An Amine-Assisted Ionic Monohydride Mechanism Enables Selective Alkyne -Semihydrogenation with Ethanol: From Elementary Steps to Catalysis.

The selective synthesis of -alkenes in alkyne semihydrogenation relies on the reactivity difference of the catalysts toward the starting materials and the products. Here we report -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 () forms a cationic monohydride, [(NCP)IrH(EtOH)]Cl, that reacts selectively with alkynes over the corresponding -alkenes, thereby overcoming competing thermodynamically dominant alkene - 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 -amine-EtOH catalytic system exhibits an unprecedented level of substrate scope, generality, and compatibility, as demonstrated by -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 -amine-EtOH and -NaOBu-EtOH underscores the fact that this base variable, albeit in catalytic amounts, leads to different mechanisms and consequently different stereoselectivity.