Studies on the Stereochemistry of 2-(Nitromethylidene)Heterocycles

Rajappa, Srinivasachari and Nagarajan, Kuppuswamy and Venkatesan, Kailasam and Kamath, Nirupa and Padmanabhan, Vipanjeri Madabhusi and Philipsborn, Wolfgang Von and Chen, Ban Chin and Muller, Raffaelo (1984) Studies on the Stereochemistry of 2-(Nitromethylidene)Heterocycles. In: Helvetica Chimica Acta, 67 (7). pp. 1669-1680.

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Abstract

The 1H-NMR spectra of 2-(nitromethylidene)pyrrolidine (7), 1-methyl-2-(nitromethylidene)imidazolidind (10) and 3-(nitromethylidene)tetrahydrothiazine (11) in $CDCl_3$ and ${({CD}_3)}_2SO$ 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 $CDCl_3$ and in ${({CD}_3)}_2SO$. 2-(Nitromethylidene)thiazolidine (3) has the (E)-configuration in $CDCl_3$ but exists in ${({CD}_3)}_2SO$ 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 ${({CD}_3)}_2SO$. 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 $NO_2$ -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.

Item Type:	Journal Article
Publication:	Helvetica Chimica Acta
Publisher:	Verlag GmbH & Co. KGaA, Weinheim
Additional Information:	The copyright belongs to Verlag GmbH & Co. KGaA, Weinheim.
Keywords:	stereochemistry;studies;nitromethylidene;heterocycles
Department/Centre:	Division of Chemical Sciences > Organic Chemistry
Date Deposited:	13 Sep 2005
Last Modified:	19 Sep 2010 04:20
URI:	http://eprints.iisc.ac.in/id/eprint/3634

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