ISSN:
0749-1581
Keywords:
13C NMR
;
1H NMR
;
Thioketone-enethiol tautomerism
;
Dihydrothiophenes
;
Intramolecular hydrogen bonding
;
Additive substituent shielding
;
Conformational analysis
;
Chemistry
;
Analytical Chemistry and Spectroscopy
Source:
Wiley InterScience Backfile Collection 1832-2000
Topics:
Chemistry and Pharmacology
Notes:
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 13C and 1H 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 13C 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 - H⃛O 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.
Additional Material:
6 Ill.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1002/mrc.1260270810
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