Highly Chemo- and Stereoselective Transfer Semihydrogenation of Alkynes Catalyzed by a Stable, Well-Defined Manganese(II) Complex

Herein
we report unprecedented manganese-catalyzed semihydrogenation
of internal alkynes to (Z)-alkenes using ammonia
borane as a hydrogen donor. The reaction is catalyzed by a pincer
complex of the earth-abundant manganese­(II) salt in the absence of
any additives, base, or superhydride. The ammonia borane smoothly
reduces the manganese precatalyst [Mn­(II)–PNP]­[Cl]2 to the catalytically active species [Mn­(I)–PNP]–hydride
in the triplet spin state. This manganese hydride is highly stabilized
by complexation with the alkyne substrate. Computational density functional
theory (DFT) analysis studies of the reaction mechanism rationalize
the origin of stereoselectivity toward formation of (Z)-alkenes.