9,10-Dehydroanthracene: p-Benzyne-Type Biradicals Abstract Hydrogen Unusually Slowly

The 9,10-dehydroanthracene biradical, a model for the p -benzyne-type biradicals implicated in DNA cleavage by the enediyne antitumor antibiotics, was prepared by photodissociation of a propellane in solution. Trapping products characteristic of biradicals, e.g. anthracene- d 2, are found. The rates of hydrogen abstraction by the biradical from acetonitrile and isopropyl alcohol are measured directly by flash photolysis/transient absorption spectroscopy, giving second-order rate constants of k MeCN,abstr = (1.1 ± 0.2) × 10 3 M -1 s -1 and k i -PrOH,abstr = (6.5 ± 0.6) × 10 3 M -1 s -1 at room temperature, which are 100−200 times lower than the corresponding rate constants for phenyl or 9-anthryl radical. A second decay route for the biradical is found, and assigned, based on thermochemical, kinetic, and trapping arguments, to a retro-Bergman reaction that converts the 9,10-dehydroanthracene biradical into the ring-opened 3,4-benzocyclodeca-3,7,9-triene-1,5-diyne. Although the retro-Bergman reaction is relatively fast, k ≈ 4 × 10 5 s -1 at room temperature, it is competitive with hydrogen abstraction by the biradical only because the hydrogen abstraction is slower than expected. Through-bond coupling in the 1,4-biradical is discussed as a rationalization for the 100- to 200-fold reduction in the abstraction rate.