Hydroboration of terminal alkenes and trans-1,2-diboration of terminal alkynes catalyzed by a Mn(I) alkyl complex

A Mn(I)-catalyzed hydroboration of terminal alkenes and the 1,2-diboration of terminal alkynes with pinacolborane (HBPin) is described. In the case of alkenes anti-Markovnikov hydroboration takes place, while in the case of alkynes the reaction proceeds with excellent trans-1,2-selectivity. The most active pre-catalyst is the bench-stable alkyl bisphosphine Mn(I) complex fac-[Mn(dippe)(CO)3(CH2CH2CH3)]. The catalytic process is initiated by migratory insertion of a CO ligand into the Mn-alkyl bond to yield an acyl intermediate which undergoes B-H bond cleavage of HBPin (in the case of alkenes) and rapid C-H bond cleavage (in the case of alkynes) forming the active Mn(I) boryl and acetylide catalysts [Mn(dippe)(CO)2(BPin)] and [Mn(dippe)(CO)2(CCR)], respectively, together with liberated butanal. The diboration is accompanied by dihydrogen liberation. A broad variety of aromatic and aliphatic alkenes and alkynes was efficiently and selectively borylated. Mechanistic insights are provided based on experimental data and DFT calculations revealing that an acceptorless reaction pathway involving dihydrogen release is operating.