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. (24th August 2022)

• Record Type: Journal Article
• Authors: Martínez‐Aguirre, Mayte A.; Medrano, Felipe; Ramírez‐Rave, Sandra; Yatsimirsky, Anatoly K.
• Is Part Of: Journal of physical organic chemistry. Volume 35:Number 12(2022)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2022-08-24
• Subjects: acidity -- boronic acid -- Brønsted‐type correlations -- cyclic ester -- diol -- stability -- strain; Chemistry, Physical organic -- Periodicals; 547.1
• DOI: 10.1002/poc.4425
• Languages: English
• ISSNs: 0894-3230

Abstract

Abstract: 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 ~10 4 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 p K a values of the reaction components shows that K tet / K trig values vary in a wide range from ~1 to ~10 6, 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. Abstract : The ratio of stability constants of trigonal and tetrahedral boronate cyclic esters K tet / K trig Δp K a = p K a B − p K a E values vary in a wide range from ~1 to ~10 6 as a function of diol acidity (p K a D ), and this variation can be satisfactorily explained by the stabilization of tetrahedral hydroxyboronate cyclic esters by more acidic (smaller p K a D ) diols without invoking of the strain release effect.