Imidazolium salts ferrocenyl

The synthesis of the unsymmetrical imidazolium salt 11 bearing a planar-chiral ferrocene was described by Bolm starting from (Rp)-[2-(trimethysilyl)-ferrocenyl] methanol 12 which afforded the salt in good yield after reaction with Ar,M-carbonyl diimidazole and methylation (Scheme 9) [18]. 

In situ derived systems, in general, performed similarly to preformed complexes, in telomerisation of butadiene with MeOH, Tables 4.1 and 4.2 [68,70,71,77,78], In situ systans may be generated from free NHC or from imidazolium salt in combination with an appropriate Pd(0) or Pd(ll) source. Typically, 2-4 equivalents of imidazolium salt relative to Pd have been nsed [68,70,77], In situ catalysts derived from mono- and bis-Fc-snbstituted (Fc = ferrocenyl) imidazohnm and benzimidazolium salts (64-68) (Table 4.2) showed interesting telomerisation activities ascribed to the steric bulk of the Fc substituents [70]. Unsymmetrical salts 65 and 66 bearing A -Fc and A -Me... 

The C2 - H oxidative addition of imidazolium salts to metal complexes was recently proved for metals other than low valent group 10. The reaction of a ferrocenyl-bisimidazolium salt to [IrCl(cod)]2 in the presence of NEt3 provided the first evidence of the preparation of a stable NHC-Ir(III)-H complex by direct oxidative addition of the imidazolium salt [96]. It was proposed that the ferrocenyl fragment may be sterically protecting the M - H from further reductive elimination, but later it was shown that this fragment was not necessary in order to obtain the desired NHC-Ir(III)-H complexes (Scheme 43) [159]. The role of the weak base (NEt3 in this case) had to be reconsidered in order to explain the overall metallation process, and it was proposed that a mechanism as that shown in Scheme 44 may better explain the process. The oxidative addition of the C2 - H bond of the imida-... 

Scheme 33 Coordination chemistry of chiral ferrocenyl phosphine/sulfide-imidazolium salts...

Scheme 36 Design of a chiral bis(ferrocenyl)imidazolium salt...

Naturally, it is possible to synthesise a similar ligand system without central chirality and in fact without the unnecessary methylene linker unit. A suitable synthesis starts with planar chiral ferrocenyl aldehyde acetal (see Figure 5.30). Hydrolysis and oxidation of the acetal yields the corresponding carboxylic acid that is transformed into the azide and subsequently turned into the respective primary amine functionalised planar chiral ferrocene. A rather complex reaction sequence involving 5-triazine, bromoacetal-dehyde diethylacetal and boron trifluoride etherate eventually yields the desired doubly ferrocenyl substituted imidazolium salt that can be deprotonated with the usual potassium tert-butylate to the free carbene. The ligand was used to form a variety of palladium(II) carbene complexes with pyridine or a phosphane as coligand. 

Pd(OAc)2 was reacted with an imidazolium salt with two ferrocenyl phosphine substituents to give a complex 122 with a chiral tridentate PGP ligand. Treatment with [Et30]PF6 gave a dicationic complex, [Pd(PGP)(NGMe)](PF6)2, which was an effective catalyst for the enantioselective addition of morpholine to... 

The fourth group of chiral imidazolium precursors is made up from planar chiral derivatives. The first representative of this type, 58, was prepared by Bohn et al Togni and co-workers described the C2-symmetrical ferrocenyl-sub-stituted imidazolium salt 59 and related derivatives are also known.The planar-chiral imidazolium salt with A-paracyclophane substituents 60 and some derivatives " are known. The enantiomerically pure rra s-l,2-diaminocy-clohexane was an useful starting material for the generation of the chiral diimi-dazolium salts and 62 and of mono-imidazolium salts. ... 

Several mono-NHC complexes were obtained. For example, transmetallation from a silver-NHC complex afforded the preparation of an Rh-NHC complex, in which the NHC ligand was obtained from caffeine. Deprotonation of chiral ferrocenyl imidazolium salts allowed the preparation of chiral Rh(l)-NHC complexes that could also be P,C and S,C chelate. Dicyclophane imidazolium carbene ligands also provided an efficient way of obtaining chiral Rh-NHC catalysts. ... 

---