Investigation of the one-flask synthesis of porphyrins bearing meso-linked straps of variable length, rigidity, and redox activity

Article abstract of DOI:10.1016/S0040-4020(97)00327-X

The reactions of 18 dialdehydes have been examined in the two-step one-flask room temperature porphyrin synthesis. Efficient alkylation methods were established for the reaction of diols and diacids with bromomethylbenzaldehyde. Dialdehydes linked at the o,o'- or m,m'-positions were converted to strapped porphyrins in yields up to 25%, while the one p,p'-linked dialdehyde that was examined failed to give porphyrin. The resulting porphyrins bear straps joining adjacent, meso-positions rather than across the face of the porphyrin. Dialdehydes incorporating rigid groups provided improved yields in some but not all cases. The yield of strapped porphyrin exhibited a maximum at 10^-2 M reactant concentrations. The o,o'-strapped porphyrins exist as atropisomers that are sufficiently stable to interconversion at room temperature to be separable chromatographically. No atropisomers of m,m'-strapped porphyrins could be separated, though some could be observed by ¹H NMR spectroscopy. For two different m,m'-strapped porphyrins, the ΔG values for interconversion of the atropisomers were found to be 66 and 68 kJ/mol. The outer rings in these strapped porphyrins range in size from 14 to 24 atoms. Five porphyrins with bridging redox-active groups (ferrocene or anthraquinone) have been prepared in one-flask reactions, including a porphyrin bearing one ferrocene and one anthraquinone in straps across adjacent meta-substituted phenyl sites.