Masked Imidazolyl−Dipyrromethanes in the Synthesis of Imidazole-Substituted Porphyrins

Imidazole-substituted metalloporphyrins are valuable for studies of self-assembly and for applications
where water solubility is required. Rational syntheses of porphyrins bearing one or two imidazol-2-yl or
imidazol-4-yl groups at the meso positions have been developed. The syntheses employ dipyrromethanes,
1-acyldipyrromethanes, and 1,9-diacyldipyrromethanes bearing an imidazole group at the 5-position. The
polar, reactive imidazole unit was successfully masked by use of (1) the 2-(trimethylsilyl)ethoxymethyl
(SEM) group at the imidazole pyrrolic nitrogen, and (2) a dialkylboron motif bound to the pyrrole of the
dipyrromethane and coordinated to the imidazole imino nitrogen. The nonpolar nature of such doubly
masked imidazolyl−dipyrromethanes facilitated handling. Selected masked dipyrromethanes were
characterized by 11B and 15N NMR spectroscopy. Five distinct methods were examined to obtain trans-A2B2-, trans-AB2C-, and trans-AB-porphyrins. Each porphyrin contained one or two SEM-protected
imidazole units. The SEM group could be removed with TBAF or HCl. Two zinc(II) porphyrins and a
palladium(II) porphyrin bearing a single imidazole moiety were prepared and subjected to alkylation
(with ethyl iodide, 1,3-propane sultone, or 1,4-butane sultone) to give water-soluble imidazolium−
porphyrins. This work establishes the foundation for the rational synthesis of a variety of porphyrins
containing imidazole units.