Copper/TEMPO Redox Redux: Analysis of PCET Oxidation of TEMPOH by Copper(II) and the Reaction of TEMPO with Copper(I)

Ryan, Michael C.; Whitmire, Lauren D.; McCann, Scott D.; Stahl, Shannon S.

doi:10.1021/acs.inorgchem.9b01326

Title: Copper/TEMPO Redox Redux: Analysis of PCET Oxidation of TEMPOH by Copper(II) and the Reaction of TEMPO with Copper(I)

Journal Article · Mon Jul 08 00:00:00 EDT 2019 · Inorganic Chemistry

DOI:https://doi.org/10.1021/acs.inorgchem.9b01326· OSTI ID:1544423

Ryan, Michael C. ^[1]; Whitmire, Lauren D. ^[1];

^[1];

^[1]

Univ. of Wisconsin, Madison, WI (United States)

Copper salts and organic aminoxyls, such as TEMPO (2,2,6,6- tetramethylpiperidine N-oxyl), are versatile catalysts for aerobic alcohol oxidation. Previous reports in the literature contain conflicting proposals concerning the redox interactions that take place between copper(I) and copper(II) salts with the aminoxyl and hydroxylamine species, TEMPO and TEMPOH, respectively. In this work, we reinvestigate these reactions in an effort to resolve the conflicting claims in the literature. Under anaerobic conditions, CuIIX2 salts [X= acetate (OAc), trifluoroacetate (TFA), triflate (OTf)] are shown to promote rapid proton-coupled oxidation of TEMPOH to TEMPO: Cu^IIX₂ + TEMPOH → Cu^IX + TEMPO + HX. In the reaction with acetate, yet, slow reoxidation of Cu^IOAc occurs. This process requires both TEMPO and HOAc and coincides with reduction of TEMPO to 2,2,6,6-tetramethylpiperidine. Analogous reactivity is not observed with trifluoroacetate and triflate species. As a whole, the facility of proton-coupled oxidation of TEMPOH by Cu^II salts suggests that this process could contribute to catalyst regeneration under aerobic oxidation conditions.

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Research Organization:: Univ. of Wisconsin, Madison, WI (United States)

Sponsoring Organization:: USDOE Office of Science (SC), Basic Energy Sciences (BES); National Institutes of Health (NIH); National Science Foundation (NSF)

Grant/Contract Number:: FG02-05ER15690; CHE-1048642; CHE-0741901

OSTI ID:: 1544423

Journal Information:: Inorganic Chemistry, Vol. 58, Issue 15; Related Information: Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.9b01326.; ISSN 0020-1669