Imidazolinium salts, preparation

These salts are obtained from 1,2 (or l,3)-diamines. The cyclization step is generally the condensation of the diamines on ethyl orthoformate in the presence of ammonium tetrafluoroborate (Scheme 19). By using enantiop-ure diamines, chiral salts 34 bearing stereogenic centers on the backbone, on the nitrogen substituents, or on both were prepared [1,2,4,5,26]. Several dicyclophane imidazolinium salts have been prepared by one-pot three-step procedure [27]. 

The ionic liquid [bmim][BF ] is known to catalyze the aza-Diels-Alder reaction in the synthesis of pyrano- and furanoquinolines [190]. This reaction was also catalyzed by the enantiopure bis-imidazolinium salt 67 in 67% yield with an endo. exo ratio of 60 40 (Scheme 69) [191]. The product was obtained as a race-mate. In addition the aza-Diels-Alder reaction with imines and Danishefsky s diene was catalyzed by the salt 67 giving racemic product. The salt and its analogues could be easily prepared via the oxidation of the corresponding aminals [192]. Investigation of the influence of the counter anion in achiral C2-substituted imidazolinium salts, which can be also described as 4,5-dihydroimidazolium or saturated imidazolium salts, in the aza-Diels-Alder reaction showed, that the catalytic activity increased, the more lipophilic the counter anion and therefore the more hydrophobic the salt was [193]. 

The imidazolinium salts, which are being used as ligand precursors, are generally prepared from C2-symmetric chiral vicinal diamines [43,44]. 

Wanzlick and Schikora were the first to attempt the preparation of saturated imidazolin-2-ylidenes by an a-elimination of chloroform from imidazoline derivatives but could only obtain the electron-rich entetraamines (see Scheme 1.2). It was Arduengo et al. who presented the first stable crystalline imidazolin-2-ylidene 86 (Figure 1.8) obtained by deprotonation of the corresponding imidazolinium salt. " Shortly thereafter, Denk et al. used the reductive desulfurization of imidazolin-2-thiones to obtain the N,N -A tert-butyl) substituted stable carbene of type 86 and the entetraamine 86=86 with the sterically less demanding A,A -dimethyl substituents (Figure 1.8). Additional imidazolin-2-ylidenes with varying A,A -substituents were obtained by... 

Related heterocyclic salts, imidazolinium chlorides, have recently been prepared through a solvent free reaction of a formamidine with dichloroethane and a base. Solvents are used in this reaction as excess dichloroethane is used and the residue is triturated and washed using either acetone or toluene. However, it is a superior route to these valuable A-heterocyclic carbene... 

Scheme 1.7 Synthetic methods for the preparation of imidazolinium and amidinium salts.

In order to complement earlier studies showing that IV-alkyl-substituted ben-zimidazolylidene-ruthenium complexes were efficient catalysts for the regiose-lective alkylation of cinnamyl carbonate by dimethyl malonate or 1,3-diketones, as well as etherification of allylic halides by phenols, Bruneau and co-workers prepared a wide range of imidazolinium 38, tetrahydropyrimidinium 39, and benzimidazolium salts 40 that were screened as NHC ligand precursors in various allylic substitution reactions (Equation (7.9)). Unfortunately, linear versus branched selectivities were only modest, and no characterisation of the [(NHC)Ru(Cp )] complexes assumed to take part in the reaction could be achieved, thereby preventing any further rational ligand modification that would have helped refine the catalytic system. 

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