Surprising Outcomes of Classic Ring‐Expansion Conditions Applied to Octaethyloxochlorin, 2. Beckmann‐Rearrangement Conditions

Abstract

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 ring‐expansion of the oxochlorin are discussed. Other Beckmann‐rearrangement 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.