- Persistent hydrogen-bonded and non-hydrogen-bonded phenoxyl radicals
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The production of stable phenoxyl radicals is undoubtedly a synthetic chemical challenge. Yet it is a useful way to gain information on the properties of the biological tyrosyl radicals. Recently, several persistent phenoxyl radicals have been reported, but only limited synthetic variations could be achieved. Herein, we show that the amide-o-substituted phenoxyl radical (i.e. with a salicylamide backbone) can be synthesised in a stable manner, thereby permitting easy synthetic modifications to be made through the amide bond. To study the effect of H-bonding on the properties of the phenolate/phenoxyl radical redox couple, simple H-bonded and non-H-bonded o,p-tBu-protected salicylamidate compounds have been prepared. Their redox properties were examined by cyclic voltammetry and showed a fully reversible one-electron oxidation process to the corresponding phenoxyl radical species. Remarkably, the redox potential appears to be correlated, at least partially, with H-bond strength, as relatively large differences (ca. 300 mV) in the redox potential between H-bonded and non-H-bonded phenolate salts are observed. The corresponding phenoxyl radicals produced electrochemically are persistent at room temperature for at least an hour; their UV/Vis and EPR characterisation is consistent with that of phenoxyl radicals, which makes them excellent models of biological tyrosyl radicals. The analyses of the experimental data coupled with theoretical calculations indicate that both the deviation from planarity of the amide function and intramolecular H-bonding influence the oxidation potential of the phenolate. The latter H-bonding effect appears to be predominantly exerted on the phenolate and not (or only a little) on the phenoxyl radical. Thus, in these systems the H-bonding energy involved in the phenoxyl radical appears to be relatively small.
- Wanke, Riccardo,Benisvy, Laurent,Kuznetsov, Maxim L.,Guedes Da Silva, M. Fatima C.,Pombeiro, Armando J. L.
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experimental part
p. 11882 - 11892
(2011/11/29)
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- Synthesis of highly hindered polyanionic chelating ligands
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Practical and efficient protocols to obtain highly hindered polyanionic chelating ligands based on bis-(3,5-di-tert-butyl-2-hydroxybenzamido) compounds are reported here. N-3,5-di-tert-Butylsalicyloyloxysuccinimide was treated with aliphatic diamines to form aliphatic hydrocarbon-linked bis-amides 4a-4g. Aromatic diamines required more powerful electrophile, thus the corresponding benzylated acid chloride was used to form aromatic hydrocarbon-linked bis-amides 8a-8d. The yields ranged from good to very good and showed that choosing the right acylating agent is a key point in this synthesis. All the compounds were characterized by elemental analysis, IR, MS and NMR.
- Jiménez, Claudio A.,Belmar, Julio B.
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p. 3933 - 3938
(2007/10/03)
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