2,2,2-Trifluoroethanol-assisted imine hydrogenation by a Ru-monohydride

• Base-free imine hydrogenation with [Ru(OAc)2(CO)2(PnBu3)(PPh3)] (1) in different reaction media.
• 2,2,2-Trifluoroethanol (TFE) emerged as the solvent of choice for the 1-catalyzed imine hydrogenation under mild conditions.
• TFE stabilizes very efficiently the monohydride of 1, which enters the catalytic cycle.
• A TFE-assisted outer-sphere reaction mechanism is favored over a classical inner-sphere reaction mechanism.

The trans heterodiphosphane Ru-based compound of the formula [Ru(OAc)2(CO)2(PnBu3)(PPh3)] proved to be a suitable precatalyst for imine hydrogenation in 2,2,2-trifluoroethanol (TFE) without the addition of an external base. High-pressure (HP) NMR investigations combined with a DFT-study, carried out on a related model precatalyst, were indicating the formation of a cationic Ru-monohydride-2,2,2-trifluoroethanol (TFE) species that catalyzes the imine hydrogenation by a TFE-assisted outer-sphere reaction mechanism.

[Ru(OAc)2(CO)2(PnBu3)(PPh3)] was used as precatalyst to hydrogenate selected imines to the corresponding amine in different reaction media and under base-free conditions. The solvent of choice revealed to be 2,2,2-trifluoroethanol (TFE) which directly participated in an TFE-assisted outer-sphere reaction mechanism by forming a Ru-monohydrate-TFE intermediate.Figure optionsDownload as PowerPoint slide