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, NO2) were allowed to decompose at 18 degreesC in C6D12, CDCl3, CD3CN, and d(6)-DMSO, and the rates of decomposition were followed by H-1 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 up 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.