Synthesis and beta-carbon functionalisation of cis- and trans- A2B2 porphyrins with push/ pull meso substituents

Abstract

Many biomedical and optical applications are based on unsymmetrically substituted porphyrins of the ABCD-type, which have different residues at the methine bridges. Systems of special interest are push-pull systems, with one group being electron donating and the other being withdrawing electrons, or amphiphilic systems with hydrophobic and hydrophilic groups. Such 5,10-A2B2-type porphyrins where two different substituents are positioned at diagonally opposite meso positions have an intramolecular dipole moment along a vector through the pyrrole rings. In contrast, the well established 5,15-A2B2 porphyrins have the dipole moment oriented through two meso-meso axes.<br />Porphyrin chemistry has advanced significantly in recent decades.<br />Initially, only symmetric 5,15-A2- and 5,10,15,20-A4-type porphyrins could easily be prepared via mixed pyrrole or dipyrromethane condensation reactions. These reactions were limited due to scrambling reactions which can occur under the acidic conditions required for the condensation. This has been overcome through total synthesis routes and through stepwise introduction of individual residues.<br />The main part of the work presents a reliable synthetic approach towards 5,10-A2B2 porphyrins with push-pull-substituents. Key steps of the synthetic pathway involve dialkylboron complexation of 1-acyldipyrromethanes and subsequent 9-acylation of 1-acyldipyrromethanes containing a dialkylboron mask for the [alpha]-acylpyrrole motif. Also a second total synthetic approach towards the same 5,10-A2B2 porphyrins via bilanes was tested, but has been found to be unsuccessful. At last a few exploratory test reactions such as bromination, Vilsmeier formylation and reduction were performed with the new 5,10-A2B2 porphyrins and their 5,15-A2B2 counterparts to test their reactivity.<br />