S. Rajappa, K. Nagarajan, K. Venkatesan
Nov 7, 1984
Citations
0
Influential Citations
15
Citations
Journal
Helvetica Chimica Acta
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.