Hydrogen Shifts in Aryl Radicals and Diradicals: The Role of m-Benzynes

Abstract

Density functional and coupled cluster results are presented for hydrogen shifts in radicals derived from polycyclic aromatic hydrocarbons (PAHs) and for rearrangement mechanisms for several phenylenes. RCCSD(T)/cc-pVDZ//UBLYP/cc-pVDZ free energy barriers for 1,4-H shifts at 298 K are consistently predicted to be ca. 25 kcal/mol, whereas barriers for 1,5- and 1,6-shifts range from 6 to 28 kcal/mol. The barriers correlate reasonably well with the distance from the radical center to the shifting hydrogen in the reactant. Proposed mechanisms (via diradical intermediates) of known rearrangements of linear [3]phenylene, benzo[b]biphenylene, and angular [4]phenylene have BD(T)/cc-pVDZ//(U)BLYP/cc-pVDZ computed barriers of 74-82 kcal/mol, consistent with pyrolysis temperatures of 900 to 1100 °C. Hydrogen shift reactions in most of the aryl diradicals arising from phenylenes produce m-benzyne intermediates which, despite being 8-15 kcal/mol more stable than other diradicals involved in the pathways, do not significantly lower the computed overall free energies of activation.

Protonated Ground-State Singlet meta-Pyridynes React from an Excited Triplet State

“…meta -Benzyne derivatives have received increasing interest − as their radical reactivity has been demonstrated to be tunable from nonexistent to predominant by minor structural changes, such as the addition of substituents to the system. , They are also important intermediates in many organic reactions and bioorganic processes. ,− However, the examination of the chemical properties of meta -benzynes in condensed phases is challenging either due to the difficulty in generating pure meta -benzynes or due to their high reactivities under such conditions. Therefore, many gas-phase reactivity studies have been carried out to explore the reactivity of protonated meta -benzyne analogues by using ion trap mass spectrometry. − …”

Unusual Scaffold Rearrangement in Polyaromatic Hydrocarbons Driven by Concerted Action of Single Gold Atoms on a Gold Surface

“…Another mechanism could be rearrangement by radical‐induced processes similar to those proposed by Kartney et al. for the rearrangement of triphenylenes [46] . Following this line of thought, the first step would involve the cleavage of an Csp 2 –Csp 3 bond of the dibenzocyclopentadiene ring resulting in the formation of a biradical.…”

Unusual Scaffold Rearrangement in Polyaromatic Hydrocarbons Driven by Concerted Action of Single Gold Atoms on a Gold Surface

“…[45] Another mechanism could be rearrangement by radicalinduced processes similar to those proposed by Kartney et al for the rearrangement of triphenylenes. [46] Following this line of thought, the first step would involve the cleavage of an Csp 2 -Csp 3 bond of the dibenzocyclopentadiene ring resulting in the formation of a biradical. This biradical would undergo a series of complex chemical rearrangements including 1,5-hydrogen shifts along with rotation(s) of molecular fragments.…”