5 years ago

Base-Catalyzed Aryl-B(OH)2 Protodeboronation Revisited: From Concerted Proton Transfer to Liberation of a Transient Aryl Anion

Base-Catalyzed Aryl-B(OH)2 Protodeboronation Revisited: From Concerted Proton Transfer to Liberation of a Transient Aryl Anion
Andrew D. Campbell, Guy C. Lloyd-Jones, Edward J. King, Paul A. Cox, Andrew G. Leach, Marc Reid
Pioneering studies by Kuivila, published more than 50 years ago, suggested ipso protonation of the boronate as the mechanism for base-catalyzed protodeboronation of arylboronic acids. However, the study was limited to UV spectrophotometric analysis under acidic conditions, and the aqueous association constants (Ka) were estimated. By means of NMR, stopped-flow IR, and quenched-flow techniques, the kinetics of base-catalyzed protodeboronation of 30 different arylboronic acids has now been determined at pH > 13 in aqueous dioxane at 70 °C. Included in the study are all 20 isomers of C6HnF(5–n)B(OH)2 with half-lives spanning 9 orders of magnitude: <3 ms to 6.5 months. In combination with pH–rate profiles, pKa and ΔS values, kinetic isotope effects (2H, 10B, 13C), linear free-energy relationships, and density functional theory calculations, we have identified a mechanistic regime involving unimolecular heterolysis of the boronate competing with concerted ipso protonation/C–B cleavage. The relative Lewis acidities of arylboronic acids do not correlate with their protodeboronation rates, especially when ortho substituents are present. Notably, 3,5-dinitrophenylboronic acid is orders of magnitude more stable than tetra- and pentafluorophenylboronic acids but has a similar pKa.

Publisher URL: http://dx.doi.org/10.1021/jacs.7b07444

DOI: 10.1021/jacs.7b07444

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