Catalysis of Organic..

126TEMPO Oxidation of Alcoholsconcentration of the dissolved oxygen in the liquid phase), becomes more importantat higher temperatures. At 43°C, the reaction is fast enough, the starting alcohol iscompletely converted to 2 over the 60 min reaction time and the aldehydesselectivity remains high in the 92-94% range (3c).O2 Uptake rate, mmol/min0.80.70.60.50.40.30.20.143C40C37C34C0.00 5 10 15 20 25 30 35Pressure, psia b c43C40C37C34C0 5 10 15 20 25 30 35Pressure, psi34C37C40C43C0 5 10 15 20 25 30 35Pressure, psi1009896949290Conversion(Selectivity) at 30min, %Fig 3. Effect of the oxygen pressure and the temperature on the rate of oxidation (a),on the conversion of Hexanol-1 (b) and on the selectivity to Hexanal (c) at 34°C (◊),37°C (+), 40°C (□) and 43 °C (○). Conditions: [Hexanol-1] = 2mL (16mmol),CH 3 COOH =10mL, [AA TEMPO] = 0.64mmol, [NBS]=0.048 mmol and Mg(NO 3 ) 2= 1.2 mmol.Reduction of the Volume of HOAc at High Substrate to Catalyst (S/C) RatiosThe next attempt to further improve the reaction efficiency was to reduce the volumeof the acetic acid solvent and to proportionally increase the initial concentration ofthe alcohol substrate while keeping the total reaction volume at constant level. Themain purpose of these studies was to determine the minimum amount acetic acidneeded to maintain a homogeneous system until complete conversion of hexan-1-olto 2. Since the oxidation reaction produces stoichiometric amounts water, it was felt,that the formation of a second aqueous phase along with the hydrophobic aldehydephase would lead to the creation of a two-phase reaction system with the inevitablepartition of the catalyst system between the two phases. In addition it was alsoimportant to determine the highest possible S/C ratio while maintaining a reasonablereaction rate.The starting point here was the 16 mmol scale oxidation reaction using thestandard conditions shown in Figure 4a (curve 1). Full conversion was achieved over22 min at a TOF of 68h -1 . In the next experiment (curve 2), the hexan-1-olconcentration was doubled (32mmol scale, 2.7M, 33.3% v/v) while the concentrationof the AA-TEMPO-MNT-NBS composition was kept at the same level. Again,