Boron trihalide compounds

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... 

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... 

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... 

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. 

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... 

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). 

These compounds are easily hydrolyzed and decompose at temperatures above — 15°. They are soluble in polar solvents. Reactions between bis[trifluoromethyl] tellurium dichloride or dibromide with boron trihalides or aluminum trihalides resulted in decomposition1. 

The boron trihalides hydrolyze quickly the bromide and chloride yield boric acid and the halogen acid but the fluoride retains the B—F bonds, forming the addition compound F3B OH2, a strong acid ... 

The chemistry of the boron trihalides has been extensively studied. These compounds are strong Lewis acids and form a wide range of simple 1 1 adducts that have served as model compounds for the study of Lewis acid-base interactions. Many reviews of their coordination chemistry have appeared (41, 60, 66,120, 174, 178) and these are summarized in the more recent reviews appearing in the mid-1960s (41,120). 

Redistribution of substituents tends to be especially facile for halides, hydrides, and alkyls of Groups I—III nontransition elements because these compounds are electron-deficient. Bridging groups are present in many of these compounds. Even in the boron trihalides that are not bridged, a bridged transition state making use of the empty valence shell orbitals is possible, so that redistribution can occur with a relatively low activation energy (113) ... 

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... 

The mixed boron trihalide adducts hold few surprises in terms of their donor-acceptor bond behavior, but provide striking examples of dependence of halogen redistribution behavior on the nonhalogen substituent. The simplicity and accessibility of these systems suggests their use as model compounds in the study of ligand redistribution reactions. Many of the features complicating ligand redistribution in, for example, metal carbonyl systems (46) are simplified or absent here. 

It is of interest that reactions of tetraorganotin compounds and boron trifluoride differ somewhat from the reactions of the other boron trihalides. The reactions of boron trifluoride proceed in a less facile manner, and yield monosubstituted organo-difluoroboranes only. Thus Stone and eoworkcrs reported the... 

The method of choice for the synthesis of borepins, benzoborepins, and heterole-fused borepins is reaction of the corresponding stannepins with boron trihalides or aryl dihaloboron compounds (see Section 14.20.10). A dibenzodi-hydroborepin has been prepared by a [6+l]-type cyclization of a naphthalene-1,8-diborane with BC13 (see Section 14.20.9.1). Yields of these methods are comparable. The saturated systems are typically prepared by reaction of 1,6-heptadienes with monoalkyl boranes such as thexyl borane (see Section 14.20.6.1). 

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