Chemoselective Oxidation of Alcohols to Aldehydes and Ketones by tert -Butyl Hydroperoxide Catalyzed by a Ruthenium Complex of N,N′,N″-Trimethyl-1,4,7-triazacyclononane

An operationally simple method based on [Cn*RuIII(CF3CO2) 3·H2O] (Cn* = N,N′,N″-trimethyl-1,4,7-triazacyclononane) catalyst and 1-1.2 equiv of tert-butyl hydroperoxide as terminal oxidant is effective for selective transformation of alcohols to aldehydes and ketones in methylene chloride. The reaction proceeds in high yield and selectivity. Preparation of benzaldehyde (98% yield) from benzyl alcohol on a 200 mmol scale can be performed without modification of the procedure such as slow addition of the oxidant or cooling to 0°C, and catalyst turnovers of 700 are achieved. Oxidation of geraniol which contains an isolated trisubstituted C=C bond leads to geranial selectively without oxidation of the C=C bond. Results from Hammett correlation studies (ρ = -0.47) and primary kinetic isotope effect (kH/kD = 4.8) for the catalytic benzyl alcohol oxidations are inconsistent with an oxoruthenium (O=Ru) based mechanism. A mechanism involving reactive tBuO·/tBuOO· radicals is also excluded based on results from previous works: Cheng, W.-C.; Fung, W.-H.; Che, C.-M. J. Mol. Catal. (A) 1996, 113, 311; absence of di-tert-butyl peroxide; and using cumyl hydroperoxide as a radical probe. A tert-butylperoxoruthenium complex is postulated to be the active intermediate.