Aluminum alkyls direct synthesis

Four methods are used for the industrial production of simple aluminum alkyls direct synthesis, the displacement... 

Organoaluminum Compounds. Apphcation of aluminum compounds in organic chemistry came of age in the 1950s when the direct synthesis of trialkylalurninum compounds, particularly triethylalurninum and triisobutylalurninum from metallic aluminum, hydrogen, and the olefins ethylene and isobutylene, made available economic organoalurninum raw materials for a wide variety of chemical reactions (see a-BONDED alkyls and aryls). 

The so-called direct synthesis of aluminum alkyls (49), particularly those of triisobutylaluminum and triethylaluminum, by adding together aluminum metal, hydrogen, and the corresponding olefin were found by a systematic but not very long search. The process is now, I believe, common knowledge. 

In the displacement reaction, reversible -hydride elimination of an alkene from an aluminum alkyl (equation 5) is followed by insertion of a different alkene into the Al-H bond of the resulting diaUcylaluminum hydride (equation 6). Three repetitions of this process yield the new trialkylaluminum compound as in equation (7). Tri-wo-butylaluminum obtained from the direct synthesis is a convenient starting aluminum alkyl for this process. 

It is noteworthy that the transition metals that serve as hydroaluminating catalysts are also active in establishing the equilibrium between aluminum alkyls and their decomposition products, aluminum, hydrogen and alkene (equation 12). Accordingly, these metals, in addition to hafnium, niobium, vanadium, scandium and lanthanum, have found use as activators for the direct synthesis of aluminum alkyls (equation 12, to the left). Probably most of these metal salts will also be capable of accelerating the hydroalumination reaction. 

Alkylation of dianions occurs at the more basic carbon. This technique permits alkylation of 1,3-dicarbonyl compounds to be carried out cleanly at the less acidic position. Since, as discussed earlier, alkylation of the monoanion occurs at the carbon between the two carbonyl groups, the site of monoalkylation can be controlled by choice of the amount and nature of the base. A few examples of the formation and alkylation of dianions are collected in Scheme 1.7. In each case, alkylation occurs at the less stabilized anionic carbon. In Entry 3, the a-formyl substituent, which is removed after the alkylation, serves to direct the alkylation to the methyl-substituted carbon. Entry 6 is a step in the synthesis of artemisinin, an antimalarial component of a Chinese herbal medicine. The sulfoxide serves as an anion-stabilizing group and the dianion is alkylated at the less acidic a-position. Note that this reaction is also stereoselective for the trans isomer. The phenylsulfinyl group is removed reductively by aluminum. (See Section 5.6.2 for a discussion of this reaction.)... 

A new synthesis of ( )-menthofuran (155) has been described which involves a three-step reaction sequence from the cyclohexanone (152) via direct C-alkylation with ethyl 2-iodopropionate to give (153) (Scheme 35). Hydrolysis of the diester (153) with hydrochloric acid afforded 3,6-dimethyl-2,4,5,6,7,7a-hexahydrobenzofuran-2-one (154). The final step in the sequence was the conversion of the a,/3-unsaturated y-lactone ring into the furan ring by reduction with lithium aluminum hydride and 2-propanol to afford (i)-menthofuran (155) in satisfactory yield (80JOC1517). 

Direct methods for large-scale synthesis stemmed from studies by K. Ziegler that showed that aluminum hydride or LiAlH reacts with olefins to give alkyls or alkyl anions—a reaction specific for B and A1 hydrides ... 

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