[2.2](2,7)-Fluorenophanediene, [2.2.2](2,7)-fluorenophanetriene, and their fluorenide ions

Abstract

The McMurry coupling of 2,7-diformylfluorene affords a dimer, anti-[2.2](2,7)-fluorenophanediene, and a trimer, [2.2.2](2,7)-fluorenophanetriene, as cyclic oligomers. X-Ray crystallography of the dimer establishes its strained, layered structure with anti-conformation. While the fluorenyl groups of the dimer do not show any sign of rotation in the variable temperature ¹H NMR spectra up to 120 °C, those of the trimer are fast rotating even at −50 °C. Treatment of the dimer and trimer with KH in DMSO furnished the respective dianion and trianion that are stable at room temperature in the solutions. While the trianion shows substantial conjugation through the etheno bridges, the dianion shows little conjugation through the etheno bridges and almost no proton–deuterium exchange of the fluorenyl C9-H with the solvent DMSO-d₆ probably due to stereospecific protonation–deprotonation associated with the tightly layered structure.