Highly efficient and cis-selective semihydrogenation of dialkyl alkynes with EtOH enforced by a catalyst-state induced color-change strategy

Synthesis of valuable（Z）-1,2-dialkyl alkenes via cis-semihydrogenation of alkynes is often plagued by side reactions such as alkene isomerization and over-reduction.Here we report a catalyst-state induced color-change（CatSICC） strategy for precise detection of reaction endpoint of transfer hydrogenation（TH）,enabling highly efficient and cis-selective semihydrogenation of dialkyl alkynes with EtOH as H-donor.A series of NHC carbene-containing pincer iridium complexes(PCCNHC)IrHX（X=Br or I） have been prepared and applied to the TH reaction.Monitoring the TH process reveals a vibrant color-change of the solution from purple to yellow-orange as a result of transition of the catalyst-state from(PCCNHC)Ir（alkyne） to(PCCNHC)IrHMe（CO）,signifying the complete conversion of alkyne substrate.Quenching the reaction timely according to the color-change allows access to（Z）-1,2-dialkyl alkenes with very high efficiency,exquisite precision,and broad functional group tolerance.The reliability and practicability of this protocol is demonstrated by cis-semihydrogenation of complex polyfunctionalized bioactive molecules.Mechanistic studies establish that the propensity of(PCCNHC)Ir to undergo facile EtOH decarbonylation to form isomerization-inactive species(PCCNHC)IrHMe（CO） only when the semihydrogenation reaction is completed is important to stereo-and chemoselectivity control.