Molecular structure of nitrophenyl O-glycosides in relation to their redox potentials

• Published in: Journal of physical organic chemistry 2010-09, Vol.23 (9), p.853-858
• Main Authors: Gubica, Tomasz, Temeriusz, Andrzej, Pawłowski, Paweł, Stroka, Jadwiga
• Format: Article
• Language: English
• Subjects: cyclic voltammetry; electroreduction; mercury electrode; nitrophenyl O-glycosides
• ISSN: 0894-3230; 1099-1395
• DOI: 10.1002/poc.1673

The effect of the non‐electroactive groups on the redox potentials of the active centres of 26 nitrophenyl O‐glycosides possessing various substituents has been studied electrochemically using cyclic voltammetry. The potentials of both redox processes, a two‐electron quasi‐reversible R‐NHOH/R‐NO (Ef) and four‐electron irreversible R‐NO2/R‐NHOH (Epc(I)) systems, have been determined and compared for all the compounds under investigations. The nitrophenyl O‐glycosides were chosen as model compounds as they significantly vary in many aspects of their structure such as: (i) the isomeric substitution of nitro group in benzene ring to the sugar moiety (ortho, meta and para isomers); (ii) the size of sugar moieties (the derivatives of mono‐ and disaccharides); (iii) the presence and absence of additional groups in saccharidic fragments (e.g. pentose and hexose); (iv) functionalisation of hydroxyl groups (free or acetylated hydroxyl groups) and (v) absolute configurations of selected sugar carbon atoms (e.g. the pairs of anomers). Among other effects, a significant variation in the increasing order of the two‐electron quasi‐reversible (Ef, ortho > meta > para) and four‐electron irreversible (Epc(I), meta > ortho > para) redox processes has been found and explained taking into account the negative inductive effect (–I) caused by the glycosidic oxygen atom that facilitates the electroreduction of the nitro group, and the positive mesomeric effect (+M) which makes the electroreduction more difficult. Copyright © 2010 John Wiley & Sons, Ltd. This paper describes a new approach to the electrochemistry of large molecules – the effect of the non‐electroactive large fragments of the electroactive molecules on their electrochemical properties. Even subtle differences in the structure of the non‐electroactive fragments, e.g. stereochemistry, influence the redox potentials of the 26 nitrophenyl O‐glycosides in a different way, depending on the type of the redox process, either 2e− quasi‐reversible or 4e− irreversible.