Synthesis and Properties of Unusually Stable Antiaromatic Porphyrinoids

Macrocycles can be aromatic, nonaromatic, or antiaromatic depending on the number of π ‐electrons involved in delocalization. If the π ‐electron count follows the (4 n + 2) rule, the macrocycle is aromatic and exhibits a diatropic ring current. In contrast, if it follows a 4 n π ‐electron count, it is antiaromatic and exhibits paratropic ring currents. In recent times, some macrocycles with [4 n + 2] π ‐electrons showed Möbius antiaromatic behavior, whereas macrocycles with [4 n ] π ‐electrons showed Möbius aromatic character. Thus, the earlier myth of antiaromatic macrocycles is, in general, difficult to synthesize because of their unstable nature has been replaced in recent past by synthesizing several stable antiaromatic macrocycles. Porphyrinoid macrocycles obeying Hückel's [4 n + 2] π ‐electron rule are generally aromatic, stable, and widely functional, whereas [4 n ] π systems are often antiaromatic, highly reactive, and challenging to isolate. Interestingly, not all [4 n ] π ‐porphyrinoids are antiaromatic in nature since many of these are also nonaromatic and stable compounds. Thus, only selected porphyrinoids with [4 n ] π ‐electrons are antiaromatic and exhibit quite distinct properties that are different from aromatic and nonaromatic porphyrinoids. This review surveys the synthesis, structures, and properties of unusually stable antiaromatic porphyrinoids, spanning from classical porphyrins to contracted porphyrins and extended up to the expanded porphyrinoids reported to date.