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The versatile electrophilic reactivity of 4,6-dinitrobenzo[d]isoxazole-3-carbonitrile

Abstract : The interaction of 4,6-dinitrobenzo[d]isoxazole-3-carbonitrile (5a) with methoxide ion has been kinetically investigated in methanol and a 20:80(v/v)MeOH-Me 2 SO mixture. In methanol, stopped-flow experiments have revealed that a monomethoxyl σ-adduct (5a-Me) is first formed, resulting from a fast MeO-addition at the unsubstituted 7-carbon. Rate and equilibrium constants for this σ-complexation process could be determined, allowing a ranking of 5a within the pK a scale established for Meisenheimer electrophiles in methanol. With a pK a value of 13.50, the electrophilicity of 5a falls in the range of 1,3,6,8-tetranitronaphthalene, 2,4-dinitrothiophene or 4-nitrobenzofuroxan. This corresponds to a two-pK a units increase in electrophilicity from that of TNB, the common reference in σ-complex chemistry but it is notably below that of so-called superelectrophilic molecules like 4,6-dinitrobenzofuroxan. In addition to its ease of σ-complexation, 5a is found to undergo a slow but thermodynamically favourable addition of MeO-to the cyano group bonded to the isoxazole ring, leading to a complete conversion of the adduct 5a-Me into a dinitroimidate 6. The reactivity of 6 could be kinetically assessed. Going to 80% Me 2 SO still afforded initially the adduct 5a-Me but this anionic species undergoes addition of a second molecule of MeO-to the CN group, giving a dianion whose structure is unprecedented in the literature. Combining the above results with synthetic observations showing that 5a can readily contribute to S N Ar reactions under appropriate experimental conditions emphasizes the multifaceted electrophilic reactivity of this electron-deficient heterocycle.
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https://hal.inrae.fr/hal-02921678
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Submitted on : Tuesday, August 25, 2020 - 2:12:27 PM
Last modification on : Tuesday, June 15, 2021 - 2:56:05 PM

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N Zelinsky, Betty Cottyn, Alexei Starosotnikov, Dominique Vichard, Régis Goumont, et al.. The versatile electrophilic reactivity of 4,6-dinitrobenzo[d]isoxazole-3-carbonitrile. Organic and Biomolecular Chemistry, Royal Society of Chemistry, 2009, 7 (6), pp.1137. ⟨10.1039/b820256g⟩. ⟨hal-02921678⟩

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