Catalysts alkyl carbene-based

Figure 10 Cyclic (alkyl) (amino)carbene-based Ru metathesis catalysts...

Interestingly, the Pd/A-heterocyclic carbene-based catalyst is effective for Sonogashira reactions of both primary alkyl bromides and iodides with terminal alkynes under mild conditions (eq 28). 2... 

The MCR toward 2//-2-imidazolines (65) has found apphcation in the construction of A(-heterocyclic carbene (NHC) complexes (74). Alkylation of the sp Af-atom with an alkyl halide followed by abstraction of the proton at C2 with a strong base (NaH, KOtBu) resulted in the formation of the free carbene species, which could be trapped and isolated as the corresponding metal complexes (Ir or Rh) [160]. The corresponding Ru-complexes were shown to be active and selective catalysts for the transfer hydrogenaticm of furfural to furfurol using iPrOH as hydrogen source [161]. 

Fischer-type (alkylthio)- and (arylthio)carbene complexes187 are available by thiolysis of the corresponding alkoxycarbene complexes in presence of a base catalyst,188 or by thiolysis of (acyloxy)carbene complexes generated in situ.10c-10d-189 192 Addition of thiols R SH (R1 = alkyl, aryl)... 

The number of alkyl substituents attached to the imidazolium cation was found to be of great importance in the telomerisation of butadiene and methanol with a palladium/phosphine catalyst system, see Scheme 8.8.[27] In the presence of imidazolium ionic liquids with an acidic proton in the 2-position, rapid deactivation of the catalyst took place and it was proposed that formation of stable and inactive palladium-carbene species occurred. In contrast, both a pyridinium-based ionic liquid as well as imidazolium ionic liquids bearing a methyl substituent in the 2-position led to active systems. 

The preferences of the various pathways are dependent on the catalyst used, specifically the electronic and steric factors involved. The electronic contribution is based on the preference of the metallacycle to have the electron-donating alkyl groups at either the a or the carbon of ftie metallacycle [23]. The steric factors involved in the approach of the olefin to the metal carbene also determine the re-giochemistry of the metallacyclobutane formed. These factors include both steric repulsion of the olefin and carbene substituents from each other and from the ancillary ligands of the metal complex. Paths (b), (c), and (e) in Scheme 6.10 are important to productive ADMET. The relative rates of pathways (c) and (e) will determine the kinetic amount of cis and trans double bonds in the polymer chain. Flowever, in some cases a more thermodynamic ratio of cis to trans olefin isomers is attained after long reaction times, presumably by a trans-metathesis olefin equilibration mechanism [31] (Scheme 6.11). 

N-Heterocyclic carbenes (NHC) are examples of ligands that possess the desired attributes [19]. In 2004, Caddick and Cloke reported that a Pd/NHC-based catalyst cross-couples primary alkyl bromides with alkyl- and vinyl-9-BBN reagents (Eq. 6) [20]. A system composed of Pd(dba)2/IPr HCl/AgOTf (IPr HCl=l,3-bis(2,6-diisopropylphenyl)imidazolium chloride) in the presence of KOt-Bu provides the most efficient catalyst. The yields of the cross-coupling products are modest (Table 3). 

Interestingly these complexes showed high activity without addition of alkyl aluminum compounds in the ionic liquid while they are almost inactive in toluene. These results are interpretable in terms of catalyst stabilization by the imidazolium-based ionic liquid. Reductive elimination of imidazolium is also possible as in toluene as in the ionic liquid but in the ionic liquid, a rapid reoxidation via addition of the solvent imidazolium cation seems possible and may prevent the formation of Ni deposits associated with catalyst deactivation. The carbene complex with R = n-Bu showed the highest activity with a dimer yield of 70.2% (TOF = 7020 h ). The preferred product of the nickel-catalyzed reaction is methylpentene. Additional phosphine ligand had no significant influence on the distribution of the products in this case. 

Ionic liquid stability is known to be a function of temperature (for details see Section 3.1) but the presence of nucleophiles/bases and the water content also have to be considered. There is no doubt that, under the conditions of a catalytic reaction, temperature stability issues are more complicated than imder the conditions of a TGA experiment. The presence of the catalyst complex, the reactants and impurities in the system may well influence the thermal stability of the ionic liquid. Basic and nucleophilic counter-ions, reactants and metal complexes may not only lead to deprotonation of 1,3-dialkylimidazolium ions (to form carbene moieties that will undergo further consecutive reactions) but will also promote thermal dealkylation of the ionic liquid s cation. If basic reaction conditions are required for the catalysis only tetraalkylphosphonium ions can be recommended as the ionic liquid s cation at this point in time. Tetraalkylphosphonium cations have been recently shown to display reasonabe stability, even under strongly basic conditions [290]. In contrast, all nitrogen-based cations suffer to some extent from either carbene formation, Hofmann elimination or rapid dealkylation (with alkyl transfer onto the nucleophilic anion). 

A -Alkylated thiazolium and benzothiazolium salts also experience base-promoted deprotonation at the 2-position to form ylides. Such compounds, often referred to as TV-heterocyclic carbene (NHC), are nucleophilic catalysts in benzoin condensation. In 1943, Ugai and co-workers reported that thiazolium salts catalyze self-condensation of benzaldehyde to generate benzoin via an umpoulong process. Breslow at Columbia University in 1958 proposed thiazolium ylide as the actual catalyst for this transformation. In this mechanism, the catalytically active species was represented as a thiazolium zwitterion, the resonance structure of an NHC, and the reaction was postulated to ensue via the enaminol or the Breslow intermediate. ... 

Similar studies involving metal-mediated carbene polymerization using diazocarbonyl monomers were reported in 2006 by de Brain. These showed that rhodium-based catalysts could be used for the stereoselective polymerization of carbenes generated from alkyl diazoacetates (Figure 31.5) [26]. Indeed, such catalysts were the first to produce high-molecular-weight, functionalized polymethylene 3, and demonstrated the first use of a mononuclear Rh(I) species (4 and 5) for carbene transfer reactions. The resulting poly(alkyl 2-ylideneacetate)s displayed... 

Tungsten hexachloride can apparently also act as a catalyst without the aluminum alkyl. In that case it is believed to be activated by oxygen [166].The propagation reaction based on the tungsten carbene mechanism can be shown as follows [162] ... 

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