Base-catalyzed aryl-B(OH)2 protodeboronation revisited : from concerted proton-transfer to liberation of a transient arylanion

Cox, Paul A. and Reid, Marc and Leach, Andrew G. and Campbell, Andrew D. and King, Edward J. and Lloyd-Jones, Guy C. (2017) Base-catalyzed aryl-B(OH)2 protodeboronation revisited : from concerted proton-transfer to liberation of a transient arylanion. Journal of the American Chemical Society. ISSN 0002-7863

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    Abstract

    Pioneering studies by Kuivila, published more than 5o 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) estimated. Using NMR, stopped-flow IR, and quench-flow techniques, the kinetics of base-catalyzed protodeboronation of 30 different arylboronic acids have now been determined at pH >13 in aqueous-dioxane at 70 °C. Included in the study are all twenty isomers of C6HnF(5-n)B(OH)2 with half-lives spanning nine orders of magnitude: <3 msec to 6.5 months. In combination with pH-rate profiles, pKa, S‡, KIEs (2H, 10B, 13C), linear free-energy relationships, and DFT, we identify 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-dinitrophenyl boronic acid is orders of magnitude more stable than tetra and penta-fluorophenyl boronic acids, but has a similar pKa.