Imidazolium salt derivative

Flexible steric bulk is a characteristic of a new class of imidazolium salts derived from bisoxazolines (IBiox) (Glorius et al. 2002 Altenhoff et al. 2004), which were tested for the organocatalytic synthesis of bu-tyrolactones (Burstein and Glorius 2004). 

In 2011, the same group reported the immobilization of an enantiomerically pure imidazolium salt derived from chiral binaphthyl on the surface of magnetite Fe304 and the formation of two catalysts derived thereof - a molecular Pd complex 84 and Pd NPs 85 (Scheme 3.47) [73]. These two catalysts were successfully... 

With the goal of exploring the efficiency of NHC-type ligands in this particular catalytic reaction, Ma and coworkers [12], in 2010, synthesized several planar chiral imidazolium salts derived from [2.2]paracyclophane (Figure 7.3). By using these imidazolium salts as carbene precursors, the rhodium-catalyzed 1,2-addition of arylboronic acids to aldehydes proceeded readily with low catalyst loadings (0.03-0.3 mol%) and afforded a variety of diarylmethanols in excellent yields but in low to moderate enantioselectivities (a maximum of 52% ee was achieved). 

Figure 7.3 The chiral imidazolium salts derived from [2.2]paracyclophane, as described by Ma and coworkers [12] and used in the arylation of aldehydes.

Yoshizawa, H. Mihara, T. Koide, N. (2004). Thermal properties and ionic conductivity of imidazolium salt derivatives having a calamitic mesogen. Mol Cryst. Liquid Cryst, 423,61-72... 

RajanBabu reported the first preparation of a bis-imidazolium salt 15 bearing a chiral linker (Scheme 11). The starting material was the enantiomerically pure (S)-l,l -bi-2-naphtol bis(trifluoromethanesulfonate) which was transformed in two steps into the dibromomethyl derivative 16 and then into the bis-imidazole. Quaternarization of this compound afforded 15 [20]. 

A bis-imidazolium salt was also prepared by Burgess, starting from the dichloride 20 derived from optically pure N,N -dimethyl-tmns-l,2-diaminocyclohexane (Scheme 13). The salt 21 was obtained by addition of this compound to several 1-alkylimidazoles [22]. 

Chiral diaminocarbene complexes of copper were used in asymmetric conjugate addition of diethylzinc to Michael acceptors. Achiral copper carbene complexes derived from imidazolium salts were synthesized and characterized for the first time by Arduengo in 1993 [43]. In 2001, Woodward reported the use of such Arduengo-type carbene in copper-catalyzed conjugate addition and showed their strong accelerating effect [44]. The same year, Alex-... 

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 use of imidazolium salts for in situ catalyst formation was shown to be optimal for the coupling of TMS-protected alkynes even with sterically demanding aryl bromides and avoids the formation of homocoupling-derived products. For this reaction, Nolan reported that the activation of chlorobenzene by this catalytic system was possible in moderate yield [125] (Scheme 6.41). 

Some other enantioselective approaches have been attempted, still with moderate enantioselectivities, by making use of in situ systems containing a chiral NHC precursor. Luo and co-workers reported on the use of the bidentate chiral imidazo-lium salt 16, derived from L-proUne, in combination with [RhCia-COCcod)], leading to an enantiometic excess of around 20% [30]. The use of chiral imidazolium salt 17 in combination with [RhCl(CH2=CHj)j]j by Aoyama afforded slightly better ee (Fig. 7.3) [31 ]. So far, Bohn and co-workers have obtained the best enantioselectivities (up to 38% ee) for the catalytic addition of phenylboronic acid to aromatic aldehydes by using planar chiral imidazolium salts 18, derived from paracyclophane, in combination with [Rh(OAc)2]2 [32]. 

Homoenolate Protonation The p-protonation of homoenolates has been observed by Scheidt and co-workers, resulting in a redox transformation of enals to afford saturated esters 48. This process is catalysed by the NHC derived from imidazolium salt 46 and utilises phenol as a proton source [14]. A range of primary and secondary alcohols, and phenol itself, are competent nucleophiles with which to trap the acylazolium intermediate 47 generated by protonation (Scheme 12.8). 

The most common methods suitable for the synthesis of different azolium compounds will be discussed here. Two routes are particularly useful for the preparation of the imidazolium salts (1) substitution reactions at the nitrogen atoms of imidazole [25] and (2) multicomponent reactions for the generation of an Af,Af -substituted heterocycle which are particularly useful for the synthesis of imidazolium salts bearing aromatic, very bulky, or particularly reactive N,N -sub-stituents (Fig. 3a,b) [26]. Both methods offer the opportunity to produce unsym-metrically substituted imidazolium salts of type 1 either by stepwise alkylation of imidazole or by the synthesis of an W-arylated imidazole derivative followed by 77 -alkylation [27]. Nevertheless, the method of choice for the preparation of the imidazolium salts 1 is the 77,77 -substitution of imidazole. Several other methods for the preparation of imidazolium salts with previously unattainable substitution patterns have also been described [28, 29]. 

By analogy, imidazolium salts, e.g. (331), can be converted with a high yield into the corresponding 2-thione or 2-selenone derivatives (332) by heating with sulfur or selenum in DMF in the presence of triethylamine (90CHE1689). 

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... 

A new type of triaryl phosphine-functionalized imidazolium salt containing cations such as (6) has been prepared. Palladium complexes of (6) generated in situ have been used successfully in Heck-type reactions of aryl halides with acrylates and of 4-bromotoluene with styrene derivatives.34 The first Heck-type reaction of aryl halides with allenes has been reported. 1,3-Double arylations were observed with 3-substituted-l,2-allenyl sulfones, while 1-monoarylation was favoured with 3,3-disubstituted-l,2-allenyl sulfones.35 It has been shown that the a-arylation of methane-sulfonamides (7) may be achieved using palladium catalysis reaction proceeds through the sulfonamide enolates.36 It is also reported that palladium cross-coupling of alkynes with /V - (3 - i odophe n y I an i I i ncs) may lead to the formation of substituted carbazoles.37... 

Quite recently, a series of N-alkyl substituted imidazolium salts has been evaluated for additive effects on the mesomorphic behavior and ensuing optical properties of HPC aqueous solutions, followed by characterization of the thermotropicity of novel cellulose derivatives with such an ionic liquid structure in the side-chains [193]. 

Chelation is not necessary to promote the abnormal metallation. When imidazolium salts with one bulky substituent ( Pr, fBu) are refluxed with pyridine and IrHs(PPh3)2 in THF, C-5-bound complexes are obtained in good yield, with the least sterically hindered of the three imidazole carbons selectively bound to Ir (Scheme 30) [125]. Infrared spectroscopy on carbonyl derivatives indicated that abnormally bound NHCs are much stronger electron donors than their ubiquitous C-2 counterparts [125]. 

Chung et al. reported the enantioselective synthesis of chiral NHCs, such as 6, using a chiral ferrocene derivative (Scheme 8) [28]. The nucleophilic substitution of the hydroxy function by an imidazole in an acidic medium gives the imidazolium salt with retention of the configuration at the chiral C-atom. 

Crabtree and Chianese have extended the scope of Hoveyda s ligand by making the imidazolium salt 39 in two steps from l,l/-diamino-2,2/-binaphthyl (Fig. 10) [80]. They prepared neutral rhodium and iridium complexes with that ligand precursor and applied these complexes in the asymmetric hydrosilylation of acetophenone. Moderate enantioselectivities were obtained with the iridium derivative (up to 60% ee) whilst the rhodium catalysts only gave low enantioselectivities. 

Scheme 34 Coordination chemistry of the tridentate PCP ligand derived from the imidazolium salt 47...

In the imidazolium salts 63, obtained by nucleophihc substitution of the iodo-derivative 61 by an imidazole 62, the oxazoline is bnked by the carbon atom in the 4-position. Coordination of the bidentate bgand to the Ir(COD) + complex fragment is then achieved by in situ deprotonation (Scheme 43). This modular design allows facile and rapid access to a large ligand library by variation of the substituents in the 2-position of the oxazoline and at the terminal N-atom of the heterocyclic carbene. 

Only recently an additional, elegant access to unsymmetrically substituted imidazolium salts has been disclosed (Fiirstner et al. 2006). Unsymmetric bisaryl-substituted IV-heterocyclic carbenes (imidazolidinium derived) can be prepared from ethyl chlorooxoacetate (Waltman and Grubbs 2004). 

Upon deprotonation, imidazolium salts 29 react both with electrophiles and nucleophiles, thereby introducing a substituent derived from these reactants at position 2 concomitantly, the 1-arylamino substituent (R = aryl, R = alkyl) is eliminated (Scheme 6) (OOTH). 

A third example comes from Clyne et al. [358] and concerns the axial chiral binaphthyl backbone [359,360], itself known from phosphorus chemistry [361]. The synthesis starts from the trifluoromethylsulfonato substituted binaphthyl with a Kumada coupling reaction [291,292] with methytmagnesiumbromide. Oxidation with NBS yields the methyl brominated derivative that can be attached to the imidazole ring. Subsequent methylation results in the bis-imidazolium salt that is deprotonated to the bis-carbene and coordinated to the transition metal halide (Pd, Ni), a rather straightforward reaction sequence (see Figure 3.113). The overall yield for the four-step reaction to the bis-imidazolium salt is surprisingly good (65%). 

The chemistry of transition metal carbene complexes with NHC derived from purines or xanthines has its roots in the synthesis of xanthinium betaines [75], the xanthine analogues of imidazolium salts. The synthesis is sttaightforward and involves the methylation of the xanthine with methyl tosylate (see Figure 6.32). 

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