Copper-Catalyzed Ring-Opening/Borylation of Cyclopropenes

Since organoboron compounds readily undergo a diverse array of transformations and are widely used in various fields, the development of C-B-bond-forming reactions have attracted considerable attention. Herein, we report a new method for forming C-B bonds by means of Cu-catalyzed ring-opening/borylation reactions of cyclopropenes. This method provides efficient access to a new type of stable allylborane-Lewis base adduct, which is a versatile synthon. The configuration of the products can be well controlled with this method, and some of the configurations we obtained are inaccessible by other catalytic methods for generating allylborons. Mechanistic studies indicated that the reactions proceed via insertion of an alkenyl Cu carbene-generated in situ by cyclopropene ring opening-into the B-H bond; the ring-opening step determines both the rate and stereochemistry. [GRAPHICS] .