Synthesis and Dynamic NMR Study of Atropisomerism in Stable 1,4-Diionic Phosphorus Compounds

Abstract
The addition of triphenylphosphine to acetylenic ketones in the presence of strong CH-acids, such as Meldrom's acid, N , N '-dimethylbarbituric acid, dimedone, or indane-1,3-dione leads to 1,4-diionic organophosphorus compounds. Dynamic NMR study of these stable betaines show an unusually high sixfold energy barrier for rotation around the sp 2 -sp 3 carbon-carbon single bond, which leads to an observable atropisomerism.