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 nonpyrrolic building block is an appealing method 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 different Baeyer-Villiger-oxidation conditions to this ketone merely led to the formation of novel and known compounds, such as porphyrinic meso-OH and N-oxide derivatives. This work demonstrates how the extraordinary structural stability of the porphyrin macrocycle redirects the reactivity patterns of classic ring-expansion reactions, which outlines some limits of the methods to convert a pyrrole in porphyrins into a nonpyrrolic building block.