A study of electronic effects on the kinetics of thermal deamination of N-nitrosoamides.
N-4-R-Benzyl-N-nitrosopivalamides (1a-d; R = MeO, Me, H, NO(2)) were allowed to decompose at 18 degrees C in C(6)D(12), CDCl(3), CD(3)CN, and d(6)-DMSO, and the rates of decomposition were followed by (1)H 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 sigma(p) was linear (R(2) = 0.986) with a rho-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.
Darbeau
R W
RW
Department of Chemistry, McNeese State University, Lake Charles, Louisiana 70609, USA. rdarbeau@mail.mcneese.edu
Pease
R S
RS
Gibble
R E
RE
D000577:Amides / Q000737:chemistry*; D003641:Deamination; D004583:Electrons; D006358:Hot Temperature; D006863:Hydrogen-Ion Concentration; D007202:Indicators and Reagents; D007700:Kinetics; D009682:Magnetic Resonance Spectroscopy; D009603:Nitroso Compounds / Q000737:chemistry*; D012997:Solvents; D013056:Spectrophotometry, Ultraviolet; D017550:Spectroscopy, Fourier Transform Infrared
D000577:Amides; D007202:Indicators and Reagents; D009603:Nitroso Compounds; D012997:Solvents