A combined computation, kinetic, and trapping study of the 2,5-didehydropyridine biradical finds the hydrogen abstraction reaction to be tunable by protonation. The observation of small amounts of pyridine products in the thermolysis of a C,N-dialkynylimine, or azaenediyne, only when the reaction occurs in the presence of moderate amounts of protic acid, is consistent with qualitative theoretical predictions as well as ab initio calculations at the CASSCF and CASMP2 levels. The implication of these findings for a more selective antitumor agent is discussed.

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) × 103 M-1 s-1 and ki -PrOH,abstr = (6.5 ± 0.6) × 103 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 × 105 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.

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