Synthesis of meso ‐Alkyl‐Substituted Norcorrole–Ni II Complexes and Conversion to 5‐Oxaporphyrins(2.0.1.0)

The synthesis of antiaromatic Ni II –norcorroles having primary, secondary, and tertiary alkyl groups at the reactive meso ‐positions was attempted. Reductive coupling of a Ni II –dipyrrin precursor provided Ni II – meso ‐dihexylnorcorrole, which underwent substantial degradation on silica gel. Introduction of tert ‐butyl groups was unsuccessful due to the difficult preparation of the corresponding dipyrrin precursor. Meanwhile, Ni II –norcorroles with isopropyl and cyclohexyl groups were isolated as stable molecules under ambient conditions. Furthermore, we found that oxidation of Ni II – meso ‐dialkylnorcorroles with hydrogen peroxide in the presence of sodium carbonate gave Ni II –5‐oxaporphyrins(2.0.1.0). In contrast, oxidation of Ni II – meso ‐dimesitylnorcorrole under the same reaction conditions gave 10‐oxaporphyrin(1.1.1.0). The contrasting reactivity can be attributed to the steric congestion around the meso ‐positions.