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

Abstract

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.Dihydrophenanthrenes (DHPs) are intermediates in the Mallory photocyclization reaction, which is an oxidative photocyclization that has been extensively used in the synthesis of polyaromatic hydrocarbons (PAHs). 1,2 It is a very versatile reaction that has been successfully exploited for the construction of both carbocyclic, 1, 3 and heterocyclic, 2, 4 PAHs and is notably the most prevalent method utilized to make carbocyclic helicenes 3 5 (Scheme 1).The mechanism of the Mallory reaction is illustrated in Scheme 2 for the reaction of trans-stilbene. In general, the reaction is not sensitized by triplet sensitizers or quenched by oxygen 1 and consequently is initiated by excitation of transstilbene to the S 1 potential energy surface (PES). The excited trans-stilbene passes over barrier a of approximately 3 kcal/