While isopropanol or formic acid derivatives are mainly applied as hydrogen sources in catalytic transfer hydrogenation reactions, there are only a few reports on the use of primary alcohols. In the present communication we report that ecologically benign and cheap ethanol can be applied with a ruthenium(II) that is completely stable against moisture and oxygen. Catalytic transfer hydrogenation is an equilibrium reaction. Therefore, a rapid removal of acetic aldehyde being formed as the product of ethanol oxidation is the key for high conversions. The reaction shows a broad substrate scope and allows the hydrogenation of ketones, aldehydes and imines.

The Griffith‐Ley oxidation of alcohols to aldehydes and ketones is performed with either RuCl3 ⋅ (H2O)x or a highly stable, well‐defined ruthenium catalyst and with cheap trimethylamine N‐oxide (TMAO) as the oxygen source. The use of n‐heptane as the solvent, which forms a second phase with TMAO and a part of the alcohol, allows the reactions to be performed with a minimum amount of catalyst. This results in high local concentrations and thus to very rapid conversions. Detailed quantum chemical calculations suggest, that the Griffith‐Ley oxidation not necessarily requires high oxidation states of ruthenium but can also proceed with RuII/RuIV species.

A series of ruthenium(II) complexes bearing tridentate N,N’‐diallyl‐2,6‐di(5‐butylpyrazol‐3‐yl)pyridine ligands was synthesized and characterized. Introduction of substituents in the 4‐position of the pyrazole rings tune the electron density at the ruthenium center, which was proved by correlation of the 31P NMR chemical shifts with the σp parameters of the Hammett equation. The structural elucidation of a phosphine‐free ruthenium(II) complex proves that one of the allyl side‐chains undergoes chelating coordination to the ruthenium site to realize a 18 VE center. This compound is the starting point for complexes of the type (N,N,N)Ru(L)(Cl)2 bearing ligands L other than triphenylphosphine. The ruthenium(II) complexes were investigated for their activity in the transfer hydrogenation with ethanol as the hydrogen source. Here the logarithms of the measured turn‐over frequencies (TOF) correlate with the σp parameters of the Hammett equation in terms of a linear free‐energy relationship.