Stereoselective ZnCl₂‑Catalyzed B–H Bond Insertion of Vinyl Carbenes Generated from Cyclopropenes for the Synthesis of Allylboranes

Zinc-catalyzed insertion of vinyl carbenes generated from cyclopropenes into the B–H bonds of Lewis base–borane adducts for concise and efficient access to allylboranes has been developed. This protocol represents the first zinc-catalyzed B–H bond insertion of carbenes for organoborane compounds. In this protocol, inexpensive ZnCl₂, with low toxicity, is used as the catalyst. This simple ligand-free catalytic system affords allylboranes in yields up to 92%, with E/Z ratios of >20:1. Besides, this new protocol offers a broad scope of Lewis base–borane adducts, which are not easily obtained by other catalytic methods for metal carbene insertion into B–H bonds. The potential synthetic applicability of this new methodology is exemplified by a gram-scale experiment and synthetic transformation of the products.