Recent Progress in Equilibrium Acidity Studies of Organocatalysts

This account summarizes our recent work on the p K _a scales of some frequently used organocatalysts, especially those of hydrogen-bond-donor catalysts and stronger Bronsted acid catalysts. Most of these p K _a values were obtained by the Bordwell overlapping indicator method, which is known to provide high accuracy. Linear free-energy relationships associated with p K _a values are discussed in relation to understanding of reaction mechanisms. 1 Introduction 2 Single Hydrogen-Bonding Donors 2.1 Proline-Type Organocatalysts 2.2 Cinchona Alkaloids Bearing a Hydrogen-Bonding Donor in the 6′-Position 3 Double-Hydrogen-Bonding Donors 3.1 Thioureas 3.2 Squaramides 3.3 BINOLs 4 Stronger Bronsted Acids 5 N-Heterocyclic Carbenes 6 Summary and Outlook.