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Karbeny N-heterocykliczne : synteza i zastosowanie

Malinowska M., Hryniewicka A.

Wiadomości Chemiczne

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2015

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[Z] 69, 3-4

227--253

EN

N-Heterocyclic carbenes (NHCs) are powerful tools in organic chemistry, with numerous applications in academic and industrial laboratories. They are usually defined as singlet carbenes, in which the divalent carbonic centre is connected directly to at least one nitrogen atom in the heterocycle [1]. They have played an important role in organic chemistry ever since the first evidence of their existence. The isolation of stable, free 1,3‑diadamantylimidazol-2-ylidene (IAd, Fig. 1) by Arduengo et al. in 1991 was a milestone in the chemistry of carbenes [2]. From the beginnings as academic curiosities, N‑heterocyclic carbenes today are very useful compounds in a variety of organic transformations (Fig. 13). NHCs are neutral σ-donors, which form very strong bonds with the majority of transition metals (stronger than phosphines). These compounds are easy-to-make ligands with great potential in homogeneous catalysis (mainly ruthenium and palladium complexes) for large number of reactions, including the coupling reactions (Heck, Negishi, Stille, Suzuki or Sonogashira reactions) and olefin metathesis [3]. Moreover, they are very useful as organocatalysts used in the benzoin condensation, the Stetter reaction and ring-opening polymerization (ROP) or transesterification [4]. In this review, we aim to give an overview of the properties and applications of NHCs, which we expect will be a useful introduction for chemists interested in studying and applying these important compounds. The first part of this review is devoted to the main synthetic routes to NHCs, their properties and reactivity. In the second part we describe the metal complexes with NHCs as homogeneous catalysts and their applications in various types of reactions. At the end of this part of the paper the use of NHCs as organocatalysts is presented.

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Hydroformylacja w środowisku cieczy jonowych

Trzeciak A. M.

Wiadomości Chemiczne

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2011

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[Z] 65, 11-12

1003-1020

EN

The hydroformylation reaction was discovered by Otto Roelen in 1938. He studied the side processes occurring during the Fischer-Tropsch synthesis with a cobalt catalyst and found some amounts of aldehydes formed from the olefin and syngas (H2/CO) [1]. The hydroformylation found application in the chemical industry, mainly for production of n-butanal from propene. Aldehydes obtained by propene hydroformylation are subsequently hydrogenated to alcohols, used as solvents. Butanal can also be condensed to C8 aldehydes and alcohols, 2-ethylhex-2-enal and 2-ethylhexanol, important components for plasticizers such as dioctylphtalate. The hydroformylation reaction can be applied not only for the synthesis of aldehydes but also for other products. In particular, successful synthesis of quaternary carbon centers by hydroformylation has been reported in which the rhodium catalyst was modified with a ligand that serves as a catalytic directing group by covalently and reversibly binding to both the substrate and the catalyst. Ionic liquids have been recognized as a novel class of solvents which can be successfully used for homogeneous catalysis [4]. Application of ionic liquids, non-aqueous and non-volatile solvents, has made it possible to construct biphasic systems in order to efficiently separate catalysts from organic products. It is also important that the properties of ionic liquids, such as solubility, acidity, or coordination ability, can be tuned by the use of different cations and anions. In the ideal case, the ionic liquid is able to dissolve the catalyst and displays partial miscibility with the substrate. If the products have negligible miscibility in the ionic liquid, they can be removed by simple decantation, without extracting the catalyst. If the products are partially or totally miscible in the ionic liquid, separation of the products is more complicated [4e, 4h]. The main problem with catalytic systems for hydroformylation containing ionic liquid phase was a significant leaching of the catalyst out of the ionic liquid phase, which can be overcome by modifying neutral phosphane with ionic groups. Examples of such systems are presented in the article. It was revealed that N-heterocyclic carbenes were formed in the biphasic hydroformylation reactions promoted by Rh complexes in an imidazolium ionic liquid [10]. Consequently, reactivity of the in situ formed Rh-carbene complexes can strongly influence on the hydroformylation reaction course [11]. The best methodology to perform the hydroformylation reaction would be a flow system in which the catalyst remains in the reactor and the substrates and products flow continuously into and out of the reactor. For the construction of such a system with soluble rhodium catalysts, ionic liquids could be considered as media used for the immobilization of the catalyst. The first example of continuous flow hydroformylation was reported by Cole-Hamilton [19, 20]. Different Supported Ionic Liquid Phase (SILP) catalysts have been examined in hydroformylation [15–17]. Interestingly, the neutral ligand can be applied efficiently in a continuous gas-phase SILP process, while in a typical biphasic system containing ionic liquid and organic solvent it would leach into the product phase.

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Nanocząsteczkowe katalizatory palladowe w reakcjach tworzenia wiązań C-C

Trzeciak A. M., Ziółkowski J. J.

Wiadomości Chemiczne

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2009

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[Z] 63, 1-2

1-15

EN

The important role of palladium nanoparticles has been recently demonstrated in many catalytic systems designed for C-C bond forming reactions [1-4]. There are examples of catalytic systems described earlier as homogeneous in which Pd(0) nanoparticles were now identified. In the article three different palladium catalytic systems are discussed. In the first one, Pd(0) nanoparticles, obtained by chemical reduction of PdCl2 and stabilized by polyvinylpyrrolidone, were used for Heck coupling in [Bu4N]Br medium. Decrease of nanoparticles size in reaction conditions was explained as a result of dissolution of Pd(0) colloid and simultaneous formation of catalytically active monomolecular anionic palladium complexes [33]. The second example presents application of Pd(II) and Pd(0) supported on alumina-based oxides in Suzuki-Miyaura reaction [36]. Reduction of Pd(II) to Pd(0) nanoparticles under reaction conditions was confirmed. In contrast to the first described case, in Suzuki-Miyaura reaction the size of Pd(0) nanoparticles was the same before and after the catalytic cycle. The catalytic activity of both palladium forms was quite high, however Pd(0) formed in situ was slightly more efficient as catalyst. In the third part of the article studies of palladium reduction in anionic complexes of [IL]2[PdX4] type are shown, where IL = imidazolium cation [37]. These complexes catalyzed well Suzuki-Miyaura cross-coupling, but they were not stable under reaction conditions and decomposed to Pd(0) nanoparticles and Pd black. Using ESI-MS method it was possible to identify polynuclear (Pd3, Pd5) intermediate forms, stabilized with imidazolium cations or N-heterocyclic carbenes. In all systems discussed in the article co-existence of Pd(0) nanoparticles and monomolecular complexes was observed. That is important for understanding of the nature of catalytically active forms in C-C bond forming reactions.

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Rola cieczy jonowych w reakcjach tworzenia wiązań C-C z udziałem katalizatorów palladowych : szczególny wpływ halogenków imidazoliowych

Trzeciak A. M.

Wiadomości Chemiczne

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2006

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[Z] 60, 11-12

819-843

EN

The strategy of modification in chemical processes in order to ensure a safer, cleaner environment in the future is one of the main goal of green chemistry. The basic twelve principles of green chemistry were formulated by P. Anastas and J.Warner in 1998 and accepted by society. Following these principles chemists designing a new process should pay special attention to select substrates and chemicals that minimize their harm to the environment and to human health. Also existing chemical technologies should be modified in a similar way. One approach to achieve this goal is replacement of traditional toxic solvents (mainly VOC,s volatile organic solvents) with ionic liquids presenting a group of liquids or low-temperature melting solid salts of no vapour pressure. Application of ionic liquids in processes catalyzed by transition metal complexes meets two (or in some cases even three) green chemistry rules. The presence of ionic liquids as a solvent in catalytic systems for C-C bond forming reactions like Heck, Sonogashira, Suzuki and carbonylation offers many spectacular advantages including facilitation of catalyst separation from organic products. Elimination even traces of metals from the products of C-C coupling reactions which are used as medicines or agricultural chemicals is extremely important. An article presents catalytic systems containing palladium catalyst precursors, both soluble and heterogenized complexes as well as palladium nanocolloids applied in C-C bond forming processes performed in ionic liquids. The applicability of ionic liquids and influence of their molecular structure on the reaction course is discussed. A special attention is paid to the reactions of ionic liquids with palladium precursors leading to the formation of new species and modification of catalytic properties of the system. It is shown that in many catalytic systems a strong inhibiting effect of imidazolium halides was observed. This fact can be explain on the basis of experimental data by the reaction of imidazolium halide with palladium - aryl intermediate leading to N-heterocyclic carbene complex of lower catalytic activity. Decomposition of palladium - aryl intermediates with formation of phosphonium salts in the presence of imidazolium halide was also observed. In both above mentioned cases a key intermediate in C-C bond forming reactions, that is palladium - aryl halide complex, is eliminated from the reaction mixture causing to decrease of the final product yield. An article presents a state of knowledge in the field of ionic liquids application in catalysis and formulates expectations for future designing of catalytically active and environmentally friendly palladium based systems for C-C bond forming reactions.