Dipyrroethanes/Dipyrroethenes: New Precursors for Porphyrinoids.

Dipyrroethanes/dipyrroethenes (DPEs) containing two pyrroles connected by two meso sp/sp carbons are very useful precursors for the synthesis of very novel porphyrinoids. Dipyrromethanes/dipyrromethenes (DPMs) that consist of two pyrrole rings connected via one meso sp/sp carbon are the most popular precursors for the synthesis of several types of porphyrinoids. Dipyrromethanes/dipyrromethenes can be readily prepared by condensing aldehyde and pyrrole under acid-catalyzed conditions, whereas dipyrroethenes (DPEs) require a few skilled synthetic steps to be obtained in good quantities. Especially, dipyrroethenes exist in E/Z-isomeric mixtures but their separation is not required for the synthesis of porphyrinoids. In the last decade, DPEs have been used as key precursors to synthesize contracted porphyrins such as triphyrins(2.1.1), porphyrin isomers such as porphycene(2.0.2.0), and several expanded porphyrins. This review describes different methods available for the synthesis of 5,6-di(alkyl/aryl/heteroaryl) dipyrroethanes/dipyrroethenes and their use in the synthesis of different porphyrinoids ranging from contracted porphyrinoids to expanded porphyrinoids. The structure, reactivity, and physico-chemical properties of various porphyrinoids which were synthesized from DPEs are also discussed.