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

Abstract A series of ruthenium-cymene derivatives subjecting [Ru(η 6 -cymene)Cl 2 ] 2 and substituted bidentate N,O donor precursors have been synthesized and characterized. Reacting [Ru(η 6 -cymene)Cl 2 ] 2 with two equivalents the lithium salt of phenyl keto-amine ligands LiL 1 ∼ LiL 3 resulted in the formation of [Ru(η 6 -cymene)(L)Cl] ( 1 – 3 ). Similarly, the reaction between [Ru(η 6 -cymene)Cl 2 ] 2 and two equivalents of LiL 4 or NaL 5 afforded compounds {Ru(η 6 -cymene)[L 4 )]Cl} ( 4 ) and [Ru(η 6 -cymene)(L 5 )Cl] ( 5 ). The 1 H 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(η 6 -cymene)Cl 2 ] 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 = NCH 2 Ph and HN(CH 2 Ph) 2 were detected, with PhCH = NCH 2 Ph 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.