Borenium-Catalyzed\n“Boron Walking” for Remote Site-Selective Hydroboration

Remote functionalization through progressive olefin isomerization\nenables site-selective modification at a distal position, diversifying\nthe synthetic approaches. However, the developed protocols have long\nrelied on transition metal catalysis. Transition metal catalysts are\ndeemed irreplaceable, albeit facing challenges in metal residue and\ncatalyst poisoning. In this work, we present a pioneering approach\nthat employs a borenium ion as a catalyst for site-selective, remote\nborylation, eliminating the need for metal catalysts. As the reaction\nprogresses, borylation isomers at different positions emerge, gradually\nand ultimately converging into the predominant α-borylation\nproduct. This process is akin to a “walking” of a boron\nmoiety along a carbon skeleton toward an aryl terminus. Detailed mechanistic\nstudies and DFT calculations substantiate the borenium-catalyzed,\nstepwise migration via a reversible B–H insertion/elimination\nsequence. This remote borylation exhibits good functional group compatibility,\ncomplementing those methods reliant on transition metals. Furthermore,\nthis metal-free protocol permits the convenient synthesis of silyl-remote-boryl\ncompounds, demonstrating an opposite regioselectivity to that observed\nin transition-metal-catalyzed tandem silylation-borylation reactions.\nThis discovery therefore contributes to site-selective, remote difunctionalization\nvia sequential C–B and C–Si derivatizations, exemplified\nby the synthesis of amino-remote-alcohol bioactive molecules.