Copper-Catalyzed Ring-Opening Hydroboration and Enantioselective Cascade Hydroboration/Protoboration of Arylidenecyclobutanes

Arylidenecyclobutanes (ACBs) are an emerging class of five-carbon synthons for the construction of structurally complex molecules. Organoboron compounds are of central importance in modern synthetic chemistry; herein, we report copper-catalyzed ring-opening hydroboration and enantioselective cascade hydroboration/protoboration reactions that use arylidenecyclobutanes as C5 building blocks to access linear (E)–1,4-alkenylboronates and chiral 1,4-diboronates, respectively. Mechanistic studies and DFT calculations reveal that the CuH-catalyzed ring-opening hydroboration of ACBs proceeds through β-carbon elimination and selectively affords (E)–1,4-alkenylboronates. In the presence of B2pin2 and a proton source, this internal alkenyl intermediate can further undergo asymmetric protoboration with a Cu–Bpin species to furnish chiral 1,4-diboronate products. These boration reactions provide strategies for the ring-opening hydrofunctionalization and asymmetric 1,4-difunctionalization of ACBs. Moreover, the resulting mono- and diboronates serve as versatile intermediates for downstream transformations, enabling the efficient synthesis of bioactive molecules and chiral ligands.