Determination of tautomeric phenotypes of β‐thioxo esters and characterization of the tautomeric enethiolic constituents by means of 13C NMR spectroscopy

Abstract

AbstractThe 13C NMR spectra of 28 enethiolizable β‐thioxo esters and 6 enethiolizable β‐thioxo thioloesters have been recorded in order to establish the tautomeric phenotypes of these compounds. All compounds investigated are essentially enethiolic. The carbonyl‐conjugated (Z)‐enethiol form is the exclusive or predominant tautomer of open‐chain β‐thioxo esters and thioloesters, thioacylmalonates and medium‐sized 2‐alkoxycarbonylcycloalkanethiones. The carbonyl‐conjugated (E)‐enethiol form is identifiable for open‐chain α‐unsubstituted β‐thioxo esters and thioloesters, and abundant for open‐chain α‐substituted β‐thioxo esters. Non‐conjugated enethiol forms [i.e. (Z)‐ and (E)‐isomeric β,γ‐unsaturated β‐mercapto esters] are abundant tautomeric constituents of ω‐substituted and higher 2‐alkoxycarbonylcycloalkanethiones. The chemical shifts of the carbon atoms directly involved in the tautomeric change have been rationalized in terms of substituent screening contributions. Deuterium isotope effects on the central carbon atoms of selected deuterio‐enethiolic compounds have been measured in order to depict the ester group rotamerism in CO‐conjugated (Z)‐enethiols. The abundance of the CO‐conjugated (E)‐enethiols, as well as the preferred population of the non‐conjugated (Z)‐enethiol form relative to the non‐conjugated (E)‐enethiol form, is rationalized in terms of the occurrence of a no‐bond interaction between the lone‐pair electrons of the enethiolic sulphur atom and the ‘chelating’ methylene hydrogen atoms of cis‐alkyl groups.