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13 C and 1 H NMR study of structure and structural dynamics of tautomeric 2‐ethoxycarbonylthiolane‐3‐thiones
Author(s) -
Duus Fritz
Publication year - 1989
Publication title -
magnetic resonance in chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.483
H-Index - 72
eISSN - 1097-458X
pISSN - 0749-1581
DOI - 10.1002/mrc.1260270810
Subject(s) - tautomer , chemistry , intramolecular force , ring (chemistry) , chemical shift , substituent , diastereomer , solvent effects , vicinal , stereochemistry , proton , nuclear magnetic resonance spectroscopy , enantiomer , proton nmr , computational chemistry , solvent , organic chemistry , physics , quantum mechanics
Abstract 2‐Ethoxycarbonylthiolane‐3‐thione (1) and its 5‐methyl, 5‐phenyl, 5,5‐dimethyl and 4,5‐dimethyl derivatives (2–5) were studied in solution by 13 C and 1 H NMR spectroscopy, using solvents of various polarity. Compounds 1–4 exist entirely in the tautomeric ( Z )‐enethiol form, i.e. they are in fact 2‐ethoxycarbonyl‐3‐mercapto‐4,5‐dihydrothiophenes (1C‐4C), apparently irrespective of their molecular surroundings. Compound 5 exists predominantly as the pair of enantiomeric (and hence NMR‐indistinguishable) 2‐ethoxycarbonyl‐3‐mercapto‐4,5‐dimethyl‐4,5‐dihydrothiophenes having a trans location of their ring‐methyl groups, i.e. 5C(4R, 5R) and 5C(4S, 5S). The diastereomeric counterparts 5C(4R, 5S)/5C(4S, 5R) are observable as minor constituents, co‐existing with the former in a double two‐step equilibrium system, involving tautomeric 2‐ethoxycarbonyl‐3‐mercapto‐4,5‐dimethyl‐2,5‐dihydrothiophenes as intermediates. The 13 C chemical shifts of C‐4 and C‐5 of the unsubstituted and mono‐ and di‐methyl‐substituted 4,5‐dihydrothiophenes (1C, 2C, 4C, 5C) are describable in terms of additive methyl‐substituent shielding. The observed solvent‐induced displacement of the mercapto proton chemical shift is interpreted in terms of the solvent's ability to effect pertubation of the intramolecular S—H ⃛O hydrogen bonding. Vicinal couplings between ring protons at C‐4 and C‐5 are influenced by substitution at C‐4 and C‐5, and also by the nature (polarity) of the solvent used. The measured coupling constants have been rationalized in terms of the conformational mobility of the 4,5‐dihydrothiophene framework. A methyl group at C‐5 has little significance for conformational preference. A phenyl group at C‐5 preferentially occupies the near‐equatorial position. The conformational equilibrium of the trans ‐4,5‐dimethyl‐4,5‐dihydrothiophenes 5C(4R, 5R)/5C(4S, 5S) apparently depends on the solvent's polarity, a more polar solvent favouring the conformer having near‐diaxial methyl groups.