Studies on the Stereochemistry of 2‐(Nitromethylidene)‐Heterocycles

Abstract

AbstractThe 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.