Oxyboration with and without a Catalyst: Borylated Isoxazoles via B–O σ-Bond Addition

Herein we report an oxyboration reaction with activated substrates that employs B–O σ bond additions to C–C π bonds to form borylated isoxazoles, which are potential building blocks for drug discovery. Although this reaction can be effectively catalyzed by gold, it is the first example of uncatalyzed oxyboration of C–C π bonds by B–O σ bonds—and only the second example that is catalyzed. This oxyboration reaction is tolerant of groups incompatible with alternative lithiation/borylation and palladium-catalyzed C–H activation/borylation technologies for the synthesis of borylated isoxazoles.