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Base-catalyzed Aryl-B(OH)2 Protodeboronation Revisited: from Concerted Proton-Transfer to Liberation of a Transient Arylanion.

Cox, PA and Reid, M and Leach, AG and Campbell, AD and King, EJ and Lloyd-Jones, GC (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.

Item Type: Article
Additional Information: This document is the unedited author's version of a Submitted Work that was subsequently accepted for publication in Journal of the American Chemical Society, copyright © American Chemical Society after peer review. To access the final edited and published work, see http://dx.doi.org/10.1021/jacs.7b07444
Uncontrolled Keywords: 03 Chemical Sciences
Subjects: Q Science > QD Chemistry
Divisions: Pharmacy & Biomolecular Sciences
Publisher: American Chemical Society
Related URLs:
Date Deposited: 12 Sep 2017 09:32
Last Modified: 14 Sep 2017 02:24
DOI or Identification number: 10.1021/jacs.7b07444
URI: http://researchonline.ljmu.ac.uk/id/eprint/7084

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