Configuration and stereodynamics of exo/endo-isomeric push-pull alkenes of pentadiene structure

Abstract

The configuration of the two double bonds in a series of alkenyl-substituted cyclic push-pull pentadienes [-1,3- (1–9, endocyclic) and -1,4-dienes (1–9, exocyclic)], the corresponding preferred ground state conformations and the stereodynamics were studied by the whole arsenal of 1D and 2D NMR spectroscopic methods, mass spectrometry and parallel molecular modelling. The configuration was found strongly dependent on the ring size (five-, six-, seven- and eight-membered). The dynamic phenomena found in the ¹H and ¹³C NMR spectra were proved to result from the sterically hindered rotation about the C2–C3 bond; the torsional barriers were determined by dynamic NMR spectroscopy and also estimated by semi-empirical and ab initio quantum chemical calculations.