Catalysis of Organic..

122TEMPO Oxidation of AlcoholsThe real breakthrough was achieved when the test oxidation was initiated withjust the homogeneous MeO-TEMPO/Mg(NO 3 ) 2 /NBS catalyst system, used in theabc0.5111100O 2 Uptake rate, mmol/min0.40.30.20.123232380604020Conversion (Selectivity), %0.00.0 0.5 1.0[MeO-TEMPO], mmol00.0 0.2 1.0 2.0 0.0 1.0 2.0 3.0[Br - ] / [MeO-TEMPO] [Mg(NO 3 ) 2 ] / [MeO-TEMPO]Fig 1. Effect of the [MeO-TEMPO] (a), the [NBS] (b) and the [Mg(NO 3 ) 2 ] (c) on therate of oxidation (○), on the conversion of Hexanol-1 (◊) and on the selectivity toHexanal (□) at T= 46°C, P (O 2 ) 15psi, solvent CH 3 COOH =10mL, [Hexanol-1] =2mL (16mmol). [NBS]=0.96 mmol and Mg(NO 3 ) 2 = 0.48 mmol (for a), [M-TEMPO]=0.48mmol and Mg(NO 3 ) 2 = 0.48 mmol (for b) , [M-TEMPO]=0.48mmoland [NBS] = 0.048 (for c).same concentration levels (entry 6). Under the test conditions the reaction wascompleted at near full conversion of the starting alcohol (99%), at very highaldehyde selectivity (95%) and at turnover frequencies of 45 h -1 at 46°C. This result,without any optimization, was much higher than the TOF’s reported in the literaturefor the TEMPO / Mn(NO 3 ) 2 / Cu(NO 3 ) 2 system of 1.7 h -1 at 40°C (20) and for theTEMPO / NaNO 2 / Br system of 27.8 h -1 at 80°C (21).Effect of the MeO-TEMPO, NBS and the MNT on the Reaction Efficiency.The initial study on the MeO-TEMPO / Mg(NO 3 ) 2 / NBS triple catalyst system in theoxidation of 1 indicated the necessity of all three components; the TEMPO basedcatalyst, the nitrate source (MNT) and the bromine source (NBS). A large number ofmetal nitrates and nitrites were screened initially and the highest activity andaldehyde selectivity under comparable reaction conditions were recorded usingMg(NO 3 ) 2 as the nitrate component. A number of organic and inorganic bromidessoluble in HOAc were also screened and high reaction rates were found when NBSwas used as the bromide source. The effect of the concentration of the individualcomponents of the new triple catalyst system on the reaction rate, on the conversionof 1 and on the selectivity to 2 over 60 min reaction time is shown in Figure 1.