Theoretical study of alkali metal pyrrolides in comparison with nH-pyrrole

Abstract

The purpose of the present work is to demonstrate the remarkable difference between Li-pyrrolide, on one hand, and Na and K ones, on the other, viz., if the former exists as two conformers, a and 7 with a significant preference to the pi, at least in the gas phase, Na- and K-pyrrolides only establish the stable pi structure. A key factor of its higher stabilization is the formation of the M+delta-N-sigma ionic pair (M = Li, Na, and K) where the alkali metal atom behaves as a cation interacting with the pyrrole ring via a typical pi-cation interaction. The formation of alkali metal pyrrolides is related to the reactions of the N-H bond-breaking H abstraction and a further hydrogen "walk" along the pyrrole ring resulting in nH-pyrrole. Both reactions are thoroughly studied, and four novel pathways for the pyrolysis of pyrrole starting, by means of the 1,2 H migration, from pyrrolenine (2H-pyrrole) and exiting along the HCN-propyne channel, are proposed.