Tandem manganese catalysis for the chemo-, regio-, and stereoselective hydroboration of terminal alkynes: precatalyst activation as a key to enhanced chemoselectivity.

The manganese(ii) complex [Mn(PNP)Cl] (PNP = 2,6-bis(diisopropylphosphinomethyl)pyridine) was found to catalyze the stereo- and regioselective hydroboration of terminal alkynes employing HBPin (pinacolborane). In the absence of activators, mixtures of alkynylboronate and -alkenylboronate esters were formed, whereas when NaHBEt was employed as the activator, -alkenylboronate esters were exclusively accessed. Mechanistic studies revealed a tandem C-H borylation/semihydrogenation 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.