Room temperature chemoselective hydrogenation of CC, CO and CN bonds by using a well-defined mixed donor Mn(i) pincer catalyst

Abstract

Chemoselective hydrogenation of CC, CO and CN bonds in α,β-unsaturated ketones, aldehydes and imines is accomplished at room temperature (27 °C) using a well-defined Mn(I) catalyst and 5.0 bar H₂. Amongst the three mixed-donor Mn(I) complexes developed, κ³-(^R2PN³N^Pyz)Mn(CO)₂Br (R = Ph, ⁱPr, ^tBu); the ^tBu-substituted complex (^tBu2PN³N^Pyz)Mn(CO)₂Br shows exceptional chemoselective catalytic reduction of unsaturated bonds. This hydrogenation protocol tolerates a range of highly susceptible functionalities, such as halides (–F, –Cl, –Br, and –I), alkoxy and hydroxy, including hydrogen-sensitive moieties like acetyl, nitrile, nitro, epoxide, and unconjugated alkenyl and alkynyl groups. Additionally, the disclosed method applies to indole, pyrrole, furan, thiophene, and pyridine-containing unsaturated ketones leading to the corresponding saturated ketones. The CC bond is chemoselectively hydrogenated in α,β-unsaturated ketones, while the aldehyde's CO bond and imine's CN bond are preferentially reduced over the CC bond. A detailed mechanistic study highlighted the non-innocent behavior of the ligand in the (^tBu2PN³N^Pyz)Mn(I) complex and indicated a metal–ligand cooperative catalytic pathway. The molecular hydrogen (H₂) acts as a hydride source, whereas MeOH provides a proton for hydrogenation. DFT energy calculations supported the facile progress of most catalytic steps, involving a crucial turnover-limiting H₂ activation.