Lewis Acidic Boranes, Lewis Bases, and Equilibrium Constants: A Reliable Scaffold for a Quantitative Lewis Acidity/Basicity Scale

A quantitative Lewis acidity/basicity scale toward boron‐centered Lewis acids has been developed based on a set of 90 experimental equilibrium constants for the reactions of triarylboranes with various O‐, N‐, S‐, and P‐centered Lewis bases in dichloromethane at 20 °C. Analysis with the linear free energy relationship log K (B)=LA (B)+LB (B) allows equilibrium constants, K (B), to be calculated for any type of borane/Lewis base combination through the sum of two descriptors, one for Lewis acidity (LA (B)) and one for Lewis basicity (LB (B)). The resulting Lewis acidity/basicity scale is independent of fixed reference acids/bases and valid for various types of trivalent boron‐centered Lewis acids. It is demonstrated that the newly developed Lewis acidity/basicity scale is easily extendable through linear relationships with quantum‐chemically calculated or common physical–organic descriptors and known thermodynamic data (ΔH [Formula: see text] ). Furthermore, this experimental platform can be utilized for the rational development of borane‐catalyzed reactions.