Electronic boron compounds

Boron creates an electron deficiency in the siHcon lattice resulting in a -type semiconductor forp—n junctions. Boron compounds are more commonly used as the dopant, however (see Boron hydrides). 

Another type of anion, confined for practical purposes to boron compounds, has no unshared electrons at the anionic site, and must be thought of as being formed by addition of hydride to a boron atom (or other atom with an incomplete valence shell). Such structures were not anticipated at the time general heterocyclic nomenclature was developed, and they are only recently being fitted into systematic nomenclature (lUPAC Provisional Recommendation 83.2). Proposals for a suffix to indicate such structures are under consideration (1982). 

It is interesting that all of these crystals except diamond are boron compounds. Note also, that most of them consist exclusively of relatively small atoms. The exception is ReB2. Since Re has a large number of valence electrons the general rule is followed that high hardness is associated with high VED (valence electron density). 

The analogy between the trivalent boron compounds and car-bonium ions extends to the geometry. Although our arguments for a preferred planar structure in carbonium ions are indirect, there is electron diffraction evidence for the planar structure of boron trimethyl and the boron trihalides.298 Like carbonium ions, the boron and aluminum analogs readily form a fourth covalent bond to atoms having the requisite non-bonding electrons. Examples are the compounds with ammonia, ether, and fluoride ion.297... 

This chapter summarizes recent developments in the expanding field of electron-deficient compounds having from three up to 13 skeletal boron and carbon atoms. In particular, the focus will be on the transition of classical organoboranes into non-classical compounds. Therefore, we first want to briefly review electron counting rules and bonding characteristics of these classes. For a more thorough discussion see Chapter 1 by King and Schleyer. 

Often Lewis acids are added to the system as a cocatalyst. It could be envisaged that Lewis acids enhance the cationic nature of the nickel species and increase the rate of reductive elimination. Indeed, the Lewis acidity mainly determines the activity of the catalyst. It may influence the regioselectivity of the catalyst in such a way as to give more linear product, but this seems not to be the case. Lewis acids are particularly important in the addition of the second molecule of HCN to molecules 2 and 4. Stoichiometrically, Lewis acids (boron compounds, triethyl aluminium) accelerate reductive elimination of RCN (R=CH2Si(CH3)3) from palladium complexes P2Pd(R)(CN) (P2= e g. dppp) [7], This may involve complexation of the Lewis acid to the cyanide anion, thus decreasing the electron density at the metal and accelerating the reductive elimination. 

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... 

Being an electron deficient compound, boron trifluoride forms complexes with Lewis bases and compounds that have unshared pair(s) of electrons. With ammonia, it forms boron trifluoride ammonia. Similar coordination compounds are formed with monoethylamine, BF3-NH2C2H5 diethyl ether, CH3CH20(BF3)CH2CH3 and methanol, BF3—OHCH3. It forms a sohd complex HNO3-2BF3 with concentrated nitric acid. 

Only for boron hydride does another type of bond formation have to be assumed. For this compound an octet structure is impossible, since boron has only three valency electrons. The compound BH3 thus would have a sextet structure... 

It has been remarked earlier that the lone electron-pair in PC13 can be used for completion of an octet in boron compounds with BC13 it forms the compound C P.BCIs, the bromide and iodide have been prepared, a less stable compound is formed from BC13 and AsC13 and there is no reaction with SbCl3. 

Hydroboration. Although hydroboration seldom requires a catalyst, hydrobora-tion with electron-deficient boron compounds, such as boric esters, may be greatly accelerated by using transition-metal catalysts. In addition, the chemo-, regio- and stereoslectivity of hydroboration could all be affected. Furthemore, catalyzed hydroboration may offer the possibility to carry out chiral hydroboration by the use of catalysts with chiral ligands. Since the hydroboration of alkynes is more facile than that of alkenes the main advantage of the catalytic process for alkynes may be to achieve better selectivities. Hydroboration catalyzed by transition-metal complexes has become the most intensively studied area of the field.599... 

Yamamoto et al. have shown that the boron compound (CgFs BOH, in a quantity as low as 1 mol%, is able to promote the oxidation of allylic and benzylic alcohols with pivalaldehyde at room temperature.43 This result is not surprising considering the similitude of the electronic structure of boron and aluminium. 

It is well recognized that electron deficiency of boron compounds can be considerably compensated by Jt-back bonding. Exploiting this principle, Noth and Staudigl584 have succeeded in obtaining borenium ions. 

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