Solvolysis in carboxamides. VII. Carboxamide solvolysis of threo-2-aryl-1-methylpropyl brosylates. Reactivity-selectivity relationships as a mechanistic probe.

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概要

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• Solvolysis of a series of <I>threo</I>-2-aryl-1-methylpropyl brosylates (<I>p</I>-MeO, <I>p</I>-Me, H, <I>p</I>-Cl, <I>m</I>-Cl, <I>m</I>-CF<SUB>3</SUB>, and <I>p</I>-NO<SUB>2</SUB>; <B>1a</B>–<B>g</B>) has been carried out in four carboxamides, <I>N</I>-methylformamide (NMF), <I>N</I>-methylacetamide (NMA), <I>N</I>,<I>N</I>-dimethylformamide (DMF), and <I>N</I>,<I>N</I>dimethylacetamide (DMA), as solvent. Fractions of <I>k<SUB>s</SUB></I> and <I>Fk</I><SUB>Δ</SUB> pathway were determined on the basis of the upward drift from the Hammett plot of the rate constant (<I>k</I><SUB>t</SUB>); the extra acceleration by <I>p</I>-MeO and <I>p</I>-Me substituents was assigned to the <I>Fk</I><SUB>Δ</SUB> fraction. The <I>Fk</I><SUB>Δ</SUB>⁄<I>k</I><SUB>t</SUB> values were not in agreement with the amount of retained products (<I>threo</I>-acetate and <I>threo</I>-alcohol) obtained in NMA and DMA solvolysis of <B>1a–c</B>, although they should be in line with each other on the basis of the bridged-ion theory. The C(2)-deuterated phenyl derivative (<B>1c</B>-2-<I>d</I>) was solvolyzed in NMF, NMA, and DMF to give retained carboxylates (acetate or formate). However, the isotope scrambling at C(2) and C(1) of the carboxylates is incomplete, indicating that a symmetrical ion, such as a phenyl-bridged ion, can not be the intermed be the intermediate of the carboxamide solvolysis. A good correlation of log<I>k</I><SUB>t</SUB> for substrates (<B>1a</B>–<B>g</B>) with log(% <I>threo</I>-acetate% <I>erythro</I>-acetate) or with log (% <I>threo</I>-acetate% olefinic products) was found for solvent systems such as NMA, DMA, and acetic acid, indicating that the reactions of all substituted substrates can be explained by intervention of a common intermediate [R<SUP>+</SUP>X<SUP>−</SUP>], a classical tight ion-pair from which the final products are derived. Such reactivity-selectivity relationship also holds between log <I>k</I><SUB>t</SUB>'s of <B>1c</B> in a series of carboxamide and logarithms of <I>threo</I>-<I>erythro</I> ratio in acetate or formate. Through-space interaction between a face of aryl ring and carbenium ion center in the transition state is considered as a possible cause for the anomalous acceleration by the activating aryl groups.

• 公益社団法人 日本化学会の論文

著者

• Saito Seiki

  Department Of Bioengineering Science Faculty Of Engineering. Okayama University

• Okamoto Kunio

  Department of Fuel Chemistry Faculty of Engineering Kyoto University

• Moriwake Toshio

  Department of Synthetics Chemistry, School of Engineering, Okayama University

• Doihara Kenji

  Department of Synthetics Chemistry, School of Engineering, Okayama University

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