Organocatalytic synthesis of dipyrromethanes by the addition of N-methylpyrrole to aldehydes

The reaction of aldehydes with N-methylpyrrole carried out in the presence of catalytic pyrrolidinium tetrafluoroborate at room temperature affords the corresponding dipyrromethanes and minor amounts of tripyrranes. This new, mild, and convenient process represents the first organocatalytic synthesis of dipyrromethanes. The products are obtained in chemical yields similar to those obtained with existing methods, which, however, require either a much larger excess of heterocycle (when aldehydes are employed as starting materials), or more drastic reaction conditions (when aldehyde equivalents are used as starting materials) than those employed here. A mechanism is proposed to explain the course of this reaction.