Proton Transfer in the Copper(II)—Tetrapyridine Catalyzed Oxidation of Acetol (Monohydroxyacetone)

The biomimetic complex copper(II)-tetrapyridine, a nonenzymic analogue of the enzyme galactose oxidase, catalyzes the oxidation of a-keto alcohols by O₂. Classical mechanistic organic protocols have been used to study this reaction by using acetol (monohydroxyacetone) as substrate. Upon the basis of steady-state kinetic studies performed at various oxygen concentrations a kinetic mechanism is proposed in which alcohol substrate binds to catalyst, C; this complex then reacts with oxygen in a kinetically bimolecular reaction yielding catalyst and products. These data can be fit to the rate expression » = k₃[C] [O₂] [S]/{(k₂+ k₃[O₂])/k_}+ [S]| Detailed analysis of the oxygen dependence of the reaction at various pH values yielded the magnitude of rate constants k_uk₂, and k$ and their pH dependence. The reaction exhibited an ascending pH-rate profile. In the presence of excess pyridine, the pAT_cs(equilibrium constant) associated with this pH dependence is 5.51 ± 0.05. In a solvent containing D₂O a shift in this equilibrium (dissociation) constant (ApAT_cs= +0.59) was observed. In addition, there was a kinetic solvent isotope effect (KSIE = 1.15 ± 0.02). A proton inventory was performed indicating that a single hydrogenic site contributed to this KSIE. Additional kinetic experiments were performed examining catalysis by reaction mixtures lacking either excess pyridine, Cu(II), or both excess pyridine and Cu(II). Visual absorbance and ESR spectral studies were performed, examining the state of the Cu(II)-Pyr₄ complex in the presence and absence of excess pyridine at various pH values. The data indicate that Cu(II)-Pyr₄ catalysis of acetol oxidation involves three pH-separable pathways, two of which include concerted catalysis by free (excess) pyridine.