Imidazolium ylide

Imidazole and its derivatives continued to play an important role in asymmetric processes. Optically active pyrroloimidazoles 26 were prepared by the cycloaddition of homochiral imidazolium ylides with activated alkenes <96TL1707>. This reaction was used in the enantioselective preparation of pyrrolidines <96TL1711>. A review of the use of chiral imidazolidines in asymmetric synthesis was published <96PAC531> and the preparation and use of a new camphor-derived imidazolidinone-type auxiliary 27 was reported < 6TL4565> <96TL6931>. 

Similar imidazolium ylides are implicated in the aroylation of TV-phenyl benzimidoyl chlorides (92CPB2627), and in reactions of 1-substituted imidazoles with cyanogen bromide to form 2-cyano- or 2-bromo-imidazoles (88S470). 

Note. Abbreviations used imH, imidazole imFL, neutral imidazolium ylide (deprotonated at C-2) glyglyhis, glycylglycyl-L-histidine bzimH, benzimidazole biimH2, 2,2 -biimidazole pyzH, pyrazole pz, pyrazine pzH, pyrazinium monocation 4-pic, 4-picoline isn, isonicotinamide py, pyridine 4-acpy, 4-acetylpyridine dmpzH. 2,6-dimethylpyrazinium monocation 4,4 -bpyH, 4,4 -bipyridinium monocation. T = 25°C unless otherwise stated. 

N-Substituted imidazolium salts, precursors for imidazolium ylide intermediates... 

Friedel-Crafts acylations are unknown for the azoles, clearly because of interaction between the basic nitrogen and the Lewis-acid catalyst. It is, however, possible to 2-aroylate 1-alkyl-imidazoles " or indeed imidazole itself by reaction with the acid chloride in the presence of triethylamine, the substitution proceeding via an iV-acyl-imidazolium ylide. Trifluoroacetylation of iV-substituted imidazoles, and some oxazoles and thiazoles, also produces 2-substituted products in good yields. In the reverse sense, 2-acyl-substituents can be cleaved by methanolysis, the mechanism again involving an imidazolium ylide. ... 

Another example of the utility of A-acyl-imidazolium ylides is a synthesis of 2-formylimidazole the electrophile which attacks the ylide is in this case an iV-benzoylimidazolium cation." ... 

Various azole ylides react with alkynes or alkenes with subsequent ring opening and recyclization. Thus pyrazolium ylide (348) gives compound (349) (Equation (188)) <93JCS(P1)883>, imidazolium ylide (350) gives (351) (Equation (189)) <91H(32)2217>, and thiazolium ylide (352) gives (353) (Equation (190)) <76JOC187>. 

Oxidation. Allylic and benzylic alcohols are selectively oxidized to carbonyl compounds by this mild reagent, a yellow-orange, air-stable powder. Its preparation (92% yield) involves treatment of the imidazolium ylide with CrOj and HCl. 

The balance of a reversible oxygen-sulfur rearrangement triggered on nucleophilic addition of thiocarbamate imidazolium ylide to benzaldehydes (Scheme 43) is sensitive to ortho substituents on the phenyl group. ° ... 

Neither pyrazoles nor pyrazolium salts react by this mechanism which has been described for imidazoles and imidazolium salts (Section 4.01.1.7.4). As exchange rates show (Section 4.04.2.1.7(iii)), it is considerably more difficult to generate an ylide from a pyrazolium salt than from an imidazolium salt (at C-2). 

Imidazolium, thiazolium and pyridinium ylides 198 also react with the acylmethylenecyclopropenes 208-210 to give the analogous heterocyclic cage compounds 212 (Table 20). 

Two parallel processes appear to be occurring rate-determining proton abstraction from the imidazolium ion by D2O and by OD to give the ylide at C-2 (137) followed by deuteration there (Scheme 66). The pD profile for 4-substitution can be accounted for by an additional path involving proton abstraction from the imidazole neutral molecule. In strongly alkaline medium imidazoles with no nitrogen substituent exchange more readily... 

There are two examples of these syntheses, collected in CHEC-I <84CHEc-I(6)973), one by reaction of an imidazolium JV-ylide with ethyl propiolate <68JA3830>, and another by reaction of imidazole-2-carbaldehyde with vinyltriphenylphosphonium bromide in presence of NaH in THE <76JHClii>. In the early 1990s, the intramolecular 1,5-dipolar cycloaddition reaction of the imidazolium N-allylides (48) to give the pyrrolo[l, 2-a]imidazole (49) has been described. Compound (48) is generated... 

The 1,3-dimethylimidazolium ylide, generated using sodium hydride, allows the introduction of electrophiles to C-2." Isolable, crystalline carbenes have been derived from 1,3-bis(adamantanyl)imidazolium chloride, " and from 1,3,4,5-tetra-phenylimidazolium chloride. A stable thiazol-2-ylidene carried methyl groups at positions 4 and 5 and a 2,6-di-/-propylphenyl substitutent on nitrogen. 

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