Imidazolium salts ruthenium complexes

A benzene imidazolium-tagged ruthenium complex immobilized in [G4GiGiIm]PF6 is a highly enantioselective IL biphasic transfer hydrogenation catalyst (isopropanol/KOH) of acetophenone as compared with conventional (untagged) complexes (Figure Along the same lines, a diimide-imidazolium salt moiety associated with... 

Imidazole is characterized mainly by the T) (N) coordination mode, where N is the nitrogen atom of the pyridine type. The rare coordination modes are T) - (jt-) realized in the ruthenium complexes, I-ti (C,N)- in organoruthenium and organoosmium chemistry. Imidazolium salts and stable 1,3-disubsti-tuted imidazol-2-ylidenes give a vast group of mono-, bis-, and tris-carbene complexes characterized by stability and prominent catalytic activity. Benzimidazole follows the same trends. Biimidazoles and bibenzimidazoles are ligands as the neutral molecules, mono- and dianions. A variety of the coordination situations is, therefore, broad, but there are practically no deviations from the expected classical trends for the mono-, di-, and polynuclear A -complexes. 

The ruthenium complexes were prepared in 50-80% yield by treatment of the imidazolium salts with potassium hexafluoro-t-butoxide, and then by (PCy3)2Cl2Ru = CHPh. A single phosphine is displaced by the carbene affording the desired complexes as air-stable solids that were purified by silica gel... 

Madsen and co-workers have reported an important extension to the amine alkylation chemistry, in which oxidation takes place to give the amide product [13]. A ruthenium NHC complex is formed in situ by the reaction of [RuCl Ccod)] with a phosphine and an imidazolium salt in the presence of base. Rather than returning the borrowed hydrogen, the catalyst expels two equivalents of H. For example, alcohol 31 and benzylamine 27 undergo an oxidative coupling to give amide 32 in good isolated yield (Scheme 11.7). 

A ruthenium complex containing a novel imidazolium salt moiety catalyses the asymmetric transfer hydrogenation of acetophenone derivatives, with a formic acid- triethylamine azeotropic mixture in an ionic liquid, [bmim][PF6]. The yields and ee are excellent.308... 

Scheme 7.2 Formation of ruthenium carbene complexes from 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 simpler architecture is the 1,1 -biphenyl scaffold, likewise introduced by Hoveyda and coworkers [19]. The synthesis of the imidazolium salt starts with a chiral diamine and a substituted, achiral biphenyl [82-84], Subsequent introduction of a Mes substituent on the remaining primary amino end and ring closure reaction yields the chiral saturated imidazolium salt after hydrolysation of the methoxy group to liberate the phenolic hydroxy group (see Figure 4.22). Reaction with silver(I) oxide and carbene transfer to a Grubbs (Hoveyda) catalyst sets up the ruthenium catalyst complex. 

This facile solution for catalyst immobilisation follows a more complex approach reported by Mayr et al. [246], This research group attached the imidazolium salt to a nor-bomene unit that was then subjected to a ring opening metathesis polymerisation (ROMP) reaction to form the polymeric NHC precursor (see Figure 4.77). The ROMP reaction had to be carried out with a molybdenum based Schrock catalyst because the ruthenium based Grubbs catalyst did not provide an endgroup that could be quantitatively capped with a suitable final endgroup. 

To overcome the problem of catalyst decomposition, Audic and coworkers [53] designed a novel alkyl imidazolium salt-snpported ruthenium carbene complex (Scheme 17.7). The resulting ionic catalyst is completely soluble in ionic liquid and allows the RCM reaction to be carried out under standard homogeneous conditions. This new catalyst system provided high conversion and yields (92-98%). Moreover, the recycled catalyst solution was used for the metathesis, and similar results were obtained up to the tenth cycle. The catalyst has shown high activity with a remarkable recyclability and can be stored in [bmijPFg for several months without loss of activity. 

The use of stoichiometric ruthenium-NHC complexes generated in situ from [Ruljd-COCKp-cymene)], an imidazohnm salt [4] or an imidizol(idin)ium-2-carboxylate [4] has been applied in the cyclopropanation of styrene 5 with ethyl diazoacetate (EDA) 6 (Scheme 5.2). No base was necessary when imidazolium-2 carboxylate were employed. The diastereoselectivity was low and the cis/trans ratio was around 50/50 (Table 5.1). Although the diastereoselectivity was moderate, the reaction was highly chemoselectivity as possible side reactions (homologation, dimerisation and metathesis) were totally or partially suppressed. 

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