3 years ago

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

Stable Oxindolyl-Based Analogues of Chichibabin's and Müller's Hydrocarbons
Jishan Wu, Hoa Phan, Tun Seng Herng, Jian Wang, Tullimilli Y. Gopalakrishna, Xingdong Xu, Guangwu Li, Jun Ding
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 (ΔES-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 (ΔES-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. A new take on an old classic: Oxindolyl-based analogues of Chichibabin's and Müller's hydrocarbons were synthesized in stable form. Their diradical character is tunable by altering the R-substituent and the molecules undergo a cis/trans isomerization process.

Publisher URL: http://onlinelibrary.wiley.com/resolve/doi

DOI: 10.1002/anie.201708612

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