E and Z Conformations of Esters, Thiol Esters, and Amides
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- 作者：Diwakar M. Pawar ; Abdelnaser A. Khalil ; Denise R. Hooks ; Kenneth Collins ; Tijuana Elliott ; Jefforey Stafford ; Lucille Smith ; and Eric A. Noe
- 刊名：Journal of the American Chemical Society
- 出版年：1998
- 出版时间：March 11, 1998
- 年：1998
- 卷：120
- 期：9
- 页码：2108 - 2112
- 全文大小：64K
- 年卷期：v.120,no.9(March 11, 1998)
- ISSN：1520-5126
文摘
Populations and free-energy differences for the E andZ conformations of S-methyl,cyclopropyl,isopropyl, and cyclopentyl thioformate were determined bylow-temperature 1H NMR spectroscopy, and free-energy barriers of 10.63 and 11.84 kcal/mol were obtained forinterconversion of E and Z conformationsofS-methyl thioformate at -52.4 
C. Populations andfree-energy differences were also determined at roomtemperature by using 13C NMR for a series of N-substitutedformamides and N-cyclopropylacetamide in 1%solutions inCD2Cl2/CH2Cl2.In both sets of compounds, electron-withdrawing groups attached tosulfur ornitrogen appear to favor the E conformations. Theelectronegativities of the groups are taken to increase inthe order methyl < vinyl ~ phenyl ~ cyclopropyl < hydrogen <ethynyl. Data from the literature are discussedin these terms, including the E-Z energydifferences for formic acid and its ethynyl, vinyl, and methylesters.
C. Populations andfree-energy differences were also determined at roomtemperature by using 13C NMR for a series of N-substitutedformamides and N-cyclopropylacetamide in 1%solutions inCD2Cl2/CH2Cl2.In both sets of compounds, electron-withdrawing groups attached tosulfur ornitrogen appear to favor the E conformations. Theelectronegativities of the groups are taken to increase inthe order methyl < vinyl ~ phenyl ~ cyclopropyl < hydrogen <ethynyl. Data from the literature are discussedin these terms, including the E-Z energydifferences for formic acid and its ethynyl, vinyl, and methylesters.