Aluminium isopropoxide

Aluminium isopropoxide is the chemical compound usually described with the formula Al(O-i-Pr)₃, where i-Pr is the isopropyl group (CH(CH₃)₂). This colourless solid is a useful reagent in organic synthesis. The structure of this compound is complex, possibly time-dependent, and may depend on solvent.

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Structure

The structure of the metal alkoxides are often complex and aluminium isopropoxide is no exception. The complexity is also reflected in the disputed melting point for the material which could reflect the presence of trace impurities, such as water, slow oligomerisation ("aging") or both. The tetrameric structure was verified by NMR spectroscopy and X-ray crystallography. The species is described by the formula Al[[(μ-O-i-Pr)₂Al(O-i-Pr)₂]]₃.^[2][3] The unique central Al is octahedral surrounded by three bidentate "[[Al(O-i-Pr)₄]]^-" ligands, each featuring tetrahedral Al. The idealised point group symmetry is D₃. The tert-butoxide is a dimeric with the formula Al₂(μ-O-t-Bu)₂(O-t-Bu)₄^[4] It is prepared analogously to the isopropoxide.^[5]

Preparation

A widely accepted method for preparing aluminium isopropoxide was published in 1936 by Young, Hartung, and Crossley.^[6] Their procedure entails heating a mixture of 100 g of aluminium, 1200 mL of Isopropyl alcohol, and 5 g of mercuric chloride at reflux. The process occurs via the formation of an amalgam of the aluminium. A catalytic amount of iodine is sometimes added to initiate the reaction, which can be quite vigorous. Young et al. achieved an 85-90% yield, after purification by distillation at 140-150 °C (5 mm Hg).

Reactions

In a MPV reduction, ketones and aldehydes are reduced to alcohols concomitant with the formation of acetone. This reduction relies on an equilibrium process, hence it produces the thermodynamic product. Conversely, in the Oppenauer Oxidation, secondary alcohols are converted to ketones,^[7] and homoallylic alcohols are converted to α,β-unsaturated carbonyls.^[7] In these reactions, it is assumed that the tetrameric cluster disagregates.

Being a basic alkoxide, Al(O-i-Pr)₃ has been also investigated as a catalyst for ring opening polymerization of cyclic esters.^[8]

History

Aluminium isopropoxide was first reported as a reducing agent by Meerwein and Schmidt in the Meerwein-Ponndorf-Verley reduction ("MPV") in 1925.^[9] The reverse of the MPV reaction, oxidation of an alcohol to a ketone, is termed the Oppenauer oxidation. The original Oppenauer oxidation employed aluminium butoxide in place of the isoproxide.^[10]

References

1. ^ Ishihara, K.; Yamamoto, H. "Aluminum Isopropoxide" in Encyclopedia of Reagents for Organic Synthesis, 2001 John Wiley. DOI: 10.1002/047084289X.ra084
2. ^ Folting, K.; Streib, W. E.; Caulton, K. G.; Poncelet, O.; Hubert-Pfalzgraf, L. G. (1991). "Characterization of aluminum isopropoxide and aluminosiloxanes". Polyhedron 10: 1639-46. doi:10.1016/S0277-5387(00)83775-4.
3. ^ Turova, N. Y.; Fozunov, V. A.; Yanovskii, A. I.; Bokii, N. G.; Struchkov, Yu T.; Tarnopolskii, B. L. Journal of Inorganic and Nuclear Chemistry 1979, volume 41, p. 5.
4. ^ Holleman, A. F.; Wiberg, E. (2001). Inorganic Chemistry. San Diego: Academic Press. ISBN 0-12-352651-5. 
5. ^ Wayne, W.; Adkins. H. (1955). "Aluminum tert-Butoxide". Org. Synth.; Coll. Vol. 3: 48. 
6. ^ Young, W.; Hartung, W.; Crossley, F. (1936). "Reduction of Aldehydes with Aluminum Isopropoxide". J. Am. Chem. Soc. 58: 100-2. doi:10.1021/ja01292a033.
7. ^ ^{a b} Eastham, J. F.; Teranishi, R. (1963). "Δ4-Cholesten-3-one". Org. Synth.: 192; Coll. Vol. 4. 
8. ^ Tian, D.; Dubois, Ph.; Jérôme, R. (1997). "Macromolecular Engineering of Polylactones and Polylactides. 22. Copolymerization of ε-Caprolactone and 1,4,8-Trioxaspiro[4.6]-9-undecanone Initiated by Aluminum Isopropoxide". Macromolecules 30: 2575 -2581. doi:10.1021/ma961567w.
9. ^ Meerwein, H.; Schmidt, R. (1925). "Ein neues Verfahren zur Reduktion von Aldehyden und Ketonen". Justus Liebig's Annalen der Chemie 444: 221-238. doi:10.1002/jlac.19254440112.
10. ^ Oppenauer, R. V. (1937). "Dehydration of secondary alcohols to ketones. I. Preparation of sterol ketones and sex hormones". Recueil des Travaux Chimiques des Pays-Bas et de la Belgique 56: 137-44.