Tandem Manganese Catalysis for the Chemo-, Regio-, and Stereoselective Hydroboration of Terminal Alkynes: In Situ Precatalyst Activation as a Key to Enhanced Chemoselectivity

The manganese(II) complex [Mn(iPrPNP)Cl2] (iPrPNP = bis(diisopropylphosphine)pyridyl)) was found to catalyze the stereo- and regioselective hydroboration of terminal alkynes employing HBPin (pinacolborane). In the absence of in situ activators, mixtures of alkynylboronate and E-alkenylboronate esters were formed, whereas when NaHBEt3 was employed as in situ activator, E-alkenylboronate esters were exclusively accessed. Mechanistic studies revealed a tan-dem C-H borylation / semihydrogenation as the pathway accounting for the formation of the products. Stoichiometric reactions hint toward reaction of a Mn-H active species with the terminal alkyne as the catalyst entry pathway to the cycle, whereas reaction with HBPin led to catalyst deactivation.