Olefins imidazolium salts

Another example for methoxy functionalised imidazolium salts comes from the group of Cetinkaya [185,186] featuring an -alkyl tether. Cetinkaya etal.me the traditional route to transition metal carbene complexes employing the electron-rich olefins as carbene source [57-59], Thermal cleavage of the olefinic double bond in the presaice of the metal precursor complex yields the desired transition metal carbene complex (see Figure 3.66). Using this method, Cetinkaya et al. synthesised rhodium(l) [185,186] and ruthenium(ll) [185] complexes. 

The hydroxy-binaphthyl functionalised saturated imidazolium salt is readily available from 1-amino-I -hydroxy-binaphthyl in a reaction with a ( oc-protected mesitylamine aldehyde [86] (see Figure 4.24). The resulting Schiff base is reduced to the diamine by Na(OAc)3BH. Subsequent deprotection and ring closure reaction with triethyl orthoformate yields the corresponding hydroxy-binaphthyl functionalised saturated imidazolium salt. Reaction with silver(I) carbonate and subsequent carbene transfer to the ruthenium(II) precursor yields the asymmetric olefin metathesis precatalyst. 

Ketz et al. synthesised a keto functionalised imidazolium salt using the standard protocol by reacting the appropriate N-aryl substituted imidazole with the respective keto halide [95], Their intention was to use the corresponding nickel(II) carbene enolate complex in olefin polymerisation reactions similar to the phosphino enolates in the SHOP process [1 ]. It proved difficult to prepare the intended precatalyst with only one carbene enolate ligand attached to the nickel centre. Initially, the homoleptic complex with two carbene enolate ligands was formed. The authors pointed out that the high proportion of n-olefins is unusual. 

The oligomerization of olefins is mostly catalyzed by cationic complexes which are very soluble in ionic liquids. The Pd-catalyzed dimerization of butadiene [36] and the Ni-catalyzed oligomerization of short-chain olefins [5, 37], which is also known as the Difasol process [1 d] if chloroaluminate melts are used, can be mn in imidazolium salts 1 [38, 39]. Here, the use of chloroaluminate melts and toluene as the co-solvent is of advantage in terms of catalyst activity, product selectivity, and product separation. Cp2TiCl2 [6] and TiCU [40] in conjunction with alkylaluminum compounds were used as catalyst precursors for the polymerization of ethylene in chloroaluminate melts. Neither Cp2ZrCl2 nor Cp2HfCl2 was catalytically active under these conditions. The reverse conversion of polyethylene into mixtures of alkanes is possible in acidic chloroaluminate melts without an additional catalyst [41]. 

There is an interest in these types of ionic liquids as green media for chemical reactions, this reaction provides a useful approach to the synthesis of finely tuned ionic liquids, using imidazolium salts as building blocks. Functionalization of heterocyclic substrates via transition metals is an important extension of this work (Equation (6.14)). Bergman has shown that these species can be successfully reacted with olefins in the presence of a rhodium catalyst via an NHC intermediate [55, 56). 

Scheme 6.8 Proposed mechanism for the reaction of olefins with imidazolium salts.

Hydroformylation of Aliphatic Olefins In 1997, Herrmann and coworkers [25] were the first to use NHCs and imidazohum salts, respectively, as hgands or preligands in rhodium-catalyzed hydroformylation. The isolated NHC-rhodium complexes 1 and 2 (Figure 2.55) and the complexes prepared in situ from the water-soluble imidazolium salts 3a-c and rhodium(lll)acetate were tested in the homogeneous and biphasic hydroformylation of propene. The catalyst derived from 1 produced >99% yield of isomeric butanals (CO/H2 = 1 1,10 MPa S/C = 100 000 1, toluene, 60 h). In the biphasic system, after 20 h of reaction time and S/C = 10000 1 in water, rhodium catalysts derived from 2 or based on hgands 3a-c allowed up... 

Abstract The manuscript describes the methods that are most often used in the preparation of N-heterocyclic carbene (NHC) complexes. These methods include (1) insertion of a metal into the C = C bond of bis(imidazolidin-2-ylidene) olefins (2) use of carbene adducts or protected forms of free NHC carbenes (3) use of preformed, isolated free carbenes (4) deprotonation of an azolium salt with a base (5) transmetallation from an Ag-NHC complex prepared from direct reaction of an imidazolium precursor and Ag20 and (6) oxidative addition via activation of the C2 - X (X = Me, halogen, H) of an imidazolium cation. 

The palladium-catalysed arylation of olefins with aryl halides, the Heck reaction, is usually performed in polar solvents such as acetonitrile or dimethyl sulfoxide, in combination with a base and a Pd(ii) pre-catalyst that may or may not be associated with a phosphorus ligand. Given that quaternary ammonium or phosphonium salts are known to increase reaction rates, ILs emerged as promising solvents for this reaction.In the case of imidazolium-based ionic liquids, the solvent, beside providing an unusual coulombic environment. 

Recently, Dupont et al. [76] have reported the hydroformylation of olefins in neat conditions using unmodified or Xantphos-modified Rh(0) nanoparticles, prepared by simple hydrogenation reduction of the chloride salts in imidazolium ionic liquids. They have shown that the particle size has a strong influence on the selectivity of the hydroformylation (Table 11.13). 

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