Journal
JOURNAL OF THE AMERICAN CHEMICAL SOCIETY
Volume 135, Issue 6, Pages 2357-2367Publisher
AMER CHEMICAL SOC
DOI: 10.1021/ja3117203
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Funding
- DOE [DE-FG02-05ER15690]
- ACS GCI Pharmaceutical Roundtable
- Camille and Henry Dreyfus Postdoctoral Program in Environmental Chemistry
- NIH [CBIT NIGMS T32 GM008505, S10 RR08389]
- NSF [CHE-9208463, CHE-9629688]
- Division Of Chemistry
- Direct For Mathematical & Physical Scien [0946901] Funding Source: National Science Foundation
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Homogeneous Cu/TEMPO catalyst systems (TEMPO = 2,2,6,6-tetramethylpiperidine-N-oxyl) have emerged as some of the most versatile and practical catalysts for aerobic alcohol oxidation. Recently, we disclosed a (bpy)Cu-I/TEMPO/NMI catalyst system (NMI = N-methylimidazole) that exhibits fast rates and high selectivities, even with unactivated aliphatic alcohols. Here, we present a mechanistic investigation of this catalyst system, in which we compare the reactivity of benzylic and aliphatic alcohols. This work includes analysis of catalytic rates by gas-uptake and in situ IR kinetic methods and characterization of the catalyst speciation during the reaction by EPR and UV-visible spectroscopic methods. The data support a two-stage catalytic mechanism consisting of (1) catalyst oxidation in which Cu-I and TEMPO-H are oxidized by O-2 via a binuclear Cu2O2 intermediate and (2) substrate oxidation mediated by Cu-II and the nitroxyl radical of TEMPO via a Cu-II-alkoxide intermediate. Catalytic rate laws, kinetic isotope effects, and spectroscopic data show that reactions of benzylic and aliphatic alcohols have different turnover-limiting steps. Catalyst oxidation by O-2 is turnover limiting with benzylic alcohols, while numerous steps contribute to the turnover rate in the oxidation of aliphatic alcohols.
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