Chemically Triggered C–ON Bond Homolysis of Alkoxyamines. 5. Cybotactic Effect

• Published in: Journal of organic chemistry Vol. 77; no. 21; pp. 9634 - 9640
• Main Authors: Audran, Gérard, Brémond, Paul, Marque, Sylvain R. A, Obame, Germain
• Format: Journal Article
• Language: English
• Published: Washington, DC American Chemical Society 02.11.2012
• Subjects: Chemical Sciences; Chemistry; diastereomers; Exact sciences and technology; Free radicals chemistry; General and physical chemistry; Heterocyclic compounds; Heterocyclic compounds with only one n hetero atom and condensed derivatives; homolytic cleavage; hydrogen bonding; Kinetics and mechanisms; nitrogen; Organic chemistry; Preparations and properties; Reactivity and mechanisms; Solutions; Solvation. Solvent properties; solvents; Solvent effect; Correlation; Nitroxyl; Polarizability; Hyperfine coupling constant; Nitrogen heterocycle; Stabilization; Organic phosphonate; Free radical reaction; Phosphorus Organic compounds; Pyridine derivatives; Amine; Solvation; Counter ion; Polarity; Homolysis; Rate constant; Solvent; Diastereomer; Hydrogen bond
• ISSN: 0022-3263; 1520-6904; 1520-6904
• DOI: 10.1021/jo3016665

In a recent work ( Org. Lett. 2012, 14, 358 ), we showed that the rate constant k d of the chemically activated C–ON bond homolysis of alkoxyamines was subject to solvent effects. However, we showed that solvent effects were weak for the nonactivated alkoxyamine 1 (diethyl (1-(tert-butyl­(1-(pyridin-4-yl)­ethoxy)­amino)-2,2-dimethylpropyl)­phosphonate) and its N+O– oxide activated version 3. On the other hand, the activated N-methylated version 2 of 1 experienced a strong solvent effect for a radical reaction, i.e., a 24-fold increase in k d from tert-butylbenzene (tBuPh) to 2,2,2-trifluoroethanol (TFE). Good correlations were observed with the normalized Reichardt solvent polarity constant E T N and the nitrogen hyperfine coupling constant of the released nitroxide a N,SG1, meaning that the stabilization of the nitroxide played an important role in C–ON bond homolysis in alkoxyamines. The Kalmet–Abboud--Taft relationships described successfully the solvent effect for each diastereoismer of 1 and 2, as for example with the minor diastereoisomer of 1 log­(k d′/s–1) = −4.84 + 0.37π* + 0.21α and that of 2 log­(k d′/s–1) = −3.11 + 0.37π* + 0.47α, with π* being the polarity/polarizability and α the hydrogen bond donor (HBD) ability of the solvent. Surprisingly, the HBD effect is larger for 2 than for 1, whereas no extra lone pair is available in 2. This amazing effect was ascribed to the solvation of the counteranion, which is expected to be better solvated in a HBD solvent.