ARC Centre of Excellence for Free Radical Chemistry and Biotechnology, School of Chemistry, University of Wollongong, Northfields Ave, Wollongong, NSW 2522, Australia.
Org Biomol Chem. 2011 Jul 7;9(13):4936-47. doi: 10.1039/c1ob05037k. Epub 2011 May 19.
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.
受阻胺光稳定剂（HALS）的失活途径已通过观察模型化合物的反应来研究，这些模型化合物基于 4-取代的 2,2,6,6-四甲基哌啶-N-氧自由基（TEMPO）衍生物。在这些反应中，在存在水溶性 TEMPO 衍生物的情况下，用紫外光辐照含有光催化纳米颗粒二氧化钛的稀水溶液。电子自旋共振（ESR）和电喷雾电离质谱（ESI-MS）数据的获取提供了这些反应的奇数和偶数电子产物的结构阐明的补充，并证明了 4-氧-TEMPO（TEMPONE）的形成。TEMPONE 是由 4-取代的 TEMPO 化合物形成的，推测是由羟基自由基在 4-位上的氢提取引发的。高水平的从头计算表明，这种氢的提取具有热力学偏好，但计算出的活化能表明，尽管可行，但它不如 TEMPO 支架上其他位置的氢提取有利。然而，如果在 4-位形成自由基，则计算阐明了两种反应途径，即与第二个羟基自由基或氧气结合后导致 TEMPONE 的形成。还考虑了通过烷氧基自由基中间体将 TEMPOL 转化为 TEMPONE 的替代机制，并发现其与其他途径竞争。ESI-MS 分析还显示，在辐照过程中类似的 4-取代哌啶的丰度增加，表明在 1-位进行竞争性修饰以产生仲胺。通过串联质谱获得的离子化哌啶的特征碎片模式证实了这种修饰。结论描述了 4-位的反应如何导致 HALS 在着色表面涂层中的逐渐耗尽，其次，氮的修饰形成相应的仲胺物种可能在 HALS 的稳定机制中发挥比以前认为的更大的作用。
