Synthesis of a Series of Tropone-Fused Porphyrinoids.

A series of tropone-fused porphyrinoids with unique spectroscopic features has been prepared. A dimethyl tropone-fused pyrrole was reacted with lead tetraacetate to give a bis(acetoxymethyl) derivative that condensed with an α-unsubstituted pyrrole -butyl ester to form a tripyrrane intermediate. Cleavage of the -butyl ester protective groups, followed by condensation with a series of aromatic dialdehydes and oxidation with DDQ, afforded the tropone-fused porphyrinoid systems. Reactions with pyrrole, furan, thiophene, and selenophene dialdehydes gave tropone-fused porphyrins and related heteroporphyrins. In addition, indene, 4-hydroxybenzene, and 3-hydroxypyridine dicarbaldehydes generated examples of carba-, oxybenzi-, and oxypyriporphyrins. The electronic absorption spectra of the tropone-fused porphyrinoids were greatly altered, showing shifts to longer wavelengths and the appearance of strong Q bands between 600 and 800 nm. The proton nuclear magnetic resonance spectra were also very unusual, as the internal protons were strongly shifted upfield, in some cases giving rise to resonances that approached -10 ppm. However, the external protons showed reduced downfield shifts compared to porphyrinoids that do not have tropone ring fusion. The profound changes observed for these macrocycles demonstrate that the introduction of fused tropone units, together with other structural changes such as core modification, can provide the means by which porphyrinoids with unique spectroscopic properties can be accessed.