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.