Stereoelectronic control of facial selectivity in the Diels-Alder cycloaddition of sterically unbiased 5,5-diarylcyclopentadienes
摘要:
This study provides strong evidence for stereoelectronic control of diastereoselectivity in the Diels-Alder cycloaddition of sterically unbiased 5-(4-X-phenyl)-5-phenylcyclopentadienes 1 (X = NO2, Cl, and N(CH3)2) with dimethyl acetylenedicarboxylate (DMAD) producing diastereomeric norbornyl diesters 2 in cis/trans ratios (diester relative to the substituted arene) varying from 68:32 to 38:62 as determined by IH NMR spectroscopy. The reactions were carried out under both thermal and Lewis acid catalyzed conditions, giving essentially identical selectivities regardless of the conditions used. Structural assignments were made by H-1 NMR lanthanide shift reagent studies of the isolated diastereomers, separable by preparative thin-layer chromatography. In each case the dienophile approached the diene from the side opposite the more electron rich aromatic ring as predicted by Cieplak's theory for explaining stereoelectronic control. The observed ratios correspond to an overall energy difference of 0.65 kcal/mol. A Hammet plot of the log (cis/trans) versus the sigma(p) parameter produced a linear relationship with a correlation coefficient of 0.98. An efficient synthesis of the diarylcyclopentadienes is described.
Stereoelectronic control of facial selectivity in the Diels-Alder cycloaddition of sterically unbiased 5,5-diarylcyclopentadienes
摘要:
This study provides strong evidence for stereoelectronic control of diastereoselectivity in the Diels-Alder cycloaddition of sterically unbiased 5-(4-X-phenyl)-5-phenylcyclopentadienes 1 (X = NO2, Cl, and N(CH3)2) with dimethyl acetylenedicarboxylate (DMAD) producing diastereomeric norbornyl diesters 2 in cis/trans ratios (diester relative to the substituted arene) varying from 68:32 to 38:62 as determined by IH NMR spectroscopy. The reactions were carried out under both thermal and Lewis acid catalyzed conditions, giving essentially identical selectivities regardless of the conditions used. Structural assignments were made by H-1 NMR lanthanide shift reagent studies of the isolated diastereomers, separable by preparative thin-layer chromatography. In each case the dienophile approached the diene from the side opposite the more electron rich aromatic ring as predicted by Cieplak's theory for explaining stereoelectronic control. The observed ratios correspond to an overall energy difference of 0.65 kcal/mol. A Hammet plot of the log (cis/trans) versus the sigma(p) parameter produced a linear relationship with a correlation coefficient of 0.98. An efficient synthesis of the diarylcyclopentadienes is described.
Stereoelectronic control of facial selectivity in the Diels-Alder cycloaddition of sterically unbiased 5,5-diarylcyclopentadienes
作者:Ronald L. Halterman、Bridget A. McCarthy、Marjorie A. McEvoy
DOI:10.1021/jo00047a009
日期:1992.10
This study provides strong evidence for stereoelectronic control of diastereoselectivity in the Diels-Alder cycloaddition of sterically unbiased 5-(4-X-phenyl)-5-phenylcyclopentadienes 1 (X = NO2, Cl, and N(CH3)2) with dimethyl acetylenedicarboxylate (DMAD) producing diastereomeric norbornyl diesters 2 in cis/trans ratios (diester relative to the substituted arene) varying from 68:32 to 38:62 as determined by IH NMR spectroscopy. The reactions were carried out under both thermal and Lewis acid catalyzed conditions, giving essentially identical selectivities regardless of the conditions used. Structural assignments were made by H-1 NMR lanthanide shift reagent studies of the isolated diastereomers, separable by preparative thin-layer chromatography. In each case the dienophile approached the diene from the side opposite the more electron rich aromatic ring as predicted by Cieplak's theory for explaining stereoelectronic control. The observed ratios correspond to an overall energy difference of 0.65 kcal/mol. A Hammet plot of the log (cis/trans) versus the sigma(p) parameter produced a linear relationship with a correlation coefficient of 0.98. An efficient synthesis of the diarylcyclopentadienes is described.