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

Paul A. Cox, Marc Reid, Andrew G. Leach, Andrew D. Campbell, Edward J. King, Guy C. Lloyd-Jones

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74 Citations (Scopus)
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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.
Original languageEnglish
Number of pages11
JournalJournal of the American Chemical Society
Early online date21 Aug 2017
DOIs
Publication statusE-pub ahead of print - 21 Aug 2017

Keywords

  • arylboronic acids
  • penta-fluorophenyl boronic acids
  • proton transfer

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