Boron compounds stabilization

Among the three subcategories, boronate compounds seemed to be the most efficient in coordinating with anions and enhancing lithium ion stability, although the number of electron-withdrawing substituents in boronate is only two. The authors thus inferred that the ability of these anion receptors to capture an anion depends not only on the electron-deficiency of the core atom but perhaps also on the steric hindrance presented by these substituents on the core. With only two substituents, the core of the boronates is obviously more exposed and therefore more easily accessible for an anion. The higher ion conductivity achieved by boronate additive therefore comes from the better balance between the electron-deficiency and steric openness of this compound as... 

Whether or not such electrophilic organometallic species can be identified, or indeed isolated, depends primarily on the stability of the counteranion. The per-fluorophenyl boron compounds B(C,sF5)3 and [B(C6F5)4] , first prepared by Stone and co-workers in 1963 [33], proved particularly useful in this respect. Their use in metallocene polymerisation catalysis [34, 35] led to significantly more active catalysts and well-defined catalyst systems that proved mechanistically informative. These results have then enabled similar species to be detected in the more complex MAO-activated catalyst systems (vide infra). 

Unlike aldehydes and ketones, allylic boron compounds are not ubiquitous, commercial organic substrates. There are several methods for the preparation of allylic boronates, however, and many of these have been developed in the past decade. This topic has been reviewed recently " so only the most common methods are emphasized in this section. As a result of the lesser stability of allylic boranes, methods to access these reagents are more limited and it is generally easier to prepare allylic boronates with a wide range of functional groups. 

The stability of the complexes also depends on the halogen ion in the boron compounds the fluorides are not stable, no compound being observed in the system BF3—PF3. The experimental data are too scarce to decide whether the bromides and iodides are more, or less, stable than the chlorides. 

One chemical property quoted in support of aromatic character is kinetic stability towards hydrolytic break-down. Many boron heterocycles have been stated to be more stable than expected for organoboranes. On the other hand, several saturated cyclic boron compounds are stable as well, suggesting that the cyclic structure itself is favorable. 

If the system contains three electrons, the two occupying 4 will be stabilized, and the other one, localized in XV2, destabilized. Here, the stability of the molecule depends upon the relative energies of 4, Tf, and the AOs thus, HHe dissociates spontaneously, but the three-electron bond in He2+ is moderately robust. Note that, in contradiction with Lewis theory, a covalent bond may be formed with one or three electrons. Electron-deficient bonds (where there are fewer than two electrons per bond) are particularly prevalent amongst boron compounds. 

Liquid sulfur trioxide is used for sulfonation, especially in the manufacture of detergents. In the past, the difficulty was the instability of the sulfur trioxide. However, under the trade name Sulfans , stabilized forms of sulfur trioxide are available several patented inhibitors such as boron compounds, methane sulfonyl chloride, sulfur, tellurium, and phosphorus oxychloride inhibit crystallization or conversion to a polymer. 

Lack of space prevents descriptions of a number of additional types of boron compounds such as the numerous interstitial metallic borides, the subhalides (of type B2CI4), the organoboron compounds (such as B(CH3)3), and the boron-nitrogen addition compounds, many of which have been prepared by H. C. Brown to study the effects of structures on the stability of Lewis acid-base adducts. ... 

The carbon-boron heterocycle, 3-phenyl-3-benzoborepin, exhibits oxidative stability upon exposure to air, an unusual feature for a trivalent boron compound. In Table XVI are recorded the chemical shift data for the vinyl protons for the benzometallepins of B, Sn, and Si. The PMR spectrum of 3-phenyl-3-benzoborepin exhibits vinyl proton resonances at lower fields than would be expected for an olefinic boron compound (compared to trivinylboron or 4,5-dihydroborepin see Table XV), and also at lower field than the benzostannepin derivative (217). The shift to lower field of 0.4 to 0.8 ppm may be consistent with the presence of a ring current, which would require the participation of the Bp orbital in the 7r-electron system. Support for increased electron density at boron might be provided from B NMR measurements, but such data have not yet been reported. Complexation of boron, which converts the... 

The examination of structure/property relations of molecular educts and resulting ceramics required the synthesis of stoichiometrically and stmcturally different precursors. A variety of synthesis routes for Si-B-N-C precursors fiom organosilanes, silazanes, and boron compounds have been reported in recent years [3 - 5]. As an example, Riedel obtained a polymeric precursor via hydroboration of methylvinyldichlorosilane and subsequent condensation of the hydroboration product with ammonia (Eq. 1). Pyrolysis led to silicoboron carbonitride ceramics exhibiting thermal stability up to 2000 °C [6]. 

Boron-stabilized carbanions. Ordinarily bases coordinate with the boron atom of organoboranes. However, Rathke and Kow report that a highly hindered lithium amide such as lithio-2,2,6,6-tetramethylpiperidine or lithio-t-butylneopentylamine2 can remove the a-proton from an organoborane to generate carbanions. Thus treatment of the boron compound B-methyl-9-borabicyclononane (1)J in benzene with the former base for 12 hr. at room temperature followed by quenching with deuterium oxide results in deuterium incorporation of 50% (equation I). 

An improvement over this earlier system was attained with the addition of boron compounds such as boric acid or borate salts [106-108], It has been hypothesized that boric acid and calcium form intramolecular bonds which effectively crosslink or staple an enzyme molecule together [107,108], The use of polyols such as propylene glycol, glycerol, and sorbitol in conjunction with the boric acid salts further enhances the stability of these enzymes [109-111], The patent literature contains numerous examples of enzyme stabilization systems that utilize borates, polyols, carboxylate salts, calcium, and ethanolamines, or combinations thereof [91,112-115],... 

In many boron compounds, the boron atoms form a fragment of the regular icosahedron, or a combination of such fragments. We have carried out free-electron calculations on such structures. The results have been checked by constructing the corresponding waves (except for B18H22). To explain the relative stabilities of various alternate structures, one has to make the delta-function potential at each nucleus positive. 

---