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.

Hydrogenation of Terminal Alkenes Catalyzed by Air-Stable Mn(I) Complexes Bearing an N-Heterocyclic Carbene-Based PCP Pincer Ligand.

Efficient hydrogenations of terminal alkenes with molecular hydrogen catalyzed by well-defined bench stable Mn(I) complexes containing an N-heterocyclic carbene-based PCP pincer ligand are described. These reactions are environmentally benign and atom economic, implementing an inexpensive, earth abundant non-precious metal catalyst. A range of aromatic and aliphatic alkenes were efficiently converted into alkanes in good to excellent yields. The hydrogenation proceeds at 100 °C with catalyst loadings of 0.25-0.5 mol %, 2.5-5 mol % base (KOt Bu) and a hydrogen pressure of 20 bar. Mechanistic insight into the catalytic reaction is provided by means of DFT calculations.