Catalytic amination of benzyl alcohol using ruthenium cymene compounds containing bidentate N,O-donor ancillary ligands

A series of ruthenium-cymene derivatives subjecting [Ru(η⁶-cymene)Cl₂]₂ and substituted bidentate N,O donor precursors have been synthesized and characterized. Reacting [Ru(η⁶-cymene)Cl₂]₂ with two equivalents the lithium salt of phenyl keto-amine ligands LiL₁∼LiL₃ resulted in the formation of [Ru(η⁶-cymene)(L)Cl] (1–3). Similarly, the reaction between [Ru(η⁶-cymene)Cl₂]₂ and two equivalents of LiL₄ or NaL₅ afforded compounds {Ru(η⁶-cymene)[L₄)]Cl} (4) and [Ru(η⁶-cymene)(L₅)Cl] (5). The ¹H NMR chemical shifts of aromatic protons of the cymene fragment for compounds 1–5 fell in the range δ 5.54–3.58, showing chemical shifts farther upfield than the corresponding protons of [Ru(η⁶-cymene)Cl₂]₂ due to the intramolecular ring current effect. The 2,6-diisopropylphenyl substituted pheny lketo-amine ligand of compound 3 shows a slow C-N bond rotation with the rotation rate constant at ca. 4.62 s⁻¹. The catalytic aminations of benzyl alcohol with benzylamine using compounds 1–3 as catalysts resulted in higher conversion than using compounds 4 and 5. Two products, PhCH = NCH₂Ph and HN(CH₂Ph)₂ were detected, with PhCH = NCH₂Ph being the major product. When benzyl alcohol was reacted with phenylamine in the presence of compounds 1–5, the conversions are all greater than 60% and PhCH = NPh was the major product. While sterically hindered 2,6-diisopropylaniline appeared in alcohol amination reactions in the presence of catalysts, no imine or amine products were detected.