Boron and aluminium

The addition of boron hydrides and of aluminium hydrides to olefins are processes of wide application. The former, known as hydroboration, provides a simple laboratory route to boron alkyls, and thence to a wide range of organic compoimds from readily available starting materials, (Chapter 3, p 71). The latter is important industrially and is discussed in the section on aluminium, (Chapter 3, p 91). Addition of boron hydrides (usually diborane or alkylboron hydrides such as (RBH2)2 or (R2BH)2) to olefins occurs under mild conditions at room temperature in an ether solvent. The role of ethers is unclear they may catalyze some reactions, whilst in other cases they are not essential. 

Recently, the corresponding addition of Ga—H bonds to olefins has been described. For example, Et2GaH adds 1-decene giving Et2Ga C10H21, and the dimer (GaHCl2)2 also adds terminal olefins. 

Addition of transition metal hydrides to olefins and the role of such reactions in catalytic processes are discussed in Chapter 9. 

The first two elements of Gp.III, although they have the configuration ns npi, are effectively tervalent since promotion to ns np occurs very readily. They form cations with an inert-gas structure much less readily than the elements of Gp.II which precede them, and their bonding is predominantly covalent. The covalent and ionic radii are given in Table 49. 

Though the first ionisation potential of boron and aluminium is fairly low, the second is high, the p electron being removed much more easily than one of 

The densities and atomic volumes are normal for the places occupied in the Periodic Table. Boron s extremely high m.p. indicates very strong binding forces the structure of several crystalline forms of pure boron have been clearly established. Crystals of the purest material are very hard, 9-10 on Mohs scale. The specific conductance increases about 100 times between 20 and 600 . Aluminium has a low m.p. compared with neighbouring elements its face-centred cubic lattice is characteristic of a true metal it is soft, and its conductance is high. 

Aluminium dissolves readily in caustic alkali solutions  

Boron and aluminium give trihalides. BF3 is most conveniently made by heating BgO with concentrated H2SO4 and a fluoride. It reacts with aluminium chloride and bromide to produce involatile AIF3 and volatile BCI3. The volatility of the boron halides decreases with molecular weight. 

Bis(trimethylstannyl)calcium can be obtained as colourless crystals by stirring hexame-thyldistannane with metallic calcium in THF. The molecule (Me3Sn)2Ca 4 THF has an octahedral structure with some distortion. The CSnC angles are approximately 96.5°, implying the use almost exclusively of p-orbitals on the tin, and, in line with this, 1.l(SnC) is very small at 106 Hz.42 With more hexamethyldistannane, further reaction occurs to give the bis[tris(trimethylstannyl)stannyl]calcium. 

The distannanes react much more rapidly if the metal is dissolved in liquid ammonia, and the main products from Ph6Sn2 and calcium, strontium, or barium are the stannides (Ph3Sn)2M. Addition of HMPA and 18-crown-6 to the barium stannide gives crystals of the ionic complex [Ph3Sn]2[Ba(18-c-6)(HMPA)2], but, under the same conditions, the calcium and strontium stannides give the more complex compounds [(Ph3Sn)3Sn]2[MII(18-c-6)(HMPA)2].43 

Me3Sr SnMe3 Me2N-B, B-NMe2 X Bu3Sn. OMe B 224.6-225.4 pm Me3Srf-BH3 Li + 

The ate complex 19-6 is obtained from the reaction of Bu3SnLi with the methoxybo-rane. The trihydroborate 19-7 is formed from Me3SnLi and BH3, and acts as a hydro-gen-transfer rather than a tin-transfer agent.46 

Stannyl derivatives have also been prepared from higher boranes.50 For example, Li+B5H8 reacts with trimethyltin chloride to give p-Me3SnB5H8 (19-8), in which the bridging nature of the stannyl groups is inferred from the H and nB NMR spectra.51 

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The discontinuities observed correspond to changes in electronic configuration. Boron and aluminium both have one electron in a... 

This is an exothermic process, due largely to the large hydration enthalpy of the proton. However, unlike the metallic elements, non-metallic elements do not usually form hydrated cations when their compounds dissolve in water the process of hydrolysis occurs instead. The reason is probably to be found in the difference in ionisation energies. Compare boron and aluminium in Group III ... 

Of the five Group III elements, only boron and aluminium are reasonably familiar elements. Aluminium is in fact the most abundant metal, the third most abundant element in nature, but the other elements are rare and boron is the only one so far found In concentrated deposits. 

Only thallium of the Group III elements is affected by air at room temperature and thalliumflll) oxide is slowly formed. All the elements, however, burn in air when strongly heated and, with the exception of gallium, form the oxide M2O3 gallium forms a mixed oxide of composition GaO. In addition to oxide formation, boron and aluminium react at high temperature with the nitrogen in the air to form nitrides (BN and AIN). 

Boron and aluminium halides show many similarities but also surprising differences. Table 7.2 gives the melting and boiling points of the MX3 halides. 

Both boron and aluminium chlorides can be prepared by the direct combination of the elements. Boron trichloride can also be prepared by passing chlorine gas over a strongly heated mixture of boron trioxide and carbon. Like boron trifluoride, this is a covalent compound and a gas at ordinary temperature and pressure (boiling point 285 K). It reacts vigorously with water, the mechanism probably involving initial co-ordination of a water molecule (p, 152). and hydrochloric acid is obtained ... 

The tribromide and triodide of both boron and aluminium can be made by the direct combination of the elements although better methods are known for each halide. The properties of each halide closely resemble that of the chloride. 

When boron and aluminium burn in air small quantities of nitride are formed. 

Many of the uses of boron and aluminium compounds have already been discussed. The elements and a number of other compounds also have important applications. 

As is the case for boron and aluminium (in Group 13), the underlying electron cores are those of the preceding noble gases and indeed, as was pointed out in Chapter 7, a much more... 

The enantioselective inverse electron-demand 1,3-dipolar cycloadditions of nitrones with alkenes described so far are catalyzed by metal complexes that favor a monodentate coordination of the nitrone, such as boron and aluminium complexes. However, the glyoxylate-derived nitrone 256 favors abidentate coordination to the catalyst, and this nitrone is an interesting substrate, since the products that are obtained from the reaction with alkenes are masked ot-amino acids (Scheme 12.81). 

These reagents demonstrate two important properties of boron and aluminium compounds. Xf you are uncertain of tbe periodic table just check to see where these two elements come. Neutral tervalent B and Al compound are electron deficient ... 

These two properties may be summarised by saying that tervalent boron and aluminium compounds will accept anions from one molecule and transfer them to another. 

Beryllium has the highest heat of combustion of the solid elements, followed by boron and aluminium. Aluminium is a relatively cheap and useful element, and is used to increase the performance of explosive compositions, such as aluminized ammonium nitrate and aluminized... 

To a solution of B(0-i-Pr)3 (5 mmol) in THF (250 ml) was added A1(0-i-Pr)3 (lmmol) and MeOH (40 ml). The mixture was refluxed for 30 min, after which Na (20 mmol) was added. The mixture was then refluxed until the sodium had dissolved. (1) (10 mmol) was introduced over 15 min. The solution was subsequently refluxed again for 60 min. The mixture was kept under reflux and a solution of (2) (lOmmol) in THF (75 ml) was added dropwise over a period of 8-10 h. After the addition was complete, stirring was continued for 4 h. The mixture was evaporated to dryness and the residue was redissolved in CH2C12- The organic layer was washed with brine to remove the boron and aluminium salts. The solvent was removed in vacuo and the residual solid was chromatographed over silica gel with CH2Cl2/hexane 2 1 as the eluent. The organic layer was evaporated to provide the product (3) in a 52% yield. 

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. 

The influence of the metal atom on mechanism is particularly well illustrated by the pronounced tendency of the alkyls of boron and aluminium to react via coordination complexes in which the metal atom is four-co-ordinate. It is no coincidence that the prime examples of substitution by mechanism SE2(co-ord) occur in reactions of alkyl-boron compounds. 

Both A1 and B are in Group 3 of the periodic table and have three valence electrons in their outer shell. These elements can form three bonds. However, there is still room for a fourth bond. For example in BF3, boron is surrounded by six electrons (three bonds containing two electrons each). However, boron s valence shell can accommodate eight electrons and so a fourth bond is possible if the fourth group can provide both electrons for the new bond. Since both boron and aluminium are in Group 3 of the periodic table, they are electropositive and will react with electron-rich molecules so as to obtain this fourth bond. Many transition metal compounds can also act like Lewis acids (e.g. TiCl4 and SnCl4). 

Other familiar cases of stable dimers are neutral boron and aluminium hydrides. DIBAL, for example, exists as two molecules linked by Al-H-Al bonds in a four-membered ring. Again, the dimer is a practical source of monomer for chemical reactions. 

Methods of Preparation—Alkali Phosphides—Alkaline Earth Phosphides— Copper, Silver and Gold Phosphides—Zino Group Phosphides—Boron and Aluminium Phosphides—Titanium Group Phosphides—Tin and Lead Phosphides—Arsenic, Antimony and Bismuth Phosphides—Chromium, Molybdenum and Tungsten Phosphides—Manganese Phosphides—Iron, Cobalt and Niokel Phosphides—Platinum Phosphides. 

An order of susceptibility to electrophilic attack may be formulated as C6H5 > CeFs > CeCls > CH3, which correlates with similar cleavage reactions of tin derivatives. These reactions have applications in the synthesis of boron and aluminium compounds. 

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