A new mechanism for halide incorporation into biomolecules involves Bergman cyclization of an enediyne (cyclodeca-1,5-diyn-3-ene) to a p-benzyne intermediate, followed by halide addition, which generates a strong base (aryl anion) capable of deprotonating not only a carboxylic acid RCOOH but also solvent acetonitrile or dimethylsulfoxide.  We now report the relative kinetic acidities among solvents and kinetic isotope effects (KIEs) on the neutralization of aryl anion, measured by assaying the deuterium content of 1-iodotetranaphtalene. The KIEs are kCH3CN/kCD3CN = 2.9 and kDMSO/kDMSO-d6 = 5.8. The relative reactivities are kCD3CN/kDMSO = 1.7, kCH3CN/kDMSO-d6 ~ 43, kCH3CN/kDMSO = 4.8, and kCD3CN/kDMSO-d6 = 9.7, showing a low selectivity for CH3CN over DMSO, despite a 104-fold difference in acidities. This is evidence for the high basicity of the aryl anion. The same relative reactivities are obtained with Bu4N+I- as with Li+I-; thus the aryl anion reacts faster than Li+ can migrate to form an aryllithium.

Kinetic isotope effects in neutralization of the aryl anion generated by iodide addition to a <i>p</i>-benzyne derived from an enediyne

245th National Meeting (2013)

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Kinetic isotope effects in neutralization of the aryl anion generated by iodide addition to a p-benzyne derived from an enediyne