E and Z Conformations of Esters, Thiol Esters, and Amides

Populations and free-energy differences for the E and Z conformations of S-methyl, cyclopropyl, isopropyl, and cyclopentyl thioformate were determined by low-temperature 1H NMR spectroscopy, and free-energy barriers of 10.63 and 11.84 kcal/mol were obtained for interconversion of E and Z conformations of S-methyl thioformate at −52.4 °C. Populations and free-energy differences were also determined at room temperature by using 13C NMR for a series of N-substituted formamides and N-cyclopropylacetamide in 1% solutions in CD2Cl2/CH2Cl2. In both sets of compounds, electron-withdrawing groups attached to sulfur or nitrogen appear to favor the E conformations. The electronegativities of the groups are taken to increase in the order methyl < vinyl ∼ phenyl ∼ cyclopropyl < hydrogen < ethynyl. Data from the literature are discussed in these terms, including the E−Z energy differences for formic acid and its ethynyl, vinyl, and methyl esters.