Boronic acids in water

Several microwave-assisted protocols for soluble polymer-supported syntheses have been described. Among the first examples of so-called liquid-phase synthesis were aqueous Suzuki couplings. Schotten and coworkers presented the use of polyethylene glycol (PEG)-bound aryl halides and sulfonates in these palladium-catalyzed cross-couplings [70]. The authors demonstrated that no additional phase-transfer catalyst (PTC) is needed when the PEG-bound electrophiles are coupled with appropriate aryl boronic acids. The polymer-bound substrates were coupled with 1.2 equivalents of the boronic acids in water under short-term microwave irradiation in sealed vessels in a domestic microwave oven (Scheme 7.62). Work-up involved precipitation of the polymer-bound biaryl from a suitable organic solvent with diethyl ether. Water and insoluble impurities need to be removed prior to precipitation in order to achieve high recoveries of the products. 

Several microwave-assisted procedures have been described for soluble polymer-supported syntheses. Polyethylene glycol) (PEG)-supported aryl bromides have been shown to undergo rapid palladium(0)-catalyzed Suzuki couplings with aryl boronic acids in water (Scheme 12.16) [63], The reaction proceeded without organic cosolvent... 

The PdCl2-EDTA complex is an efficient catalyst for the Suzuki-Miyaura reactions of aryl and heteroarylhalides with aryl-(heteroaryl)boronic acids in water. Aryl iodides and bromides provide coupled products with TON up to 97,000 (Table 14.3) [194]. 

The groups of Veith, Braunstein, and Bedford reported the use of Pluronic P123 (a polymeric material) as template for the pore structure [52]. Hydrolysis and co-condensation of l,3-bis(3-trimethoxysilylpropyl)imidazolium chloride and tetramethoxysilane in the presence of these vesicles under acidic conditions, followed by immobilization of palladium precursors, resulted in a highly ordered mesoporous material with a two-dimensional hexagonal pore structure with palladium nanoparticles inside the pores. These palladium particles have been successfully utilized for the Suzuki cross-coupling of aryUiaUdes with aryl-boronic acids in water, as well as the aerobic oxidation of alcohols under oxygen and air atmospheres as shown in Scheme 4.10 [53]. 

Water-insoluble iodoarenes react with boronic acids in water in the presence of PEG-6000 as phase-transfer agent, giving biaryls in good yields. ... 

Figure1.6 Ionization equilibrium of boronic acids in water.

Table 33 Transition-metal-free Suzuki-type coupling of aryl halides and boronic acids In water using microwave heating.

Table 1 The binding constants calculated by Lorand and Edwards for phenyl-boronic acid in water at 25°Cf The four monosaccharides most routinely used in the evaluation of sensors for saccharides are highlighted in RED.

In another study, Gallon et al. used polyaniline (PANI) nanofiber-supported palladium nanoparticles as catalysts for the coupling of aryl chlorides with phenyl boronic acid in water [98]. Typically, the aryl chlorides were ineffective in most Suzuki coupling reactions but, by subshtuhng the aryl halide appropriately, it could be shown that Pd nanoparticles were capable of carrying out the transforma-hon in the aqueous phase. 

Table 1 Hsts some of the physical properties of duoroboric acid. It is a strong acid in water, equal to most mineral acids in strength and has a p p o of —4.9 as compared to —4.3 for nitric acid (9). The duoroborate ion contains a neady tetrahedral boron atom with almost equidistant B—F bonds in the sohd state. Although lattice effects and hydrogen bonding distort the ion, the average B—F distance is 0.138 nm the F—B—F angles are neady the theoretical 109° (10,11). Raman spectra on molten, ie, Hquid NaBF agree with the symmetrical tetrahedral stmcture (12).

Standard boron solution. Dissolve 0.7621 g boric acid in water and dilute to 1 L. Take 50 mL of this solution and dilute to 1 L the resulting solution contains 6.667 Ug B per mL. 

Fig. 3 Above Spontaneous gelation mechanism between the phenylboronic acid moiety (boronate ion) in water-soluble PMBV and the hydroxyl groups (diol units) in PVA. Below Photoimages of spontaneously forming PMBV/PVA hydrogel, before gelation (left), after gelation...

Compound LII, on the other hand, can be made readily. It can have either the planar tricovalent boron structure or the "triptych tetra-covalent structure. In the latter structure the nitrogen is attached to boron and should be considerably less basic and nucleophilic than usual. It does in fact react unusually slowly with methyl iodide and with acids. The neutralization reaction with acids in water is not only slow but of zero order with respect to the acid. It is believed to have a rate-determining transformation from the triptych to the more basic form as the first step. 

For all the other halides, Eh-pH conditions have no influence. Boron occurs in water mainly as boric acid H3BO3 and its progressive ionization products at increasing pH. Redox conditions do not affect the speciation state of boron. 

Cotton soaked with boron trifluoride in water and placed on the skin for a day or so resulted in a typical acid burn there was no... 

The proposed mechanism of the boron-catalyzed amidation is depicted in the Figure. It has been ascertained by H NMR analysis that monoacyloxyboronic add 1 is produced by heating the 2 1 mixture of 4-phenylbutyric add and [3,5-bis(trifluoromethyl)phenyl]boronic acid in toluene under reflux with removal of water. The corresponding diacyloxyboron derivative is not observed at all. When 1 equiv of benzylamine is added to a solution of 1 in toluene, the amidation proceeds even at room temperature, but the reaction stops before 50% conversion because of hydrolysis of 1. These experimental results suggest that the rate-determining step is the generation of 1. 

Sires I., P.L. Cabot, F. CenteUas, J.A. Garrido, R.M. Rodriguez, C. Arias, and E. Brillas (2006). Electrochemical degradation of clofibric acid in water by anodic oxidation Comparative study with platinum and boron-doped diamond electrodes. Electrochimica Acta 52 75-85. 

Pinanediol esters cannot be readily displaced by treating with diethanolamine. A convenient method of removing either pinanediol esters or pinacol esters for boronic acids that are insoluble in ethers and are readily soluble in water has been developed. 34 The boronic acid ester, e.g. 21, is incubated with a hydrophobic boronic acid such as phenylboronic acid 34 in a rapidly stirred mixture of water and ether (Scheme 8). After 3 hours, the phases are separated and the aqueous phase is concentrated to give the free boronic acid 22. In this case the reaction is driven to completion by the greater solubility of the free boronic acid in the aqueous phase and the greater solubility of the pinacol ester of phenylboronate in the ether phase. This procedure has also been used to remove the pinanediol ester from Ac-D-Phe-Pro-boroArg-pinanediol 34 (see Section 15.1.7.5). 

One drawback of the Stille coupling is that the tin by-products are toxic and are not easily removed from the product. A solution to this problem developed by Suzuki uses a boronic acid in place of the organ-otin compound. The boron-containing by-products are innocuous and are easily removed because of their solubility in water. The Suzuki coupling has found widespread use in organic synthesis (Scheme 10.20). These reactions are extremely important and the methodology is extensively used, particularly in the search for new pharmaceutical products. 

Oxazaborolidines substituted at boron (1 R = alkyl, aryl) are prepared from a chiral p-amino alcohol and the corresponding boronic acid in a two-step process (eq S). Heat and an efficient method of water removal (i.e. azeotropic distillation, molecular sieves) are required to drive the second step. When R and R are connected, more forcing conditions are necessary, both to complete the second step and to prevent the intermediate from proceeding to an alternate disproportionation product. Alternative procedures using bis(diethylamino)-phenylborane (eq trisubstituted boroxines (eq 7), - and... 

Clathrochelate iron(II) complexes derived from alicyclic dioximes (nioxime, heptoxime, 4-methylnioxime, and octoxime) are of particular interest. The first clathrochelate FeNx3(BOH)2 and FeNx3(BF)2 compounds of this type, prepared by template cross-linking of three dioxime molecules on the Fe2+ ion with a boric acid in water and BF3-(C2H5)20 in re-butanol, respectively, were described in Ref. 40. The syntheses with alicyclic dioximes have been realized for a number of boron-containing cross-linking agents in many cases, in a quantitative yield, and in dilute solutions well [49, 54-61]. 

The boronic acid in 1200 ml of water in a 2 L beaker is heated and stirred for 15-20 min to azeotropically remove a trace of bitolyl and other organic materials. The hot solution is filtered with suction to remove any insoluble matter, and then the beaker and filter cake is rinsed with two 30 mL portions of hot water. The filtrate is slowly cooled to room temperature and then to 0 °C for a few hours. The precipitated white crystals are collected in a Buchner funnel with suction, and are then allowed to stand overnight in air or in a desiccator to obtain p-tolylboronic acid. 50 g, 80-90% yield, mp 240 °C. ... 

In this manner, unprotected nucleosides can be coupled to boronic acids in moderate yields. In the Stille coupling reaction, the palladium catalyst formed in situ from PdCl2 and four equivalents of TPPMS was used effectively to couple various aryl halides with R groups on RSnCl3 (Eq. 3) [16]. The use of this catalyst improved the yield and reproducibility compared to the use of PdCl2 alone. The So-nogashira coupling of alkynes with aryl halides is an additional route for the formation of sp—sp2 bonds. The use of this water-soluble phosphine with Pd(OAc)2... 

Although problems associated with boron (B) in water supplies have been recognized for years in many sectors of the agricultural community, until recently these problems have been limited primarily to arid areas. With the widespread use of boric acid and its salts in domestic and industrial detergent and cleansing formulations, the presence of boron in our waters has taken on the aspects of a more general problem. 

Boron, atomic number 5, occurs naturally as two isotopes, and B, with natural abundances of 19.9% and 80.1%, respectively, (a) In what ways do the two isotopes differ from each other Does the electronic configuration of differ from that of B (b) Draw the orbital diagram for an atom of B. Which electrons are the valence electrons (c) Indicate three major ways in which the Is electrons in boron differ from its 2s electrons, (d) Elemental boron reacts with fluorine to form BF3, a gas. Write a balanced chemical equation for the reaction of solid boron with fluorine gas. (e) AHf for Bp3(g) is —1135.6 kl/mol Calculate the standard enthalpy change in the reaction of boron with fluorine, (f) When BCI3, also a gas at room temperature, comes into contact with water, the two react to form hydrochloric acid and boric add, H3BO3, a very weak acid in water. Write a balanced net ionic equation for this reaction. 

The related direct oxidation of trialkylboranes has been studied 178), as well as the brominolysis and iodinolysis of benzeneboronic acid in aqueous acetic acid (50%) and nt-chlorobenzeneboronic acid in aqueous solution. The latter reveals a pattern involving tetracovalent boron 179). In water solutions, and acetate buffers at constant ionic strength with the w-chloro derivative, plots of log k vs. pH were linear with unit slope (from pH 2 to 5), suggesting specific lyate ion catalysis. In both cases catalysis by fluoride ion was observable, and catalysis by hydroxy acids or diols, which form coordination complexes with boron, was seen in the latter case. Indeed in water solvent, the catalytic constant for fluoride ion is some 6000 times that of the uncatalyzed case. 

The starting point for the development of the Suzuki coupling was a procedure described by Carrera and Sheppard [9]. The protocol was chosen due to the technical feasibility and the structural similarity of the products. Carrera and Sheppard reacted 7-brom-indoles with boronic acids in a biphasic mixture of toluene, ethanol, and water. Sodium carbonate was used as the base, and palladium (O)tetrakis (triphenylphospine) as the catalyst. 

Fig. 1. Comparison between experimental and calculated critical buckllngs as functions of boron concentrations in water moderator. LAttlce description 3.745% enriched VO rods, 0.7544 cm diam, pitch = 1.2522 cm (square), clad thickness 0.0406 cm, poison is boric acid.

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