Abstract
Nitrosation of N-carbamoylamino acids (CAA) by gaseous NO+O2, an interesting synthetic pathway to amino acid N-carboxyanhydrides (NCA), alternative to the phosgene route, was investigated on N-carbamoyl-valine either in acetonitrile suspension or solventless conditions, and compared to the classical nitrosating system NaNO2+CF3COOH (TFA), the latter being quite less efficient in terms of either rate, stoichiometric demand, or further tractability of the product. The rate and efficiency of the NO+O2 reaction mainly depends on the O2/NO ratio. Evaluation of the contribution of various nitrosating species (N2O3, N2O4, HNO2) through stoichiometric balance showed the reaction to be effected mostly by N2O3 for O2/NO ratios below 0.3, and by N2O4 for O2/NO ratios above 0.4. The relative contribution of (subsequently formed) HNO2 always remains minor. Differential scanning calorimetry (DSC) monitoring of the reaction in the solid phase by either HNO2 (from NaNO2+TFA), gaseous N2O4 or gaseous N2O3 , provides the associated rate constants (ca. 0.1, 2 and 10^8 s-1 at 25°C, respectively), showing that N2O3 is by far the most reactive of these nitrosating species. From the DSC measurement, the latent heat of fusion of N2O3, 2.74 kJ.mol-1 at -105°C is also obtained for the first time. The kinetics was investigated under solventless conditions at 0°C, by either quenching experiments or less tedious, rough calorimetric techniques. Auto-accelerated, parabolic-shaped kinetics was observed in the first half of the reaction course, together with substantial heat release (temperature increase of ca. 20°C within 1-2 min in a 20-mg sample), followed by pseudo-zero-order kinetics after a sudden, important decrease in apparent rate. This kinetic break is possibly due to the transition between the initial solid-gas system and a solid-liquid-gas system resulting from water formation. Overall rate constants increased with parameters such as the specific surface of the solid, the O2/NO ratio, or the presence of moisture (or equivalently the hydrophilicity of the involved CAA), however without precise relationship, while the last two parameters may directly correlate to the increasing acidity of the medium.