Conformation and circular dichroism of some N-nitropyrrolidines

Abstract

The results of a theoretical investigation (ab initio calculations) of the parent nitropyrrolidine 1 and its (R)-2-methyl and (R,R)-2,5-dimethyl derivatives 2, 3 are in good agreement with the CD, UV and ¹H NMR spectra of 2, 3 and the X-ray analysis of 3. In the ground state of nitropyrrolidines l, 2 both nitrogen atoms of the nitroamino group are pyramidal and both nitrogen atoms of 3 are planar. In all cases, a small twisting of the nitroamino group about the N–N bond is observed. An envelope conformation with the C4 atom at the tip is predicted for nitropyrrolidine 1 and the more stable isomer 2a, which has a pseudoaxial methyl group. The less stable isomer 2b, (with a pseudoequatorial methyl group) and both isomers of 3 (2,5-di-pseudoaxial 3a and 2,5-di-pseudoequatorial 3b) adopt a half-chair conformation. In solutions and the crystal state, 3 exists as isomer 3a. Four Cotton effects are observed in the CD spectra of 2, 3. The signs of the Cotton effect at 270, 240 and 200 nm (the n_o⁺–π*, π_N–π* and π_o–π* electronic transitions) are determined by the configuration of the α-carbon chiral centres. The intrinsic chirality of the nitroamino chromophore and the chirality of the five-membered ring influence, mostly, the Cotton effect of the n_o^––π* transition at 300 nm.