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)

Abstract

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.

Authors:

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

^[1];

^[1]

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

Publication Date:: Mon Jul 08 00:00:00 EDT 2019

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

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

OSTI Identifier:: 1544423

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

Resource Type:: Accepted Manuscript

Journal Name:: Inorganic Chemistry

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

Publisher:: American Chemical Society (ACS)

Country of Publication:: United States

Language:: English

Subject:: 37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY

Citation Formats


                    Ryan, Michael C., Whitmire, Lauren D., McCann, Scott D., and Stahl, Shannon S. Copper/TEMPO Redox Redux: Analysis of PCET Oxidation of TEMPOH by Copper(II) and the Reaction of TEMPO with Copper(I).  United States: N. p., 2019. 
Web.  doi:10.1021/acs.inorgchem.9b01326.

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                    Ryan, Michael C., Whitmire, Lauren D., McCann, Scott D., & Stahl, Shannon S. Copper/TEMPO Redox Redux: Analysis of PCET Oxidation of TEMPOH by Copper(II) and the Reaction of TEMPO with Copper(I).  United States.  https://doi.org/10.1021/acs.inorgchem.9b01326

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                    Ryan, Michael C., Whitmire, Lauren D., McCann, Scott D., and Stahl, Shannon S. Mon .  
"Copper/TEMPO Redox Redux: Analysis of PCET Oxidation of TEMPOH by Copper(II) and the Reaction of TEMPO with Copper(I)".  United States.  https://doi.org/10.1021/acs.inorgchem.9b01326.  https://www.osti.gov/servlets/purl/1544423.

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@article{osti_1544423,

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

  author       = {Ryan, Michael C. and Whitmire, Lauren D. and McCann, Scott D. and Stahl, Shannon S.},

  abstractNote = {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: CuIIX2 + TEMPOH → CuIX + TEMPO + HX. In the reaction with acetate, yet, slow reoxidation of CuIOAc 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 CuII salts suggests that this process could contribute to catalyst regeneration under aerobic oxidation conditions.},

  doi          = {10.1021/acs.inorgchem.9b01326},

  journal      = {Inorganic Chemistry},

  number       = 15,

  volume       = 58,

  place        = {United States},

  year         = {Mon Jul 08 00:00:00 EDT 2019},

  month        = {Mon Jul 08 00:00:00 EDT 2019}

}

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Cited by: 24 works

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Scheme 1: Simplified Mechanism for Cu/TEMPO-Catalyzed Alcohol Oxidation.

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