Hydroboration of Terminal Alkynes Catalyzed by a Mn(I) Alkyl PCP Pincer Complex Following Two Diverging Pathways

A stereo- and regioselective
Mn(I)-catalyzed hydroboration of terminal
alkynes with pinacolborane (HBPin) is described. The hydroboration
reaction is highly Z-selective in the case of aryl
alkynes and E-selective in the case of aliphatic
alkynes. The reaction requires no additives or solvents and proceeds
with a catalyst loading of 1 mol % at 50–70 °C. The most
active precatalyst is the bench-stable alkyl Mn(I) complex cis-[Mn(PCP-iPr)(CO)2(CH2CH2CH3)]. The catalytic process is initiated
by the migratory insertion of a CO ligand into the Mn-alkyl bond to yield an acyl intermediate. This species undergoes
C–H and B–H bond cleavage of the alkyne (aromatic alkynes)
and HBPin (in the case of aliphatic alkynes) forming the active Mn(I)
alkynyl and boryl catalysts [Mn(PCP-iPr)(CO)(CCR)]
and [Mn(PCP-iPr)(CO)(BPin)], respectively. A broad variety of aromatic and aliphatic
alkynes was efficiently and selectively borylated. Mechanistic insights
are provided based on experimental data and DFT calculations. The
functionalized alkenes can be used for further applications in cross-coupling
reactions.