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
resulted stable in the whole range of temperature employed, except those having
a benzyl or a t-butyl group at the b -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
(E
a
). 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 E
a
and BDH, whereas in the case of t-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.
