Cu-Catalyzed Chemoselective Borylcupration of Borylated (<i>Z</i>)-Skipped Dienoates: A Case Study for the Synthesis of <i>gem</i>-diborylcyclobutanes

Chemoselective borylcupration of borylated (<i>Z</i>)-skipped dienoates is controlled by the ester group to access 3,3-di-(pinacol)-borylalkenoates. Electrophilic trapping with H⁺, D⁺, alkyl-, benzyl-, or allyl halides, as well as isocyanates has proved to be efficient for α-functionalized products. The Cu-catalyzed borylcupration of skipped dienoates containing C-Br bonds resulted in concomitant ring closing sequences toward alkylidene <i>gem</i>-diborylcyclobutane scaffolds. We performed DFT calculations to characterize the reaction mechanism of the formation of <i>gem</i>-diborylcyclobutanes. The key steps of the proposal comprise a selective borylcupration directed by alkene substituents, followed by an intramolecular C-C coupling toward strained four-membered rings assisted by the potassium cation. We also analyzed the effect of the nature of the halogen leaving group on the selectivity. The versatility of alkylidene cyclobutanes has been demonstrated through postfunctionalization reactions.