Regioselective Substituent Effects upon the Synthesis of Dipyrrins from 2-Formyl Pyrroles

Abstract

The synthesis of symmetric α-free meso-H-dipyrrin hydrobromides from 5-H-2-formyl pyrroles was investigated. The self-condensation produces regioisomeric dipyrrins through the adoption of two mechanistic pathways. The key difference between the two pathways lies in which position of the pyrrole directs nucleophilic attack. Through a systematic study involving various substituted and/or isotopically labelled 5-H-2-formyl pyrroles, we herein provide evidence to suggest that not only do two mechanistic pathways exist but that the steric bulk of the substituent adjacent to the 5-unsubstituted position influences which pathway dominates.