Boron halides trihalides

BC13 has a hexagonal bimolecular cell, Cj , P63, with a = 6.08 and c = 6.55 A. The trigonal planar molecules are in A and B positions as for SbCls and Sb(CH3)3Br2 (Figures 4.26 and 4.27). All other boron halides are also planar. Phosphorus trihalides, like NH3 and nitrogen halides, are pyramidal because of the lone electron pair on P and N. 

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. 

Olah and Kuhn S found that fluorochloro-, fluorobromo- and fluoroiodo-alkanes are effective chloro-, bromo- and iodo-aikylating agents, respectively, in Friedel-Crafts alkylations of arenes in the presence of boron halide catalysts (equation 86). Boron trihalides catalyzq reactions of only the C—F bonds, but not of the C—Cl, C—Br or C—I bonds. The order of reactivity of the catalysts was found to be BI3 > BBr3 > BCI3 > BF3, and that of the carbon-halogen bonds C—F > C—Cl > C—Br > C—I. 

Boron Halides.—N.m.r. data have been recorded for the adducts formed by (XC6H4)3 PC1 with boron trihalides." A correlation was found between the shielding of the orfho-protons and the stability of the adducts. 

Two further methods of preparing organoboron compounds have been the subject of fairly recent detailed study, namely, reaction of boron halides with hydrocarbons and addition of boron hydrides to olefins. For instance, arylboron dihalides are formed when boron trihalides are heated with aromatic hydrocarbons, aluminum chloride, and aluminum powder,217 and phenyl-boron dibromide has been prepared analogously.218 Further, boron halides such as diboron tetrachloride and tetrafluoride add to olefins, yielding bis-(dichloroboryl) and bis(difluoroboryl) compounds 219... 

The boron trihalides, BXj, are Lewis acids (Chapter 6). These compounds are monomeric and planar—unlike diborane, B2H5, and the aluminum halides, AI2X5 (Section 3.1.4). As Lewis acids, boron trihalides can accept an electron pair from a halide to form tetrahalobo-rate ions, BX4. Boron halide catalysts act as halide ion acceptors, as in the Friedel-Crafts alkylation of aromatic hydrocarbons (in margin). 

Boron forms trihalides with all the four halogens. Boron trifluoride is the most important halide of boron. 

Boron trihalide-mediated coupling reactions of aryl aldehydes and alkynes have been reported (Scheme 23.21). ° Interestingly, the stereochemistry of the diene product depends on the boron halide used. If boron trichloride is used, the reactions afford (Z, )-l,4-pentadienes as major products, while (Z,Z)-l,4-dienes are obtained using boron tribromide. In order to avoid dihalogenation reactions of the aryl aldehydes, reactions must be carried out at low temperatures. 

AsH3, primary and secondary amines, and lower alcohols, BC13, BBr3, and BI3 react to liberate the corresponding hydrogen halide. Tertiary alcohols and the boron trihalides yield the alkyl halide and boric acid. The boron trihalides hydrolyze readily in water or moist air to produce boric acid and hydrogen halides. 

The strength or coordinating power of different Lewis acids can vary widely against different Lewis bases. Thus, for example, in the case of boron trihalides, boron trifluoride coordinates best with fluorides, but not with chlorides, bromides, or iodides. In coordination with Lewis bases such as amines and phosphines, BF3 shows preference to the former (as determined by equilibrium constant measurements).66 The same set of bases behaves differently with the Ag+ ion. The Ag+ ion complexes phosphines much more strongly than amines. In the case of halides (F, CP, Br, and P), fluoride is the most effective base in protic acid solution. However, the order... 

Boron-sulfur bonds, addition, to alkynes, 10, 778 Boron trihalides, in boron compound synthesis, 9, 146 Boron-zinc exchange and copper-catalyzed substitutions, 9, 518 for organozinc halide preparation, 9, 89 Borostannylation, enynes, 10, 334... 

A third basic procedure of synthesizing 1,3,2-diazaboracyloalkanes involves the interaction of aliphatic a, co-diamines with boron trihalides in the presence of a tertiary amine as hydrogen halide acceptor as is illustrated by Eq. 5 15>17). 

Very little is yet known about the specific chemistry of 1,3,2-diaza-boracycloalkanes. At room temperature, these compounds add hydrogen halide readily in a 1 2 molar ratio 1>. In a similar manner, boron trihalide is added at low temperatures 13>. The solid addition products of the latter reaction decompose near room temperature with cleavage of the original B—N linkages presumably leading to linear products (Eq. 9). 

Diorgano tellurium dihalides form complexes with iodine and interhalogen compounds organic compounds with N, P, O, S, and Se donor atoms boron, aluminum, and gallium trihalides antimony pentachloride and mercury(II) halides. 

---