Organoaluminium

An organoaluminium compound is an organometallic compound with a carbon to aluminium bond.^{[1] [2]} Relevant organoaluminium compounds are trimethylaluminium, triethylaluminium and Tebbe's reagent. The first organoaluminium compound Et₃Al₂I₃ was discovered in 1859 ^[3]

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

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

• Diisobutylaluminium hydride is prepared by hydride elimination:

(i-Bu₃Al)₂ → (i-Bu₂AlH)₂ + 2 (CH₃)₂C=CH₂

Reactions

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

AlR'₃ + ROH → (R'₂Al−OR)_n + R'H

• Hydroalumination of C=C double bonds
• Industrially, organoaluminium compounds can be used as catalysts for alkene polymerization to polyolefins, for example the catalyst methylaluminoxane.

Low valent organoaluminium

The first organoaluminium compound with a Al-Al bond was synthesized in 1988 as (((Me₃Si)₂CH)₂Al)₂ (a dialane) from reduction of ((Me₃Si)₂CH)₂Al by metallic potassium. Another notable group of alanes are tetraalanes having a tetrahedrane core such as (Cp*Al)₄ or ((Me₃Si₃C)Al)₄

See also

References