(E)-1-(4-Methylthiophenyl)-2-nitrobutene and (Z)-1-(4-Methylthiophenyl)-2-nitrobutene

Fig. 1. Structures and numbering of the (E)- and (Z)-1-(4-methylthiophenyl)-2-nitroalkenes mentioned in this study: 1a, R² = CH₂CH₃; 1b, R² = CH₃; 1c, R² = H.


The (E) and (Z) isomers of 1-(4-methylthiophenyl)-2-nitrobutene were formed in the reaction mixture of 4-methylthiobenzaldehyde and 1-nitropropane in refluxing toluene, using N,N-dimethylethylenediamine as catalyst, in an approximately 92:8 molar ratio, judging from the ¹H NMR spectrum of the crude product. Both products [(E)-1a and (Z)-1a, respectively] were separated chromatographically and fully characterized by ¹H- and ¹³C-NMR spectroscopy, using HMBC, HMQC and NOESY experiments for complete assignment of the signals. Tables 1 and 2 allow direct comparison of the ¹H and ¹³C chemical shifts of stereoisomers (E)-1a and (Z)-1a.

The ¹H-NMR spectrum of (E)-1-(4-methylthiophenyl)-2-nitrobutene [(E)-1a] (Figure 2a) shows the H1 resonance shifted further downfield (7.98 ppm) than either of the aromatic ring proton resonances (7.29 and 7.36 ppm), while in the (Z) isomer [(Z)-1a] (Figure 2b) the H1 nucleus resonates upfield (6.29 ppm) from the aromatic ring protons (7.16 ppm). This striking difference prompted us to study the two isomers in detail.

Fig. 2a. NOESY spectrum of (E)-1-(4-methylthiophenyl)-2-nitrobutene [(E)-1a] (CDCl₃).

Fig. 2b. NOESY spectrum of (Z)-1-(4-methylthiophenyl)-2-nitrobutene [(Z)] (CDCl₃).

(E)-1-(4-Methylthiophenyl)-2-nitrobutene [(E)-1a] and (Z)-1-(4-Methylthiophenyl)-2-nitrobutene [(Z)-1a].
A mixture of 4-methylthiobenzaldehyde (1.3 ml, 0.010 mol), N,N-dimethylethylenediamine (1.3 ml, 0.010 mol), 1-nitropropane (4.5 ml, 0.041 mol) and toluene (10 ml) was refluxed for 24 h with continuous water removal under a Dean-Stark trap. All volatiles were removed under reduced pressure and the residue was fractionated chromatographically over silica gel, eluting with CHCl₃, to afford the E [(E)-1a] (1.08 g, 92% in the mixture) and Z isomers [(Z)-1a] (0.096 g, 8% in the mixture) as viscous orange colored liquids.
(E)-1a: ¹H-NMR (CDCl₃) δ 1.28 (t, 3H, J = 7.4 Hz, CH₂CH₃), 2.52 (s, 3H, S-CH₃), 2.88 (q, 2H, J = 7.4 Hz, CH₃CH₂C=CH), 7.29 (d, 2H, J = 8.5 Hz, H3' and H5'), 7.36 (d, 2H, J = 8.5 Hz, H2' and H6'), 7.98 (s, 1H, Ar-CH=C). HREIMS m/z (M⁺) = 223.06621; calc. for C₁₁H₁₃NO₂S = 223.06670.
(Z)-1a: ¹H-NMR (CDCl₃) δ 1.20 (t, 3H, J = 7.4 Hz, CH₂CH₃), 2.46 (s, 3H, S-CH₃), 2.67 (q, 2H, J = 7.4 Hz, CH₃CH₂C=CH), 6.29 (s, 1H, Ar-CH=C), 7.16 (apparent s, 4H, J = 9.2 Hz, Ar-H). HREIMS m/z (M⁺) = 223.06622; calc. for C₁₁H₁₃NO₂S = 223.06670.

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