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

Two bench-stable Fe(II) alkyl complexes [Fe(κ 3 PCP-PCP- i Pr)(CO) 2 (R)] (R = CH 2 CH 2 CH 3 , CH 3 ) were obtained by the treatment of [Fe(κ 3 PCP-PCP- i Pr)(CO) 2 (H)] with NaNH 2 and subsequent addition of CH 3 CH 2 CH 2 Br and CH 3 I, respectively. The reaction proceeds via the anionic Fe(0) intermediate Na[Fe(κ 3 PCP-PCP- i Pr)(CO) 2 ]. The catalytic performance of both alkyl complexes was investigated for the transfer hydrogenation of terminal and internal alkynes utilizing PhSiH 3 and i PrOH 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 n Pr > Me. Accordingly, [Fe(κ 3 PCP-PCP- i Pr)(CO) 2 (CH 2 CH 2 CH 3 )] is the more active catalyst. The reaction takes place at 25 °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.