a catalyst and with KOH as the base in DMSO at1108C was reported by Rao et al.[88]This procedure was utilized for the synthesis of a variety of aryl selenides in excellent yields from the readily available aryl halides and diaryl diselenides.In a preliminary experiment,when iodoben-zene was treated with diphenyl diselenide in the presence of 2mol%of CuO nanoparticles and KOH(2.0equiv)in DMSO (2.0mL)for12h,the corresponding diaryl selenide was ob-tained in94%yield(Table25).This is the first CuO nanoparti-cle-catalyzed coupling reaction of aryl halides with diphenyl diselenide to form diaryl selenide.

In general,all reactions are clean,and the diaryl selenides are obtained in high yields.This procedure is compatible with diphenyl diselenide with electron-rich,electron-deficient,and sterically crowded ortho-substituted aryl iodides.KOH and Cs2CO3worked best with this system and gave moderate to ex-cellent yields compared to K3PO4,KO t Bu,and NaOMe.Solvents other than DMSO,such as DMF and toluene,were less effec-tive.The effect of different combinations of various metal oxide nanoparticles on the diaryl selenide formation with KOH in DMSO at1108C using different substituted aryl iodides was investigated.However,their catalytic activities were considera-bly lower than that of CuO nanoparticles.The yields were highly dependent on the reaction temperature and the base.

3.5.2.Synthesis of symmetrical diaryl selenides using sele-nourea and aryl halides/boronic acids

Selenourea is used as an effective selenium surrogate in the CàSe cross-coupling reaction catalyzed by CuO nanoparticles under ligand-free conditions.Rao et al.[89]reported the synthe-sis of a variety of symmetrical diaryl selenides in good to excel-lent yields by the reaction of aryl halides/boronic acids and se-lenourea.The best result was obtained when the reaction was performed at808C using3.0mol%of the CuO nanoparticles in the presence of KOH(2.0equiv)and DMSO(2.0mL,Table26). The catalyst maintained its good level of activity even after being recycled four times.

Iodobenzenes with electron-donating groups(e.g.,Me,Et, and OMe)reacting with selenourea produced the correspond-ing selenides in excellent yields,but for electron-withdrawing groups a decrease in the yield of the diaryl selenide was ob-served.Iodobenzene was more reactive than bromo-and chlorobenzenes,thus requiring shorter reaction times.This protocol was also applied to the cross-coupling reaction of aryl boronic acids with selenourea,and the corresponding diaryl selenides were obtained in high yields with the same catalyst. This CuO nanoparticle-catalyzed CàSe cross-coupling reaction

24.CuO nanoparticle-catalyzed synthesis of aryl sulfides.

Yield

[%]

Product

87

90

75

[a]Reagents and reaction conditions:Aryl iodide(2mmol),

(1.2mmol),CuO NPs(5mol CO3(2equiv.),DMSO(2mL); 15h.CuO nanoparticle-catalyzed CàSe coupling reaction.

Product Yield

[%]

85

89

82

and reaction conditions:Aryl

equiv.),CuO NPs(2equiv.),

h,N2atmosphere.

G

r

e

e

n

C

h

e

m

i

s

t

r

y

b

y

N

a

n

o

c

a

t

a

l

y

s

i

s

Copper-Catalyzed Bond Formation with a Greener Perspective

of aryl halides and selenourea proceeds through the oxidative addition of aryl halide to the CuO nanoparticles leading to the formation of I ,which undergoes reaction with selenourea to give intermediate II .This intermediate provides III through re-ductive elimination,which produces a benzeneselenate moiety and urea upon hydrolysis in the reaction mixture (Scheme 21).In this step,urea is hydrolyzed in situ to produce carbon diox-ide and ammonia.Oxidative addition of aryl iodide to CuO nanoparticles leads to the formation of I ,which undergoes re-action with benzeneselenate anion to give intermediate IV ,which produces diaryl selenide through reductive elimination.

3.5.3.Cross-coupling reactions of organic diselenide with aryl boronic acid

CuO nanoparticle-catalyzed coupling reactions of organic dise-lenides with aryl boronic acids to form organic selenides has been reported by Alves et al.[90]This ligand-free coupling reac-tion involved the use of CuO nanoparticles (3mol %),diphenyl diselenide (0.25mmol),phenyl boronic acid (1.5equiv),and DMSO as a solvent.The heterogeneous reaction mixture was

stirred for 24h at 1008C under air to afford the corresponding diaryl selenide in high yields (Scheme 22).

The reaction is not sensitive to electronic effects of the sub-stituents attached to the aromatic ring of the boronic acid,al-though the nature of substitution in the aromatic ring of the diaryl diselenide has a considerable influence on the reaction.Aryl boronic acids bearing both electron-donating and elec-tron-withdrawing groups gave uniform yields of the product.However,the diaryl diselenides bearing electron-donating groups gave lower yields than those with electron withdraw-ing groups.A decrease in the yield of the product was also ob-served in the coupling reaction of hindered boronic acid.

3.5.

4.Phenyl-selenylation of aryl iodides and vinyl bromides in water

Ranu et al.[21]developed a green and efficient method for the coupling reaction of aryl iodides/vinyl bromide with diphenyl diselenides in the presence of Cu 0nanoparticles and zinc in water.The reaction proceeds well in water under reflux giving the best yield when using 20mol %of Cu 0nanoparticles.With a lesser amount of Cu 0nanoparticles,the reaction remained in-complete within a reasonable time https://www.360docs.net/doc/1011790454.html,bination of zinc with Cu 0nanoparticles is essential for this coupling reac-tion (Table 27).

The reaction proceeded without any difficulty with both electron-donating and electron-withdrawing substituents on the aromatic ring of the aryl iodides.The reactions with (E )-styrenyl bromides were highly stereoselective giving (E )-sele-nides.However,the (Z )-vinyl bromides produced mixtures of (E )-and (Z )-isomers.A change of the solvent from water to DMF or THF did not improve the stereoselective distribution of the product.The aliphatic vinyl bromide also participated in this reaction.Regarding the mechanism of this reaction,it was suggested that Cu 0nanoparticles readily undergo oxidative ad-dition with diphenyl diselenide to form the intermediate (PhSe)2Cu II .Upon reduction by zinc,this intermediate leads to the key intermediate PhSeCu I ,which reacts with ArI to give the product ArSePh via a transient Cu III intermediate (Scheme 23).The CuI generated in the reaction of PhSeCu I with ArI is re-duced by zinc to regenerate Cu 0nanoparticles,which initiate the next cycle.The Zn(SePh)2formed in the reduction process of Cu(SePh)2with zinc reacts with CuI to give PhSeCu I ,which undergoes further reaction.Thus,both PhSe moieties of PhSeSePh are used up in the process.

26.Cu-catalyzed synthesis of symmetrical diaryl selenides.Product

Yield [%]Product

91

86[a]Reagents and reaction Aryl iodide (2equiv.),selenourea (1equiv.),CuO NPs (3equiv.),equiv),

DMSO (2mL);10020

h.

Scheme 21.Probable mechanism for the selenylation reaction.

Scheme 22.Cross-coupling reactions of organic diselenides and aryl boronic acids catalyzed by CuO nanoparticles.Reagents and reaction conditions:Diphenyl diselenide (0.5mmol),aryl boronic acid (1.5equiv.),DMSO (1mL).

Green Chemistry by Nanocatalysis

B.C.Ranu et al.

3.6.Miscellaneous

3.6.1.Reduction of the nitro group

Recently,Ranu et al.[91]demonstrated that Cu nanoparticles in combination with ammonium formate offered chemoselective reduction of aromatic nitro compounds to the corresponding amino derivatives (Table 28)at 1208C in EG.Although the re-duction of the nitro to the amino group is a classical reaction and can be performed by using a variety of reducing agents,it was often observed that selective reduction of the nitro group in the presence of other reducible functional groups is not sat-isfactory.Thus,an alternative efficient,simple,and chemoselec-tive procedure developed by these authors is of considerable significance.

The reductions are successfully performed in the presence of a wide variety of other reducible functional groups in the molecules,such as àCl,àI,àOCH 2Ph,àNHCH 2Ph,àCOR,àCOOR,or àCN.To perform the reduction,Cu nanoparticles and ammonium formate are complementary to each other.5equiv.

of ammonium formate are required for complete conversion,and this might be attributable to the fact that a portion of the ammonium formate,deposited by sublimation on the wall of the condenser with the progress of the reaction,remained un-available for the reaction,thus requiring a higher than stoichio-metric amount.The reactions are clean and high yielding.

The cost effectiveness of Cu nanoparticles compared to other noble metals,such as Au [92]and Pt [93]nanoparticles re-ported earlier for this reduction,make it more useful for indus-trial purpose.

3.6.2.Hydrogenolysis of glycerol

As glycerol is a main byproduct in the production of biodiesel obtained through transesterification of vegetable oils and animal fats,large quantities of glycerol are available.[94]Thus,the appropriate utilization of glycerol would be appreciated.One of the attractive uses of glycerol is to produce glycols,es-pecially propanediols,by using an alternative route,which in-volves selective hydrogenolysis of glycerol.Recently,Chen and Xia et al.[26]developed silica-supported Cu nanoparticles as effi-cient catalysts for glycerol hydrogenolysis (Scheme 24).This process provides a clean and economically competitive route for the production of these useful chemicals from glycerol.

3.6.3.Three-component coupling reactions

The three-component coupling reactions of thiazolidine-2,4-dione (TZD),aromatic aldehyde,and ammonium acetate (Scheme 25)using Cu nanoparticles as catalyst in an ionic liquid at room temperature was reported by Chandra et al.[95]

Yield [%]Product

87

8886[c]

[a]Reagents and reaction conditions:Styryl bromide/aryl iodide diphenyl diselenides (0.5equiv.),Zn dust (1.5equiv.)Cu NPs H 2O;reflux,10–12h.[b]E /Z =20:80.[c]E /Z =

32:68.

Scheme 23.Probable mechanism for the selenylation reaction.

nanoparticle-catalyzed chemoselective nitroreduction.Yield [%]Product 83

8675[a]Reagents and reaction conditions:Nitroarenes (1mmol),(3mmol),HCOONH 4(5mmol),ethylene glycol (10mL);120

8C,Scheme 24.Cu nanoparticle-catalyzed hydrogenolysis of glycerol.Reagents and reaction conditions:Aqueous solution of glycerol (80g of 80wt %),cat-alyst (4g),H 2pressure 9.0MPa;12h.

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Copper-Catalyzed Bond Formation with a Greener Perspective