Aluminums boron halides

The most commonly used traditional Lewis acids are halides of aluminum, boron, titanium, zinc, tin, and copper. However, there are also more complex Lewis-acids that are quite effective catalysts that can be easily modified for carring out enantioselective processes, by incorporating chiral ligands. These can overcome some limitations associated with the use of classical Lewis acids [47]. 

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. 

Various boron compounds have been used as rocket fuels, diamond substitutes, and additives to aluminum alloys to improve electrical and thermal conductivity, as well as for grain refining. Boron hydrides are sensitive to shock and can detonate easily. Boron halides ate corrosive and toxic. 

Boron. Boron hydride, dl-, penta- and deca-, and the boron halides, trichloride, trifluoride, are the chief compounds of interest. No adequately documented collection method exists for any of these compounds, and most recent work in the area has been futile. Boron oxide is the only compound that can be analyzed with certainty by P CAM 173. This compound is a nuisance dust like aluminum oxide, and the AAS method provides a means of identifying the compound. 

Although the Friedel-Crafts halides which are listed include the typical halides of aluminum, boron, iron, titanium, and zinc, as well as others, all examples except one utilize zinc chloride as the complexing agent. 

Aluminum and boron halides are sometimes used to dealkylate alkyl aryl ethers to phenols. Boron tribromide cleaves aliphatic ethers to alcohols and alkyl halides, but the reaction has no preparative value in the aliphatic series. Aluminum halide and the ether first form a complex from which a molecule of alkyl halide is eliminated upon heating. 

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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