Formation, structure, and reactivity of meso-tetraaryl-chlorolactones, -porpholactams, and -chlorolactams, porphyrin and chlorin analogues incorporating oxazolone or imidazolone moieties

Reaction of known meso-tetraarylporpholactone free bases 3, made from the corresponding porphyrins, with hydrazine produces three products: It converts the lactone functional group into an N-aminolactam moiety, generating porphyrin-like N-aminoporpholactams 8. It also reduces regioselectively the beta,beta'-double bond of the pyrrolic moiety opposite to the imidazolone in both the starting material and the N-aminoporpholactam, thus forming the chlorin-like chlorolactones 7 and N-aminochlorolactams 9. An equivalent set of reaction products is also derived from the reaction of porpholactones 3 with tosylhydrazide. Reductive N-N cleavage of the N-aminoporpholactams 8 generated the parent porpholactams 10. The molecular structures of all key compounds were shown by single crystal X-ray diffraction to be essentially planar. Porpholactam 10a can be converted in two steps (enolization and halogenation alpha to the imine, followed by reductive removal of the halogen) to known imidazoloporphyrin 5a, thus constituting the third independent pathway to replace a beta-carbon of a tetraphenylporphyrin by a nitrogen. All these transformations show the flexibility of our 'porphyrin breaking and mending' strategy toward the synthesis of novel porphyrin and chlorin analogues incorporating non-pyrrolic heterocycles that carry functionalities at their periphery.