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-live s of the nitrosoamides were found to vary in a systematic way with the natu re of the R group on the aromatic nucleus. Electron-releasing groups were f ound to decrease the stability of the starting nitrosoamide, whereas electr on-withdrawing ones increased the nitrosoamides' thermal stability. A Hamme tt-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 significan t positive charge at the benzylic position in the transition state of the r ate-determining step. The thermal stability of the nitrosoamides was also f ound to be systematically affected by the polarity of the solvent: as the s olvent polarity increased, so did the lability of the nitrosoamides. This o bservation of intra- and intermolecular electronic perturbations of the kin etics of nitrosoamide decomposition appears to be novel. A closer look at t he rate-determining step of nitrosoamide thermolysis is made, and a mechani stic framework is proposed that accounts for both steric and electronic mod ulation of nitrosoamide stability as well as the greater thermal stabilitie s of the related N-nitrocarboxamides and N-nitrosotosylamides.