Selective Transfer Semihydrogenation of Alkynes Catalyzed by an Iron PCP Pincer Alkyl Complex
Heiko Schratzberger, Berthold Stöger, Luı́s F. Veiros, Karl Kirchner
Abstract
High Resolution Image Download MS PowerPoint Slide 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.