Selective Transfer Semihydrogenation of Alkynes Catalyzed by an Iron PCP Pincer Alkyl Complex

Two bench-stable Fe(II) alkyl complexes [Fe(kappa(PCP)-P-3-PCP-iPr)(CO)(2)(R)] (R = CH2CH2CH3, CH3) were obtained by the treatment of [Fe(kappa(PCP)-P-3-PCP-iPr)(CO)(2)(H)] with NaNH2 and subsequent addition of CH3CH2CH2Br and CH3I, respectively. The reaction proceeds via the anionic Fe(0) intermediate Na[Fe(kappa(PCP)-P-3-PCP-iPr)(CO)(2)]. The catalytic performance of both alkyl complexes was investigated for the transfer hydrogenation of terminal and internal alkynes utilizing PhSiH3 and iPrOH as a hydrogen source. Precatalyst activation is initiated by migration of the alkyl ligand to the carbonyl C atom of an adjacent CO ligand. In agreement with previous findings, the rate of alkyl migration follows the order nPr > Me. Accordingly, [Fe(kappa(PCP)-P-3-PCP-iPr)(CO)(2)(CH2CH2CH3)] is the more active catalyst. The reaction takes place at 25 degrees C with a catalyst loading of 0.5 mol%. There was no overhydrogenation, and in the case of internal alkynes, exclusively, Z-alkenes are formed. The implemented protocol tolerates a variety of electron-donating and electron-withdrawing functional groups including halides, nitriles, unprotected amines, and heterocycles. Mechanistic investigations including deuterium labeling studies and DFT calculations were undertaken to provide a reasonable reaction mechanism.

Automated summary

In this study, two stable Fe(II) alkyl complexes were successfully synthesized and their catalytic performance in transfer hydrogenation reactions was investigated. The results showed that these complexes exhibited good activity and selectivity for hydrogenation of various alkynes. Through both experimental and theoretical investigations, the researchers also revealed the reaction mechanism.