Babak Karimi

Ordered Mesoporous Organosilica with Ionic‐Liquid Framework: An Efficient and Reusable Support for the Palladium‐Catalyzed Suzuki–Miyaura Coupling Reaction in Water

The preparation of a novel palladium-supported periodic mesoporous organosilica based on alkylimidazolium ionic liquid (Pd@PMO-IL) in which imidazolium ionic liquid is uniformly distributed in the silica mesoporous framework is described. Both Pd@PMO-IL and the parent PMO-IL were characterized by N(2)-adsorption-desorption, diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), TEM, and solid-state NMR spectroscopy. We have demonstrated that Pd@PMO-IL is an efficient and reusable catalyst for the Suzuki-Miyaura coupling reaction of various types of iodo-, bromo-, and even deactivated aryl chlorides in water. It was also found that although the PMO-IL nanostructure acts as reservoir for soluble Pd species, it can also operate as a nanoscaffold to recapture the Pd nanoparticles into the mesochannels thus preventing extensive agglomeration of Pd. This observation might be attributed to the isolated ionic liquid units that effectively control the reaction mechanism by preventing Pd agglomeration and releasing and recapturing Pd nanoparticles during the reaction process. The catalyst can be recovered and reused for at least four reaction cycles without significant loss of activity.

DESIGN OF A HIGHLY EFFICIENT AND WATER-TOLERANT SULFONIC ACID NANOREACTOR BASED ON TUNABLE ORDERED POROUS SILICA FOR THE VON PECHMANN REACTION

Among a number of different sulfonic acid nanoreactors prepared, 5 having both acidic sites and phenyl groups located inside the mesochannels of SBA-15 was shown to be the most active and reusable catalyst in the von Pechmann reaction. The mesochannels, and covalently anchored organic groups, provide a synergistic means of an efficient approach of the reactants to acidic sites, enough space for the subsequent cyclization, and suitable hydrophobicity to drive out the water byproduct.

A study on applications of N-substituted main-chain NHC-palladium polymers as recyclable self-supported catalysts for the Suzuki-Miyaura coupling of aryl chlorides in water.

The preparation and characterization of a number of main-chain organometallic polymers (NHC-Pd MCOP) with different N-alkyl substituted groups such as benzyl (3a), n-hexyl (3b), and n-dodecyl (3c) are described. Among these polymers, 3c bearing the more lipophilic group n-dodecyl was found to be a more reactive and recoverable catalytic system in the Suzuki-Miyaura cross-coupling reaction of chloroarenes, including both deactivated and hindered aryl chlorides with different types of arylboronic acids under aqueous conditions. While the catalysts seem to be highly recyclable, on the contrary, we have provided much compelling evidence, such as kinetic monitoring, poisoning experiments, and average molecular weight determination before and after catalysis, that shows that the described organometallic polymers might be indeed the source of production of active soluble Pd species in the form of either Pd nanoparticles or fragmented NHC-Pd complexes. Our studies showed that in order to assess whether the catalysts are functioning in a heterogeneous pathway or they are simply a source of production of active Pd species, it is crucial to devise a suitable and highly efficient poison that could capture essentially soluble catalytic species. In this regard, we interestingly found that among a variety of well-known catalyst poisons such as Hg(0), SBA-15-PrSH, and cross-linked poly(4-vinylpyridine) (PVP), only PVP could efficiently quench catalysis, thus providing clear evidence of the formation soluble Pd species in our protocol. In addition, several experiments such as bright-field microscopy, dynamic light scattering (DLS) of the reaction mixture, and kinetic monitoring of the reaction at an early stage confirm not only that the described organometallic polymers could be a source of production of trace amounts of Pd nanoparticles but the capsular structures of these lipophilic polymers in water provides a means of entrapment of nanoclusters in a hydrophobic region, thus accelerating the reaction in pure water in the absence of any co-organic solvent.

Aerobic oxidation of alcohols using various types of immobilized palladium catalyst: the synergistic role of functionalized ligands, morphology of support, and solvent in generating and stabilizing nanoparticles

Preparation and characterization of a variety of immobilized palladium catalyst, based on either ligand functionalized amorphous or ordered mesoporous silica, is described. The resulting Pd-loaded materials act as efficient catalyst for the oxidation of a variety of alcohols using molecular oxygen and air. Our studies show that in the case of supported palladium catalyst on hybrid amorphous silica, the nature of ligand and the solvent could effectively control the generation of nanoparticles. Furthermore, we have found that nanoparticles with smaller size and higher activity were generated from the anchored palladium precursor when the aerobic oxidation of alcohols was carried out in α,α,α-trifluorotoluene (TFT) instead of toluene. On the other hand, in the case of aerobic oxidation reactions by using supported palladium catalyst on hybrid SBA-15, the combination of organic ligand and ordered mesoporous channels resulted in an interesting synergistic effect that led to enhanced activity, prevention of Pd nanoparticles agglomeration, and finally generation of a durable catalyst.

A Highly Recyclable Magnetic Core‐Shell Nanoparticle‐Supported TEMPO catalyst for Efficient Metal‐ and Halogen‐Free Aerobic Oxidation of Alcohols in Water

The selective oxidation of primary and secondary alcohols to the corresponding carbonyl compounds is one of the most challenging reactions in organic chemistry. Many oxidizing reagents such as stoichiometric Cr salts, DMSOcoupled reagents, and hypervalent iodine have been traditionally used to accomplish this transformation. However, these reagents show poor atom efficiency and are often toxic; their widespread use thereby causes significant environmental concerns that render them impractical. As a consequence, due to ever-increasing environmental standards and economic pressures, there is substantial interest towards the use of heterogeneous and recyclable catalysts to achieve the efficient oxidation of alcohols with molecular oxygen or air as the oxidant. Whereas methodologies for the improvement of catalytic activities and selectivities have been developed considerably, they may possibly leave toxic traces of heavy metals in the products. Moreover, many of the metal-based catalyst systems are often deactivated because of the occupation of coordination sites with the by-produced water and so they generally require the use of organic solvents. Therefore, it seems that there is still a great interest in developing efficient and non-metallic catalysts for the aerobic oxidation of alcohols from the viewpoint of socalled green and sustainable chemistry. Of particular interest in this area is the application of the stable nitroxyl radical 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) in combination with various types of non-metallic, stoichiometric co-oxidants as an alternative to metal-based oxidants. Accordingly, the use of TEMPO-based catalyst systems in combination with transition metals such as Ru, Cu, and Mn–Co, as co-catalysts has been well documented for the oxidation of alcohols using molecular oxygen as terminal oxidant. However, these methods still require transitionmetal co-catalysts, which are intrinsically difficult to separate from the product and recycle. Moreover, many of these methods are inefficient for the oxidation of aliphatic alcohols and/or require high reaction temperatures, which are not suitable for thermally unstable substrates. Meanwhile, the use of TEMPO in the absence of transition metals has received increasing attention as a possible alternative to the widely used traditional TEMPO/Metal/O2 in the aerobic oxidation of alcohols. Although, these findings presented an interesting breakthrough in the field of metal-free aerobic oxidation of alcohols, the methods still require homogeneous TEMPO, which is quite expensive, and therefore the effective recovery of TEMPO is highly desirable for many practical applications. In addition, these methods require acidic additives such as HBr, HCl, Br2, and chlorinated solvents in most cases. To address the recovery issues, TEMPO was chemically anchored onto various supports such as silica, organic polymers, mesoporous silica, functionalized ionic liquids, and perfluoroalkyl systems with multiple triazole moieties. Although these methods have partly addressed the recycling problem of the homogeneous TEMPO, only a few of them have shown acceptable activities in the aerobic oxidation of alcohols under transition-metal-free conditions. 15] Nevertheless, these methods mainly employ acidic conditions in which oxidation of acid sensitive substrates such as allylic alcohols can be problematic. Furthermore, although these protocols represent considerable advances, they still need halogen containing co-reagents, and more importantly, the supported TEMPO catalyst is difficult to separate from the reaction solution by classical methods such as extraction, filtration, or centrifugation. A possible strategy to circumvent these problems is to change the traditional supported matrix to materials that have magnetic properties, thus allowing easy separation of the catalysts by simply applying an external magnetic field. Besides the easy separation, an interesting feature of magnetically functionalized catalysts is that they show identical and sometimes even higher activity than their corresponding homogeneous analogues. Quite recently, this approach has been applied for anchoring TEMPO on graphene-coated nanobeads with a magnetic cobalt core through a “click-chemistry” approach. This catalytic material was then successfully applied to the TEMPO-mediated oxidation of alcohols under the standard Anelli s oxidation and magnetically separated and recycled. However, to the best of our knowledge there has been no report on the use [a] Prof. Dr. B. Karimi, E. Farhangi Department of Chemistry, Institute for Advanced Studies in Basic Sciences (IASBS) Zanjan 45137-6731 (Iran) Fax: (+98)241-415-3225 E-mail : karimi@iasbs.ac.ir [**] TEMPO=2,2,6,6-tetramethylpiperidine-1-oxyl. Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201100047.

Recent Applications of Magnetically Recoverable Nanocatalysts in CC and CX Coupling Reactions

Magnetic nanoparticles have emerged recently as an alternative for the easy separation of nanosized catalysts from reaction mixtures by employing an external magnetic field. These magnetic nanoparticles have been used as supports for catalysts and/or as part of an active catalytic site. Herein, special attention is given to identify the main synthetic steps required to develop both precious‐ and nonprecious‐metal‐catalyzed CC and CX coupling reactions by using magnetically separable catalysts.

A highly water-dispersible/magnetically separable palladium catalyst based on a Fe3O4@SiO2 anchored TEG-imidazolium ionic liquid for the Suzuki–Miyaura coupling reaction in water

A novel ionic liquid functionalized magnetic nanoparticle was prepared by anchoring an imidazolium ionic liquid bearing triethylene glycol moieties on the surface of silica-coated iron oxide nanoparticles. The material proved to be an effective host for the immobilization of a Pd catalyst through a subsequent simple ion-exchange process giving a highly water dispersible, active and yet magnetically recoverable Pd catalyst (Mag-IL-Pd) in the Suzuki–Miyaura coupling reaction in water. The as-prepared catalyst displayed remarkable activity toward challenging substrates such as heteroaryl halides and ortho-substituted aryl halides as well as aryl chlorides using very low Pd loading in excellent yields and demonstrating high TONs. Since the catalyst exhibited extremely low solubility in organic solvent, the recovered aqueous phase containing the catalyst can be simply and efficiently used in ten consecutive runs without significant decrease in activity and at the end of the process can be easily separated from the aqueous phase by applying an external magnetic field. This novel double-separation strategy with negligible leaching makes Mag-IL-Pd an eco-friendly and economical catalyst to perform this transformation.

Zinc Chloride Catalyzed Silylation of Alcohols and Phenols by Hexamethyldisilazane. A Highly Chemoselective Reaction

Abstract Hexamethyldisilazane (HMDS) in presence of a catalytic amount of anhydrous zinc chloride silylates efficiently and selectively different types of alcohols and phenols in refluxing acetonitrile or benzene-acetonitrile. This reagent in presence of anhydrous zinc chloride discriminates absolutely amines and thiols from alcohols.

Lithium triflate (LiOTf) catalyzed efficient and chemoselective tetrahydropyranylation of alcohols and phenols under mild and neutral reaction conditions

Abstract Different types of alcohols and phenols were effectively converted to the corresponding THP ethers in the presence of DHP and a catalytic amount of lithium trifluoromethanesulfonate (LiOTf) in refluxing 1,2-dichloroethane under essentially neutral reaction conditions. The method also shows good chemoselectivity for mono-tetrahydropyranylation of symmetrical diols.

Synthesis and Characterization of Alkyl‐Imidazolium‐Based Periodic Mesoporous Organosilicas: A Versatile Host for the Immobilization of Perruthenate (RuO4−) in the Aerobic Oxidation of Alcohols

The preparation and characterization of a set of periodic mesoporous organosilicas (PMOs) that contain different fractions of 1,3-bis(3-trimethoxysilylpropyl)imidazolium chloride (BTMSPI) groups uniformly distributed in the silica mesoporous framework is described. The mesoporous structure of the materials was characterized by powder X-ray diffraction, transmission electron microscopy, and N(2) adsorption-desorption analysis. The presence of propyl imidazolium groups in the silica framework of the materials was also characterized by solid-state NMR spectroscopy and diffuse-reflectance Fourier-transform infrared spectroscopy. The effect of the BTMSPI concentration in the initial solutions on the structural properties (including morphology) of the final materials was also examined. The total organic content of the PMOs was measured by elemental analysis, whereas their thermal stability was determined by thermogravimetric analysis. Among the described materials, it was found that PMO with 10% imidazolium content is an effective host for the immobilization of perruthenate through an ion-exchange protocol. The resulting Ru@PI-10 was then employed as a recyclable catalyst in the highly efficient aerobic oxidation of various types of alcohols.