Transfer C-H Borylation of Alkenes under Rh(I)-Catalysis: In-sight into the Mechanism, Selectivity-Control & Synthetic Capacity

Transfer C-H borylation of alkenes bears the potential to unlock a range of attractive transformations for modular synthe-sis and late-stage derivatization of complex molecules. However, its scarce precedence associated with a limited mechanistic understanding hinders the development of practical synthetic protocols. Here we report a Rh(I)-catalyzed transfer C-H borylation reaction that is founded on an unprecedented yet crucial elementary step of the beta-boryl elimination engaging the Rh(I)-(beta-borylalkyl) intermediate. A thorough mechanistic investigation involving a series of catalytic and stoichiometric experiments and complementary computational studies revealed that this step proceeds with a considerably low free energy barrier, further elucidated the full catalytic cycle, and provided insight into the features con-trolling the activity and the selectivity. Driven by this mechanistic understanding, we devised a protocol that is compatible with a plethora of functional groups, including often problematic motifs, and applicable not only to terminal but also inter-nal alkenes and varied electronic and steric properties. The method proved also to be effective in complex settings of the late-stage borylation of derivatives of macrocyclic mycoestrogen Zearalenol, bioactive Brompheniramine, Chlorpromazine, and CD3254, and the synthesis of the boronic acid bio¬isostere of the drug Ozagrel. Besides the valuable new method, these mechanistic investigations set the stage for the development of other hydrogen-for-functional group exchange reactions undergoing a similar pathway.