Magnetic nanoparticle-supported organocatalysts – an efficient way of recycling and reuse

Abstract

Recycling of organocatalysts is an important aspect in green chemistry. Several techniques have been applied to address this issue ranging from traditional separation techniques (extraction, chromatography) to immobilization on solid supports. Magnetic separation, i.e. attraction of the catalyst by an external magnet and decantation of the supernatant appeared as a new way for separation of organocatalysts omitting problems connected to filtration when solid supported catalysts are used. In this publication, the state of the art of magnetic nanoparticle-supported organocatalysts is reviewed demonstrating a wide range of applications but at the same time hitherto unaddressed fields awaiting future exploration are discussed.

---

1. Introduction

Catalysis is one of the most important fields in organic synthesis and is still in the focus of contemporary research with applications in academia and industries. While this field was dominated by metal catalysts for many years, organocatalysis has seen an enormous development in the last 13 years in particular in asymmetric synthesis.^1,2 Organocatalysts are devoid of metals and can have a number of advantages over metal catalysts, such as robustness, lacking toxicity, and lower price. Nevertheless recycling and reuse of organocatalysis is an important issue from the point of view of green chemistry and industrial application even in cases where the organocatalysts are comparatively cheap. Several techniques have been developed for recycling of organocatalysts. Besides traditional ways like extraction, salt formation or chromatography, fixation of organocatalysts on solid supports or soluble polymers has gained importance.^3–5 Recently, ionic liquid-tagging has found interesting application in organocatalysis. The ionic liquid tag provides a useful solubility, e.g. in ionic liquids and allows easy separation by either distillation off the organic materials or by separation after changing the solvent polarity. As a very recent development in recycling of organocatalysts, fixation at magnetic nanoparticles emerged. Here, the organocatalysts are linked to superparamagnetic nanoparticles (NP), which form colloidal suspensions thus providing quasi-homogeneous reaction conditions. After the reaction has come to completion the magnetic organocatalyst (MOC) is easily separated by an external magnet and decantation of the supernatant (Fig. 1). Linkage of the organocatalyst to magnetic nanoparticles can also help to avoid unpleasant or poisonous vapors appearing by the free organocatalyst.

Fig. 1 Separation of magnetically supported organocatalysts by an external magnet.

Since superparamagnetic NP show magnetism only in the presence of an external magnetic field they normally do not attract each other magnetically and thus catalysts linked to them can be redispersed after magnetic separation and reused. This technology has been applied in several fields of catalysis since about 10 years and was reviewed in the literature already.^6–16 However, these reviews mainly focus on magnetic fixation of metal catalysts which are either adsorbed in the organic shells of superparamagnetic NP or organic ligands are covalently bound to the nanoparticles forming complexes with the catalyst metal. Reviews about magnetic nanoparticle-supported organocatalysts do not exist so far. This gap is filled by the present publication. Although application of magnetic organocatalysts is more challenging in asymmetric syntheses also catalytic systems were included in this review wherein simple acids, bases or other catalytic moieties are covalently linked to magnetic nanoparticles (MNP) for application in non-asymmetric organocatalysis. In fact, these applications are more abundant so far. The chapters are chosen according to the types of catalytic units linked to MNP. Since the mechanism of the catalysis normally does not change upon linking of the organocatalyst with a magnetic support mechanistic aspects are not discussed in this review but can be studied in publications dealing with respective non-supported organocatalysts. The content of this publication is as follows:

2.1 Synthesis of magnetic nanoparticles as supports for organocatalysts.

2.2 Types of organocatalysts supported by magnetic nanoparticles and their application.

2.2.1 Non-chiral amines and other bases.

2.2.2 Chiral amines.

2.2.3 Amino acids and derivatives.

2.2.4 Brønsted acids.

2.2.5 Crown ethers and quarternary salts as phase transfer catalysts.

2.2.6 Miscellaneous.

2. Discussion

2.1. Synthesis of magnetic nanoparticles as supports for organocatalysis

MNP used so far as supports for organocatalysts are mainly magnetite (Fe₃O₄), maghemite (γ-Fe₂O₃) and cobalt spinel ferrite (CoFe₂O₄).^17–22 They combine several advantageous issues such as low price, easiness of preparation also in large scale, stability and high saturation magnetization values. Sometimes, also MNP were used consisting of Fe cores, covered by magnetite or carbon as well as Co cores covered with carbon. Various methods exist for the preparation of magnetite nanoparticles and most of them can be applied to produce magnetic supports for organocatalysts. The most often used and straight forward way to magnetite and maghemite NP is coprecipitation of FeCl₂ and FeCl₃ under aqueous basic conditions.²³ Thermal decomposition of iron complexes or flame spray pyrolysis is less often used.^18,21 The MNP are usually stabilized by surfactants (e.g. fatty acid salts, dodecylsulfate, oleylamine), catechols (e.g. in dopamine), functionalized dialkylphosphates or phosphonates, polymers (siloxanes, polyacrylates, polystyrene, poly ionic liquid, polyaniline, polyurethane), inorganic materials like silica, hydroxyapatite, carbon or ionic liquids.^17,19,20,22 Sometimes the coverage is implemented after prior formation of aggregates. A number of magnetite nanopowders are commercially available. The catalytic functions of MOCs are usually covalently linked to the shell of the nanoparticles via a silane, a polymer, hydroxyapatite, an alkylphosponate or a dialkyl phosphate. In other cases, the catalytic function (cysteine, glutathione, sulfuric acid, sulfonic acids) is directly attached to the surface of unmodified magnetite NP. Particle sizes of magnetically supported organocatalysts depend on the thickness of the shells and usually ranges between 5 and 100 nm but sometimes even reach 200 nm.

2.2. Types of organanocatalysts supported by magnetic nanoparticles and their application

2.2.1. Nonchiral amines and other bases. A variety of amines were connected to MNP in order to get basic MOCs, which can easily recycled by magnetic decantation. Thus, magnetite NP were functionalized with piperidine moieties by treatment with piperidine-4-carboxylic acid. The carboxylic groups anchor at the NP exhibiting the basic amino function to the surrounding. The catalyst was applied in Knoevenagel-reactions of aldehydes with nitroethane in dichloromethane at 25 °C or 80–100 °C (??) providing moderate to high yields and showing 95% conversion after 3 recyclings (Scheme 1).²⁴

Scheme 1 Synthesis of magnetite NP functionalized with piperidine and its application in Knoevenagel reactions.

In most of the examples of anchoring N-bases to MNP, silica coating was used. In this way, an DABCO-derivatized propyltriethoxysilane was used to furnish magnetite NP with quinucleidine.²⁵ This catalytic system was successfully applied in Baylis–Hillman reactions at room temperature resulting in high yields (Scheme 2).

Scheme 2 DABCO-loaded MNP and their application in Baylis–Hillman reactions.

Recycling was demonstrated for 6 times wherein the initial yield dropped from 88% to 80%. The MOC performed better than silica bound quinucleidine devoid of MNP.

In a similar way, reaction with aminopropyltriethoxysilane gave magnetite NP useful in a special type of a 3-component reaction of salicylaldehydes, malononitrile and amines affording chromeno[2,3-d]pyrimidines (Scheme 3). High yields were achieved under solvent-free conditions at room temperature. Yields above 90% were maintained after 4 recyclings.²⁶

Scheme 3 Aminoalkylsilanyloxy functionalized magnetic nanoparticles and their application in 3-component reactions leading to chromenopyrimidines

Analogous diaminoalkyl-functionalized magnetite²⁷ and cobalt spinel ferrite NP²⁸ were applied in Knoevenagel reactions (Scheme 4).

Scheme 4 Magnetic aminoalkylsiloxy-functionalized NP as MOCs for Knoevenagel reactions.

In one case it was demonstrated that the catalyst could be magnetically recycled and reused still providing quantitative conversion. However the catalytic activity decreased slightly. It was observed that the application of polar solvents is crucial in some of those cases.²⁷

A similar catalyst was established on the basis of cobalt spinel ferrite (CoFe₂O₄) NP. It was applied in multistep reaction networks starting from benzaldehydedimethylacetal controlled by combinations of different solid catalysts.²⁹ 100% conversion was achieved and the catalysts were fully recovered (Scheme 5).

Scheme 5 3-step reaction with application of a magnetic aminoalkylsilyloxy NP as one out of three reported solid catalysts.

Hydroxyapatite coated maghemite core–shell NP were used as support for a dual catalyst consisting of a nucleophilic amine and a phenol moiety (Scheme 6).³⁰ Linkage was performed via primary reaction with 3-aminopropyltrimethoxysilane. The aminofunction was condensed with salicylaldehyde and the resulting imine reduced to a benzylamine moiety. The resulting catalytic system was used in the synthesis of anellated 4(1H)-pyranes by Knoevenagel condensation of arylaldehydes with malononitrile followed by reaction with 4-hydroxybenzopyrane-2-one or dimedone in a one-pot procedure. The catalytic function of the amino group was proposed in terms of intermediate iminium salt formation with the aldehyde in the first step and with the carbonyl group in the final step while the phenolic OH group acted as a proton acceptor. The reaction was performed in boiling water and provided high yields within short times. No drop of the performance of the catalyst was observed within ten subsequent runs.

Scheme 6 Synthesis of dual MOC and application in synthesis of anellated 2-aminopyranes.

4-Dimethylaminopyridine (DMAP, Steglich catalyst) and analogues have found widespread application as organocatalysts in organic synthesis and also were attached to MNP. Thus, magnetite NP were covered with a silica shell by reaction with TEOS (Stöber-process) and then treated with a triethoxysilane containing the 4-aminopyridine moiety (Scheme 7).³¹ This catalyst was applied to various reactions in sequence sometimes even in as little as 1 mol% including acylations, phenoxycarbonyl-migration and acetalization, wherein the catalyst was recycled 30 times still providing excellent yields.

Scheme 7 4-Dimethylaminopyridine motif linked to silica-coated MNP applied in acylation reactions, acetalization and acyl migration.

The same catalytic system was used in ring opening reaction of 1,2-epoxyhexane by phenol with high regioselectivity (93 : 7).³² Here, the initial yield of 83% dropped considerably during recycling and reached only 42% in the third run. It was assumed that the silanol units interfere with the DMAP catalysis (Scheme 8).

Scheme 8 Regioselective ring opening of epoxides catalyzed by DMAP-analogous MOC.

Pyridine-based MOCs were obtained from magnetite NP by alkylsilanization and subsequent reaction with 2-aminopyridine (Scheme 9). Their application in chromone synthesis in water–ethanol under microwave conditions led to high yields. The initial yield of 92% dropped to 83% after the 3^rd recycling.³³

Scheme 9 Application of 2-aminopyridine-functionalized magnetic NP in 3-component synthesis of condensed 2-aminopyranes.

Pyridine was also linked to silica-coated maghemite NP by silanization. The MOC was applied in phospha-Michael additions of dialkylphosphites to α,β-unsaturated malonic acid derivatives (Scheme 10). In this way, not only the catalyst could easily be recycled (9 recycling steps without a significant decrease in yield) but also the unpleasant smell and toxicity of pyridine was circumvented.³⁴

Scheme 10 Application of silica-coated, pyridine linked magnetic organocatalyst in the synthesis of phosphonates.

Hydroxyapatite-encapsulated maghemite NP were functionalized with diethylaminoethylimidazolium salt by silanization (Scheme 11).³⁵ The high efficiency of the resulting catalytic system in Knoevenagel reactions was explained by cooperative effects of the basic sites generated by the hydroxyapatite (HAP) framework and the supported ionic liquid. The catalyst was recycled 4 times and reused after washing and careful drying under vacuum while the yield remained virtually unchanged. A problem related to the catalyst could be toxicity included by the PF₆ anion. Similar results were achieved if an imidazolium catalyst was used wherein the hydroxide ion acted as basic site rather than a tertiary amino group (Scheme 11).³⁶

Scheme 11 Imidazolium salts linked to MNP and application in Knoevenagel reactions.

Linking to ionic liquids also was applied to CoFe₂O₄-NP that were coated with silica affording mesoporous systems, which can be transformed to nanoparticles by treatment with sodium hydroxide.³⁷ Each of these magnetic materials was further functionalized by treatment with trimethoxy-(3-mercaptopropyl)silane, polymerization of 1-allyl-3-dodecylimidazolium bromide in the presence of AIBN and final treatment with sodium hydroxide. The resulting catalytic systems consist of an ionic liquid coating with the basic hydroxide as counterion. They were applied to transesterification of trioleoylglycerol with methanol affording 86% yield of methyloleate, a process being interesting in biofuel production. However the yield went down after recycling (Scheme 12).

Scheme 12 Magnetic mesoporous material or silica coated NP containing imidazolium hydroxides and application in transesterification of triglycerides.

Guanidine-functionalized magnetite nanoparticles were obtained by reaction of magnetite NP with 3-chloropropyl-triethoxysilane followed by substitution with guanidine hydrochloride.³⁸ They were applied as catalyst in reactions of arylaldehydes with CH-acidic nitriles and eventually with α-naphthol or cyclohexanedione or dimedone affording cinnamonitriles or condensed pyranes, respectively (Scheme 13).

Scheme 13 Guanidine-containing MNP and their application in Knoevenagel reactions and 3-component synthesis of condensed 2-aminopyranes.

2.2.2. Chiral amines. There are only a few reports about linking of magnetic nanoparticles to chiral amines and their application in asymmetric synthesis. In a pioneering work C₂-symmetric 1,2-diaminocyclohexane was fixed to magnetite NP by silanization.³⁹ The resulting MOC performed well in direct asymmetric aldol reactions of aromatic aldehydes with acetone or cyclohexanone in the presence of trifluoro actic acid as co-catalyst in water at room temperature. The catalyst was recycled 10 times in a model reaction wherein it did not show a significant drop in yield and stereoselectivity during the first seven runs (Scheme 14).

Scheme 14 1,2-Diaminocyclohexane-based MOC and its application in asymmetric direct aldol reaction.

(S)-Diphenylprolinoltrimethylsilylether is known to be one of the most powerful organocatalyst (Jørgensen–Hayashi catalyst) with a wide scope of application. This catalyst was fixed on magnetic nanoparticles either via the phenyl or the pyrrolidine moiety. In the first case (Scheme 15, MOC A) vinylated Jørgensen–Hayashi catalyst was transformed into a trimethoxysilane by thiol–ene-click reaction and then linked to silica coated magnetite NP.⁴⁰ The resulting MOC (diameter 200 nm) performed very well in Michael addition of several aldehydes to nitrostyrenes, however, not as perfect as the original Jørgensen–Hayashi catalyst. Recycling via magnetic separation was possible for 4 times without significant loss of catalytic efficiency. In another approach (Scheme 15, MOC B),⁴¹ linkage of Jørgensen–Hayashi catalyst was implemented by a CuAAC click reaction of its propargyl ether with azidopropylsilyloxy coated magnetite NP. MOC B (5 nm size) was applied to Michael reaction of propanal to nitrostyrenes and performed better in comparable cases than MOC A linked via the phenyl unit, although the former was even applied in less amounts (10 mol% versus 20 mol%, respectively). Successful recycling was demonstrated for 4 subsequent runs while a considerable drop of yield from 81 to 57% occurred after the 3rd recycling. It was believed that the drop was caused by hydrolytic loss of the silyl group attached to the catalytic moiety and to some extent by ligand leaching. Remarkably, Michael addition of acetaldehyde to nitrostyrene gave rise to a racemic mixture of products.

Scheme 15 Jørgensen–Hayashi-type catalysts linked to magnetite MNP and their application in asymmetric nitro-Michael reaction.

Alternatively to magnetite NP, carbon-coated cobalt NP were used as supports for a Jørgensen–Hayashi-type catalyst. Here, the number of catalytic units was high because it was included in the monomer unit of polymer chains linked to the carbon shell (Scheme 16). The MOC was applied to nitro-Michael reaction giving very good results in terms of yields, enantioselectivity and diastereoselectivity. However, the performance in recycling tests was less convincing since the yield dropped from 99% to 34% already in the fourth run.⁴²

Scheme 16 Jørgensen–Hayashi-type catalyst linked to carbon-coated cobalt MNP and their application in asymmetric nitro-Michael addition.

A chiral Steglich base type organocatalyst was synthesized on magnetite NP by covering them first with N-methyldopamine followed by nucleophilic substitution at the 4-chloropyridine moiety (Scheme 17).⁴³ The MOC was applied in kinetic resolution of some cycloalkanols or 1,2-dihydroxycycloalkanes giving very good enantioselectivity even at conversions higher than 50%. Its recyclability was excellent indicating a complete absence of degradation. However, the application seems to be limited to cycloalkanols with a spacious group adjacent to the OH-function.

Scheme 17 Chiral DMAP-derivatives linked to MNP used in kinetic resolution of alcohols.

Dual catalytic systems consisting of a chiral amine and a chiral Brønsted acid, such as BINOL phosphates or thioureas have seen a wide advent in organocatalysis in recent years.^44,45 Despite the fact that many of them are not straight forward available and thus expensive, it is somewhat surprising that only a few examples of supporting on magnetic NP have been reported so far. Cinchonine-derived ureas or sulfonamides were linked to magnetite NP via silanization and thiol–ene click reaction.⁴⁶ The urea derivatives were successfully applied in Michael addition of dimethyl malonate to β-nitrostyrene but the enantioselectivity was modest (max. 71% ee) and the catalytic activity dropped in the fourth recycling to 46% conversion (Scheme 18). The performance of the sulfonamide MOC (MOC II) in desymmetrization of meso esters with methanol or allylic alcohol was better. Here, excellent yields (up to 97%) and ee up to 82% were obtained. The catalytic systems could be recycled many times (28 recyclings did not show any diminished performance). It also was applied to different reactants in different subsequent runs (Scheme 18).

Scheme 18 Application of cinchonine-based MOC in Michael-addition and desymmetrization of meso-anhydrides.

Cinchonine-derived thioureas were linked to magnetite NP by thiol–ene reaction. The MOCs were applied to Diels–Alder-reactions of inversed electron demand. Depending on the diene component either cyclohexene cycloadducts or azlactones were obtained in excellent yields and enantioselectivities, wherein MOC I gave better results. Recycling experiments did not show a remarkable drop in yield and enantioselectivity up to 10 runs (Scheme 19).⁴⁷

Scheme 19 MNP functionalized by cinchonine-derived thioureas and their application as dual MOC in Diels–Alder type reactions.

2.2.3. Amino acids and derivatives. Amino acids and their derivatives are typical organocatalysts for asymmetric syntheses. Surprisingly, the majority of amino acid functionalized MNP did not meet this issue but either was applied to non-stereogenic reactions or information was not given if the reactions were enantioselective or not (more likely).

One of the first examples for applications in asymmetric synthesis concerned (S)-proline as catalyst for aldol reaction.⁴⁸ Different types of acrylate and methacrylate monomers containing the (S)-proline moiety differing in length of linkers were submitted to polymerization reactions in the presence of magnetite NP covered with acrylic acid or a methacryloyl-containing phosphate (Scheme 20). The resulting MOCs were successfully applied in 10 mol% to direct asymmetric aldol reactions of arylaldehydes with CH-acidic ketones. The application of benzoic acid as co-catalyst was crucial in order to achieve high yields and excellent enantioselectivities. The catalyst tolerated a variety of different substrates and could easily be separated by magnetic decantation, recycled and reapplied in further aldol reactions. It maintained its excellent catalytic performance till 10 recyclings.

Scheme 20 MOC based on polyacrylates functionalized with (S)-proline and application in asymmetric direct aldol reaction.

The methodology to fix multiple organocatalytic moieties in a polymer at MNP seems to be more efficient than only having one catalytic unit at one surface anchor as demonstrated with a 4-hydroxyproline.⁴⁹ Here, 4-hydroxyproline was attached to silica-coated maghemite NP by a triethoxysilylpropanecarbamate (Scheme 21). Moderate to good yields, diastereoselectivities and enantioselectivities were achieved in aldol reaction of cyclohexanone with several benzaldehydes. But the results were not as good as in cases of the polymer coated MNP mentioned above (Scheme 20) even when 20 mol% catalyst was used instead of 10 mol% (Scheme 21). Recycling studies showed that the catalyst maintained its performance in five subsequent runs without significant changes.

Scheme 21 Application of 3-hydroxyproline linked to magnetite@silica NP as MOC in aldol reactions.

(S)-Proline was also linked to MNP via the carboxyl group by an amide bond starting with silica coated magnetite NP (Scheme 22). Either they were first coated with N-[3-(triethoxysilyl)propyl]-4,5-dihydroimidazole and then alkylated with a bromopropyl derivative of proline amide (formation of MOC I) or by treatment with aminopropyltriethoxysilane followed by acylation with N-boc-proline (MOC II). When applied in asymmetric direct aldol reaction the ionic liquid modified catalyst MOC I performed better giving high yields and enantioselectivities in selected cases. Its recycling revealed that the yield slightly diminished from initially 92% to 89% while the ee remained constant at 85% after the fifth run.⁵⁰

Scheme 22 Ionic liquid modified proline-containing MOC in asymmetric direct aldol reaction.

Proline-based MOCs were also obtained by chemisorptions of proline to MNP containing shells of highly cross-linked vinylimidazolium salt monomer units (Scheme 23). As compared with MNP coated with proline-containing polyacrylates (see Scheme 20), much lower ee were achieved in asymmetric direct aldol reaction with these MOCs.⁵¹

Scheme 23 Proline adsorbed at magnetite@silicaNP covered with polymeric ionic liquids and its application to asymmetric direct aldol reaction.

Amino-protected prolines (boc, fmoc) were bound to maghemite nanosupports equipped with –NH₂ groups (γ-Fe₂O₃@alendronate) through peptide formation and finally deprotected (Scheme 24).⁵² Surprisingly, the publication did not contain any application of the resulting magnetic catalyst system.

Scheme 24 Synthesis of proline amides linked to maghemite NP via alendronate.

The immobilization of a MacMillan type catalyst, based on (S)-phenylalanine, at MNP was demonstrated via CuAAC click chemistry (Scheme 25).⁵³ The starting MNP were obtained by thermal decomposition, covered with an azidopropylsilyloxy-shell and finally submitted to Cu-catalyzed cycloaddition with the respective propargyl ether. The as prepared catalyst was used in asymmetric Friedel–Crafts alkylation (Michael type reaction) of N-substituted pyrroles with α,β-unsaturated aldehydes. In addition, the same catalyst immobilized on polystyrene was prepared and applied. In all cases, these polystyrene based catalysts led to higher yields and enantioselectivities in shorter times than the one based on magnetic nanoparticles. However, the activity of the catalyst attached on MNP retained until the fifth recycling while the polystyrene based catalyst led to decreased yields after the third cycle. In both cases ee's were unchanged until the sixth cycle. Thus, modified MNP were better from the practical point of view because they could be easier separated and revealed more stability in the recycling test.

Scheme 25 MacMillan-type organocatalyst linked to MNP and application in asymmetric Friedel–Crafts alkylation.

The Mannich reaction between cyclohexanone, aniline derivatives and benzaldehydes was catalyzed by magnetite NP functionalized with cysteine (Scheme 26).⁵⁴ Products were obtained under neat conditions with 5 mol% catalyst in moderate to excellent yields, however, in unsatisfactory diastereoselectivities. Since nothing was mentioned about eventual enantioselectivity it is assumed that the reaction was not enantioselective regardless the fact that a chiral catalyst was used. The catalyst was recycled and used in nine consecutive cycles. The same MOC was applied in 3-component synthesis of hydrogenated quinoline carboxylates.

Scheme 26 MNP covered with cysteine as MOC for Mannich reactions.

Recently, a series of papers were published wherein glutathione was fixed on MNP via metal–S bonds (Scheme 27).^55–57 This MOC was checked in various reactions. Considering the fact that the catalyst is chiral it is surprising that asymmetric reactions were not included. The Paal–Knorr synthesis of a variety of 1-substituted pyrroles was implemented in water under microwave conditions in high yields. In the same manner, aza-Michael reactions and pyrazole syntheses were catalyzed affording high yields. Recycling tests revealed that the catalyst could be used in three to five runs depending on the type of reaction. Interestingly, the catalyst also catalyzed homocoupling of aryl boronic acid derivatives under metal free condition (Scheme 27).⁵⁷

Scheme 27 Glutathione-functionalized MNP as MOC for Paal–Knorr synthesis of pyrroles, Michael-type addition of amines to acrylate and homo-coupling of boronic acids.

Cysteine was attached to vinyl modified SBA-15 via thiol–ene click chemistry. The resulting mesoporous materials were mixed with magnetic nanoparticles in order to incorporate them into the pores. The final Fe₃O₄@mesoporous MOC was applied in Biginelli condensation of aldehydes, acetoacetate and urea to obtain dihydropyrimidones in good yields (Scheme 28). The MOC was used 7 times without a significant loss of activity. Surprisingly, the issue of enantioselectivity, which can be expected with this chiral catalyst was not addressed.⁵⁸

Scheme 28 Mesoporous SBA-15 containing MNP and cysteine as MOC for 3-component synthesis of dihydropyrimidones.

2.2.4. Brønsted acids. Magnetic nanoparticle-supported achiral Brønsted acids have been introduced to organocatalysis relatively often. These systems were applied to reactions, which were hitherto catalyzed by either normal Brønsted acids or solid supported acids. The acid function can be directly linked to the surface of magnetic NP, e.g. by reaction with chlorosulfonic acid. In this way, maghemite NP were obtained and applied in a high yielding (above 90%) Hantzsch synthesis of a variety of 1,4-dihydropyridines (Scheme 29).

Scheme 29 Application of sulfonic acid modified maghemite NP in the synthesis of 1,4-dihydropyridines.

No significant loss of activity was observed within five runs.⁵⁹ In this context it is necessary to mention that unmodified maghemite NP catalyzed the same Hantzsch synthesis even in a more efficient manner (shorter reaction times) than the sulfonic acid modified analogues.⁶⁰ The question appears why the authors modified the maghemite nanoparticles with sulfonic acid if the performance of the catalyst was even reduced.

An analogous coverage of magnetite by chlorosulfonic acid led to a MOC, which was applied in Ritter reactions, α-aminonitrile syntheses (first step of Strecker-synthesis) and tetrahydroacridone syntheses (Friedländer reaction) under solvent-free conditions assisted by microwave in the latter case (Scheme 30). High yields were obtained and successful recycling was demonstrated for 4 times in an example of a Ritter reaction wherein the initial yield of 84% only slightly dropped to 80%.⁶¹

Scheme 30 Application of sulfonic acid-modified magnetite NP in the synthesis of α-aminonitriles and tetrahydroacridones.

Sulfonic groups can also be directly introduced into maghemite NP by reaction with sulfolane (Scheme 31). The resulting MOC was applied in the synthesis of β-phosphomalonic acid derivatives under solvent free conditions. The catalytic system kept its performance at least over 5 runs.⁶²

Scheme 31 Functionalization of maghemite NP with sulfonylpropoxy groups and their application as MOC in the synthesis β-phosphomalonic acid derivatives.

The same MOC was used in reactions of indoles with Michael-acceptors or carbonyl compounds giving high yields of a variety of 3-substitution products under solvent free conditions (Scheme 32). As demonstrated in the reaction of indole with β-nitrostyrene the catalyst could be recycled 4 times without a significant decrease in yield (86% diminished to 84%).⁶³

Scheme 32 Application of sulfonylpropoxy-functionalized maghemite NP in C-alkylation of indoles.

Tethering of sulfonic acid to magnetite NP was often implemented via siloxane anchoring. Since the acid function can damage the magnetic core, coating of the nanoparticles is usually performed first in a separate step. Thus, aminopropyltriethoxysilane was condensed at the surface of magnetite NP and finally treated with chlorosulfonic acid to establish a sulfamic acid function (Scheme 33). The resulting MOC was used in a Strecker synthesis of α-aminonitriles in water⁶⁴ and in a 3-component synthesis of imidazoles.⁶⁵ In the latter case ultrasound irradiation turned out to be advantageous and recycling of the catalyst was performed demonstrating unchanged activity over 6 runs.

Scheme 33 Sulfamic acid functionalized MOCs and their application in Strecker-type synthesis, synthesis of imidazoles and α-aminophosphonates.

Aminopropylsilyloxy-modified magnetite NP were also used as starting material for magnetic nanoparticle-supported dendritic sulfamic acids MOC II (Scheme 33) by first performing addition of the amino group to methyl acrylate, followed by reaction with 1,2-diaminoethane and final sulfamic acid formation.⁶⁶ This MOC allowed synthesizing α-arylaminophosphonates, which are interesting in pharmaceutical chemistry. Recycling of the MOC was demonstrated in 7 subsequent runs.

Moreover, the sulfamylation strategy was used to obtain sulfamic acid organocatalysts linked to MNP consisting of iron cores and magnetite shells. These systems were applied to epoxide ring opening of epoxy fatty esters by methanol and provided the same high yields as common strong acids (Scheme 34).⁶⁷

Scheme 34 Ring opening of glycidates by methanol.

Sulfonic acids fixed to carbon skeletons were attached to cobalt spinel ferrite NP by silanization in three different ways (Scheme 35).⁶⁸ The resulting MOC were tested in the hydrolysis of benzaldehyde dimethylacetal (Scheme 35). Kinetic investigations revealed an activity at low catalyst loading better than with other commercial heterogeneous or even homogeneous catalysts. While catalysts MOC I, MOC II, and MOC IV showed good recyclability, catalyst MOC III lacking a silica-coating failed in catalysis after magnetic recycling, i.e. it probably served as a source of a homogeneous catalyst formed by disconnection from the MNP in the first run.

Scheme 35 Application of sulfonic acid-based Co–ferrite MOCs in hydrolysis of acetals.

A dual magnetic nanoparticle-supported organocatalyst consisting of a sulfonic acid moiety and an ionic liquid was obtained starting from magnetite NP, which were first covered by reaction with 3-chloropropyltrimethoxysilane. The resulting NP alkylated triphenylphosphane giving rise to the formation of triphenylphosphonium salts which were finally sulfonated at the phenyl rings. The resulting MOC was applied in acetalizations of ketones and aldehydes with ethylene glycol affording excellent yields, which were maintained over 5 runs (Scheme 36).⁶⁹

Scheme 36 Dioxolane-synthesis catalyzed by aryl sulfonic containing MOC.

A magnetic catalytic system with sulfonic acid as catalytic moiety and imidazolium units as poly-ionic liquid was synthesized by reaction of magnetite NP with a triethoxysilylpropylmethacrylate followed by copolymerization of a imidazolium-containing propanesulfonic acid with 1,4-bis(3-vinylimidazol-1-ylium)butane as cross linker in the presence of AIBN (Scheme 37).⁷⁰ This MOC was applied in the synthesis of germinal diacetates from aldehydes and acetic anhydride under neat conditions. Recycling of the catalyst did not show a significant drop of yields within 10 runs. The same MOC was also applied to cleave germinal diacetates with methanol to regenerate the aldehydes. Thus, the system is suitable for protective group methodology. It is worth mentioning that the diacetate derived from acetophenone remained unchanged under these conditions thus providing chemoselectivity.

Scheme 37 Polyvinylimidazolium coated MNP as catalysts for formation and cleavage of germinal diacetates.

As an alternative way to sulfonic acids tethered to magnetic nanoparticles by silanization another two step methodology has to be mentioned consisting of primary coating with non-functionalized silica by reaction with TEOS followed by a reaction with a functionalized tetraalkoxysilane (Scheme 38). In this way, a catalytic system was obtained that had dual character: sulfonic acid as acid catalyst and imidazolium salt as ionic liquid.⁷¹ The system was applied in the synthesis of benzoxanthenes. High yields up to 94% were obtained and kept within 5 recyclings.

Scheme 38 3-component synthesis of anellated pyranes catalyzed by MNP covered by imidazolium salts containing sulfonic acids.

Sulfonic acid moieties were also fixed to magnetite NP embedded in mesoporous silica by co-condensation of TEOS and 3-mercaptopropyl-trimethoxysilane in the presence of cetyltrimethylammonium bromide as a template reagent and final oxidation of the thiol group (Scheme 39).⁷² The template reagent caused formation of mesoporous silica MCM-41 wherein the magnetic nanoparticles were deposited. The catalyst was applied in the synthesis of condensed pyranes by a 3-component reaction under milder conditions (solvent free at room temperature) than used before with other catalysts. It afforded high yields and was recycled four times in one case without a drop in the yield.

Scheme 39 3-component synthesis of anellated pyranes catalyzed by sulfonic acid-functionalized magnetic mesoporous silica.

Moreover, the methodology of using silica-coated magnetic NP to fix Brønsted acids as organocatalysts was applied to adsorb the respective acid without forming covalent bonds. Thus, maghemite NP first were treated with TEOS followed by interaction of the resulting silica–maghemite core–shell NP with, perchloric acid,⁷³ trifluoromethane sulfonic acid⁷⁴ or 4-dodecylbenzenesulfonic acid (Scheme 40).⁷⁵ The ClO₄-containing catalyst was applied to Ritter reaction under mild neat conditions and could be reused after magnetic separation, washing and drying at 110 °C without a significant decrease in activity after 4 runs.⁷³ The triflic acid loaded maghemite NP catalyzed the synthesis of various germinal dihydroperoxides starting from ketones or aldehydes and hydrogen peroxide in acetonitrile at room temperature (Scheme 40). In case of cyclohexanone the catalyst could be used 7 times yielding 92% of the product in the last run. 4-Dodecylbenzene sulfonic acid containing MOC allowed to synthesize a library of spirooxindolopyrimidines by refluxing 1,3-cyclohexandiones, barbiturates and isatins in water. Recycling was demonstrated at least 6 times.⁷⁴

Scheme 40 Adducts of acids to silica-coated MNP as MOC for reaction of nitriles with alcohols, synthesis of germinal dihydroperoxides and 3-component synthesis of condensed pyranopyrimidines.

Covalent linking of hydrosulfate units to magnetite NP after prior silica coating was achieved with chlorosulfonic acid (Scheme 41).^76,77 The resulting catalyst was originally applied to reaction of aromatic aldehydes with cyclic 1,3-dicarbonyl compounds, such as barbituric acid, indandione etc. affording either tetraketones or, if the aryl ring is very electron rich (amino, methoxy), Knoevenagel products that are considered to be intermediates in the formation of the tetraketones (Scheme 41). Magnetic separation and reuse of the catalyst was possible, however the yield diminished to half in the fifth run.⁷⁶

Scheme 41 Sulfuric acid derivatives of silica–magnetite NP as catalysts for Knoevenagel reaction and formation of tetraketones.

The same catalyst was applied to a 3-component reaction of arylaldehydes, 1,3-diketones and phthalazine under solvent-free conditions to synthesize imidazophthalazintriones as a very special type of tetracyclic heterocycles (Scheme 42). As shown with one example, the MOC maintained high yield (83%) when used in six subsequent runs.⁷⁷

Scheme 42 Sulfuric acid derivatives of silica–magnetite NP as catalysts for 3-component synthesis of anellated phthalazines.

Furthermore, silica pre-coating by TEOS turned out to be useful with cobalt spinel ferrite NP. Reaction of the resulting NP with 3-sulfanylpropyltrimethoxysilane and further oxidation with hydrogen peroxide gave the magnetic propanesulfonic acid catalysts (Scheme 43).^78,79 Alternatively, carboxylic acid containing magnetic NP were obtained by reaction with 3-cyanopropyltriethoxysilane followed by acid hydrolysis of the cyano groups. Ring opening of a perfluorinated sultone by CoFe₂O₃@SiO₂ NP afforded a sulfonic acid organocatalyst connected to the silica shell by fluoroalkyl groups (Scheme 43).⁷⁹ All these MOC were applied in hydrothermolysis of disaccharides^78,79 and polysaccharides,⁷⁸ which is interesting for potential industrial application exploiting biomasses (Scheme 43). Furthermore, cellulose hydrolysis was also catalyzed by sulfonic acids linked to the carbon shell of magnetite–carbon core–shell NP. The catalyst system MOC IV was shown to be recyclable but lost some of its sulfonic groups in this process.⁸⁰

Scheme 43 Application of sulfonated or carboxylated MNP as catalysts in hydrolysis of di- and polysaccharides.

Besides silica also other inorganic shells were introduced for primary coating of magnetic NP, such as hydroxyapatite. These core–shell NP were transformed into hydrosulfates by reaction with chlorosulfonic acid in a direct way (MOC I)⁸¹ or in two steps (MOC II) by aminopropylsilanization and sulfamidation by chlorosulfonic acid (Scheme 44).⁸² The former catalyst was applied in formamide synthesis by reaction of 85% aqueous formic acid with amines. After using the catalyst in five consecutive runs the initial yield dropped from initial 100% to 94%.⁸¹ As shown with many examples, the same MOC was successfully applied in the reductive amination of aldehydes or ketones in the presence of sodium borohydride (Scheme 44). High yields were obtained that maintained 93% after 5 times recycling of the catalyst.⁸³ The sulfamic acid containing MOC II was applied in Friedländer reaction of o-aminoarylketones with enolizable ketones under neat conditions (Scheme 44).

Scheme 44 Sulfamidic acid derivatized maghemite–apatite-NP as MOC for formamide formations, reductive amination, Friedländer quinoline synthesis and preparation of α-aminophosphonates.

The reaction afforded high yields even at room temperature and the catalyst could be reused 9 times, still affording 85% of the product.⁸² Furthermore, the same catalyst allowed a high yielding synthesis of α-aminophosphonates starting from non-enolizable aldehydes, amines and dimethyl phosphate (Scheme 44).⁸²

As demonstrated with poly(divinylbenzene-4-vinylpyridine) copolymer, commercial magnetite NP can be covered by an organic polymer giving microbeads, which were later on sulfonated by sulfuric acid.⁸⁴ The resulting MOC was applied in esterification of propionic acid with methanol and was recycled and reused 4 times with negligible loss of activity (Scheme 45). Although magnetite NP protected by fatty acids are widely used, they were only once reported in the preparation of core–shell nanoparticles functionalized with Brønsted acids.⁸⁵ Oleic acid stabilized magnetite NP were either functionalized by propanesulfonic acids via silanization or by coating with sulfonated organopolymers (Scheme 46). Polymerization of glycidyl methacrylate and ring opening with sulfite or 3-aminopropanesulfonic acid gave corresponding polymethacrylate shells. However, the loading with sulfonic acid units was unsatisfactory in the latter case.

Scheme 45 Adducts of sulfuric acids to polymer coated magnetite NP as esterification catalyst.

Scheme 46 Polymer coated magnetite NP containing sulfonic acid functionalities as catalysts in transesterification of triglycerides.

Alternatively, styrene was polymerized at the oleic acid stabilized NP and finally sulfonated with sulfuric acid (Scheme 46). The resulting magnetic nanoparticle-supported sulfonic acid catalysts were applied to transesterification of natural fats (triglycerides of fatty acids) with methanol. 96% conversion of the fat was achieved within 2 hours and no loss of productivity was observed with MOC IV after ten reaction cycles. Magnetite NP were grafted to carbonaceous material obtained by incomplete hydrothermal carbonization of cellulose. After calcination the as-prepared superparamagnetic carbonaceous material was treated with mercaptoacetic acid and finally oxidized by hydrogen peroxide giving rise to a catalytic material possessing sulfonic acid, carboxylic acid and phenolic OH groups. It was applied in hydrolysis of cellulose in either aqueous phase or in ionic liquids at 130–180 °C for 3 to 9 h providing reducing sugars in a total yield of 51 or 69%, respectively. Recycling of the catalyst resulted in decreasing yields from 68% to 43% in the third run.⁸⁶

A MOC suitable for the synthesis of β-acetylaminoketones was obtained from silica-coated CoFe₂O₄ nanoparticles by chloropropylsilanization, reaction with imidazole and final treatment with trifluoroacetic acid (Scheme 47).⁸⁷

Scheme 47 β-Acetylaminoketone synthesis catalyzed by imidazolium coated Co–ferrite NP.

2.2.5. Crown ethers and quaternary salts as phase transfer catalysts. Phase transfer catalysis (PTC) has developed as a powerful tool to achieve asymmetric C–C-bond formation. Here, often chiral quaternary ammonium salts such as cinchona-alkaloid derivatives or spirobinaphthyl derivatives were applied.⁸⁸ In contrast, asymmetric PTC has not yet been applied to magnetically supported PTC. Just a few non-chiral applications of PTC were reported. Magnetite NP were functionalized with crown ethers (Scheme 48),⁸⁹ tetraalkylammonium or tetraalkylphosphonium moieties (Scheme 49)⁹⁰ by silanization. These MOC were applied in two-phase Finkelstein exchange reactions at alkyl halides and in alkylation of phenols and acetate (Schemes 48 and 49). In the latter case the crown ether systems could be recycled 8 times without any loss of activity (yield >99% throughout all runs), while the quaternary ammonium salt faced a drop in yield from 94 to 89% already in the 4^th recycling.

Scheme 48 Crown-ether functionalized magnetite NP as phase transfer catalysts for Finkelstein reactions.

Scheme 49 MNP functionalized by quaternary ammonium or phosphonium units as PT-catalysts in Finkelstein reaction and Williamson ether synthesis.

Magnetite NP covered by β-cyclodextrine-polyurethane polymer brushes⁹¹ served as solid–liquid phase-transfer catalysts in nucleophilic substitution of chloride or bromide in benzylhalides by SCN⁻, CN⁻, N₃⁻ and AcO⁻ in water as solvent (Scheme 50). High yields were achieved in short reaction times and the catalyst was reused 10 times with a drop of the yield from 85% to 73%.

Scheme 50 Cyclodextrine-modified MNP as MOC in Finkelstein reactions.

2.2.6. Miscellaneous. TEMPO in the presence of stoichiometric quantities of co-oxidants is an important catalyst for selective oxidation of alcohols to carbonyl compounds. It was also fixed to magnetic nanoparticles. Thus carbon coated cobalt NP obtained by reducing flame synthesis were functionalized with benzylazido groups by diazonium-mediated substitution and Mitsunobu reaction (Scheme 51).

Scheme 51 TEMPO functionalization of carbonized Co NP as catalyst in oxidation of alcohols to aldehydes.

CuAAC-click reaction with propargyl-containing TEMPO afforded a MOC, which was applied to the oxidation of a series of primary and secondary alcohols under mild conditions. The catalyst was recycled after each oxidation and provided yields as high as 96% after the 6^th run.⁹² Using click-chemistry, TEMPO was also fixed to magnetite NP either in a step-wise fashion or directly (Scheme 52). The MOC performed very well in oxidation of primary or secondary alcohols to carbonyl compounds under aerobic conditions or with hypochloride as co-oxidant. Yields above 85% were achieved and the catalyst was recycled 22 times. A considerable drop in conversion occurred as late as in the 23^rd run. It was found out that leaching and degradation of TEMPO was responsible for the deactivation of the catalytic system. Remarkably, thioethers were not affected under the aerobic oxidation conditions.⁹³

Scheme 52 TEMPO functionalized magnetite NP as MOC in oxidation of alcohols to carbonyl compounds.

Another magnetic TEMPO catalyst was obtained from silica-coated magnetite NP by silanization with aminopropyltriethoxysilane followed by reductive amination with a TEMPO-derived ketone (Scheme 53). It was applied in the presence of t-butylnitrite as co-catalyst and oxygen as co-oxidant. A variety of primary and secondary alcohols, amongst them highly sterically hindered ones, were transformed into the corresponding carbonyl compounds in high yields. The catalyst exhibited consistent efficiency in 20 subsequent runs.⁹⁴ The same catalyst was applied in many examples of oxidative Passerini reaction wherein alcohols were used instead of the usually employed carbonyl compounds (Scheme 53). Again, t-butylnitrite was used as co-catalyst and oxygen as co-oxidant. Yields are higher in case of primary alcohols and can reach 94%. Recyclability was tested in the case of a primary alcohol and resulted in 75% product yield in the 14^th run starting with initial 92%.⁹⁵

Scheme 53 TEMPO functionalized silica-coated magnetite NP as catalyst in oxidation of alcohols to carbonyl compounds and in oxidative Passerini reaction.

TEMPO catalysts supported by magnetic polystyrene nanospheres were synthesized from magnetic fluid by miniemulsion copolymerization of styrene, 1,4-divinylbenzene and 4-chloromethylstyrene affording a Merrifield type polystyrene followed by nucleophilic substitution of chloride by a TEMPO-derived alcohol (Scheme 54).⁹⁶

Scheme 54 TEMPO-containing magnetic polystyrene nanospheres as catalysts for the oxidation of alcohols to carbonyl compounds.

The MOC allowed the oxidation of a variety of primary and secondary alcohols with NaOCl as co-oxidant. It provided 95% conversion even in the 20^th run.

In an alternative method, oxidation of alcohols to carbonyl compounds was performed with catalytic amounts of dissolved TEMPO and diacetoxyiodoniumbenzene as co-oxidant fixed to MNP (Scheme 55). Again, this combination gave rise to high yields and selectivities. After magnetic separation the resulting iodobenzene-functionalized MNP was re-oxidized with peracetic acid and reused without a significant drop in yield (first run: 94%, 8^th run: 88%).⁹⁷

Scheme 55 Application of hypervalent iodo compounds fixed to MNP in TEMPO-catalyzed oxidation of alcohols.

MNP coated with covalently linked ionic liquids can also function as MOC. Thus, imidazolium units were fixed to magnetite NP by silanization and the resulting magnetic ionic liquid was used to catalyze the cycloaddition of carbon dioxide and oxiranes giving high yields of cyclic carbonates under solvent-free conditions at 140 °C (Scheme 56). The catalyst maintained it activity after 10 recyclings.⁹⁸ Another MOC (MOC P) for transforming oxiranes into cyclic carbonate was obtained be encapsulating silica-coated maghemite NP with polymers consisting of highly cross-linked vinylimidazolium monomer units (Scheme 56). Recycling was demonstrated 4 times. However the yields were generally low.

Scheme 56 Imidazolium-functionalized MNP as MOC for the synthesis of cyclic carbonates from oxiranes.

Cyanuric chloride chemistry was applied to link imidazolium salts to silica encapsulated magnetite NP. The NP were used as MOC in Betti synthesis of diarylmethane amines providing high yields (Scheme 57). Involvement of both imidazolium units at one triazine moiety was assumed in the transitions state wherein one acts as a Lewis acid and the other as a Brønsted acid. The performance of the catalyst did not change in 6 subsequent runs.⁹⁹

Scheme 57 Betti synthesis of diarylmethane amines catalyzed by imidazolium-functionalized magnetite NP.

DABCO-derived magnetic nanomaterial (xerogel) was prepared be treatment of magnetite NP with a triethoxysilane-containing derivative. The material was applied as MOC in 3-component reactions of arylaldehydes, malononitrile and hydroxypyranones or dimedone (Scheme 58). The anellated 2-aminodihydropyranes were obtained in water as solvent in high yields (>80%). It was shown that the yield decreased from initial 90% to 85% after five recyclings of the catalyst. It remains unclear what acts as catalytic moiety in this double positively charged catalyst with chloride as counterion.¹⁰⁰ The authors cite reports of a singly charged DABCO-derivative where the uncharged N-atom acts as a base, i.e. the situation is totally different.¹⁰¹

Scheme 58 Application of doubly alkylated DABCO-magnetite NP in 3-component synthesis of condensed 2-aminopyranes.

1,4-Dihydropyridine-functionalized MNP were obtained by alkylsilanization of magnetite NP or silica-coated magnetite NP. They served as MOCs for the reduction of epoxyketones with Na₂S₂O₄ to β-hydroxyketones (Scheme 59). High yields were achieved after 5 recyclings of the catalyst.¹⁰²

Scheme 59 Reductive ring opening of glycidates catalyzed by dihydropyridine-functionalized magnetite NP.

A MOC for highly efficient cross coupling of aryl chlorides with phenols and alcohols was obtained via a synthetic sequence comprising aminoalkylsilanization and aniline oxidation (Scheme 60). The magnetic mesoporous polyaniline material exhibited a wide scope of application and led to high yields of aryl ether. Recycling of the catalyst was demonstrated wherein the yield in the first run (97%) remained the same in the 5^th run. Concerning the mode of action of the catalyst the authors propose and dissociative addition of chloroarens to the polyaniline surface.¹⁰³

Scheme 60 Catalysis of cross-coupling of chloroarenes with phenols or alcohols catalyzed by magnetic mesoporous polyaniline.

3. Conclusions and outlook

The concept of magnetic nanoparticle-supporting of organocatalysts has rapidly developed in the last few years. Since such catalysts work under quasi-homogeneous conditions, their performance is usually very good. Reactions are often performed under mild conditions without using solvents or by applying water. The catalysts can be easily separated by magnetic decantation and reused in many cases. In principle, solvents are not necessary for the separation step, but are eventually used for washing the catalyst prior to subsequent use. All these issues render the application of magnetically supported organocatalysts an interesting tool for green chemistry and probably for industrial application. On the other hand, one has also to take into consideration that a number of the hitherto involved, mostly non-asymmetric reactions can be easily implemented by traditional homogeneous catalysis, e.g. by addition of a few drops of a mineral acid. In such cases the application of magnetically supported catalysts can be questioned because one has also to take into consideration that such catalytic systems have to be prepared prior to application, which consumes extra manpower and chemicals. In addition, considerable examples of the application of MOC concern very specific and less interesting cases often in heterocyclic chemistry. Therefore, future research in this area should focus on either more general and more important applications or on organocatalysts, which are difficult to prepare and expensive, e.g. chiral catalysts, and thus are in particular worth to get recycled. Thus, there is a high need for more focused research in this field in the future.

In the course of processing the manuscript the following publications appeared:

• S. Mondini, A. Puglisi, M. Benaglia, D. Ramella, C. Drago, A. M. Ferretti and A. Ponti, J. Nanopart. Res. 2013, 15:2025 (Magnetic nanoparticles conjugated to chiral imidazolidinone as recoverable catalyst).

• S. Pagoti, D. Dutta, and J. Dash, Adv. Synth. Catal., 2013, 355, 3532 (A Magnetoclick Imidazolidinone Nanocatalyst for Asymmetric 1,3-Dipolar Cycloadditions).

• S. Sobhani and M. Honarmand, Appl. Catal., A, 2013, 467, 456 (Ionic liquid immobilized on γ-Fe2O3 nanoparticles: A new magnetically recyclable heterogeneous catalyst for one-pot three-component synthesis of 2-amino-3,5-dicarbonitrile-6-thio-pyridines).

The first two concern chiral MOCs and their successful application in asymmetric cycloaddition reactions.

Acknowledgements

Financial support by Romanian Ministry of Education and Research and European Union within the POS-CCE-METAVASINT, project no 550/2010 and PN-II-RU-TE-2011-3-0130 is gratefully acknowledged.

References

1. Enantioselective Organocatalysis: Reactions and Experimental Procedures, ed. P. I. Dalko, WILEY-VCH Verlag GmbH & Co. KGaK, Weinheim, 2007.
2. B. List, Chem. Rev., 2007, 107, 5413 , and other publications in this special issue.
3. M. Benaglia, A. Puglisi and F. Cozzi, Chem. Rev., 2003, 103, 3401.
4. T. E. Kristensen and T. Hansen, Eur. J. Org. Chem., 2010, 3179.
5. A. Puglisi, M. Benaglia and V. Chiroli, Green Chem., 2013, 15, 1790.
6. A. Schätz, O. Reiser and W. J. Stark, Chem.–Eur. J., 2010, 16, 8950.
7. S. Shylesh, V. Schunemann and W. R. Thiel, Angew. Chem., Int. Ed., 2010, 49, 3428.
8. K. V. S. Ranganath and F. Glorius, Catal. Sci. Technol., 2011, 1, 13.
9. V. Polshettiwar and R. S. Varma, Green Chem., 2010, 12, 743.
10. C. W. Lim and I. S. Lee, Nano Today, 2010, 5, 412.
11. S. Roy and M. A. Pericas, Org. Biomol. Chem., 2009, 7, 2669.
12. Y. H. Zhu, L. P. Stubbs, F. Ho, R. Z. Liu, C. P. Ship, J. A. Maguire and N. S. Hosmane, ChemCatChem, 2010, 2, 365.
13. R. B. N. Baig and R. S. Varma, Chem. Commun., 2013, 49, 752.
14. V. Polshettiwar, R. Luque, A. Fihri, H. Zhu, M. Bouhrara and J.-M. Basset, Chem. Rev., 2011, 111, 3036.
15. R. B. Nasir Baig and R. S. Varma, Green Chem., 2013, 15, 398.
16. M. B. Gawande, P. S. Branco and R. S. Varma, Chem. Soc. Rev., 2013, 42, 3371.
17. N. A. Frey, S. Peng, K. Cheng and S. Sun, Chem. Soc. Rev., 2009, 38, 2532–2542.
18. A.-H. Lu, E. L. Salabas and F. Schüth, Angew. Chem., Int. Ed., 2007, 46, 1222–1244.
19. J. Gao, H. Gu and B. Xu, Acc. Chem. Res., 2009, 42, 1097–1107.
20. T. D. Schladt, K. Schneider, H. Schild and W. Tremel, Dalton Trans., 2011, 40, 6315–6343.
21. S. Laurent, D. Forge, M. Port, A. Roch, C. Robic, L. Vander Elst and R. N. Muller, Chem. Rev., 2008, 108, 2064–2110.
22. M. Faraji, Y. Yamini and M. Rezaee, J. Iran. Chem. Soc., 2010, 7, 1–37.
23. R. Massart and V. Cabuil, J. Chim. Phys. Phys.-Chim. Biol., 1987, 84, 967–973.
24. E. Karaoglu, A. Baykal, M. Senel, H. Sozeri and M. S. Toprak, Mater. Res. Bull., 2012, 47, 2480.
25. S. Z. Luo, X. X. Zheng, H. Xu, X. L. Mi, L. Zhang and J. P. Cheng, Adv. Synth. Catal., 2007, 349, 2431.
26. H. R. Shaterian and M. Aghakhanizadeh, Catal. Sci. Technol., 2013, 3, 425.
27. R. Abu-Reziq and H. Alper, Appl. Sci., 2012, 2, 260.
28. N. T. S. Phan and C. W. Jones, J. Mol. Catal. A: Chem., 2006, 253, 123.
29. N. T. S. Phan, C. S. Gill, J. V. Nguyen, Z. J. Zhang and C. W. Jones, Angew. Chem., Int. Ed., 2006, 45, 2209.
30. M. Khoobi, L. Ma'mani, F. Rezazadeh, Z. Zareie, A. Forournadi, A. Ramazani and A. Shafiee, J. Mol. Catal. A: Chem., 2012, 359, 74–80.
31. C. O'Dalaigh, S. A. Corr, Y. Gun'ko and S. J. Connon, Angew. Chem., Int. Ed., 2007, 46, 4329.
32. N. A. Brunelli, W. Long, K. Venkatasubbaiah and C. W. Jones, Top. Catal., 2012, 55, 432.
33. D. Girija, H. S. B. Naik, B. V. Kumar and C. N. Sudhamani, Am. Chem. Sci. J., 2011, 1, 97.
34. S. Sobhani, M. Bazrafshan, A. A. Delluei and Z. P. Parizi, Appl. Catal., A, 2013, 454, 145.
35. Y. Zhang and C. G. Xia, Appl. Catal., A, 2009, 366, 141.
36. Y. Zhang, Y. W. Zhao and C. G. Xia, J. Mol. Catal. A: Chem., 2009, 306, 107.
37. Y. P. Zhang, Q. Z. Jiao, B. Zhen, Q. Wu and H. S. Li, Appl. Catal., A, 2013, 453, 327.
38. A. Rostami, B. Atashkar and H. Gholami, Catal. Commun., 2013, 37, 69.
39. S. Z. Luo, X. X. Zheng and J. P. Cheng, Chem. Commun., 2008, 5719.
40. B. G. Wang, B. C. Ma, Q. Wang and W. Wang, Adv. Synth. Catal., 2010, 352, 2923–2928.
41. P. Riente, C. Mendoza and M. A. Pericas, J. Mater. Chem., 2011, 21, 7350.
42. M. Keller, A. Perrier, R. Linhardt, L. Travers, S. Wittmann, A.-M. Caminade, J.-P. Majoral, O. Reiser and A. Ouali, Adv. Synth. Catal., 2013, 355, 1748.
43. O. Gleeson, R. Tekoriute, Y. K. Gun'ko and S. J. Connon, Chem.–Eur. J., 2009, 15, 5669.
44. Q. S. Guo, M. Bhanushali and C. G. Zhao, Angew. Chem., Int. Ed., 2010, 49, 9460.
45. L. Q. Lu, X. L. An, J. R. Chen and W. J. Xiao, Synlett, 2012, 490.
46. O. Gleeson, G. L. Davies, A. Peschiulli, R. Tekoriute, Y. K. Gun'ko and S. J. Connon, Org. Biomol. Chem., 2011, 9, 7929.
47. X. X. Jiang, H. Zhu, X. M. Shi, Y. Zhong, Y. F. Li and R. Wang, Adv. Synth. Catal., 2013, 355, 308.
48. Z. Yacob, A. Nan and J. Liebscher, Adv. Synth. Catal., 2012, 354, 3259.
49. H. L. Yang, S. W. Li, X. Y. Wang, F. W. Zhang, X. Zhong, Z. P. Dong and J. T. Ma, J. Mol. Catal. A: Chem., 2012, 363, 404.
50. Y. Kong, R. Tan, L. Zhao and D. Yin, Green Chem., 2013, 15, 2422.
51. P. Agrigento, M. J. Beier, J. T. N. Knijnenburg, A. Baiker and M. Gruttadauria, J. Mater. Chem., 2012, 22, 20728.
52. E. Nehlig, L. Motte and E. Guénin, Catal. Today, 2013, 208, 90.
53. P. Riente, J. Yadav and M. A. Pericas, Org. Lett., 2012, 14, 3668.
54. M. B. Gawande, A. Velhinho, I. D. Nogueira, C. A. A. Ghumman, O. M. N. D. Teodoro and P. S. Branco, RSC Adv., 2012, 2, 6144.
55. V. Polshettiwar, B. Baruwati and R. S. Varma, Chem. Commun., 2009, 1837.
56. V. Polshettiwar and R. S. Varma, Tetrahedron, 2010, 66, 1091.
57. R. Luque, B. Baruwati and R. S. Varma, Green Chem., 2010, 12, 1540.
58. J. Mondal, T. Sen and A. Bhaumik, Dalton Trans., 2012, 41, 6173.
59. N. Koukabi, E. Kolvari, M. A. Zolfigol, A. Khazaei, B. S. Shaghasemi and B. Fasahati, Adv. Synth. Catal., 2012, 354, 2001.
60. N. Koukabi, E. Kolvari, A. Khazaei, M. A. Zolfigol, B. Shirmardi-Shaghasemi and H. R. Khavasi, Chem. Commun., 2011, 47, 9230.
61. M. B. Gawande, A. K. Rathi, I. D. Nogueira, R. S. Varma and P. S. Branco, Green Chem., 2013, 15, 1895.
62. S. Sobhani, Z. P. Parizi and N. Razavi, Appl. Catal., A, 2011, 409, 162.
63. S. Sobhani and R. Jahanshahi, New J. Chem., 2013, 37, 1009.
64. M. Z. Kassaee, H. Masrouri and F. Movahedi, Appl. Catal., A, 2011, 395, 28.
65. J. Safari and Z. Zarnegar, Ultrason. Sonochem., 2013, 20, 740.
66. A. Pourjavadi, S. H. Hosseini, S. T. Hosseini and S. A. Aghayeemeibody, Catal. Commun., 2012, 28, 86.
67. B. K. Ahn, H. W. Wang, S. Robinson, T. B. Shrestha, D. L. Troyer, S. H. Bossmann and X. S. Sun, Green Chem., 2012, 14, 136.
68. C. S. Gill, B. A. Price and C. W. Jones, J. Catal., 2007, 251, 145.
69. P. Wang, A. G. Kong, W. J. Wang, H. Y. Zhu and Y. K. Shan, Catal. Lett., 2010, 135, 159.
70. A. Pourjavadi, S. H. Hosseini, M. Doulabi, S. M. Fakoorpoor and F. Seidi, ACS Catal., 2012, 2, 1259.
71. Q. Zhang, H. Su, J. Luo and Y. Y. Wei, Green Chem., 2012, 14, 201.
72. N. Saadatjoo, M. Golshekan, S. Shariati, H. Kefayati and P. Azizi, J. Mol. Catal. A: Chem., 2013, 377, 173.
73. L. Ma'mani, A. Heydari and M. Sheykhan, Appl. Catal., A, 2010, 384, 122–127.
74. Y. H. Liu, J. Deng, J. W. Gao and Z. H. Zhang, Adv. Synth. Catal., 2012, 354, 441.
75. J. Deng, L. P. Mo, F. Y. Zhao, Z. H. Zhang and S. X. Liu, ACS Comb. Sci., 2012, 14, 335.
76. F. Nemati, M. M. Heravi and R. S. Rad, Chin. J. Catal., 2012, 33, 1825.
77. A. R. Kiasat and J. Davarpanah, J. Mol. Catal. A: Chem., 2013, 373, 46.
78. A. Takagaki, M. Nishimura, S. Nishimura and K. Ebitani, Chem. Lett., 2011, 40, 1195.
79. L. Pena, M. Ikenberry, B. Ware, K. L. Hohn, D. Boyle, X. S. Sun and D. Wang, Biotechnol. Bioprocess Eng., 2011, 16, 1214.
80. C. B. Zhang, H. Y. Wang, F. D. Liu, L. Wang and H. He, Cellulose, 2013, 20, 127.
81. L. Ma'mani, M. Sheykhan, A. Heydari, M. Faraji and Y. Yamini, Appl. Catal., A, 2010, 377, 64.
82. M. Sheykhan, L. Ma'mani, A. Ebrahimi and A. Heydari, J. Mol. Catal. A: Chem., 2011, 335, 253.
83. J. Deng, L. P. Mo, F. Y. Zhao, L. L. Hou, L. Yang and Z. H. Zhang, Green Chem., 2011, 13, 2576.
84. A. Kara and B. Erdem, J. Mol. Catal. A: Chem., 2011, 349, 42.
85. Zillillah, G. W. Tan and Z. Li, Green Chem., 2012, 14, 3077.
86. H. Guo, Y. Lian, L. Yan, X. Qi and R. L. Smith, Green Chem., 2013, 15, 2167.
87. L. Torkian, S. Vahabzadeh and M. Dabiri, in 4th International Conference on Nanostructures (ICNS4), Sharif University of Technology, Kish Island, I. R. Iran, 2012, p. 1058.
88. T. Ooi and K. Maruoka, in Enantioselective Organocatalysis: Reactions and Experimental Procedures, ed. P. I. Dalko, WILEY-VCH Verlag GmbH & Co. KGaK, Weinheim, 2007, p. 121.
89. M. Kawamura and K. Sato, Chem. Commun., 2007, 3404.
90. M. Kawamura and K. Sato, Chem. Commun., 2006, 4718.
91. A. R. Kiasat and S. Nazari, J. Mol. Catal. A: Chem., 2012, 365, 80.
92. A. Schätz, R. N. Grass, W. J. Stark and O. Reiser, Chem.–Eur. J., 2008, 14, 8262.
93. A. K. Tucker-Schwartz and R. L. Garrell, Chem.–Eur. J., 2010, 16, 12718.
94. B. Karimi and E. Farhangi, Chem.–Eur. J., 2011, 17, 6056.
95. B. Karimi and E. Farhangi, Adv. Synth. Catal., 2013, 355, 508.
96. Z. Zheng, J. L. Wang, M. Zhang, L. X. Xu and J. B. Ji, ChemCatChem, 2013, 5, 307.
97. C. J. Zhu and Y. Y. Wei, Adv. Synth. Catal., 2012, 354, 313.
98. X. X. Zheng, S. Z. Luo, L. Zhang and J. P. Cheng, Green Chem., 2009, 11, 455.
99. M. Shafiee, A. R. Khosropour, I. Mohammadpoor-Baltork, M. Moghadam, S. Tangestaninejad and V. Mirkhani, Catal. Sci. Technol., 2012, 2, 2440.
100. J. Davarpanah, A. R. Kiasat, S. Noorizadeh and M. Ghahremani, J. Mol. Catal. A: Chem., 2013, 376, 78.
101. A. Hasaninejad, M. Shekouhy, N. Golzar, A. Zare and M. M. Doroodmand, Appl. Catal., A, 2011, 402, 11.
102. H. J. Xu, X. Wan, Y. Y. Shen, S. Xu and Y. S. Feng, Org. Lett., 2012, 14, 1210.
103. R. Arundhathi, D. Damodara, P. R. Likhar, M. L. Kantam, P. Saravanan, T. Magdaleno and S. H. Kwon, Adv. Synth. Catal., 2011, 353, 1591.

---