RESUMO
2-Ferrocenyl- and 2-phenyl-N-n-butylbenzimidazoles were synthesized to evaluate the influence of the benzimidazole functional group upon their directed lithiation. The regiochemistry of lithiation was studied, as well as the effect of stabilization of the lithiated species by diamine coordination using tetramethyl- ethylenediamine and (-)-sparteine. The lithiations were followed by reaction with a variety of electrophiles to give the disubstituted 2-ferrocenyl- and 2-phenyl-N-n-butylbenzimidazoles compounds. This study showed that despite a simple n-butyl function on the benzimidazole, directed lithiation was readily achieved with high regiocontrol on the ferrocenyl and phenyl groups. (-)-Sparteine failed to provide asymmetric induction in the ferrocene system, and its inefficiency is explained by intramolecular coordination of the lithiated species by the benzimidazole nitrogen, which is preferred over sparteine coordination.
Assuntos
Benzimidazóis/química , Ferro/química , Lítio/química , Nitrogênio/química , Benzimidazóis/síntese química , Ácidos Borônicos/química , Estrutura MolecularRESUMO
N-Methyl and N-n-butyl-2-(2-boronophenyl)benzimidazoles are accessed from the corresponding mono-N-alkyl-ortho-phenylenediamines, either using a polyphosphoric acid-mediated cyclisation with ortho-bromobenzoic acid, or preferably using an Oxone-mediated cyclisation of the corresponding aldehyde, followed by a lithium-exchange and borylation sequence. The resulting boronic acids show unusual physical and chemical properties, as shown by 11B NMR and X-ray crystallography.
