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After investigating what makes the persistent radical TEMPO stable, I was wondering if another compound which is more feasible for my needs would be comparable to it.

Would oxidizing 2,2’-dimethyl-diphenylamine result in a compound similar to TEMPO? It will have an Aminoxyl group and have the steric hindrance of the methyl group next to it. By similar I mean if it will have a stable radical in the aminoxyl group.

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    $\begingroup$ Electronically it is a rather different species with the phenyl groups in conjugation. $\endgroup$ – Waylander Feb 25 at 19:10
  • $\begingroup$ By similar I mean if it will have a persistent/stable radical on the aminoxyl group $\endgroup$ – Evamentality Feb 25 at 20:42
  • $\begingroup$ This doesn't seem like the best question to ask here. I think a literature search would be more comprehensive. $\endgroup$ – Zhe Feb 25 at 21:27
  • $\begingroup$ @Evamentality - being electronically different will greatly affect the stability of the radical you propose. $\endgroup$ – Waylander Feb 25 at 22:10
  • $\begingroup$ I have tried to search for information regarding this compound and I can’t seem to find any on this specific configuration. The phenyl groups should make this more stable due to resonance, however organic chemistry can always surprise you $\endgroup$ – Evamentality Feb 25 at 23:47
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Aminoxyl (trivial name: a nitroxyl radical or a nitroxide) denotes a radical functional group with general structure $\ce{R2N–O^•}$. It is well known that sterically unhindered aminoxyls baring $\alpha$‐hydrogens are unstable and undergo rapid disproportionation to nitrones and hydroxylamines (Ref.1: A new mechanism for the decomposition of $\alpha$‐hydrogen nitroxides is proposed). For example, decomposition of one of such compound is displayed below (The scheme is from Ref.1):

A new mechanism for the decomposition of nitroxides

Sterically hindered aminoxyls without $\alpha$‐hydrogens such as TEMPO are stable radicals. However, it is not clear that steric hindrance at carbon $\alpha$ to nitrogen atom is important in the mechanism. Yet, the lone pair of electrons on nitrogen has played a significant role in the mechanism (Ref.1). Therefore, I speculate that 2,2'-dimethyldiphenylamine, when oxidized to relevant aminoxyl radical, may not be as effective as TEMPO, mainly due to the delocalization of the lone pair of electrons on nitrogen between two 2-methylphenyl groups.

Nonetheless, there are few available aminoxyl radicals, which are more efficient than TEMPO (Both following images are from Ref.2):

nitroxyl radicals

Efficiencies of nitroxyl radicals

The research in Ref.2 has performed catalytic oxidation using few N-oxyl radicals, namely 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO), 2-azaadamantane N-oxyl (AZADO), 1-methyl-AZADO (1-Me-AZADO), 9-azanoradamantane N-oxyl (Nor-AZADO), and 1,5-dimethyl-9-azanoradamantane N-oxyl (DMN-AZADO), in water at $\mathrm{pH} \ 10$ on wood cellulose to determine the influence of different N-oxyl radical chemical structures on the reaction kinetics and structures of oxidized celluloses. The reaction time required for complete oxidation of wood cellulose using AZADO, 1-Me-AZADO, or Nor-AZADO was less than $\pu{20 min}$, while DMN-AZADO and TEMPO required $\pu{70 min}$ and $\pu{120 min}$, respectively.

There is also some comparative work has been done in Ref.3, which also showed the superiority of other N-oxyl radicals over TEMPO (Synthetic procedure to prepare Nor-AZADO has been given in the reference; see attached PDF).

References:

  1. Aaron Nilsen, Rebecca Braslau, "Nitroxide decomposition: Implications toward nitroxide design for applications in living free‐radical polymerization," J. Polymer Sci., Part A: Polymer Chem. 2006, 44(2), 697-717 (DOI:https://doi.org/10.1002/pola.21207).
  2. Hiromasa Hondo, Tsuguyuki Saito, Akira Isogai, "Preparation of oxidized celluloses in a $\ce{NaBr/NaClO}$ system using 2-azaadamantane N-oxyl (AZADO) derivatives in water at $\mathrm{pH} \ 10$," Cellulose 2019, 26, 1479–1487 (DOI:10.1007/s10570-018-2177-5).
  3. Masaki Hayashi, Yusuke Sasano, Shota Nagasawa, Masatoshi Shibuya, Yoshiharu Iwabuchi, "9-Azanoradamantane N-oxyl (Nor-AZADO): a highly active organocatalyst for alcohol oxidation," Chem. Pharm. Bull. 2011, 59(12), 1570–1573 (DOI: https://doi.org/10.1248/cpb.59.1570)(PDF).
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    $\begingroup$ Cool, but what about diaryl radicals? I think what OP mentions isn't nearly as good, but still... $\endgroup$ – Mithoron Feb 26 at 16:41
  • $\begingroup$ @Mithoron: I'd agree Waylander on his comment. It won't be nearly as good as TEMPO. OP is looking for another compound which is more feasible for his/her needs and would be comparable to TEMPO. Diaryl compound is one of compound he/her wonder. $\endgroup$ – Mathew Mahindaratne Feb 26 at 17:25
  • $\begingroup$ Thing is OP's issue is if the one he mentions (or similar, I guess) is viable replacement - perhaps he has this amine already - not what's actually better then TEMPO. $\endgroup$ – Mithoron Feb 26 at 18:29
  • $\begingroup$ @Mithoron: See my addition to the answer. $\endgroup$ – Mathew Mahindaratne Feb 26 at 20:56
  • $\begingroup$ Thanks for the contribution (Love being called OP). Attaching TEMPO to my compound is not feasible due to the synthesis route meetings things, so I was wondering if this could work in creating a stable aminoxyl radical. It doesn't have to be as efficient for the time being. As a matter of fact, the diary in my compound is actually part of a much bigger aromatic chain, but I simplified it to two benzene rings. The main idea is to see how it operates. I will improve the efficiency when I cross that bridge, now I just need to see if the proof of concept is solid. $\endgroup$ – Evamentality Feb 27 at 0:36

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