Electrophilic Displacement Reactions: Xv. Kinetics and Mechanism of the Base-Catalyzed Protodeboronation of Areneboronic Acids

Abstract

.In investigation of the lrinetics of the protodeboronation of benzeileboronic acid in water in the pH range 2.0 to 6.7 is described. In addition to the acid-catalyzed reaction studied earlier a reaction whose rate is independent of pH and one whose rate increases linearly with hydroxide ion concentration have been observed. The effect of malonate buffer concentration a t low pH confirms the earlier observations of general acid catalysis. Changes in buffer concentration a t pH 6.70 have no effect on rate indicating specific hydroxide ion catalysis. Effect of substituents in the ortho, meta, and para position of the benzene ring on the rate of protodeboronation have been examined. Ortho-para ratios for this reaction are high; possible reasons for this are discussed. The Hainmett equation using u correlates the rates for meta and para substituents.Among the reactions of areneboronic acids in which the dihydroxyboron group is subjected to electrophilic displacement the simple hydrolysis or protodeboronation (eq. [I]) is particularly interesting. Ainley and Challenger (1)showed that the hydrolysis of boronic acid proceeds in water a t 150°, and is catalyzed by concentrated sodium hydroxide, concentrated hydrochloric acid, as well as zinc and cadinium bromides. An~moniacal silver nitrate has been shown t o be an effective catalyst for the protodeboronation of benzeneboronic acid and others of the arene series (2, 3, 4). An understanding of the mechanisms by which these catalysts function would be desirable, and we have embarked upon a series of kinetic investigations toward this end.A fairly extensive and detailed study of the kinetics of acid-catalyzed protodeboronation has been described (5,6,7). Observation of the variation of rate constants in several acidic media, and in deuterium, as well as protium, solvents, along with determination of substituent effects led to the conclusion that the reaction is an electrophilic displacement in which the rate-determining step is the transfer of a proton from an acid t o the boronic acid. In view of this an examination of the same reaction catalyzed by base was in order, especially because the strongest electrophile which might be present in significant concentration in aqueous base might well be water. The possibility that a SE1 type of reaction (eqs. 12-41) might be involved also provided some attraction.

Synthesis and Preclinical Evaluation of11C-UCB-J as a PET Tracer for Imaging the Synaptic Vesicle Glycoprotein 2A in the Brain

“…It is unclear whether this deborylated product is formed as a side reaction during the radiolabeling of the boronic acid precursor, which is known to occur particularly with electron-rich boronic acids (26), or by decomposition of the boronic acid precursor during work-up. Both base-and metal-catalyzed deboronation of boronic acids may occur under typical Suzuki-Miyaura cross-coupling reaction conditions (27,28).…”

19F NMR Spectroscopic Investigation of the Reaction of Bis(pentafluorophenyl)borinic Acid with a “Proton Sponge”: Deprotonation, Trimerization and Stepwise Dearylation

“…[24] In the upper part of Scheme 6 both of the initial steps hypothesized in the literature [24b,25] for base-catalyzed hydrodeboration, involving either direct deprotonation of 1 m or deprotonation of its water adduct, have been reported. Furthermore, the dearylation from 2 to 3 observed in the second context, when 1 has already been completely con-sumed, might occur via dearylation of 1 m .…”

Suzuki Polycondensation: Polyarylenes à la Carte

“…[48] In any case, free boronic acids tend to be more reactive than their ester analogs which shows in the strengths of bases needed in both cases, with the stronger ones for the latter. There are, however, limits to this because protodeboronation, [49] a detrimental side reaction, is also facilitated by stronger bases. [50] It should not be hidden that the aspect of relative reactivity of boronic acid versus ester is also influenced by the rather complicated solubility issue.…”