A Less Radical Pathway

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Science  06 Apr 2007:
Vol. 316, Issue 5821, pp. 19-21
DOI: 10.1126/science.316.5821.19d

Enediyne molecules, in which two doubly bonded carbons tether two sets of triply bonded carbons, have been known for over 30 years to cyclize to the intriguing para-benzyne biradical. This species has been observed in many cases to behave as a benzene ring with two diametrically opposed trivalent carbons, which each react rapidly with hydrogen or halogen atom sources.

Perrin et al. have observed a surprisingly different mode of reactivity, which is more consistent with nucleophilic attack at one of the unsaturated carbons than with radical atom abstraction. Their studies show that slight heating of an enediyne in the presence of lithium halide salts and acid results in a halide and proton adding to opposite ends of the resultant benzene ring. Isotopic labeling reveals that even as weak an acid as dimethylsulfoxide can serve as the proton donor, implicating a highly basic phenyl anion intermediate formed after halide attack. The reaction is high-yielding for chloride, bromide, and iodide salts, and shows kinetics consistent with p-benzyne formation as the rate-limiting step. These findings offer a compelling rationale for the puzzling isolation from marine sources of monochlorinated cyanosporaside isomers whose structures were inconsistent with established radical or electrophilic chlorination pathways. — JSY

J. Am. Chem. Soc. 129, 10.1021/ja070023e (2007).

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