Surprising Outcomes of Classic Ring-Expansion Conditions Applied to Octaethyloxochlorin, 2.[‡] Beckmann-Rearrangement Conditions

The step-wise conversion of synthetic porphyrins to derivatives that contain a non-pyrrolic building block is an appealing method to generate functionalized porphyrinoids. Applied to octaethyloxochlorin oxime, a Beckmann rearrangement is potentially suited to generate a nitrogen-expanded porphyrinoid. Unexpectedly, this reaction led to a ring-expansion by an oxygen atom. The mechanism -an abnormal Beckmann reaction, followed by an intramolecular ring-closing reaction and hydrolysis -was conclusively derived by trapping and structural characterization of the key secochlorin intermediate. The structural and UV/Vis spectral changes observed upon ringexpansion of the oxochlorin are discussed. Other Beckmannrearrangement conditions failed to produce ring-expanded products altogether. This work demonstrates in many ways how the extraordinary structural stability of the porphyrin macrocycle redirects the reactivity patterns of classic ring-expansion reactions, which outlines the limits of the " breaking and mending of porphyrins" approach toward pyrrole-modified porphyrins.