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

The ruthenium compounds [((6)-p-cymene)RuCl{N-2-(HCNR)(2)}]NO3 (R = 4-C6H4Me, [1]NO3; 4-C6H4OH, [2]NO3; C6H11?Cy, [3]NO3; 4-C6H10OH, [4]NO3; Bu-t, [5]NO3) were prepared in high yields from [(p-cymene)RuCl2](2), AgNO3 and the appropriate -diimine. Compounds [2]PF6 and [4]PF6 were obtained by a straightforward reaction of [((6)-p-cymene)RuCl(MeCN)(0.66)]PF6, [6]PF6, with -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 -valerolactone in isopropanol solution under microwave irradiation. [4]BPh4 was revealed to be the best catalytic precursor, affording -valerolactone in 62% yield under optimized experimental conditions.