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Friday, 18 September 2015

(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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