Protodeboronation of heteroaromatic, vinyl and cyclopropyl boronic acids: pH-rate profiles, auto-catalysis and disproportionation

Paul Alan Cox, Andrew G. Leach, Andrew D. Campbell, Guy C. Lloyd-jones

Research output: Contribution to journalArticlepeer-review

Abstract

pH-rate profiles for aqueous-organic protodeboronation of 18 boronic acids, many widely viewed as unstable, have been studied by NMR and DFT. Rates were pH dependent, and varied substantially between the boronic acids, with rate maxima that varied over 6 orders of magnitude. A mechanistic model containing five general pathways (k1-k5) has been developed, and together with input of [B]total, KW, Ka and KaH, the protodeboronation kinetics can be correlated as a function of pH (1-13) for all 18 species. Cyclopropyl and vinyl boronic acids undergo very slow protodeboronation, as do 3- and 4-pyridyl boronic acids (t0.5 > 1 week, pH 12, 70 °C). In contrast, 2-pyridyl and 5-thiazolyl boronic acids undergo rapid protodeboronation (t0.5 ≈ 25-50 sec, pH 7, 70 °C), via fragmentation of zwitterionic intermediates. Lewis acid additives (e.g. Cu, Zn salts) can attenuate (2-pyridyl) or accelerate (5-thiazolyl, and 5-pyrazolyl) fragmentation. Two additional processes compete when the boronic acid and the boronate are present in sufficient proportions (pH = pKa ± 1.6): i) self- /auto-catalysis and ii) sequential disproportionations of boronic acid to borinic acid and borane.
Original languageEnglish
Pages (from-to)9145–9157
Number of pages13
JournalJournal of the American Chemical Society
Volume138
Issue number29
Early online date29 Jun 2016
DOIs
Publication statusPublished - 27 Jul 2016

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