Organoaluminium

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History

Organoaluminium compounds, little known until the 1950s, have become widely accepted and increasingly important in the field of industry and in the laboratory, particularly after the work of Karl Ziegler and colleagues, who discovered the direct synthesis of trialkylaluminiums and their application to olefin polymerization. A comprehensive review of organoaluminium chemistry appeared in 1972, covering a variety of literature up to late 1971. Other important review articles have been published since then.

Special properties

Generally Al prefers to have >3 coordination due to electron deficiency unless R groups are very bulky, e.g. Al(Mes)₃ (Mes = 2,4,6-Me₃C₆H₂ or mesityl). The chemistry of organoaluminium compounds can be understood in terms of the dynamic nature of the C−Al bond and the high Lewis acidity of the monomeric species.

A good example of the dynamic nature of the C−Al bond is the dimer, Al₂Me₆.

Al−C−Al bridging persists in non-donor solvents with fast Al-Me exchange which can be confirmed by proton NMR. At −25°C the ¹H NMR of Me₆Al₂ comprises two signals in 1:2 ratio, as expected from the solid state structure. At 20°C, only one signal is observed because exchange of terminal and bridging methyls is too fast to be resolved by NMR. The high Lewis acidity of the monomeric species is directly related to the pronounced tendency of the aluminium atom to build up an octet of electrons.

Laboratory Preparation of organoaluminium

R₃Al can be prepared by either metathesis or transmetalation in the laboratory.

• Metathesis with RLi or RMgX

AlCl₃ + 3 BuLi → Bu₃Al + 3 LiCl

• Transmetalation

2 Al + 3 HgPh₂ → 2 AlPh₃ + 3 Hg

• Industrially, simple aluminium alkyls (Me, Et) can be prepared by a direct process.

4 Al + 6MeCl → 2 Me₃Al₂Cl₃ → Me₄Al₂Cl₂ + Me₂Al₂Cl₄ → Me₄Al₂Cl (l) + 2 Na[MeAlCl₃] (s) → Me₃Al + Al + 6 NaCl

Reactions of organoaluminium compounds

• Organoaluminium compounds are hard acids and readily form adducts with bases such as pyridine, THF and amines.
• Reaction with protic reagents:

AlR₃ + ROH → (R₂Al−OR)_n + RH

• Hydroalumination of C=C double bonds
• Industrially, organoaluminium compounds can be used as catalysts for alkene polymerisation to polyolefins.

See also

References