(<i>Z</i>)-Selective Isomerization of 1,1-Disubstituted Alkenes by Scandium-Catalyzed Allylic C–H Activation

The isomerization of 1,1-disubstituted alkenes through 1,3-hydrogen shift is an atom-efficient route for synthesizing trisubstituted alkenes, which are important moieties in many natural products, pharmaceuticals, and organic materials. However, this reaction often encounters regio- and stereoselectivity challenges, typically yielding <i>E</i>/<i>Z</i>-mixtures of the alkene products or thermodynamically favored (<i>E</i>)-alkenes. Herein, we report the (<i>Z</i>)-selective isomerization of 1,1-disubstituted alkenes to trisubstituted (<i>Z</i>)-alkenes via the regio- and stereospecific activation of an allylic C-H bond. The key to the success of this unprecedented transformation is the use of a sterically demanding half-sandwich scandium catalyst in combination with a bulky quinoline compound, 2-<i>tert</i>-butylquinoline. Deuterium-labeling experiments and density functional theory (DFT) calculations have revealed that 2-<i>tert</i>-butylquinoline not only facilitates the C═C bond transposition through hydrogen shuttling but also governs the regio- and stereoselectivity due to the steric hindrance of the <i>tert</i>-butyl group. This protocol enables the synthesis of diverse (<i>Z</i>)-configured acyclic trisubstituted alkenes and endocyclic trisubstituted alkenes from readily accessible 1,1-disubstituted alkenes. It offers an efficient and selective route for preparing a new family of synthetically challenging (<i>Z</i>)-trisubstituted alkenes with broad substrate scope, 100% atom efficiency, high regio- and stereoselectivity, and an unprecedented reaction mechanism.