Boron lower halides

Boron trichloride undergoes several exchange reactions with dihorane, other boron halides and trialkyl-, triaryl-, or triaryloxy boranes (Lower, L.D. 1978. Boron compounds (halides). In Kirk-Othmer Encyclopedia of Chemical Technology, 3rd. ed. pp 129-135. New York Wiley Interscience). The reactions occur at ambient or lower temperatures ... 

Boron forms two simple series of lower halides, BX3 and B2X4(X2B—BX2). The highly reactive trihalides are volatile covalent compounds whose molecules have the expected trigonal planar geometry. The molecules of the other series are also planar in the solid state however, in the gaseous state, the chloride and the bromide... 

Lower LD Boron halides. In M. Grayson (ed) Kirk-Othmer Concise Encyclopedia of Chemical Technology, p 180. New York, Wiley-Interscience, 1985... 

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. 

If the donor molecule contains reactive hydrogen, as H2O, NH3, PH3, AsHs, R2NH, RNH2, and lower alcohols, hydrogen halides are produced (equations 17 -19). In these reactions the initial addncts formed with boron trifluoride are much more stable to protonolysis than with the other trihalides, and these reactions proceed slowly or not at all. 

The reaction is specific for aldehydes. Aromatic ketones, esters, nitriles, halides (Cl, Br) were unreactive as evidenced by the recovery of these substrates. The steric hindrance around the boron atom retarded the reaction (entry 13), but those reactions of ort/io-monosubstituted arylboronic acids proceeded smoothly at 80 C (entry 12). The additions to aliphatic aldehydes such as hex anal and cyclohexanecarbaldehyde were very slow at 80 C due to their lower electrophilicity than that of aromatic aldehydes, but the reaction at 95 T2 in dioxane/H20 achieved high yields of the carbinols (entries 16 and 17). ( )- 1-Hexenylboronic acid also participated in the catalytic reaction, retaining its stereochemistry (entry 18). 

One practical disadvantage of these systems lies in the fact that relatively high concentrations of Lewis acids are needed to aehieve alternation. Thus methyl methaerylate and styrene alternate perfeetly with azodiisobutyronitrile initiation at 50°C when the molar ratio of ZnCL to methyl methacrylate is 0.4. Alternation is less exact, however, when this ratio is 0.25. Alkyl boron halides like ethyl boron diehloride are effective at lower concentrations and act also as initiators if oxygen is present. 

The first moderate-yield process for the formation of B—H bonds from the reaction of Hj with boron halides utilizes passage of mixtures of and BClj through an electrical discharge between Cu electrodes. When H BClj is 10 at 2.67 X 10 Pa, yields of < 60% B H, based on BClj consumed in 2 h, are available. This approach is more convenient when BBrj is used instead of BClj. The lower volatility of the bro-mo species makes the process simpler and separation of products more convenient. This discharge process proceeds in steps ... 

Croft explains the results in terms of the acceptor property of the boron atoms. They tend to take an electron pair from a donor molecule to form sp bonds. This is why only halides in lower oxidation states react with boron nitride (e.g., CuCl and SbCls but not Cuds or SbCls) This is different from the case of graphite in which the carbon planes are a source of electrons. The intercalation of aluminum and ferric chlorides is attributed by Croft to bonding by the electron pairs of the nitrogen atoms of boron nitride. It is surprising, however, that boron trifluoride and trichloride, which are particularly good electron acceptors, appear unable to become intercalated. 

P. Gross, C. Hayman, D. L. Levi and R. H. Lewln, "Stability of Lower Boron Halides", Pinal Technical Report, August 22, 1963, Fulmer Research Institute Limited, England. 

Born tribromide is used as received. It is photosensitive, but can be purified by storage over elemental mercury, followed by storage in the dark. The use of the boron halides BF, and BCl, results in much lower yields and dilTiculties with purification, and they are therefore considered to be unsatisfactory for this reaction. 

Alkyl derivatives of metals such as aluminum, boron and zinc are fairly active Friedel-Crafts catalysts. However, hyperconjugative effects result in a lowering of the electron deficiency. In the case of metal alkoxides this effect is even stronger, and, as a result, they are fairly weak Lewis acids. Metal alkyls, such as alkylaluminums, alkylaluminum halides and sesquihalides are also vital components of Ziegler-Natta catalyst systems which sometimes are utilized for Friedel-Crafts-type reactions. For example, alkylations of aromatics with alkenes in the presence of a Ziegler-Natta catalyst such as AIR3 -1- TiCU results in lower-chain alkylates. Even alkylaluminum halides and sesquihalides serve as Friedel-Crafts catalysts. 

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