Nonplanar porphyrins: Synthetic and dynamic studies

The Suzuki coupling reaction is employed in the synthesis of several novel dodecaarylporphyrins. Steric, electron and metalation effects are examined for the boronic acid and porphyrin reactants. The advantages and limitations of the Suzuki coupling reaction in comparison to other methodologies, such as the Adler-Longo and Lindsey condensations, are also explored. Variable temperature 1H NMR studies of these novel dodecaarylporphyrins reveal that the rotational barriers of the peripheral aryl groups exhibit a marked dependence on the substituents in the porphyrin core, with activation energies of approximately 40 and 90 kJ mol-1 for beta and meso phenyl rotation in the dications versus approximately 55 kJ mol--1 for rotation of both kinds of phenyl groups in the nickel complexes. Crystallographic data and molecular mechanics calculations suggest that this modulation of the rotational barriers occurs via an unusual conformational effect, whereby the core substituents alter the conformational preference of the porphyrin macrocycle. The knowledge gained from these studies has allowed for the synthesis and characterization of a novel chiral nonplanar porphyrin that demonstrates enantioselective binding with a chiral amine ligand.