Origins, and formulation implications, of the pK_a difference between boronic acids and their esters: A density...

The purpose of this study is to try to identify the etiology and formulation implications of the significant pK_a drop in aqueous solution that occurs when boronic acids are reversibly esterified by reaction with alcohols, especially 1,2-diols. Experimental studies have shown that conversion of a boronic acid to a boronic acid ester, both Lewis acids, is accompanied by an increase in the acidity, that is, a lowering of the pK_a value in aqueous solution. The drop in pK_a value has significant implications for the formulation of boronic acid-based drugs. Specifically, in the presence of 1,2-diols, a ΔpK_a, a drop, of about 3 pK_a units has been observed for several boronic acids, including boron-based drugs that have demonstrated value as active pharmaceutical ingredients. Boric acid itself shows an even larger drop of about 4 units. Here, electronic structure calculations are used to investigate the origins of this pK_a change. Specifically, density functional theory (DFT) is used to predict the ΔpK_a for a model boronic acid, methylboronic acid, to two of its esters, the dimethyl ester and the cyclic ester with ethylene glycol. The approach is validated by accurately predicting the experimentally observed increased acidity of the cyclic boronic acid ester relative to the corresponding boronic acid. The origins of the acidity difference are examined in detail. Analysis shows that the primary origin is an electronic effect with a smaller contribution due to the reduced structural flexibility of the boronic ester.