Switchable Magnesium Catalysis: Dehydrogenative Borylation Versus Hydroboration of Terminal Alkynes

Organoboranes are key intermediates in modern synthetic chemistry, yet their preparation often relies on precious metal catalysts and multistep protocols. We report, for the first time, the dehydrogenative borylation of terminal alkynes using a three‐coordinate magnesium complex. Uniquely, the reaction outcome is tunable, switching between dehydrogenative borylation and hydroboration depending on the ligand framework and solvent environment. Mg‐complex bearing oxygen‐bridged spacer selectively promotes dehydrogenative borylation with high efficiency, whereas ethylene‐ or phenyl‐linked analogues favour hydroboration. Solvent polarity further modulates chemo‐selectivity, offering control over divergent alkyne borylation pathways with a single catalytic platform. Mechanistic studies, supported by Density Functional Theory (DFT) calculations, reveal that the change in selectivity arises from distinct energy barriers for CH activation versus alkyne insertion, which are governed by the ligand framework's steric and electronic features. Controlled experiments enabled the isolation of the active catalyst and intermediate, which was spectroscopically detected, validating the proposed catalytic cycle. The borylated products are versatile precursors for further derivatization, demonstrating the method's broad synthetic utility. The reaction proceeds efficiently on a gram‐scale, delivering alkynyl boronates in high yield. This work showcases magnesium as a sustainable platform for CB bond formation, in which fine‐tuning of the ligand and solvent medium unlocks new main‐group reactivity.