C-Substitution via electrophilic attack at N, deprotonation and rearrangement

1-Substituted imidazoles can be acylated at the 2-position by acid chlorides in the presence of triethylamine. This reaction proceeds by proton loss from the TV-acylated intermediate (311). An analogous reaction with phenyl isocyanate gives (312), probably via a similar mechanism. Benzimidazoles react similarly (87CHE284), but most pyrazoles do not (80AHC(27)24i) (cf. Section 3.4.1.4.6). 

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). 

Acid chlorides convert l-methylbenzimidazol-2-yl-silanes and -stannanes into 2-acylbenzimida-zoles. The reaction also works with imidazoles, with the stannanes being more reactive than the silanes. The mechanism is believed to involve initial N-acylation, then loss of the silicon or tin substituent to give a zwitterion, and finally N — C-migration of the acyl group (83JHC1011). 

Reactivity of Five-membered Rings with Two or More Heteroatoms 

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