Zirconacyclopentadiene Transformations to Phenanthrene-Annulated Antiaromatic Compounds

This contribution describes methods for transformations of zirconacyclopentadienes to access three different antiaromatic systems. First, zirconium–boron exchange generated a phenanthrene-fused borole that displays an absorption onset much higher than previously reported borole derivatives. In addition, a new synthetic route to a phenanthrene-fused pentalene involves an unsymmetrical zirconacyclopentadiene intermediate containing alkynyl substituents on the α-positions. Finally, a multiantiaromatic system was developed, involving a 1,4-naphthalenediyl-bridged bis-cyclobutadiene, which displays an absorption onset significantly red-shifted compared to its monocyclobutadiene counterpart. The high antiaromaticity of all three compounds is corroborated by calculations that show strong paratropicity at the antiaromatic unit.