Boron species

Vapor phases ia the B2O3 system iaclude water vapor and B(OH)3(g) at temperatures below 160°C. Appreciable losses of boric acid occur when aqueous solutions are concentrated by boiling (43). At high (600—1000°C) temperatures, HB02(g) is the principal boron species formed by equiUbration of water vapor and molten B2O3 (44). At stiU higher temperatures a trimer (HB02)3(g) (2) is formed. 

Better results can be obtained by genetaling die boronate species widi die aid of sodium medioxide. In diis case, satisfactory transmetalalion occurs on treatment widi Cut. Hills, die ftinctionalized copper reagent 55 can be alkyuylaied widi 1-bromo-l-bexyne at -40 C, ftirnisbing die etiyue 56 in 7596 yield (Sdieme 2.15) [32]. 

This means that although Be2 is unstable, both its 1+ cation and 1- anion are somewhat stable. It also shows that the stability of the boron species is in the order B2+ < B2 < B2. ... 

In order to prove or disprove this hypothesis, you need to find hydrogen in the bulging drum headspace and/or find evidence of sodium borohydride in the EO. The latter might be shown by measuring the element, boron, or by specifically measuring one of the boron species shown in reaction 21.6. 

Better results can be obtained by generating the boronate species with the aid of sodium methoxide. In this case, satisfactory transmetalation occurs on treatment with Cul. Thus, the functionalized copper reagent 55 can be alkynylated with 1-bromo-l-hexyne at —40 °C, furnishing the enyne 56 in 75% yield (Scheme 2.15) [32]. 

Iminoboranes may be identified as a class of molecules with an imino group NR and a varying group X (e.g., F, RO, R2N, R3C) bonded to boron. Iminoboranes (XBNR) belong to the family of neutral two-coordinated boron species which may be arranged systematically in the following way ... 

Alkoxides are usually more difficult to hydrolyze than halides, although hydrolysis can be rapid in activated systems. Pyrimidinethiones can sometimes be hydrolyzed directly to pyrimidinones, but it is often better to convert the thiones into alkylsulfenyl, alkylsulfmyl, or alkylsulfonyl derivatives before hydrolysis <1994HC(52)1, 1996CHEC-II(6)93>. The formation of 5-hydroxypyrimidines is not normally performed using hydrolytic procedures, although it can be achieved by the oxidation of boronate species in aqueous solution <1996CC2719, 2006TL7363>. 

Direct aminomethylation at the 5-position of 5-bromopyrimidine 226 to afford 229 has recently been achieved using potassium iV-(trifluoroboratomethyl)piperidine 228 as the boron species <20070L1597>. 

In water, boron readily hydrolyzes to form the electrically nentral, weak monobasic acid H3BO3 and the monovalent ion B(OH)4. Waterborne boron may be adsorbed by soils and sediments (USPHS 1991). The predominant boron species in seawater is boric acid (Thompson et al. 1976) concentrations are higher at higher salinities and in proximity to indnstrial waste discharges (Liddicoat et al. 1983 Narvekar et al. 1983). In seawater, borate or boric acid occurs naturally at... 

The most important attributes of this invention are high impulse performance coupled with high exit temperature on primary combustion and favorable boron species in the primary motor exhaust. The system is also insensitive to impact and possesses excellent thermal stability at elevated temperatures. Additionally, the system is readily castable since the addition of solid oxidizers is not required. Further, high flexibility in the ballistic properties of the gas generator can be achieved by the addition of solid oxidizers such as ammonium nitrate, ammonium perchlorate, hydroxylammonium perchlorate, potassium perchlorate, lithium perchlorate, calcium nitrate, barium perchlorate, RDX, HMX etc. The oxidizers are preferably powdered to a particle size of about 10 to 350 microns [13]. 

The results of these studies were then used to demonstrate the versatility of the reaction by coupling a variety of boronic species with the same resin-bound iodobenzoate (Scheme 30). Catalyst suitability was found to depend upon the specific reaction performed, and the yields were again moderate to excellent. Of particular note is the success of the Pd(II) catalyst [presumably generating Pd(0) in situ], in contrast to the studies outlined in... 

Various neutral and, especially, anionic boron species are well known as ligands in transition metal complexes. In the majority of cases, however, one or more H, N or other basic atom of the boron compounds is coordinated to the transition metal (e.g. Chapters 13.8, 13.6 and 19). In the last 20 years, however, several complexes containing boron-transition metal bonds have been prepared with ligands such as sBX, -BX2 or BX3 (Table 14). 

B NMR was used to quantify the concentrations of the various boron species in solution. Borate diesters exhibit signals distinct from those of borate esters, and both are distinct from unbound boron. Therefore, with knowledge of the solution pH — to partition the unbound boron signal into boric acid and the active borate anion — as well as total boron concentration, the actual species concentrations can be determined. 

With these assumptions, the distribution of boron species (among boric acid, borate anion, borate esters, and borate diesters) reduces to a function of boron concentration, polymer concentration, radius of gyration and solvated volume fraction of a polymer unit, solution pH, and the association constants for the borate esters of the functional groups employed. 

From all the above information, we can describe the important modification during the boronation of zeolites (3 as follows very limited boron atoms are inserted into the (3 framework by treating the sample with an alkaline solution containing boron species. Accompanied by this insertion, a considerable amount of silicon atoms are extracted from the lattice, resulting in the micropores in crystallites are enlarged into the mesopores and the smaller mesopores are developed into larger intracrystalline mesopores. Meanwhile, the corrosion of outer layer of crystallite makes the size of crystal particle reduce. 

In conclusion, decrease in cyclohexanone oxime yield and caprolactam selectivity with time on stream is a major factor in the use of boria on alumina catalyst in the rearrangement reaction. Coke deposition and basic by-product adsorption have been suggested as a means of deactivation. In addition the conversion of water soluble boron, which is selective to lactam formation, to an amorphous water insoluble boron species is another factor that can account for the catalyst deactivation. 

Equilibrium studies in dilute aqueous solutions cannot alone help to distinguish between species that differ only by varying amounts of water. One may learn, for example, that a boron species exists with one boron atom and no charge, and another with one boron atom and the charge —1. One may write them (because of other evidence) as B(OH) and B(OH)4", but as far as the equilibrium data are concerned what we call [B(OH)4 ] might well be a sum of the concentration of B02, H2BO3, and B(OH)4". This must also be understood when we use conventional formulas for the concentrations of, say H+ and H2CO ... 

The mode of dissolution of metal borates in aqueous solution is complex. Hydrolysis of the borate anion can result in completely different boron species that are stable only under particular conditions of pH, temperature, and concentration. 

Fig. 5. Variation in the distribution of boron species with solution pH at 25°C. Total boron concentration is 0.4 M (188).

Vibrational spectroscopy has proved to be the most useful technique yet for identifying which boron species are present in solution. Close agreement between solution spectra and those of crystalline borates of known structure have confirmed the presence of hitherto postulated polyborate ions. Details of the IR spectra of the NajO-BaOs-H O system at 26°C with absorptions assigned to polyborate species are shown in Table III (416, 417). In a more recent study (126), the major ions in 0.5 M boron solution were identified as BsOefOH), B405(0H)4 , and B(OH)4-. 

Other papers by the same authors (208, 399, 400, 403) discuss the partial and complete hydrolysis of many borates and give details of various resulting boron species. 

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