E-CONFIRMATION AND Z-CONFORMATION 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 H-1 NMR spectroscopy, and free-energy ba rriers of 10.63 and 11.84 kcal/mol were obtained for interconversion o f E and Z conformations of S-methyl thioformate at -52.4, degrees C. P opulations and free-energy differences were also determined at room te mperature by using C-13 NMR for a series of N-substituted formamides a nd N-cyclopropylacetamide in 1% solutions in CD2Cl2/CH2Cl2. In both se ts of compounds, electron-withdrawing groups attached to sulfur or nit rogen appear to favor the E conformations. The electronegativities of the groups are taken to increase in the order methyl < vinyl similar t o phenyl similar to cyclopropyl < hydrogen < ethynyl. Data from the li terature are discussed in these terms, including the E-Z energy differ ences for formic acid and its ethynyl, vinyl, and methyl esters.