ja5b09940_si_001.pdf (3.57 MB)
Mechanism of Copper/Azodicarboxylate-Catalyzed Aerobic Alcohol Oxidation: Evidence for Uncooperative Catalysis
journal contribution
posted on 2016-01-13, 00:00 authored by Scott
D. McCann, Shannon S. StahlCooperative
catalysis between CuII 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 CuII/DBAD pathway, in
which DBAD serves as the oxidant, and a slow CuII-only
pathway, in which CuII 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.