title: Mechanism of Copper/Azodicarboxylate-Catalyzed Aerobic Alcohol Oxidation: Evidence for Uncooperative Catalysis.
abstract: Cooperative catalysis between Cu(II) and redox-active organic cocatalysts is a key feature of important chemical and enzymatic aerobic oxidation reactions, such as alcohol oxidation mediated by Cu/TEMPO and galactose oxidase. Nearly 20 years ago, Markó and co-workers reported that azodicarboxylates, such as di-tert-butyl azodicarboxylate (DBAD), are effective redox-active cocatalysts in Cu-catalyzed aerobic alcohol oxidation reactions [Markó, I. E., et al. Science 1996, 274, 2044], but the nature of the cooperativity between Cu and azodicarboxylates is not well understood. Here, we report a mechanistic study of Cu/DBAD-catalyzed aerobic alcohol oxidation. In situ infrared spectroscopic studies reveal a burst of product formation prior to steady-state catalysis, and gas-uptake measurements show that no O2 is consumed during the burst. Kinetic studies reveal that the anaerobic burst and steady-state turnover have different rate laws. The steady-state rate does not depend on [O2] or [DBAD]. These results, together with other EPR and in situ IR spectroscopic and kinetic isotope effect studies, reveal that the steady-state mechanism consists of two interdependent catalytic cycles that operate in sequence: a fast Cu(II)/DBAD pathway, in which DBAD serves as the oxidant, and a slow Cu(II)-only pathway, in which Cu(II) is the oxidant. This study provides significant insight into the redox cooperativity, or lack thereof, between Cu and redox-active organic cocatalysts in aerobic oxidation reactions.
authors.affiliation: Department of Chemistry, University of Wisconsin-Madison , 1101 University Avenue, Madison, Wisconsin 53706, United States.
authors.forename: Scott D
authors.initials: SD
authors.lastname: McCann
authors.affiliation: Department of Chemistry, University of Wisconsin-Madison , 1101 University Avenue, Madison, Wisconsin 53706, United States.
authors.forename: Shannon S
authors.initials: SS
authors.lastname: Stahl
mesh_terms: D000332:Aerobiosis; D000438:Alcohols / Q000737:chemistry*; D001391:Azo Compounds / Q000737:chemistry*; D002264:Carboxylic Acids / Q000737:chemistry*; D002384:Catalysis; D003300:Copper / Q000737:chemistry*; D007700:Kinetics; D010084:Oxidation-Reduction
chemical_list: D000438:Alcohols; D001391:Azo Compounds; D002264:Carboxylic Acids; D003300:Copper