Catalyst cationic carbene

Both rhodium and osmium porphyrins are active for the cyclopropanation of alkenes. The higher activity of the rhodium porphyrin catalysts can possibly be attributed to a more reactive, cationic carbene intermediate, which so far has defied isolation. The neutral osmium carbene complexes are less active as catalysts but the mono- and bis-carbene complexes can be isolated as a result. 

In a general (and generalizing) view on this issue it can be stated that suitably selected ionic liquids are very likely to form catalytically active ionic catalyst solutions with a given transition metal catalyst if the latter is neither extremely electrophilic (acidic) nor extremely nucleophilic (basic). While extremely electrophilic catalyst complexes are likely to coordinate strongly even with those anions of the ionic liquid solvent which are generally regarded as weakly coordinating, extremely nucleophilic catalytic centers are likely to react with the ionic liquid s cation. Carbene complex formation by oxidative addition as well as dealkylation of the cation are possible deactivation pathways of the catalyst in such a case. 

Chen et cd. reported their screening methodology for investigating highly active, cationic carbene catalysts. The system of Hofmarm ruthenium-catalyzed olefin metathesis (Scheme 4.10) was successfully studied through systematic variation of structural features of the catalyst and ESI-MS/MS by probing metathesis activity by reaction with vinyl ethers (Scheme 4.11) [33]. 

An interesting class ot covalent Inflates are vin l and ar>/ or heteroaryl Inflates Vinyl inflates are used for the direct solvolytic generation of vinyl cations and for the generation of unsaturated carbenes via the a-elimination process [66] A triflate ester of 2-hydroxypyridine can be used as a catalyst for the acylation of aromatic compounds with carboxylic acids [109] (equation 55)... 

The ease of formation of the carbene depends on the nucleophilicity of the anion associated with the imidazolium. For example, when Pd(OAc)2 is heated in the presence of [BMIM][Br], the formation of a mixture of Pd imidazolylidene complexes occurs. Palladium complexes have been shown to be active and stable catalysts for Heck and other C-C coupling reactions [34]. The highest activity and stability of palladium is observed in the ionic liquid [BMIM][Brj. Carbene complexes can be formed not only by deprotonation of the imidazolium cation but also by direct oxidative addition to metal(O) (Scheme 5.3-3). These heterocyclic carbene ligands can be functionalized with polar groups in order to increase their affinity for ionic liquids. While their donor properties can be compared to those of donor phosphines, they have the advantage over phosphines of being stable toward oxidation. 

The hydrosilylation of acetophenone by diphenylsilane in CH2CI2 at rt was used as a test reaction to compare the selectivity obtained with the carbene ligands (Scheme 36). The reactions were performed in the presence of a sUght excess of AgBp4 (1.2% mol). In these conditions, the N-mesityl-substituted catalyst 57c (1% mol) gave the highest selectivity (65% ee). The in situ formation of square-planar cationic rhodium species 58 as active catalysts appears to be crucial since the same reaction performed without silver salt gave both poor yield (53%) and enantioselectivity (13%). 

In 2007, Fernandez et al. demonstrated that transition-metal complexes with heterobidentate S/C ligands based on imidazopyridin-3-ylidene and thioether functionalities could be readily prepared from the corresponding azolium salts by reaction with Ag20 and transmetalation of the resulting silver carbenes with appropriate metal sources. The cationic Pd(allyl)(carbene-S) complexes have proven to be active catalysts in the test reaction, reaching enantioselectivities of... 

The possibility of adjusting acidity/coordination properties opens up a wide range of possible interactions between the ionic liquid solvent and the dissolved transition metal complex. Depending on the acidity/coordination properties of the anion and on the reactivity of the cation (the possibility of carbene ligand formation from 1,3-dialkylimidazolium salts is of particular importance here [37]), the ionic liquid can be regarded as an innocent solvent, as a ligand precursor, as a co-catalyst or as the catalyst itself. 

As opinions regarding the metathesis reaction pathway have shifted in recent years to favor a nonpairwise carbene-to-metallocyclobutane transformation, increasing attention has been given to the mechanistic significance of cyclopropane olefin interconversions. This interconversion process seems to occur primarily when certain relatively inefficient catalysts are employed, which in itself raises questions. Under ideal conditions, "good metathesis catalysts are remarkably efficient promoters of transalkylidenation, and consequently are well suited for olefin and polymer syntheses. Thus, most early studies focused primarily on applications. When side reactions did occur, they were usually ignored or presumed to be trivial cationic processes. 

Another strategy was successfully implemented by synthetic deprotonation of the acidic C2 group of the imidazolium cation by basic ligands of metal complexes, forming carbenes (Scheme 12). When Pd(OAc)2 was heated in the presence of [BMIM]Br, a mixture of palladium imidazolylidene complexes formed. The palladium carbene complexes have been shown to be active and stable catalysts for the... 

The catalysts must supply the system with lipophilic cations in order to form, with required anions, ion pairs able to enter nonpolar media. The most typical catalysts are tetraalkyl ammonium (TAA) salts R4N+X, particularly those having at least 16 carbon atoms in the four R groups. Similar lipophilic catalysts are tetraalkylphosphonium and -arsonium or trialkylsulfonium salts, which are less available and usually less stable. They are therefore of negligible practical use. There are a few reports on the use of trialkylamines as catalysts in some two-phase reactions. Usually these amines are qua-ternized by a reactant actually these reactions are catalyzed by TAA salts. More complicated is the generation of dihalocarbenes with trialkylamines. The amines form, with the carbene, an ammonium ylide, which acts as a base in the organic phase. 

In the thiazolium cation the proton in the 2-position is acidic and its removal gives rise to the ylide/carbene 227. This nucleophilic carbene 227 can add, e.g., to an aldehyde to produce the cationic primary addition product 228. The latter, again via C-deprotonation, affords the enamine-like structure 229. Nucleophilic addition of 229 to either an aldehyde or a Michael-acceptor affords compound(s) 230. The catalytic cycle is completed by deprotonation and elimination of the carbene 227. Strictly speaking, the thiazolium salts (and the 1,2,4-triazolium salts discussed below) are thus not the actual catalysts but pre-catalysts that provide the catalytically active nucleophilic carbenes under the reaction conditions used. This mechanism of action of thiamine was first formulated by Breslow [234] and applies to the benzoin and Stetter-reactions catalyzed by thiazolium salts [235-237] and to those... 

However, the Rh-catalysed hydrosilylation of terminal alkynes affords the thermodynamically unfavorable anti product 583 as the main product [224], The symanti ratio changes depending on the catalysts and solvents. The syn adduct 584 is obtained by a cationic Rh complex in MeCN [225]. The Ru catalyst gives the anti adduct. Formation of the anti adducts is explained by the following mechanism [224, 226]. Insertion of alkyne to the R3S-RI1 bond generates 585 which, due to steric repulsion, isomerizes to 588 via the carbene species 586, or the metallacyclopropene 587, and gives the anti adduct 583. 

With this experimental set-up, highly active, cationic ruthenium-carbene catalysts are used in ring-opening metathesis polymerization (ROMP). Four different structural features of the catalyst [ R2P(CH2) PR2-k2P XRu=CHR]+ (the halogen... 

The remarkable stability of the 1,1-dichlorocyclopropane system has been recognized ever since the discovery of the carbenes has enabled them to be prepared easily (6). The structure is, however, destroyed by Lewis acids (27). Therefore, polymerizations were first carried out using cationic catalysts and then Ziegler-Natta catalysts which give better results. A first group of monomers consists of 2,2-dimethyl-, 2,2,3-tri-... 

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