Catalysis of Organic..

144Bromine-Free TEMPO-Based Catalyst Systemaddition of the hypochlorite solution, the reaction composition remainedhomogeneous and no phase separation occurred. The reaction rates were high andfull conversion to aldehyde was recorded within the first 5 minutes of the reaction.The last option was to test the system by eliminating the solvent completely. Thegraph 2b shows the progress of the reaction when the toluene fraction in the organicphase was gradually reduced until no solvent was present. Carrying out the reactionunder solvent free conditions led to an almost quantitative oxidation of 1 to thealdehyde, 2 in 91.6% yield.Yield DMBALD, %100908070605040302010061(a)1 - Acetone2 - CH 2Cl 23 - Toluene4 - MeOAc5 - THF6 - Pentane7 - DEE0 5 10 15 20 25 30 35Time, minFig 2. Effect of the solvent (a) and initial alcohol concentration (b) on the reactionefficiency. Reaction conditions, same as for Fig 1 but the reaction solvent is toluene(20mL).Finding a viable alternative to the KBr co-catalyst23457[3,3-DMBALD], mol/l0.00 15 30 45 60 75A number of soluble oxymetal salts were screened, particularly those of Mo, W, V,Ti and Zr, which are known to act via peroxometal and/or oxometal mechanisms (8).Unfortunately, no alternative to the existing KBr based protocol was found, so it wasdecided to, at least, improve the aqueous buffer system to significantly reduce thevolume of the aqueous phase.When the optimized reference oxidation was run without solvent, using MeO-TEMPO, KBr and NaHCO 3 buffer a 91% aldehyde yield was obtained over a 90 minreaction time (Table 1). In the second run, the NaHCO 3 buffer was replaced withNa 2 BB4O 7 , at ten times lower concentration, using the catalyst composition of MeO-TEMPO/KBr. The aldehyde yield was 95% over the same reaction time. Thisfavorable result was achieved mostly through gains in the product selectivity.Next, the oxidation reaction was run under identical conditions just with aNa 2 B4B O 7 additive and a NaHCO 3 buffer, but, this time, in the absence of the KBr co-6.05.04.03.02.01.0(b)[C]=100%Y = 91.6%[C]=75%Y = 91.8%[C]=50%Y = 90.6%[C]=25%Y = 88.8%[C]=12.5%Y = 82.2%Post Addition Time, min