Organoaluminium chemistry

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The ball-and-stick model of diisobutylaluminium hydride, showing aluminium as pink, carbon as black, and hydrogen as white.

Organoaluminium chemistry is the study of compounds containing bonds between

Lewis acidity of the three-coordinated species. Industrially, these compounds are mainly used for the production of polyolefins
.

History

The first organoaluminium compound (C2H5)3Al2I3 was discovered in 1859.

olefin polymerization
. This line of research ultimately resulted in the Nobel Prize to Ziegler.

Structure and bonding

Aluminium(III) compounds

Organoaluminium compounds generally feature three- and four-coordinate Al centers, although higher

mesityl) or isobutyl.[4]

Structure of trimethylaluminium, a compound that features five-coordinate carbon.

Ligand exchange in trialkylaluminium compounds

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

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
.

Low oxidation state organoaluminium compounds

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

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.

Preparation

From alkyl halides and aluminium

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)

Hydroalumination

See also: alkenylaluminium compounds

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

Carboalumination

Main article:
Reactions of alkenyl- and alkynylaluminium compounds

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

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.

Laboratory preparations

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

Reactions

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

Lewis acidity

Organoaluminium compounds are

tertiary amines
. These adducts are tetrahedral at Al.

Electrophiles

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 organo

aluminum 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

halogenated hydrocarbons can be violent.[18][19]

Applications

Organoaluminium compounds are widely used in the production of alkenes, alcohols, and polymers. Some relevant processes include the

polyolefins, for example the catalyst methylaluminoxane
.

References

  1. ISBN 978-0199264636
    .
  2. ^ M. Witt; H. W. Roesky (2000). "Organoaluminum chemistry at the forefront of research and development" (PDF). Curr. Sci. 78 (4): 410. Archived from the original (PDF) on 2014-10-06.
  3. doi:10.1002/jlac.18591090214
    .
  4. ISBN 978-3-527-29390-2
    .
  5. ISBN 978-0-471-02775-1
    .
  6. ISBN 9780120311514
    .
  7. doi:10.1002/14356007.a01_543
  8. ISBN 9780080405957
    .
  9. hdl:2027/spo.5550190.0012.803
    .
  10. PMID 20465291
    .
  11. ISBN 978-0471984160
    .
  12. doi:10.1021/jo00333a041
    .
  13. ^
    doi:10.1007/BF00923507
    .
  14. doi:10.1002/ange.19560682302
    .
  15. ^ Zakharkin, L.I.; Gavrilenko, V.V.; Ivanov, L.L. (1967). Zh. Obshch. Khim. 377: 992. {{cite journal}}: Missing or empty |title= (help)
  16. Continental Oil
  17. PMID 18283706
    .
  18. ^ Cameo Chemicals SDS
  19. ^ Handling Chemicals Safely 1980. p. 929
  20. ISBN 9783527306732
    .
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