Synthesis of 2,3-dihydro-1H-pyrrolo[1,2-a]benzimidazoles via the cyclopropyliminium rearrangement of substituted 2-cyclopropylbenzimidazoles

2-Cyclopropylbenzimidazole derivatives with various substituents in the small ring undergo cyclopropyliminium rearrangement into 2,3-dihydropyrrolobenzimidazoles substituted at positions 1, 2 or 3. The substrates containing a functional group in position 1 of the cyclopropane ring form products substituted at position 3. Substituents at position 2 in most cases lead to the formation of a mixture of isomers. The reaction can be directed to yield one of the isomers predominantly by varying the solvent polarity.