Boron compounds, trihalides

Boron-based Lewis acids are often used in organic syntheses. Since the boron atom has an empty / -orbital, many boron compounds can function as Lewis acids. Typical boron Lewis acids are boron trihalides, for which Lewis acidity increases according to the order of fluoride < chloride < bromide < iodide, the reason for this order being the relative abilities of the different halogens to act as 7r-donors to boron. 

The reaction of 1 with the boron trihalides BC13 and BBr3 turned out to be even more complex. At least three different types of compounds were formed, and the product ratio depended on the polarity of the solvent.30 In the reaction with BBr3 in dichloromethane/hexane (2 1), the boron compound 50 (X = Br) was isolated as the main product (Scheme 12) X-ray crystal structure analysis revealed the presence of a novel arachno-type cluster possessing a BC4 framework (Fig. 8). 

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... 

Atmospheric boron is in the form of particulates or aerosols of borides, boron oxide, borates, boranes, organoboron compounds, trihalide boron compounds, or borazines. The half-time persistence of airborne boron particles is short, usually on the order of days (USPHS 1991). 

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... 

Halogen exchange in the coordinatively saturated boron trihalide adducts requires either a preliminary dissociative step or an associative interaction different from the doubly halogen-bridged transition state available to three-coordinate boron compounds [Eq. (1)] (113,114). The reactions can be classified as follows. 

Correlations of NMR parameters to electronic structure have been carried out for a wide range of three-coordinate boron compounds (12, 188). However, the extensive NMR data that exist for boron trihalide adducts (70) are less easily interpreted. Among the factors leading to difficulties in correlating NMR parameters with structure... 

Some of the more active catalysts for the polymerization of styrene are shown in Figure 22.17. Monocyclopentadienyltitanium halides, such as CpTiXj, CpTiXj, and CpTiXj, in combination with MAO, were more active catalysts than simple tetrahalides. Of these Cp-ligated trihalide complexes, the fluoride complexes were most active, followed by alkoxides and then chlorides. Moreover, cyclopentadienyl-ligated alkyltitanium compounds, such as Cp TiRj (R = hydrocarbyl) activated by boron compounds B(C Fj)j... 

The boron trihalides are volatile, highly reactive, monomeric molecular compounds which show no detectable tendency to dimerize (except perhaps in Kr matrix-isolation experiments at 20K). In... 

The boron trihalides form a great many molecular addition compounds with molecules... 

Scheme 3.2-30. Classical spirotin compounds 57 react with boron trihalides to give the 1,6-dihalogeno-substituted nido-C4B2 clusters 56d (X = Cl, Br, I R = Et, Bu).

Boron trihalides, particularly BBr3, add to alkenes in a reaction (haloboration) that is usually more complicated than hydroboration.465 Several compounds can be isolated as a result of secondary transformation of the primary addition product 466... 

Boron subhalides are binary compounds of boron and the halogens, where the atomic ratio of halogen to boron is less than 3. The boron monohalides, BC1, [20583-55-5], bromoborane(l) [19961-29-6], BBr, and iodoborane(l) [13842-56-3], BI, are unstable species that have been observed spectroscopically when the respective trihalides were subjected to a discharge (5). Boron dihalide radicals have been studied, and structural and thermochemical data for these species ( BX2) have been deduced (5). 

Pyridine readily forms stable coordination compounds. Thus, boron, aluminum and gallium trihalides react at 0°C in an inert solvent to give 1 1 adducts (cf. 85). Steric factors are important, and a-substituents decrease the ease of reaction. This is illustrated by the heats of reaction of pyridine, 2-methylpyridine and 2,6-dimethylpyridine with boron trifluoride which are 101.3, 94.1 and 73.2 kJ mol-1, respectively. The marked decrease in exothermicity here should be contrasted with the small steric requirement of the proton as shown by the pA., values of substituted pyridines (see Section 3.2.1.3.4). 

Derivatives of l,2-dihydro-l,3,2-diazaborines are prepared from 2-aminobenzonitrile or 2-aminobenzamides. Compounds (147) were obtained from 2-aminobenzonitrile (148) and boron trihalides (equation 12). However, it was stated that dichlorophenylborane or... 

Thorough studies of solutions of xenon in boron trichloride and boron iribromide were undertaker. A phase study of the melting point of these systems sis a function of composition showed no evidence of compound formation. The Raman spectra of these mixtures are identical to those of pure BX, indicating no noble gas-boron trihalide interactions. 

Lewis acids such as boron trihalides will also form stronger bonds with fluoride than with the other halogens, and can be used to abstract fluoride selectively from halogenated compounds. Some examples of Friedel-Crafts alkylations with fluoro-haloalkanes in which only fluoride is displaced are sketched in Scheme 4.14. As shown by the last example, however, hydride migrations can readily occur under such strongly acidic reaction conditions. 

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). 

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