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

Abstract

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.

Matrix Isolation of 3,4-Benzocyclodeca-3,7,9-triene-1,5-diyne

“…Based on the estimated activation barrier of 18 kcal/mol and an activation entropy of Ϫ3.7 cal mol Ϫ1 K Ϫ1 (B3LYP/ 6-31G*) for the Bergman cyclization leading back to the aromatic diradical 4, the half-life of 3 should be of the order of a few seconds at 20°C. Thus, in accordance with the LFP experiments, [5] enediyne 3 is predicted to be unstable at room temperature.…”

“…[5] Two transient species were observed in acetonitrile, a fast transient with a lifetime of 2.6 µs (λ max ഠ 295 nm), and a slow transient with a lifetime of 1.0 ms (λ max ഠ 335 nm). Based on kinetic data, quenching studies, and ab initio calculations, these species were assigned as 4 and 3, respectively.…”

meta-Benzyne Reacts as an Electrophile

“…A small fraction of these intermediates is able to make the transition to long-term stability and mass spectrometric detectability. A similar (albeit slower) adduct formation was observed in the absence of the tert-butyl groups (i.e., for the reaction of the N-(3,5-didehydrophenyl)pyridinium ion (5) with pyridine to form the unsubstituted bis(pyridinium)phenide cation) (9). No addition was observed under the same conditions for related ions without a meta-benzyne moiety (e.g., the N-phenyl-4-tertbutylpyridinium and N-phenylpyridinium ions), suggesting that the observed adduct formation is not electrostatic clustering and that the addition is the result of reaction at the meta-benzyne moiety.…”

Energetics of 1,n‐didehydro‐polyene diradicals and performance of reduced multireference coupled‐cluster method

“…13 and 14) that the aryne-type diradicals participate in the double-stranded cleavage of DNA by enediyne and related antibiotics. In fact, the reactivity of singletstate diradicals correlates with the magnitude of their S-T splitting: [15][16][17] the larger the splitting, the greater the barrier for the hydrogen atom abstraction and, thus, more selective the diradical. For all these reasons theoretical tools are essential for a study of the molecular and electronic structure of these systems, particularly of their stability and of the S-T separation between their lowest-lying singlet and triplet states.…”