Kinetic isotope effects aerobic oxidation

Various mechanisms for the aerobic oxidation of alcohols catalysed by (NHC)Pd (carboxylate)2(H20) complexes [NHC = l,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene] were investigated using DFT combined with a solvent model. Of these, reductive j3-hydride elimination, in which the -hydrogen of a palladium-bound alkoxide is transferred directly to the free oxygen of the bound carboxylate, provided the lowest-energy route and explained the published kinetic isotope effect, activation enthalpy, reaction orders, and dependence of rate on carboxylate pKa.26S... 

Chiral (nitrosyl)ruthenium(salen) complexes have been found to be efficient catalysts for aerobic oxidative desymmetrization of mc.vo-diols under photoirradiation to give optically active lactols. With the suitable catalysts, high enantioselectivities up to 93% has been achieved. The kinetics of the oxidation depend on the nature of the ligand. On the basis of kinetic parameters and the kinetic isotope effect, it has been suggested that when a ligand with an apical hydroxy group is used, the hydrogen atom... 

The kinetics of the aerobic oxidation of alcohols catalysed by Pd(OAc)2-triethylamine have been studied experimentally and computationally. Measurement of various kinetic isotope effects and the activation parameters and also rate law derivation support a rate-limiting deprotonation of the palladium-coordinated alcohol, contrary to the previously proposed rate-limiting /3-hydride elimination.234 The catalytic efficiency of Pd(OAc)2-triethylamine and palladium alkoxides in the aerobic oxidation of alcohols has been evaluated. A new catalyst, Pd(IiPr)(OPiv)2, is found to operate efficiently at room temperature.235... 

SemmeUiack et al. [104] reported that the combination of CuCl and 4-hydroxy TEMPO catalyzes the aerobic oxidation of alcohols. However, the scope was limited to active benzyhc and allylic alcohols and activities were low (10 mol% of catalyst was needed for smooth reaction). They proposed that the copper catalyzes the reoxidation of TEMPO to the oxoammonium cation. Based on our results with the Ru/TEMPO system we doubted the validity of this mechanism. Hence, we subjected the Cu/ TEMPO to the same mechanistic studies described above for the Ru/TEMPO system [105]. The results of stoichiometric experiments under anaerobic conditions, Hammett correlations and kinetic isotope effect studies showed a similar pattern to those with the Ru/TEMPO system, i.e., they are inconsistent with a mechanism involving an oxoammonium species as the active oxidant. Hence, we propose the mechanism shown in Scheme 4.18 for Cu /TEM PO-catalyzed aerobic oxidation of alcohols. 

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