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

✍ Scribed by F. Duus; P. E. Hansen

Publisher
John Wiley and Sons
Year
1984
Tongue
English
Weight
806 KB
Volume
22
Category
Article
ISSN
0749-1581

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✦ Synopsis

The -C NMR spectra of 28 enethiolizable p-thioxo esters and 6 enethiolizable 0-thioxo thioloesters have been recorded in order to establish the tantomeric phenotypes of these compounds. All compounds investigated are essentially enethiolic. The carbonyl-conjugated (2)-enethiol form is the exclusive or predominant tautomer of open-chain 6-thioxo esters and thioloesters, thioacylmalonates and medium-sized 2-alkoxycarbonylcycloalkanethiones. The carbonyl-conjugated (0-enethiol form is identifiable for openchain a-unsubstituted p-thioxo esters and thioloesters, and abundant for open-chain a-substituted s-thioxo esters. Non-conjugated enethiol forms 1i.e. (2)and (E)-isomeric p,y-unsaturated p-mercapto esters] are abundant tautomeric constituents of w-substituted and higher 2-alkoxycarbonylcycloalkanethiones. The chemical shifts of the carbon atoms directly involved in the tautomeric change have been rationalized in t e r n 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 (2)-enethiols. The abundance of the CO-conjugated (E)-enethiols, as well as the preferred population of the non-conjugated (2)-enethiol form relative to the non-conjugated (Q-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. 'anomalous' enethiol forms D and F (R1 = CHR4R5# Me, R ' = a l k ~l ) ~ or, in extreme cases, eventually the thioxo form B (e.g. R' = Me, R2 = Pr' or cyclopentyl).'

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