Tandem manganese catalysis for the chemo-, regio-, and stereoselective hydroboration of terminal alkynes: in situ precatalyst activation as a key to enhanced chemoselectivity.
Abstract
The manganese(ii) complex [Mn(iPrPNP)Cl2] (iPrPNP = 2,6-bis(diisopropylphosphinomethyl)pyridine) 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 the in situ activator, E-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.
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