The selective synthesis of Z-alkenes in alkyne
semihydrogenation relies on the reactivity difference of the catalysts
toward the starting materials and the products. Here we report Z-selective semihydrogenation of alkynes with ethanol via
a coordination-induced ionic monohydride mechanism. The EtOH-coordination-driven
Cl– dissociation in a pincer Ir(III) hydridochloride
complex (NCP)IrHCl (1) forms a cationic monohydride,
[(NCP)IrH(EtOH)]+Cl–, that reacts selectively
with alkynes over the corresponding Z-alkenes, thereby
overcoming competing thermodynamically dominant alkene Z–E isomerization and overreduction. The challenge
for establishing a catalytic cycle, however, lies in the alcoholysis
step; the reaction of the alkyne insertion product (NCP)IrCl(vinyl)
with EtOH does occur, but very slowly. Surprisingly, the alcoholysis
does not proceed via direct protonolysis of the Ir–C(vinyl)
bond. Instead, mechanistic data are consistent with an anion-involved
alcoholysis pathway involving ionization of (NCP)IrCl(vinyl) via EtOH-for-Cl
substitution and reversible protonation of Cl– ion
with an Ir(III)-bound EtOH, followed by β-H elimination of the
ethoxy ligand and C(vinyl)–H reductive elimination. The use
of an amine is key to the monohydride mechanism by promoting the alcoholysis.
The 1–amine–EtOH catalytic system exhibits
an unprecedented level of substrate scope, generality, and compatibility,
as demonstrated by Z-selective reduction of all alkyne
classes, including challenging enynes and complex polyfunctionalized
molecules. Comparison with a cationic monohydride complex bearing
a noncoordinating BArF– ion elucidates the beneficial
role of the Cl– ion in controlling the stereoselectivity,
and comparison between 1–amine–EtOH and 1–NaOtBu–EtOH underscores the
fact that this base variable, albeit in catalytic amounts, leads to
different mechanisms and consequently different stereoselectivity.