Catalysis of Organic..

Tanielyan et al. 123Fig 1a shows the near first order relationship between the MeO-TEMPOconcentration and the reaction rate (curve 3) and at the same time it shows a gradualdeterioration of the reaction selectivity to the aldehyde 2 at higher MeO-TEMPOconcentrations in the region 0.5-1.0 mmol (3 - 6 mol %), (curve 2). The only productdetected along with the desired aldehyde 2 is the product of the over-oxidation,hexanoic acid.In Fig 1b, the [Br - ]/[MeO-TEMPO] ratio was varied over a relatively wide rangeat equimolar concentrations for MeO-TEMPO and Mg(NO 3 ) 2 of 0.48mmol (3mol%). Three distinct regions are clearly seen: i) a region of low Br concentrationwhere increasing the NBS concentration leads to dramatic rate increase and wherethe conversion of 1, measured at 60 min reaction time reached 100% at selectivity to2 of 97-98%, ii) a region, where increasing the concentration of the NBS did notproduce a significant change in the reaction effectiveness and iii) a region, wherefurther increase in the [Br]/[MeO-TEMPO] ratio led to a rate reduction (curve 3).O RCHO2NO 2 BrO -+NH +E D C B O AHO -1/2O 2NORCH 2OHBr - NScheme 2.In the next set of experiments, the MeO-TEMPO concentration was fixed at 0.48mmol (3 mol%) and the NBS concentration at 0.048 mmol (0.3 mol%) while theMg(NO 3 ) 2 / [MeO-TEMPO] ratio was gradually increased in the range shown inFigure 1c. The first region of relatively low concentration of [Mg(NO 3 ) 2 ] ischaracterized by a first order relationship between the rate of oxidation and thenitrate concentration and in the same region, at Mg(NO 3 ) 2 / [MeO-TEMPO] ≅ 1, acomplete conversion of 1 was observed with high aldehyde selectivity. Furtherincrease in the nitrate concentration, however, leads to severe inhibition, resulting ingradual deactivation of the catalyst system and rapid decline in the reaction rates(curve 2 and 3).At the current initial concentrations of 1 in the acetic acid solvent, (1.33M, 16.6v/v %), the optimum ratio between the three components of the catalyst systemseams to be MeO-TEMPO:Mg(NO 3 ) 2 :NBS = 1:1:0.1. As we will show in the nextsection, the higher the initial concentration of the alcohol substrate, the more NBS isneeded for achieving high reaction rates in the selective oxidation of 1 to 2.A plausible reaction sequence including a multi step cascade of four redoxcycles is shown in Scheme 2. Since we have no experimental data to support eitherthe nature of the intermediate species involved or their oxidation state, nor have weOH