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

Chichibabin's and Müller'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 (y₀ =11.1 %) and large singlet-triplet gap (ΔE_S-T =-10.8 kcal mol^-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 (ΔG^≠_298K =15-16 kcal mol^-1 ). OxR-3 exhibits a much larger diradical character (y₀ =80.6 %) and a smaller singlet-triplet gap (ΔE_S-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.