Organoaluminium chemistry

Organoaluminium chemistry is the study of compounds containing bonds between carbon and aluminium. It is one of the major themes within organometallic chemistry.^[1][2] Illustrative organoaluminium compounds are the dimer trimethylaluminium, the monomer triisobutylaluminium, and the titanium-aluminium compound called Tebbe's reagent. The behavior of organoaluminium compounds can be understood in terms of the polarity of the C−Al bond and the high Lewis acidity of the three-coordinated species. Industrially, these compounds are mainly used for the production of polyolefins.

The first organoaluminium compound (C₂H₅)₃Al₂I₃ was discovered in 1859.^[3] Organoaluminium compounds were, however, little known until the 1950s when Karl Ziegler and colleagues discovered the direct synthesis of trialkylaluminium compounds and applied these compounds to catalytic olefin polymerization. This line of research ultimately resulted in the Nobel Prize to Ziegler.

Organoaluminium compounds generally feature three- and four-coordinate Al centers, although higher coordination numbers are observed with inorganic ligands such as fluoride. In accord with the usual trends, four-coordinate Al prefers to be tetrahedral. In contrast to boron, aluminium is a larger atom and easily accommodates four carbon ligands. The triorganoaluminium compounds are thus usually dimeric with a pair of bridging alkyl ligands, e.g., Al₂(C₂H₅)₄(μ-C₂H₅)₂. Thus, despite its common name of triethylaluminium, this compound contains two aluminium centres, and six ethyl groups. When the organoaluminium compound contain hydride or halide, these smaller ligands tend to occupy the bridging sites. Three coordination occurs when the R groups is bulky, e.g. Al(Mes)₃ (Mes = 2,4,6-Me₃C₆H₂ or mesityl) or isobutyl.^[4]

The trialkylaluminium dimers often participate in dynamic equilibria, resulting in the interchange of bridging and terminal ligands as well as ligand exchange between dimers. Even in noncoordinating solvents, Al-Me exchange is fast, as confirmed by proton NMR spectroscopy. For example, at −25 °C the ¹H NMR spectrum 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 methyl groups is too fast to be resolved by NMR.^[5] The high Lewis acidity of the monomeric species is related to the size of the Al(III) center and its tendency to achieve an octet configuration.

The first organoaluminium compound with an Al-Al bond was reported in 1988 as (((Me₃Si)₂CH)₂Al)₂ (a dialane). They are typically prepared reduction of the dialkylaluminium chlorides by metallic potassium:^[6]

(R₂AlCl)₂ + 2 K → R₂Al-AlR₂ + 2 KCl

Another notable group of alanes are tetraalanes containing four Al(I) centres. These compounds adopt a tetrahedrane core, as illustrated by (Cp*Al)₄ and ((Me₃Si₃C)Al)₄. The cluster [Al₁₂(i-Bu)₁₂]²⁻ was obtained from related investigations on the reduction of organoaluminium compounds. This dianion adopts an icosahedral structure reminiscent of dodecaborate ([B₁₂H₁₂]²⁻). Its formal oxidation state is less than one.

Industrially, simple aluminium alkyls of the type Al₂R₆ (R = Me, Et) are prepared in a two-step process beginning with the alkylation of aluminium powder:

2 Al + 3 CH₃CH₂Cl → (CH₃CH₂)₃Al₂Cl₃

The reaction resembles the synthesis Grignard reagents. The product, (CH₃CH₂)₃Al₂Cl₃, is called ethylaluminium sesquichloride. The term sesquichloride refers to the fact that, on average, the Cl:Al ratio is 1.5. These sesquichlorides can be converted to the triorganoaluminium derivatives by reduction:

2 (CH₃CH₂)₃Al₂Cl₃ + 6 Na → (CH₃CH₂)₆Al₂ + 2 Al + 6 NaCl

This method is used for production of trimethylaluminium and triethylaluminium.^[7]

The overall reaction for the production of these simple alkylaluminium compounds is thus as follows:

2Al + 6RX + 6M → Al₂R₆ + 6MX (where M is an alkali metal and X is a halogen)

Aluminium powder reacts directly with certain terminal alkenes in the presence of hydrogen. The process entails two steps, the first producing dialkylaluminium hydrides. Such reactions are typically conducted at elevated temperatures and require activation by trialkylaluminium reagents:

6 Al + 3 H₂ + 12 CH₂=CHR → 2 [HAl(CH₂CHR)₂]₃

For nonbulky R groups, the organoaluminium hydrides are typically trimeric. In a subsequent step, these hydrides are treated with more alkene to effect hydroalumiunation:

2 [HAl(CH₂CHR)₂]₃ + 3 CH₂=CHR → 3 [Al₂(CH₂CHR)₃

Diisobutylaluminium hydride, which is dimeric, is prepared by hydride elimination from triisobutylaluminium:

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

Organoaluminum compounds can react with alkenes and alkynes, resulting in the net addition of one organyl group and the metal fragment across the multiple bond (carboalumination). This process can proceed in a purely thermal manner or in the presence of a transition metal catalyst. For the uncatalyzed process, monoaddition is only possible when the alkene is substituted. For ethylene, carboalumination leads to a Poisson distribution of higher alkylaluminum species. The reaction is regioselective for 1-alkenes.^[8] The so-called ZACA reaction first reported by Ei-ichi Negishi is an example of an asymmetric carboalumination of alkenes catalyzed by a chiral zirconocene catalyst.^[9]

The methylalumination of alkynes in the presence of Cp₂ZrCl₂^[10][11] is employed for the synthesis of stereodefined trisubstituted olefin fragments, a common substructure in terpene and polyketide natural products. The synthesis of (E)-4-iodo-3-methylbut-3-en-1-ol^[12] shown below is a typical application of this reaction:

For terminal alkynes, the reaction generally proceeds with good regioselectivity (>90:10 rr) and complete syn selectivity, even in the presence of propargylic or homopropargylic heteroatom substituents. Unfortunately, extension of the zirconocene-catalyzed methylalumination to alkylalumination with higher alkyls results in lower yields and poor regioselectivities.

Although the simple members are commercially available at low cost, many methods have been developed for their synthesis in the laboratory, including metathesis or transmetalation.

• Metathesis of aluminium trichloride with RLi or RMgX gives the trialkyl:

AlCl₃ + 3 BuLi → Bu₃Al + 3 LiCl

• Transmetalation:

2 Al + 3 HgPh₂ → 2 AlPh₃ + 3 Hg

The high reactivity of organoaluminium compounds toward electrophiles is attributed to the charge separation between aluminium and carbon atom.

Organoaluminium compounds are hard acids and readily form adducts with bases such as pyridine, THF and tertiary amines. These adducts are tetrahedral at Al.

The Al–C bond is polarized such that the carbon is highly basic. Acids react to give alkanes. For example, alcohols give alkoxides:

AlR'₃ + ROH → 1/n (R'₂Al−OR)_n + R'H

A wide variety of acids can be employed beyond the simple mineral acids. Amines give amido derivatives. With carbon dioxide, trialkylaluminium compounds give the dialkylaluminium carboxylate, and subsequently alkyl aluminium dicarboxylates:

AlR₃ + CO₂ → R₂AlO₂CR

R₂AlO₂CR + CO₂ → RAl(O₂CR)₂

The conversion is reminiscent of the carbonation of Grignard reagents.^[13][14][15]

Similarly, the reaction between trialkylaluminum compounds and carbon dioxide has been used to synthesise alcohols, olefins,^[13] or ketones.^[16]

With oxygen one obtains the corresponding alkoxides, which can be hydrolysed to the alcohols:

AlR₃ + 3/2 O₂ → Al(OR)₃

A structurally characterized organoaluminum peroxide is [{HC[C(Me)N-C₆H₅]₂}Al(R)-O-O-CMe₃] [R=CH(SiMe₃)₂].^[17]

The reaction between pure trialalkylaluminum compounds and water, alcohols, phenols, amines, carbon dioxide, sulfur oxides, nitrogen oxides, halogens, and halogenated hydrocarbons can be violent.^[18][19]

Organoaluminium compounds are widely used in the production of alkenes, alcohols, and polymers. Some relevant processes include the Ziegler Process for the production of alcohols from ethylene. Several technologies exist for the oligomerization of ethylene to give alpha-olefins.^[20] Organoaluminium compounds are used as catalysts for alkene polymerization to polyolefins, for example the catalyst methylaluminoxane.