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Studies on the Stereochemistry of 2‐(Nitromethylidene)‐Heterocycles
Author(s) -
Rajappa Srinivasachari,
Nagarajan Kuppuswamy,
Venkatesan Kailasam,
Kamath Nirupa,
Padmanabhan Vipanjeri Madabhusi,
von Philipsborn Wolfgang,
Chen Ban Chin,
Müller Raffaelo
Publication year - 1984
Publication title -
helvetica chimica acta
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.74
H-Index - 82
eISSN - 1522-2675
pISSN - 0018-019X
DOI - 10.1002/hlca.19840670702
Subject(s) - chemistry , ring (chemistry) , stereochemistry , pyrrolidine , lone pair , conjugated system , nmr spectra database , nuclear magnetic resonance spectroscopy , chemical shift , crystallography , derivative (finance) , proton nmr , spectral line , molecule , organic chemistry , physics , astronomy , financial economics , economics , polymer
The 1 H‐NMR spectra of 2‐(nitromethylidene)pyrrolidine ( 7 ), 1‐methyl‐2‐(nitromethylidene)imidazolidind ( 10 ) and 3‐(nitromethylidene)tetrahydrothiazine ( 11 ) in CDCl 3 and (CD 3 ) 2 SO 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 ) 2 SO. 2‐(Nitromethylidene)thiazolidine ( 3 ) has the ( E )‐configuration in CDCl 3 but exists in (CD 3 ) 2 SO 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. 15 N‐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 ) 2 SO. 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 13 C‐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.