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Enediynes undergo cycloaromatization to p-benzyne diradicals. A new reaction of p-benzynes is here presented. Instead of their usual radical reactivity, they add nucleophiles, followed by protonation. Kinetic evidence showed that the rate-limiting step is the cycloaromatization, followed by rapid addition of a nucleophile. The unusual feature of nucleophilic addition to a radical center in a singlet diradical is discussed. Calculations that ignore solvation suggest that the addition has no activation barrier, but one is created by the necessity for desolvation of the anionic nucleophile. Relative reactivities of several nucleophiles have been measured under competition conditions. The observation of deuterium incorporation from DMSO-d6 or CD3CN shows that the species that is protonated is the aryl anion, not the p-benzyne. The extent of deuterium incorporation increases as the nucleophile is changed from iodide to bromide to chloride. This remarkable instance of selectivity is discussed. These studies of p-benzyne reactivity are exploring the nonradical chemistry of this diradical. Copyright © 2012 John Wiley & Sons, Ltd. |
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