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.