Hydroxyboronate anion

Although there is no direct evidence that the boronate anions, such as RB(OH)3, are capable of effecting the transmetafl tiQQjit is quite reasonable to assume a similar effect of the base for the transmetallation of organoboronic acids. The cross-coupling reaction of arylboronic acids with aryl halides at pH 7-8.5 is retarded relative to the reaction at pH 9.5-11 [51]. The of phenylboronic acid is 8.8, thus suggesting the formation of the hydroxyboronate anion [RB((OH)3 at pH > pA and its transmetallation to the palladium(II) halides. The formation of ArB(OH)3 at pH 11-12 has been reported [52]. 

The reversible formation of boronic esters by the interaction of boronic acids and polyols in water was first examined in the seminal study of Lorand and Edwards [49]. This work followed an equally important study on the elucidation of the structure of the borate ion [124]. By measuring the complexation equilibrium between phenylboronic acid and several model diols and monosaccharides using the method of pH depression, ester formation was shown to be more favorable in solutions of high pH where the boronate ion exists in high concentrations (Equation 19, Figure 1.12). This study also confirmed the Lewis acid behavior of boronic acids and the tetracoordinate structure of their conjugate base, i.e., the hydroxyboronate anion (Section 1.2.2.4). Another conclusion is that free boronic adds have lower Lewis add strengths than their neutral complexes with 1,2-diols. For example, the pJC, of PhB(OH)2 decreases... 

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