Catalysis of Organic..

Tanielyan et al. 125TEMPO concentrations is able to promote the further oxidation to an acid once thealcohol is completely converted to 2. The same is true for the best performing systemin Figure 2c. At the optimum concentration for the AA-TEMPO catalyst of 0.6 mmol(3.7 mol%), the alcohol substrate is completely consumed at an aldehyde selectivityof 95+ %. Further increase in the AA-TEMPO concentration doesn’t lead to furtherrate gains and the only visible result is the deterioration in the aldehyde selectivity.O2 Uptake rate, mmol/mina b c0.900.800.70111220.6020.503330.400.300.200.10MeO-TEMPOTEMPOAA-TEMPOS/C=13 S/C=13 S/C=22300.00200.0 0.5 1.0 1.5 0.0 0.5 1.0 1.5 0.0 0.5 1.0 1.5[MeO-TEMPO], mmol [TEMPO], mmol [AA-TEMPO], mmol110100908070605040Conversion (Selectivity), %Fig 2. Effect of the [MeO-TEMPO] (a), the [TEMPO] (b) and the [AA-TEMPO] (c)concentrations on the rate of oxidation (○), on the conversion of the Hexanol-1 (◊)and on the selectivity to Hexanal (□) at T= 46°C, P (O 2 ) 15psi, solvent CH 3 COOH=10mL, [Hexanol-1] = 2mL (16mmol), [NBS]=0.048 mmol and Mg(NO 3 ) 2 = 1.2mmol.Pressure-Temperature MatrixUnder these standard reaction conditions, the effect of the temperature and theoxygen pressure was also investigated. The reaction temperature was varied in arelatively narrow range of 34-43°C by an incremental increase of 3°C at each stepand the oxygen pressure was sequentially increased in the range 5-30 psi (Fig 3).Increasing the oxygen pressure led to a linear increase in the initial reaction rate forall four temperatures (Figure 3a) clearly showing that the rate limiting step (at theconcentration of the AA-TEMPO currently employed) has shifted to one of thecycles of the cascade in which the concentration of the dissolved oxygen affects therate. For two of the pressures in this matrix, (10 psi and 30 psi) activation energies of32.2 kJ/mol and 31.4 kJ/mol, respectively, were calculated. These values areremarkably low for an overall oxidation process and are an indication for a multi stepcascade type interaction, such as that shown in Scheme 2.The changes in the conversion of 1 at different temperatures as a function of theoxygen pressure are shown in Figure 3b. As expected, the oxygen pressure (or the