Abstract Herein, we report the development of new Co complexes that have cyclopropane‐based diphosphine ligands and can catalyze highly chemo‐, regio‐, and stereoselective hydroboration reactions of unsymmetrical internal alkynes. These reactions exhibited unusual regioselectivity: specifically, reactions of aryl alkyl internal alkynes showed excellent cis‐β‐addition selectivity, and reactions of dialkyl internal alkynes gave excellent cis‐α‐addition selectivity. Highly regioselective hydroboration of unsymmetrical dialkyl internal alkynes cannot be achieved by other known methods. The reactions described herein are highly synthetically useful, particularly for the stereoselective synthesis of trisubstituted alkenylborates and alkenes. Mechanistic studies indicate that a CoI?H species is a plausible active catalyst and the rigid structure of the cyclopropane skeleton of the ligands and the crowded reaction pocket were responsible for the unprecedented regioselectivity.
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