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 enticing modality 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 intra¬molecular ring-closing reaction and hydrolysis—was conclusively derived by the trapping and structural characterization of the key secochlorin intermediate. The UV-vis spectral and structural changes observed upon ring-expansion of the oxochlorin are discussed. Other Beckmann reaction conditions failed to produce ring-expanded products altogether. The work demonstrates in many ways how the extraordinary structural stability of the porphyrin macrocycle redirects the reactivity of classic ring expansion reactions, delineating the limits of the "breaking and mending of porphyrins" approach toward pyrrole-modified porphyrins.

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