Magnetic Circular Dichroism Studies. 66. Synthesis of Demethyl Monosubstituted Porphyrins. The Effect of Substituent Conformation on the Magnetic Circular Dichroism Spectra of Ethoxycarbonyl Porphyrins.

Article abstract of DOI:10.1021/jo00192a015

The synthesis of a series of demethyl monosubstituted (acetyl, vinyl, formyl, cyano, and ethoxycarbonyl) free-base porphyrins (6b-f) is described.The key intermediates, 5-formyl-5'-methyldipyrrylmethanes 16a and 26, used in this synthesis are prepared in high yields by an improved procedure which entails decarboxylation of the 5-carboxy-5'-methyldipyrrylmethanes 15a and 25 in trifluoroacetic acid and subsequent formylation of the decarboxylated dipyrrylmethane with a mixture of dimethylformamide and p-nitrobenzoyl chloride.The preparation of the demethylformylporphyrin 6d from the demethylvinylporphyrin 6c was succesfully accomplished by the use of thallium(III) as a "protecting group" for the macrocycle.This series of monosustituted porphyrins allows, or the first time, the assessment of the electronic and optical consequences of the substituent effects on the porphyrin macrocycle on the same sterically unconstrained basis as now exists for a wide variety of other cyclic ?-electron systems.This is illustrated by comparing the MCD spectra of the methyl and demethyl ethoxycarbonyl free-base porphyrins.The observed sign variations of the MCD bands for these two porphyrins are explained with the perimeter model approach previously elaborated for substituted porphyrins.