Dihydrophenanthrene Open-Shell Singlet Diradicals and Their Roles in the Mallory Photocyclization Reaction.

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, y o , 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.