The Halides of Boron

Boron trifluoride is most important of all the halides of boron and is obtained as a colourless highly pungent gas by heating boron trioxide with CaF2 and Cone. H2S04. 

The mechanism of initiation of cationic polymerisations by metal halides was clarified and systematized to some extent by the discovery of the phenomenon of co-catalysis or co-initiation. But, whereas there was, by the mid-1960s, good evidence that at any rate in many systems the halides of boron, titanium, and tin required a co-initiator, the position with regard to the best-known and most popular initiator, and the one which was of greatest economic significance, aluminium chloride, remained obscure. Of the vast number of published experiments on the system, aluminium chloride + isobutylene, hardly any could provide evidence concerning the initiation reaction, because they were almost exclusively concerned with measurements of yields and degree of polymerisation (DP). 

The halides of boron are readily hydrolyzed by the moisture in air. They react with a wide variety of electron-pair donors to form coordination complexes of the type RsN iBXs. In general, the halogens are not progressively replaced by other groups to form mixed halides except by means of the Grignard reagent. 

Typical examples of this class of Lewis acids are electron-deficient molecules such as the halides of boron, beryllium and aluminium, for example, BCI3, BeCl2 and AICI3. 

The halides of boron, particularly BF3 and BCI3, provide examples of the Lewis acid behavior of boron compounds. For example, BF3 can react with diethyl ether, (C2H5)20, to form an adduct ... 

From Boron Halides. Using boron haUdes is not economically desirable because boron haUdes are made from boric acid. However, this method does provide a convenient laboratory synthesis of boric acid esters. The esterification of boron haUdes with alcohol is analogous to the classical conversion of carboxyUc acid haUdes to carboxyUc esters. Simple mixing of the reactants at room temperature or below ia a solvent such as methylene chloride, chloroform, pentane, etc, yields hydrogen haUde and the borate ia high yield. 

Stoichiometric organocoppcr reactions become more y selective in the presence of boron trifluoride or trialkylboranes1Thus, RCu BF3 gives y substitution with allylic halides, alcohols, carboxylates and mesylates1119. 

At about die same time, die application of the Suzuki coupling, the crosscoupling of boronic acids widi aryl-alkenyl halides in die presence of a base and a catalytic amount of palladium catalyst (Scheme 9.12),16 for step-growth polymerization also appeared. Schliiter et al. reported die synthesis of soluble poly(para-phenylene)s by using the Suzuki coupling condition in 1989 (Scheme 9.13).17 Because aryl-alkenyl boronic acids are readily available and moisture stable, the Suzuki coupling became one of die most commonly used mediods for die synthesis of a variety of polymers.18... 

The hydrogen reduction of the halides of nonmetallic elements, such as silicon (Reaction 8) and boron, is an essential process in the production of semiconductors and high-strength fibers. 

Coatings and monolithic components of h-BN are usually produced by CVD by the reaction of a boron halide with ammonia. MOCVD and plasma-CVD are also used. The reaction of boron trichloride and ammonia is as follows ... 

The formation of boron-group IB bonds succeeds in two ways by transfer of a boryl group from metal-boron compounds to other metals, and by reaction of anionic boranes or carboranes with transition-metal halides. 

In addition, arylthiophene 70 was obtained by a one-pot Suzuki coupling of p-methoxyiodobenzene and 3-bromothiophene via an in situ boronate formation using one equivalent of the thermally stable diborane 69 [55], This method avoids the isolation of boronic acids and is advantageous when base-sensitive groups such as aldehyde, nitriles and esters are present. However, the cross-coupling yields are low when both aryl halides are electron-poor because of competitive homocoupling during the reaction. 

Bidentate boranes with an o-phenylene backbone constitute some of the simplest examples of polydentate Lewis acids with rigid backbones. They can be prepared by the reaction of boron halides with a 1,2-dimetallated benzene derivative. Thus, the reaction of 1,2-bis(chloromercurio)benzene (5) with boron trichloride affords 1,2-bis(dichloroboryl)benzene (6, Scheme 3). ... 

When the distannyl reagent 7 is employed, the fate of the reaction apparently depends on the reaction conditions. Indeed, Eisch observed that the reaction of 7 with boron trichloride at —40 °C in hydrocarbon solvent is not always selective and leads to the formation of both a monoborylated intermediate (10) that slowly converts into 6 (Scheme 5). The outcome of these reactions is apparently further complicated by a fast methyl group transfer from the tin to the boron centers yielding the undesired monoborylated derivative 11. Eisch also found that such reactions are not limited to the case of boron trialide but can be performed with dialkyl boron halide starting materials. For example, the reaction of the distannane... 

Reagents of choice for reduction of epoxides to alcohols are hydrides and complex hydrides. A general rule of regioselectivity is that the nucleophilic complex hydrides such as lithium aluminum hydride approach the oxide from the less hindered side [511, 653], thus giving more substituted alcohols. In contrast, hydrides of electrophilic nature such as alanes (prepared in situ from lithium aluminum hydride and aluminum halides) [653, 654, 655] or boranes, especially in the presence of boron trifluoride, open the ring in the opposite direction and give predominantly less substituted alcohols [656, 657,658]. As far as stereoselectivity is concerned, lithium aluminum hydride yields trans products [511] whereas electrophilic hydrides predominantly cis products... 

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>. 

Halo boronic esters have long been known to undergo anti elimination of boron and halide to form alkenes on treatment with bases as mild as water13, l4. Thus, synthetic schemes involving bases in the presence of boronic esters must avoid any structure incorporating a /(-halogen. 

In the halide group the most active lone pair is found in the phosphortrihalides. The stability of the boron complexes AXZ.BXZ decreases in the sequence PXj -> AsXz SbX3, no antimony compounds being observed. The halides of the sulphur group do not form compounds at all it seems improbable that the only reason for this behaviour is the disproportionation of these halides. 

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