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.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.