Functionalization of corroles: Formylcorroles

Porphyrins and metalloporphyrins can be readily functionalized at the meso-(5,10,15,20)-positions by using a variety of reagents, the Vilsmeier formylation reaction being universally regarded as the prototypical example. On the other hand, the literature shows that corroles and metallocorroles are unusually resistant to most functionalization procedures which are common to porphyrins. The first example of peripheral functionalization of the corrole ring is now reported. Octamethyl-corrole 1 readily reacts with the Vilsmeier reagent (POCl3/DMF), but the product is the 10-dimethylaminomethene derivative 2, and not the expected meso-formyl derivatives 3 or 4, as confirmed by a single crystal X-ray crystallographic study. Corrole 5, where the 10-meso-position is substituted, affords the similar product 6. When the reaction is extended to corrole 7 two different products, 8 and 9, are obtained wherein the substitution is performed at both 5- and 10-meso-positions. Unlike the case of porphyrins, no reaction occurs at the β-unsubstituted pyrrolic positions. Coordination of cobalt to the dimethylaminomethenylcorroles 2 and 5 causes both metalation and tautomerization/hydrolysis to afford the originally targeted meso-formylcorrolato complexes in almost quantitative yields. Attempts to formylate cobalt complex 14 using TFA/trimethylorthoformate affords the corresponding β-formyl complexes 15 and 16, the ratio of which depends on the reaction time. meso-Formylcorroles are also obtained by way of a new direct synthesis, described herein, using a prefunctionalized a,c-biladiene 22.