Allylic C‒H Alkylation of Internal and Terminal Alkenes with Alkenes by Rare-Earth Catalysts

Allylic C−H functionalization of simple alkenes presents a step-economical route to access functionalized alkenes. However, current approaches are generally focused on the transformation of α-alkenes, the conversion of internal alkenes was challenging. Herein, we present that rare-earth catalysts enable to activate the allylic C‒H bond, allowing for the atom-efficient allylic alkylation of internal or terminal alkenes with styrene derivatives as well as dimerization of internal or terminal alkenes. In the presence of cationic imidazolin-2-iminato scandium alkyl complexes, various of alkene-coupling products were afforded in moderate to good results (64 examples, 16−99% yield, 1:3.4−>19:1 E/Z ratio). In this process, both catalyst precursors and additives impact on the reactivity and stereoselectivity. Additionally, the newly developed strategy was applied to benzylic C‒H alkylation of toluene derivatives. Mechanistic studies suggested that allylic rare-earth metal species was the key intermediate. A possible catalytic cycle with the participation of Lewis base was provided to understand the reaction mechanism and E/Z selectivity.