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

The synthesis of antiaromatic NiII–norcorroles having primary, secondary, and tertiary alkyl groups at the reactive meso‐positions was attempted. Reductive coupling of a NiII–dipyrrin precursor provided NiII–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, NiII–norcorroles with isopropyl and cyclohexyl groups were isolated as stable molecules under ambient conditions. Furthermore, we found that oxidation of NiII–meso‐dialkylnorcorroles with hydrogen peroxide in the presence of sodium carbonate gave NiII–5‐oxaporphyrins(2.0.1.0). In contrast, oxidation of NiII–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. Antiaromaticity: The synthesis of antiaromatic NiII–norcorroles with primary, secondary, and tertiary alkyl groups at the reactive meso‐positions was attempted. Norcorroles having secondary alkyl groups were found to satisfy both stability and synthetic accessibility. Oxidation of meso‐dialkylnorcorroles with hydrogen peroxide in the presence of sodium carbonate gave 5‐oxaporphyrin(2.0.1.0) selectively (see figure).