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

Research output: Contribution to journalArticle

36 Citations (Scopus)

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.
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

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Proton transfer
Boronic Acids
Protons
Protonation
Acids
Rubiaceae
Acidity
Discrete Fourier transforms
Isomers
Free energy
Nuclear magnetic resonance
Association reactions
Kinetics

Keywords

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

Cite this

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title = "Base-catalyzed aryl-B(OH)2 protodeboronation revisited: from concerted proton-transfer to liberation of a transient arylanion",
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.",
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Base-catalyzed aryl-B(OH)2 protodeboronation revisited : from concerted proton-transfer to liberation of a transient arylanion. / Cox, Paul A.; Reid, Marc; Leach, Andrew G.; Campbell, Andrew D.; King, Edward J.; Lloyd-Jones, Guy C.

In: Journal of the American Chemical Society, 21.08.2017.

Research output: Contribution to journalArticle

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AU - Cox, Paul A.

AU - Reid, Marc

AU - Leach, Andrew G.

AU - Campbell, Andrew D.

AU - King, Edward J.

AU - Lloyd-Jones, Guy C.

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AB - 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.

KW - arylboronic acids

KW - penta-fluorophenyl boronic acids

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