Studies on the stereochemistry of 2-(nitromethylidene)-heterocycles

Rajappa, Srinivasachari ; Nagarajan, Kuppuswamy ; Venkatesan, Kailasam ; Kamath, Nirupa ; Padmanabhan, Vipanjeri Madabhusi ; Philipsborn, Wolfgang von ; Chen, Ban Chin ; Muller, Raffaelo (1984) Studies on the stereochemistry of 2-(nitromethylidene)-heterocycles Helvetica Chimica Acta, 67 (7). pp. 1669-1680. ISSN 0018-019X

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Official URL: http://onlinelibrary.wiley.com/doi/10.1002/hlca.19...

Related URL: http://dx.doi.org/10.1002/hlca.19840670702

Abstract

The 1H-NMR spectra of 2-(nitromethylidene)pyrrolidine (7), 1-methyl-2-(nitromethylidene)imidazolidind (10) and 3-(nitromethylidene)tetrahydrothiazine (11) in CDCl3 and (CD3)2SO indicate that these compounds have the intramolecularly H-bonded structures (Z)-7, (E)-10 and (Z)-11 while the N-methyl derivative 8 of 7 is (E)-configurated in both solvents. 1-Benzylamino-1-(methyltio)-2-nitroehtylene (13), an acylic model, has the H-bonded configuration (E)-13 in CDCl3 and in (CD3)2SO. 2-(Nitromethylidene)thiazolidine (3) has the (E)-configuration in CDCl3 but exists in (CD3)2SO as a mixture of (Z)- and (E)-isomers with the former predominating. Both species are detected to varying proportions in a mixture of the two solvents. 15N-NMR spectroscopy of 3 ruled out unambiguously the nitronic acid structure 6 and the nitromethyleimine structure 5. The N-methyl derivative 4 of 3 is (Z)-configurated in (CD3)2SO. Comparison of the olefinic proton shifts of (Z)-3 and (Z)-4 with those of analogues and also of 1,1-bis(methylti)-2-nitroethylene (12) shows decreased conjugation of the lone pair of electrons of the ring N-atom in (Z)-3 and (Z)-4. This is also supported by 13C-NMR studies. Plausible explanations for the phenomenon are offered by postulating that the ring N-atoms are pyramidal in (Z)-3 and (Z)-4 and planar in other cases or, alternatively, that the conjugated nitroenamine system gets twisted due to steric interaction between the NO2-group and the ring S-atom. Single-crystal X-ray studies of 3 and 8 show that the former exists in the (Z)-configuration and the latter in (E)-configuration; the ring N-atom in the former has slightly more pyramidal character than in the latter.

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