posted on 2024-01-02, 06:06authored byElliott
B. Hulley, Edward L. Clennan
A computational
study (ωB97X-D/6-31G(d))
of the
Mallory photocyclization
reaction has revealed that the well-established dihydrophenanthrene
(DHP) intermediates can adopt either closed-shell (CS) or open-shell-diradical
(OS) singlet ground states. A detailed study of the properties of
DHPs allowed their classifications as OS, borderline-OS, borderline-CS,
or CS intermediates. The triplet electronic state and higher energy
CS* isomer of all the OS singlet diradicals were computationally located,
and the expected relationship between the diradical index, yo, and the triplet energy and the OS-CS* energy
gaps was established. The importance of aromaticity in stabilizing
the OS singlet diradicals was confirmed by using the Harmonic Oscillator
Model of Aromaticity (HOMA). The thermal decompositions of DHPs by
cycloreversions to regenerate the Mallory starting materials were
also studied. The cycloreversion mechanism was described as a homolytic
cleavage characterized by an anchimeric assistance continuum promoted
by bis-β-homolytic cleavage.