Methylation of Sydnone N-Oxides: Kinetic and Thermodynamic Control in the Alkylation Site of an Electron-Rich Heterocycle

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• 作者：D. Scott Bohle ; Lindsey E. McQuade ; Inna Perepichka ; Lijuan Zhang
• 刊名：Journal of Organic Chemistry
• 出版年：2007
• 出版时间：May 11, 2007
• 年：2007
• 卷：72
• 期：10
• 页码：3625 - 3631
• 全文大小：177K
• 年卷期：v.72,no.10(May 11, 2007)
• ISSN：1520-6904

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Methylation of the anionic 4-methylcarboxy 1,2,3-oxadiazolate 3-oxide occurs at either the disubstitutedring nitrogen or the oxygen of the N-oxide depending upon the conditions and reagents employed.Alkylation with methyl iodide leads to N-alkylation, while dimethyl sulfate gives O-alkylation andtrifluoromethanemethylsulfonate gives a mixture of the two products. The regioselectivity of thesemethylations has been confirmed by X-ray diffraction of the two products, and these are in turn correlatedwith their theoretically predicted (B3LYP//6-311++G**) relative energies and vibrational spectra.Theoretically, N-alkylation is expected to give an isomer that is over 10 kcal mol^-1 more stable than theO-alkylated product. As a neat melt the kinetic O-alkylation product cleanly isomerizes in 2 h whenheated to 140 C to give the thermodynamic N-methylated isomer. Taken together the results illustratethe remarkable new sydnone N-oxide derivatives which are readily accessed in this chemistry, with theN-alkylation of the sydnone N-oxide, corresponding to the first case of such an N-alkylation for a diazeniumdiolate.