N‐Fused Pentaphyrins and Their Rhodium Complexes: Oxidation‐Induced Rhodium Rearrangement

Abstract

meso‐Aryl‐substituted pentaphyrins were isolated in the modified Rothemund–Lindsey porphyrin synthesis as a 22‐π‐electron N‐fused pentaphyrin ([22]NFP₅) and a 24‐π‐electron N‐fused pentaphyrin ([24]NFP₅), which were reversibly interconvertible by means of two‐electron reduction with NaBH₄ or two‐electron oxidation with dichlorodicyanobenzoquinone (DDQ). Judging from ¹H NMR data, [22]NFP₅ is aromatic and possesses a diatropic ring current, while [24]NFP₅ exhibits partial anti‐aromatic character. Metalation of [22]NFP₅ 1 with a rhodium(I) salt led to isolation of rhodium complexes 9 and 10, whose structures were unambiguously characterized by X‐ray diffraction analyses and were assigned as conjugated 24‐π and 22‐π electronic systems, respectively. In the rhodium(I) metalation of 1, the complex 9 was a major product at 20 °C, but the complex 10 became preferential at 55 °C. Upon treatment with DDQ, compound 9 was converted to 10 with an unprecedented rearrangement of the rhodium atom.