Boron halides adducts

Boron-11 resonance has been rather frequently investigated in boron containing phosphorus compounds which, in most instances, were borane- or boron halide-adducts (see Section IV). Some data are nevertheless available for typical boron-phosphorus compounds which may be discussed in relation to available nB literature (for a general review see 1969, 24). 

M. Hargittai, I. Hargittai, Electron diffraction investigation of the molecular structures of two trimethylamine-boron halide adducts in the vapour phase. J Mol. Struct. 1977, 39, 79-89. 

The monomers are electron pair acceptors, and donor molecules are often able to split the dimeric halide molecules to form adducts thus, whilst the dimeric halides persist in solvents such as benzene, donor solvents such as pyridine and ether appear to contain monomers since adduct formation occurs. Aluminium halides, with the one exception of the fluoride, resemble the corresponding boron halides in that they are readily hydrolysed by water. 

As has already been mentioned, boron halides are electron-deficient molecules. As a result, they tend to act as strong Lewis acids by accepting electron pairs from many types of Lewis bases to form stable acid-base adducts. Electron donors such as ammonia, pyridine, amines, ethers, and many other types of compounds form stable adducts. In behaving as strong Lewis acids, the boron halides act as acid catalysts for several important types of organic reactions (see Chapter 9). 

The organic analogues of the reactions to be discussed here are the borane reductions of aldehydes and ketones and the addition of metal alkyls across ketonic carbonyls, equation 15. In contrast to the ease of these organic reactions, qualitative data which has accumulated in our laboratory over the last decade demonstrates that the carbonyl group in organometallies is fairly resistant to addition across CO. For example, many stable adducts of organometallie carbonyls with aluminum alkyls are known, eq. lc, but under similar conditions a ketone will quickly react by addition of the aluminum alkyl across the CO bond. A similar reactivity pattern is seen with boron halides. 

In reactions between Lewis acids and bases such as amines and boranes or boron halides, bulky substituents on one or both species can affect the stability of the acid-base adduct. Perhaps the most straightforward type of effect is simple steric hindrance between substituents on the nitrogen atom and similar large substituents on the boron atom. Figure 9.3 is a diagrammatic sketch of the adduct between molecules of tripropylamine and triethylborane. This phenomenon is known as front or F-... 

Only in the case of X equals fluorine is the polymeric boron halide-cyanide sufficiently stable to allow isolation of a pure sample. The borane adducts, R3SiCNBH3, require elevated temperatures to yield the polymeric compound HjBCN. Di-n-butylboron chloride and trimethylsilyl cyanide... 

Salts containing these ions can be isolated as solids with large cations such as R4P+. Because they have an unshared pair of electrons on the Sn atom, these ions are Lewis bases that form adducts with Lewis acids such as boron halides ... 

Dialkyl ether adducts are typical of many boron trihalide adduct systems in that the BF3 adducts tend to be stable, whereas the BC13, BBr3, and BI3 adducts become successively more reactive, decomposing to alkyl halide and alkoxyboron dihalide (60,120). This in-... 

Detection of BF C14 species by NMR has been used as evidence for the formation of ionic boron trihalide adducts D2BF2+BFnCl4 from simple covalent adducts (82, 83) and as evidence for the abstraction of chloride ion from (C6H5)3PC12 by BF3 (32). Boron trihalides are often used as halide abstraction reagents, and when two different halogens are involved the mixed-halogen species should be formed, but often only indirect evidence has been obtained (51). 

In a study of the B2Cl4-trichloroethylene system, no isolable adduct was obtainable and all the haloolefin could be recovered upon treatment of the reaction mixture with excess ammonia to remove the boron halide 35). It was reported that peaks attributable to an addition product and to an intermediate compound retaining the double bond were observed in the proton NMR spectrum, but later workers 28, 84, 117) were unable to verify this finding. 

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. 

F N.m.r. spectra of the adducts formed by p-F- and m-F-phenyl-phosphines and -arsines with boron halides reveal a correlation between the chemical shift, J(F,H) and J(P,F), and the stabilities of the adducts formed. ... 

However, despite dimerization of the halides of Al and Ga and of the alkyls of Al, they behave as acceptors and form adducts similar to those given by boron halides and alkyls, for example, Cl3 A1N(CH3)3. Aluminum, like boron, also forms volatile alkoxides such as Al(OC2H5)3, which are similar to borate esters, B(OR)3. 

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