Benzimidazolium salts, preparation

The abstraction of a hydride is an additional route for the preparation of ben-zimidazolium salts Treatment of 2,3-dihydro-1 /f-benzimidazoles with tritylium tetrafluoroborate generates the benzimidazolium salt and triphenylmethane [Eq. (6)].44... 

Figure 15.9. Strategies for the preparation of benzimidazoles and mechanism of benzimidazolium salt formation. X O, S.

Methylbenzimidazole with DMAD in acetonitrile gave134 the yellow 360, readily oxidized by bromine in perchloric acid to a pyrido-[l,2-a]benzimidazolium salt, and an isomeric red compound originally thought to have structure 357, and consequently it was surprising that oxidation did not follow the same course as for 360. The red compound is best prepared from the reactants in toluene and has now been identi-... 

The most common way to prepare N-heterocyclic carbenes is the deprotonation of the corresponding azolium salts, like imidazolium, triazolium, tetrazolium, pyrazolium, benzimidazolium, oxazolium or thiazolium salts or their partly saturated pendants, with the help of suitable bases. The pJCa value of imidazolium and benzimidazolium salts was determined to be between 21 and 24, which puts them right in between the neutral carbonyl carbon acids acetone and ethyl acetate [41,42], Arguably, imidazolium-based carbenes have proven to be especially versatile and useful and their synthesis should be discussed in more detail. The synthesis of imidazolium salts has been developed over many decades and numerous powerful methods exist [43]. 

The reduction of the benzimidazolium salts 271 under controlled conditions with LAH in THF has been described. The resulting benzimidazo-lines 272 may be similarly prepared, using LAH in ether or NBH in water. 

Ketone syntheses. Acyl derivatives that favor the arrestment of Grignard reactions beyond the first round include A-acylpyrazoles, acyl hemiacetals, and acyl tributylphosphonium chlorides (generated in situ from RCOCl and Bu,P). The protocol involving Al-methoxy-Al-methyl carboxamides has been extended to the preparation of a-chloro ketones, a-keto amides, and a-diketones (the last two from the oxalyl diamides). Symmetrical diketones are obtained by the Grignard reaction of bis(benzimidazole) methiodides. Note that an analogous reaction of 1,3-disubstituted benzimidazolium salts furnishes aldehydes. ... 

Palladium(ii) complexes of imidazoline-2-ylidenes with pendant 1- and 1,3-di-methylferrocenyl substituents, 78 and 79, were prepared by the reaction of the relevant benzimidazolium salt with Pd(OAc)2 and a crystal structure of 79 obtained. " In common with other related carbene complexes, the plane of the benzimidazole rings are twisted with respect to the palladium coordination plane the dihedral angle is 85.17(23)°. The closely related l-ferrocenyl-3-methyl-benzimidazolin-2-ylidene complex (in which there is no methyl linker between the N and the ferrocenyl group) has also been prepared by a similar route. " l-Ferrocenylmethyl-3-benzylimidazolidinium and 1-ferrocenyl-methyl-3-(2,4,6-trimethylbenzyl)imidazolidinium iodide salts were prepared and complexed to Pd by an in situ deprotonation route. " Cyclic voltammetry studies on the salts and the Pd complex show a number of redox processes, indicating that the ferrocenyl substituents are electronically isolated from the remaining molecular framework. 

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. 

The heterocyclic quaternary salt l,3-diethyl-2-methyl-5-(iV-trifluoromethylsul-fonyl -trifluoromethylsulfoximido)-l -benzimidazolium tosylate, containing the superstrong electron acceptor (ap = 1.4) group CF3S(0)=NS0 2CF 3, was prepared and used as an intermediate for carbocyanine, merocyanine, and styryl dyes. The spectra of these dyes showed a large bathochromic shift compared with the... 

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