2017/08/21 by Paul Alan Cox, Marc Reid, Andrew G. Leach +3 · 1 citation
Chemistry · Biochemistry, Genetics and Molecular Biology · #Organoboron and organosilicon chemistry #Catalytic Cross-Coupling Reactions #Chemical Synthesis and Analysis
paper · doi:10.1021/jacs.7b07444
openalex publication_date 2017/08/21 · openalex created_date 2017/08/31 · openalex updated_date 2026/08/04
Pioneering studies by Kuivila, published more than 50 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 ( K a ) were estimated. By means of NMR, stopped-flow IR, and quenched-flow techniques, the kinetics of base-catalyzed protodeboronation of 30 different arylboronic acids has now been determined at pH > 13 in aqueous dioxane at 70 °C. Included in the study are all 20 isomers of C 6 H n F (5– n ) B(OH) 2 with half-lives spanning 9 orders of magnitude: <3 ms to 6.5 months. In combination with pH–rate profiles, p K a and Δ S ⧧ values, kinetic isotope effects ( 2 H, 10 B, 13 C), linear free-energy relationships, and density functional theory calculations, we have identified 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-dinitrophenylboronic acid is orders of magnitude more stable than tetra- and pentafluorophenylboronic acids but has a similar p K a .