Noncovalent Interactions in Asymmetric Reactions Catalysed by Chiral Phosphoric Acids
Noncovalent interactions play an important role in enzyme catalysis, helping to stabilize transition states. Favourable interactions, including hydrogen bonding, π-stacking, CH⋯π and lone pair⋯π interactions, amongst others, may also be responsible for the regioselectivity and high degrees of stereoselectivity that can be achieved with some small-molecule catalysts. Noncovalent interactions often compete with steric effects and identifying and understanding them is not always straightforward. Nevertheless, this can provide a basis for the rational design of new catalysts, to be applied in the synthesis of single enantiomers needed for various applications ranging from pharmaceuticals to advanced materials. This chapter provides an overview of recent findings in the area of chiral phosphoric acid-catalysed reactions. It considers the types of reactions and the modes of activation possible, the models used to predict stereoselectivity and the types of interactions involved. A few case studies selected from the recent literature illustrate the state-of-the-art in this important branch of catalysis.