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Nucleophilic additions to a para-benzyne derived from an enediyne : Exploring the non-radical reactivity of a diradical


A new reaction of para-benzyne diradicals with anionic nucleophiles is different from their usual homolytic atom abstraction. Our studies show cyclodeca-1,5-diyn-3-ene undergoing rate-limiting cycloaromatization to a para- benzyne, which rapidly adds nucleophiles to produce an aryl anion, which is then quenched by solvent or water to form 1-(Nu)tetrahydronaphthalenes. Our results represent the first example of anionic nucleophiles, other than halides, reacting towards a para-aryne. Our reactivity scale reflects the ease of Nu- desolvation, with the smaller, more highly solvated ions being slower in their reaction towards the para-benzyne diradical. Experimental results are in good agreement with computational data, and these suggest Nu- additions to para-benzyne to be governed by solvation other than basic strength or nucleophilic character. Deuterium can be incorporated from aryl anion reacting with such weak acids as water, DMSO or CH₃CN. The question addressed here is the relative reactivity of these two solvents and water, which can be measured by competition experiments with a mixture of labeled and non- labeled solvent. The relative reactivities kH₂O/kSolvent-d , kD₂O/kSolvent, and kSolvent/kSolvent-d were measured, and these values were combined to evaluate other relative reactivities including some which could not be measured directly, because there is no way to determine the source of the H or D in product. The low selectivity for CH₃CN over DMSO, despite a difference in acidities of nearly 10⁴ -fold, is evidence for high basicity of the aryl anion. Moreover, the observation that the same relative reactivities are obtained with Bu₄NI as with LiI is evidence that the aryl anion reacts more rapidly than the Li⁺ can reach the para carbon to form an aryllithium. To our knowledge, this is the first example of a metal-free aryl carbanion in solution. Finally, this new para-benzyne reactivity could represent an alternative mechanism, besides homolytic atom abstraction, for detoxifying enediyne antibiotics

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