Catalysis of Organic..

142Bromine-Free TEMPO-Based Catalyst Systemeconomical oxidation methods which do not require bromine based catalysts, can becarried out with environmentally friendly oxidants and avoid the use of organicsolvents.RN .H HO(2mol%)+ NaOClOHNaBr (10mol%)1 2CH 2 Cl 2 , O o Caq. NaHCO 3 , pH 8.6H+ NaCl + H 2 OOScheme 1R=H, TEMPO (3); R=OCH 3 , MeO TEMPO (4); R= NHCOCH 3 , AATEMPO (5)Here we report on a new TEMPO based catalyst system for the oxidation ofprimary and secondary alcohols selectively to aldehydes and ketones using NaOCl asa terminal oxidant.Results and DiscussionIn the standard Anelli protocol for the oxidation of a primary alcohol such as 3,3-dimethylbutanol (1) to form an aldehyde, i.e. 3,3-dimethylbutanal (2) the oxidationtakes place in a two-phase system (4). The alcohol substrate and the MeO-TEMPOcatalyst, 4, are dissolved in the dichloromethane phase while the KBr co-catalyst isin the aqueous phase buffered at pH=8.6 with sodium bicarbonate. The most criticalparameters for an efficient oxidation to take place are the continuous control of thepH, maintaining the temperature in a relatively narrow range between 0 and –2 o C,efficient stirring and the metered addition of the NaOCl solution. The aldehydeyields, plotted against the time at various Br - : 4 ratios are shown in Figure 1. At highconcentrations of the KBr co-catalyst, the alcohol is completely oxidized within thefirst 5 minutes, but the selectivity to aldehyde was surprisingly low, due to the overoxidationto the corresponding carboxylic acid. On lowering the Br - : 4 ratios, thealdehyde selectivity passes through a maximum, which is slowly shifted to longerreaction times. At a Br - : 4 ratio of 1:1, full conversion of the alcohol at 92%aldehyde selectivity was attained over a 20 min reaction time.A profile of a hypothetical reactor containing all the reactants is shown in Figure1b. The top section of the bar is the volume of the bleach to be introduced, thehatched dark section is the volume of the CH 2 Cl 2 solvent, the gray area is the volumeof the aqueous buffer and at the bottom of the bar graph is the volume of the alcoholsubstrate, which, under these standard conditions is just 2.3% of the total volume ofthe liquid phase. The deficiencies in the standard reaction protocol are obvious andthey make the application of the oxidation procedure not feasible economically.Particularly, it could be pointed out that the low substrate to catalyst ratio (less than100), the large volume of dichloromethane solvent, the large amount of buffer andthe need of KBr as a co-catalyst are all detrimental to commercial applications. Asignificantly improved method would have to be far more efficient in the use of the