A Quantitative Map of Boryl Substituent Effects Through Hammett Parameters and Electronic Analysis

Boryl groups are among the most versatile functionalities in modern chemistry, yet their electronic effects remain poorly quantified. Hammett substituent parameters provide a powerful framework for describing substituent effects, but are available for only a limited number of boryl groups. Herein, we combine experimental measurements and computational approaches to establish a comprehensive set of Hammett constants for the most commonly used boryl substituents. Experimental determination was achieved through acidity measurements of meta- and para-substituted benzoic acids and complemented by computational methods for systems inaccessible to direct measurement. The resulting parameters reveal an unexpectedly broad range of electronic behaviors, spanning strongly electron-donating tetracoordinate borates to highly electron-withdrawing dihaloboranes and boranes. Analysis of inductive and mesomeric contributions, supported by Natural Bond Orbital (NBO) calculations, identifies the key structural factors governing these trends, including boron coordination state, heteroatom-to-boron donation, and π-acceptor ability. These results establish a quantitative framework for understanding, comparing, and predicting boryl substituent effects, providing a foundation for the rational design of boron-containing reagents and functional molecules.