sp²-sp cross-carbanion coupling at a rare-earth center leading to the formation of carbon-carbon double bonds

In contrast to transition metals, rare-earth metal-promoted sp(2)-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 sp(2)-sp cross-carbanion coupling has been realized in the reaction of rare-earth azametallacyclopentadienes 2 with alkyl and aryl terminal alkynes. The reaction of 2, which are in situ generated from rare-earth metallacyclopropenes 1 and nitriles, with terminal alkynes provides a new class of seven-membered rare-earth metallacycles 3 containing cumulated double bonds. The structures of complexes 3 have been fully characterized. Mechanistic studies using DFT calculations and principal interacting orbital (PIO) analysis reveal that the formation of 3 proceeds through three steps: activation of the alkynyl C-H bond in terminal alkynes, followed by sp(2)-sp cross-carbanion coupling and subsequent propargyl-to-allenyl isomerization to form a new C-C double bond, providing metallacycles 3.