Conformational Dependence of the Circular Dichroism of N-Nitrosopyrrolidines

Authors:

• Tadeusz Poloński,
• Maria J. Milewska,
• Andrzej Katrusiak

Abstract

Several substituted N-nitrosopyrrolidines were prepared and their chiroptical spectra studied. The circular dichroism of monocyclic compounds depends on substituents, solvents, and temperature changes. Some of these N-nitrosamines exhibit bisignate CD curves in the region of n-π^* transition. On the contrary, conformationally restricted bi- and tricyclic compounds show monosignate Cotton effects, the magnitudes of which are almost solvent independent. Bisignate CD curves result from two half-chair conformers of the pyrrolidine ring being in equilibrium and contributing with the opposite CD signs. The Cotton effect signs can be predicted with our “lowered symmetry” sector rule developed earlier. Molecular geometries were calculated by the molecular mechanics (MM2) method, and the resulting torsional angles were used to estimate the vicinal coupling constants in the ¹H NMR spectra. The crystal structure of (R)-N-nitroso-3-phenylpyrrolidine ((R)-6), solved by us, is very close to the calculated minimum energy conformation. © 1993, American Chemical Society. All rights reserved.