Homogeneous dioxoruthenium(VI) catalysts for the oxidation of alcohols.
Compounds of the form RuO2(Y-py)2CI2 1 (py = pyridine, Y = H, 4-t-Bu, 3-CN, 4-CN, 2-Br, 3-Br, 4-Br, 4-Cl, 4-Me, 4-C(O)C6H5, 3-COOH, 4-COOH), RuO2ZCI2 2(Z = bipyridine, phenanthroline) and RuO2(OH)2(Y-py)2 3 (py = pyridine, Y = H, 4-CN, 4-t-Bu) were prepared and the effects of the various substituents on the properties of the compounds were investigated. The position of the v(Ru=O) bands of the complexes 1 correlate with the Hammett σ1 (or Taft σ*) and Pauling-type electronegativity values of the substituents on the pyridine ring. A comparative study of most of these complexes as both stoichiometric and catalytic oxidants for the oxidation of 1-hexanol to hexanal was carried out. The known dioxoruthenium(VI) compound [PPh4][RuO2(OCOCH3)CI2] (a) and a range of new compounds [PPh4][RuO2(OCOR)CI2] (R = CF3 b, C6H5 c, C6F5 d, C5H11 e) were synthesized and characterized. High synthetic yields were obtained for the fluorinated complexes b and d relative to their protonated equivalents a and c, possibly due to the varying charge distributions within such complexes. No clear trend in catalytic activity was noticed on varying the ligand on the ruthenium for the oxidation of 1-hexanol and 2-hexanol to hexanal and 2-hexanone respectively, with a range of co-oxidants. The complexes c and d were used very successfully in the catalytic oxidations of a variety of alcohols with the co-oxidants tetrabutylammonium periodate and 4-methylmorpholine N-oxide. The novel compounds [PPh4][RuO2(OCOC6H5)CI2 ] and [PPh4][RuO2(OCOC6F5)CI2] were supported on poly-4-vinylpyridine and used catalytically in oxidizing cyclohexanol to cyclohexanone, using 4-methylmorpholine N-oxide and tetrabutylammonium periodate as co-oxidants.