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Oxidation of 4-substituted TEMPO derivatives reveals modifications at the 1- and 4-positions

Journal Article


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Abstract


  • Potenital pathways for the deactivation of hindered amine light stabilisers (HALS) have been investigated by observing reactions of model compounds - based on 4-substituted derivatives of 2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPO) - with hydroxyl radicals. In these reactions, dilute aqueous suspensions of photocatalytic nanoparticulate titanium dioxide were irradiated with UV light in the presence of water-soluble TEMPO derivatives. Electron spin resonance (ESR) and electrospray ionisation mass-spectrometry (ESI-MS) data were acquired to provide complementary structural elucidation of the odd- and even-electron products of these reactions and both techniques show evidence for the formation of 4-oxo-TEMPO (TEMPONE). TEMPONE formation from the 4-substituted TEMPO compounds is proposed to be initiated by hydrogen abstraction at the 4-position by hydroxyl radical. High-level ab initio calculations reveal a thermodynamic preference for abstraction of this hydrogen but computed activation barriers indicate that, although viable, it is less favoured than hydrogen abstraction from elsewhere on the TEMPO scaffold. If a radical is formed at the 4-position however, calculations elucidate two reaction pathways leading to TEMPONE following combination with either a second hydroxyl radical or dioxygen. An alternate mechanism for conversion of TEMPOL to TEMPONE via an alkoxyl radical intermediate is also considered and found to be competitive with the other pathways. ESI-MS analysis also shows an increased abundance of analogous 4-substituted piperidines during the course of irradiation, suggesting competitive modification at the 1-position to produce a secondary amine. This modification is confirmed by characteristic fragmentation patterns of the ionised piperidines obtained by tandem mass spectrometry. The conclusions describe how reaction at the 4-position could be responsible for the gradual depletion of HALS in pigmented surface coatings and secondly, that modification at nitrogen to form the corresponding secondary amine species may play a greater role in the stabilisation mechanisms of HALS than previously considered. © The Royal Society of Chemistry 2011.

Authors


  •   Marshall, David L. (external author)
  •   Christian, M L. (external author)
  •   Gryn'ova, Ganna (external author)
  •   Coote, M L. (external author)
  •   Barker, Phil J.
  •   Blanksby, Stephen J. (external author)

Publication Date


  • 2011

Citation


  • Marshall, D. L., Christian, M. L., Gryn'ova, G., Coote, M. L., Barker, P. J. & Blanksby, S. J. (2011). Oxidation of 4-substituted TEMPO derivatives reveals modifications at the 1- and 4-positions. Organic and Biomolecular Chemistry, 9 (13), 4936-4947.

Scopus Eid


  • 2-s2.0-79959785773

Ro Full-text Url


  • http://ro.uow.edu.au/cgi/viewcontent.cgi?article=7422&context=scipapers

Ro Metadata Url


  • http://ro.uow.edu.au/scipapers/4080

Number Of Pages


  • 11

Start Page


  • 4936

End Page


  • 4947

Volume


  • 9

Issue


  • 13

Abstract


  • Potenital pathways for the deactivation of hindered amine light stabilisers (HALS) have been investigated by observing reactions of model compounds - based on 4-substituted derivatives of 2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPO) - with hydroxyl radicals. In these reactions, dilute aqueous suspensions of photocatalytic nanoparticulate titanium dioxide were irradiated with UV light in the presence of water-soluble TEMPO derivatives. Electron spin resonance (ESR) and electrospray ionisation mass-spectrometry (ESI-MS) data were acquired to provide complementary structural elucidation of the odd- and even-electron products of these reactions and both techniques show evidence for the formation of 4-oxo-TEMPO (TEMPONE). TEMPONE formation from the 4-substituted TEMPO compounds is proposed to be initiated by hydrogen abstraction at the 4-position by hydroxyl radical. High-level ab initio calculations reveal a thermodynamic preference for abstraction of this hydrogen but computed activation barriers indicate that, although viable, it is less favoured than hydrogen abstraction from elsewhere on the TEMPO scaffold. If a radical is formed at the 4-position however, calculations elucidate two reaction pathways leading to TEMPONE following combination with either a second hydroxyl radical or dioxygen. An alternate mechanism for conversion of TEMPOL to TEMPONE via an alkoxyl radical intermediate is also considered and found to be competitive with the other pathways. ESI-MS analysis also shows an increased abundance of analogous 4-substituted piperidines during the course of irradiation, suggesting competitive modification at the 1-position to produce a secondary amine. This modification is confirmed by characteristic fragmentation patterns of the ionised piperidines obtained by tandem mass spectrometry. The conclusions describe how reaction at the 4-position could be responsible for the gradual depletion of HALS in pigmented surface coatings and secondly, that modification at nitrogen to form the corresponding secondary amine species may play a greater role in the stabilisation mechanisms of HALS than previously considered. © The Royal Society of Chemistry 2011.

Authors


  •   Marshall, David L. (external author)
  •   Christian, M L. (external author)
  •   Gryn'ova, Ganna (external author)
  •   Coote, M L. (external author)
  •   Barker, Phil J.
  •   Blanksby, Stephen J. (external author)

Publication Date


  • 2011

Citation


  • Marshall, D. L., Christian, M. L., Gryn'ova, G., Coote, M. L., Barker, P. J. & Blanksby, S. J. (2011). Oxidation of 4-substituted TEMPO derivatives reveals modifications at the 1- and 4-positions. Organic and Biomolecular Chemistry, 9 (13), 4936-4947.

Scopus Eid


  • 2-s2.0-79959785773

Ro Full-text Url


  • http://ro.uow.edu.au/cgi/viewcontent.cgi?article=7422&context=scipapers

Ro Metadata Url


  • http://ro.uow.edu.au/scipapers/4080

Number Of Pages


  • 11

Start Page


  • 4936

End Page


  • 4947

Volume


  • 9

Issue


  • 13