Analysis of the relative stability of trigonal and tetrahedral boronate cyclic esters in terms of boronic acid and diol acidities and the strain release effect

Higher stability of tetrahedral hydroxyboronate cyclic esters of 1,2-diols in comparison with the respective trigonal esters and the associated increased acidity of the trigonal esters in comparison with free boronic acids are well-known and important aspects of the thermodynamics of boronate ester formation usually attributed to the strain in O–B–O angle created in the trigonal esters and released in the tetrahedral hydroxyboronates. The stability constants of tetrahedral (K_tet) and trigonal (K_trig) boronate esters of several aliphatic and aromatic 1,2- and 1,3-diols with a series of five boronic acids of variable acidity were determined by potentiometric titrations in water. The ratio K_tet/K_trig varies from ~10 for cis-1,2-cyclopentanediol to ~10⁴ for catechol entirely due to the increase in K_tet for more acidic diols and is weakly affected by the acidity of boronic acids. Analysis of DFT minimized structures of trigonal and tetrahedral esters of phenylboronic acid with cis-1,2-cyclopentanediol, 1,4-anhydroerythritol, and catechol demonstrates the absence of postulated strain release in the tetrahedral esters. A global analysis of new and previously reported relevant data on K_tet, K_trig, and K_tet/K_trig values for 1,2- and 1,3-diols and different boronic acids in terms of the Brønsted-type correlations with pK_a values of the reaction components shows that K_tet/K_trig values vary in a wide range from ~1 to ~10⁶, and this variation can be satisfactorily explained by the stabilization of tetrahedral hydroxyboronate cyclic esters by more acidic diols without invoking of the strain release effect.