text: It is often difficult to distinguish between hydrogen-bond catalysis and general acid catalysis. Hydrogen-bond donors can have varying acidity, from mild to essentially strong Brønsted acids like phosphoric acids. Looking at the extent of proton transfer over the course of the reaction is challenging and has not been investigated thoroughly in most reactions. Nevertheless, strong acid catalysts are often grouped with hydrogen-bond catalysts as they represent an extreme on this continuum and their catalytic behaviors share similarities. The mechanism of activation for these reactions involves initial protonation of the electrophilic partner. This has the effect of rendering the substrate more electrophilic and creating an ion pair, through which it is possible to transfer stereochemical information. 
  Asymmetric catalysis involving nearly complete protonation of substrate has been effective in Mannich reactions of aromatic aldimines with carbon nucleophiles. In addition, aza-Friedel-Crafts reactions of furans, amidoalkylations of diazocarbonyl compounds, asymmetric hydrophosphonylation of aldimines and transfer hydrogenations have also been reported. Chiral Brønsted acids are often easily prepared from chiral alcohols such as BINOLs, and many are already present in the literature due to their established utility in molecular recognition research.