sp²-sp Cross-Carbanion Coupling at a Rare-Earth Center Leading to the Formation of Carbon-Carbon Double Bonds

Zhengqi Chai, Ze-Jie Lv, Wei Liu, Jinxiao Yang, Junnian Wei, and Wen-Xiong Zhang*

Inorg. Chem. Front. 2024, 11, 2312-2318. 

https://doi.org/10.1039/d4qi00178h

　　In contrast to transition metals, rare-earth metal promoted sp²-sp carbon-carbon coupling through reductive elimination cannot be realized due to the inaccessibility of two-electron metal-based redox couples. Herein, we report that sp2-sp cross-carbanion coupling has been realized in the reaction of rare-earth azametallacyclopentadienes toward alkyl and aryl terminal alkynes. The reaction of rare-earth azametallacyclopentadienes, which are in situ generated from rare-earth metallacyclopropenes and nitriles, with terminal alkynes provides a new class of seven-membered rare-earth metallacycles containing cumulated double bonds. The structure of seven-membered rare-earth metallacycles has been fully characterized. Mechanistic studies by DFT calculations and principal interacting orbital (PIO) analysis reveal that the formation of seven-membered rare-earth metallacycles proceeds through three steps: the activation of the alkynyl C−H bond in terminal alkynes, followed by the sp²-sp cross-carbanion coupling and subsequent alkynyl-to-allenyl isomerization to form a new C−C double bond, providing the seven-membered rare-earth metallacycles.