Chirale Butadiene 1. H-NMR-Spektroskopischer Nachweis der behinderten Rotation bei hexasubstituierten Butadienen

Abstract

The skew conformation of butadiene has the point group C2 and is therefore chiral. The chirality of butadienes has been demonstrated spectroscopically for the first time with four compounds, 7, 8, 9 and 11, whose preparation is described. Their bulky substituents slow down the rotation around the central carbon single bond, by which the interconversion of the enantiomers is effected through an achiral coplanar conformation. The barriers to rotation ΔGmath image, as determined by 1H-n.m.r. spectroscopy, are 17.6 ± 0·3 kcal/mol (at 75°) for 11,∼21 kcal/mol (at ∼140°) for 7, >22kcal/mol for 8, and >24 kcal/mol for 9.