Stable Oxindolyl-Based Analogues of Chichibabin's and Muller's Hydrocarbons

Chichibabin's and Muller's hydrocarbons are classical open-shell singlet diradicaloids but they are highly reactive. Herein we report the successful synthesis of their respective stable analogues, OxR-2 and OxR-3, based on the newly developed oxindolyl radical. X-ray crystallographic analysis on OxR-2 reveals a planar quinoidal backbone similar to Chichibabin's hydrocarbon, in accordance with its small diradical character (y(0)=11.1%) and large singlet-triplet gap (Delta ES-T=-10.8 kcalmol(-1)). Variable-temperature NMR studies on OxR-2 disclose a slow cis/trans isomerization process in solution through a diradical transition state, with a moderate energy barrier (Delta G(298K)(double dagger)= 15-16 kcal mol(-1)). OxR-3 exhibits a much larger diradical character (y(0)= 80.6%) and a smaller singlet-triplet gap (DES-T=-3.5 kcal mol(-1)), and thus can be easily populated to paramagnetic triplet diradical. Our studies provide a new type of stable carbon-centered monoradical and diradicaloid.