Ruthenium p-cymene complexes with α-diimine ligands as catalytic precursors for the transfer hydrogenation of ethyl levulinate to γ-valerolactone

Abstract

The ruthenium compounds [(η⁶-p-cymene)RuCl{κ²N-(HCNR)₂}]NO₃ (R = 4-C₆H₄Me, [1]NO₃; 4-C₆H₄OH, [2]NO₃; C₆H₁₁Cy, [3]NO₃; 4-C₆H₁₀OH, [4]NO₃; ^tBu, [5]NO₃) were prepared in high yields from [(p-cymene)RuCl₂]₂, AgNO₃ and the appropriate α-diimine. Compounds [2]PF₆ and [4]PF₆ were obtained by a straightforward reaction of [(η⁶-p-cymene)RuCl(MeCN)_0.66]PF₆, [6]PF₆, with α-diimine, whereas [4]BPh₄ was obtained by metathesis between [4]NO₃ and NaBPh₄. All the ruthenium products were characterized by analytical methods, IR, NMR and UV-Vis spectroscopy; in addition, the structure of [1]NO₃ was ascertained by an X-ray diffraction study. Compounds [1–4]NO₃, [4]PF₆ and [4]BPh₄ were investigated as catalytic precursors in the transfer hydrogenation reaction of ethyl levulinate to γ-valerolactone in isopropanol solution under microwave irradiation. [4]BPh₄ was revealed to be the best catalytic precursor, affording γ-valerolactone in 62% yield under optimized experimental conditions.