Boronate anion

The overall mechanism is closely related to that of the other cross-coupling methods. The aryl halide or triflate reacts with the Pd(0) catalyst by oxidative addition. The organoboron compound serves as the source of the second organic group by transmetala-tion. The disubstituted Pd(II) intermediate then undergoes reductive elimination. It appears that either the oxidative addition or the transmetalation can be rate-determining, depending on reaction conditions.134 With boronic acids as reactants, base catalysis is normally required and is believed to involve the formation of the more reactive boronate anion.135... 

The boron in the complex boron anion ends up as BF4e, but the details of this reaction need not concern you. Write the steps that you expect to be involved in the reaction to form R30 and that you can support by analogy with other reactions discussed in this chapter. 

Ogner [1] has described an automated analyser method for the determination of boron-containing anions in plants. This is based on the formation of a fluorescent complex between these anions and carminic acid at pH 7. The plant tissues are ashed at 550 °C and the residue dissolved in 0.5 N hydrochloric acid prior to adjustment to pH 6-7 with sodium carbonate solution. The solution is excited at 470 nm and fluorescence intensities measured at 585 nm. Interferences by the reaction of some cations with carminic acid are overcome by passing the solution through an ion exchange column to exchange the cations for sodium ions. Analytical recoveries of boron anions were in the range 98-104%. The detection limit of the method was 5 xg/l boron. 

The first fluorescence PET sensors for saccharides were based on fluorophore boronic acids. Czarnik and Yoon showed that 2- and 9-anthryboronic acid [50] 19 and 20 could be used to detect saccharides. However, the fluorescence change was small [/ (in the presence of saccharide)// (in the absence of saccharide) = ca. 0.7], The pA/a of the fluorophore boronic acids are shifted by saccharide present in the medium. The extent of the effect is in line with the inherent selectivity of phenylboronic acid [49], The PET from the boronate anion is believed to be the source of the fluorescence quenching. Although... 

The structure determination of crystalline borates is well advanced, and it is hoped that the present enthusiasm will continue. There are many candidates for further investigation, particularly where spectroscopic or dehydration characteristics have suggested a particular boron anion. For example, the borate 2ZnO - 3B203 - 3.5H20 loses its water at temperatures in excess of 260°C, implying that its formula water is in the form of hydroxyl groups. 

Fig. 16.19. Palladium-cata lyzed, stereoselective alkenylation of an arylboronic acid (preparation according to Figure 5.39) with a variety of iodoalkenes. The boronic acid is converted into the boronate anion A. The ion A reacts with the Pd(II) intermediate B via transmetalation subsequent reductive elimination leads to the coupling products.

For instance, GMCSC calculations on the boron anion [2] and on the dilithium molecule [26], both in their ground states, have shown how singleconfiguration wavefunctions, including spin-coupled ones, can be hard-put to provide a robust description of certain highly-symmetric systems. By robust description , we mean one that will not change, at least qualitatively, as more configurations are added to the wavefunction. 

The OBS-GMCSC method offers a practical approach to the calculation of multiconfiguration electronic wavefunctions that employ non-orthogonal orbitals. Use of simultaneously-optimized Slater-type basis functions enables high accuracy with limited-size basis sets, and ensures strict compliance with the virial theorem. OBS-GMCSC wavefunctions can yield compact and accurate descriptions of the electronic structures of atoms and molecules, while neatly solving symmetry-breaking problems, as illustrated by a brief review of previous results for the boron anion and the dilithium molecule, and by newly obtained results for BH3. 

The oxyanion produced in the first step can help stabilize the electron-deficient BH3 molecule by adding to its empty p orbital. Now we have a tetravalent boron anion again, which could transfer a second hydrogen atom (with its pair of electrons) to another molecule of aldehyde. 

The synthesis of boron-substituted ligands starts with the desired R group already attache to the boron. Anions of the type [R Bfpzy are prepared by the following reactions... 

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]. 

Finally, liquids bearing chiral boronate anions have been synthesized starting from boric acid, sodium hydroxide and chiral hydroxyl acids (1-malic acid, mandelic acid and diethyl tartrate) followed by the exchange of the sodium ions with methyltrioctylammonium (Scheme 21). ... 

Scheme 21. Ionic liquids bearing chiral boronate anions.

Boron has a high affinity for fluoride, and boronic acids can be converted, via reaction with KHF2, into trifluoroborates (RBF3K), the fluorine analogues of the boronate anion. These compounds are very stable, but can be reactive under the appropriate conditions and are very useful in palladium-catalysed couplings. 

For boronic acids, coordination of the boron with a nucleophile, such as hydroxide, fluoride or an amine, giving a tetrahedral boronate anion, is necessary to drive transmetallation. 

In this way, tetra(organosilyl)boron anions can be derived from B(OMe)3 ... 

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