Theoretical Study on the Transfer Hydrogenation of Alkynes Using Ethanol Catalyzed by a NCP Pincer Iridium Complex

The reaction mechanism of Ir(NCP)-catalyzed stereoselective hydrogenation of alkyne to alkenes with ethanol as a hydrogen source was investigated via density functional theory (DFT). The theoretical study was carried out to uncover the origins of chemo- and stereoselectivity in the iridium-catalyzed stereodivergent semihydrogenation of alkynes. The predicted reaction mechanism is composed of the following two processes in general: 1) Ir(NCP) combined with the substrate or EtOH to form the catalyticall active species; 2) the catalytic cycle reaction from the catalyst activate species. Calculations on the alkyne system indicates that there exist four competing reaction paths. For the optimal reaction mechanism, the rate-determining step (RDS) is identified as the step corresponding to the oxidative addition of the O‐H bond of the hydrogen source to the iridium center of the catalytic complex. The computational results show that over-reduction of the alkene is forbidden in this catalytic system because the alkyliridium intermediate formed by alkene insertion prefers to undergo β‐H elimination rather than proton transfer from the Ir‐H bond. Mechanism investigation reveals that the active iridium hydride species formed by percatalyst strong catalytic activity for alkene isomerization. Subsequent free energy reveals that fast deactivation of the catalyst with free energy reactivity of the cis-alkene intermediate in the presence of the alkyne substrate prevents Z/E alkene isomerization using precatalyst. The predominant product with the E-configuration is reproduced theoretically, which is consistent with the experimental observations. Reduced density gradient (RDG) analysis of the transition states confirm the significant influence of ethanol on the mechanism and stereoselectivity.