Synthesis of annulated rosarins via iminium activation

Abstract

Applications of expanded porphyrins, pyrrole-based macrocycles known for their unique optoelectronic properties, have been hampered by challenges associated with their synthesis. Typically, these compounds are prepared via condensation–macrocyclization reactions under acidic conditions, using pyrrolic nucleophiles and aldehydes as electrophiles. The yield of these transformations is heavily influenced by the electrophilicity of the aldehydes, thus limiting the scope of the reaction. In this study, we demonstrate that activation of aldehydes by pre-conversion to the corresponding iminium species can be exploited for the construction of expanded porphyrins. Specifically, we employ iminium-based activation to prepare a series of naphthorosarins (NRos) using various aldehydes as electrophiles, resulting in different meso substituents. This approach has proved to be more efficient than classical methodologies, paving the way for the synthesis of previously inaccessible systems. Based on a combination UV-vis spectroscopy, electrochemistry, and density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations, the meso-substituent affect slightly the optoelectronic properties of the NRos considered in this study. Theoretical calculations provide support for the conclusion that the iminium pathway makes the condensation reaction both kinetically and thermodynamically more favorable. The present approach may provide a general tool useful in the synthesis of expanded porphyrins and other pyrrolic macrocycles.