Synthesis and Thermal Stability of Benzoxazine Nitroxides

A new class of stable nitroxides (aminoxyls) having a 1,4-benzoxazine structure were synthesized and the corresponding thermal stability tested. All derivatives were stable in the entire range of temperatures employed, except those having a benzyl or a tert-butyl group at the β-position with respect to the aminoxyl function, which underwent radical fragmentation. Such a behavior allowed a kinetic study, carried out by means of EPR spectroscopy, to determine the corresponding rate constants and activation parameters (Ea). Appropriate DFT calculations were performed for all nitroxides including also the thermally stable ones, in order to study the geometries of the fragmentation transition States as well as to compute the corresponding bond dissociation enthalpies (BDH), useful for further modeling purposes. The data obtained were interpreted on the basis of the relative stability of the leaving radical, according to the corresponding Ea and BDH, whereas in the case of tert-butyl derivatives steric hindrance should play a determinant role, as evidenced by a comparison of the geometric, kinetic, and thermodynamic parameters upon the whole series.