π‐Extended Bisallyls Through Valence Isomerism: From Transient Diradicals to Persistent Anions

ABSTRACT

Bisallyl diradicals are central intermediates in allene dimerization, yet they are normally too short‐lived to observe under thermal conditions. Here, π‐extension of Staudinger‐type allene dimers weakens the central cyclobutane bond sufficiently to make reversible ring opening accessible near room temperature. Variable‐temperature NMR spectroscopy and ROESY exchange correlations reveal repeated cleavage and reformation of this bond, with activation free energies of 15.77 and 13.79 kcal mol ⁻¹ for the Staudinger dimer and its π‐expanded analogue, respectively. Calculations reproduce this trend and identify low‐population open‐shell singlet bisallyls as the intermediates, while explaining the absence of neutral‐state EPR signals. Reduction reverses the relative stability of the closed and open valence isomers. Voltammetry and chemical reduction support a sequence of ring‐opened radical‐anion, dianion, and tetraanion states. The dianion is characterized by NMR spectroscopy and x‐ray diffraction, while spectroscopy and calculations reveal progressive bond redistribution from mixed‐valence allyl‐radical/allyl‐anion character to two allyl‐anion units and, finally, four fluorenyl‐anion‐like termini. These results connect hidden neutral diradicals with persistent reduced bisallyl frameworks through charge‐controlled covalent‐bond reorganization.