Surprising Outcomes of Classic Ring Expansion Conditions Applied to Octaethyloxochlorin. 1. Baeyer-Villiger Oxidation Conditions

The conversion of synthetic porphyrins to derivatives that contain a non-pyrrolic building block is an enticing modality to generate porphyrinoids with functional groups at their periphery for sensing applications or chromophores with fine-tuned electronic properties for a number of technical and biomedical applications. Baeyer-Villiger oxidations are potentially suited to generate such pyrrole-expanded porphyrinoids from known octaethyloxochlorin. However, the application of a number of Baeyer-Villiger oxidation conditions to this ketone merely led to the formation of novel or known porphyrinic meso-OH or N-oxide derivatives. The work demonstrates how the extraordinary structural stability of the porphyrin macrocycle redirects the reactivity of classic ring expansion reactions, delineating some limits of the methodology to convert a pyrrole into a non-pyrrolic building block.

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