A Study of Electronic Effects on the Kinetics of Thermal Deamination of N-Nitrosoamides

• Published in: Journal of organic chemistry 2001-07, Vol.66 (15), p.5027-5032
• Main Authors: Darbeau, Ron W, Pease, Rebecca S, Gibble, Rebekah E
• Format: Article
• Language: English
• Subjects: Amides - chemistry; Deamination; Electrons; Hot Temperature; Hydrogen-Ion Concentration; Indicators and Reagents; Kinetics; Magnetic Resonance Spectroscopy; Nitroso Compounds - chemistry; Solvents; Spectrophotometry, Ultraviolet; Spectroscopy, Fourier Transform Infrared
• ISSN: 0022-3263; 1520-6904
• DOI: 10.1021/jo001741l

N-4-R-Benzyl-N-nitrosopivalamides (1a−d; R = MeO, Me, H, NO2) were allowed to decompose at 18 °C in C6D12, CDCl3, CD3CN, and d 6-DMSO, and the rates of decomposition were followed by 1H NMR spectroscopy. The half-lives of the nitrosoamides were found to vary in a systematic way with the nature of the R group on the aromatic nucleus. Electron-releasing groups were found to decrease the stability of the starting nitrosoamide, whereas electron-withdrawing ones increased the nitrosoamides' thermal stability. A Hammett-type plot of log(rate constants of deamination) vs σp was linear (R2 = 0.986) with a ρ-type value of −0.90 indicating development of significant positive charge at the benzylic position in the transition state of the rate-determining step. The thermal stability of the nitrosoamides was also found to be systematically affected by the polarity of the solvent:  as the solvent polarity increased, so did the lability of the nitrosoamides. This observation of intra- and intermolecular electronic perturbations of the kinetics of nitrosoamide decomposition appears to be novel. A closer look at the rate-determining step of nitrosoamide thermolysis is made, and a mechanistic framework is proposed that accounts for both steric and electronic modulation of nitrosoamide stability as well as the greater thermal stabilities of the related N-nitrocarboxamides and N-nitrosotosylamides.