Aluminium isopropoxide

Chembox new
Name = Aluminium isopropoxide
ImageFile = Al4(OiPr)12.png ImageName = One enantiomorph of Al₄(OiPr)₁₂
IUPACName = Aluminium Isopropoxide
OtherNames = Triisopropoxyaluminium
Aluminium isopropanolate
Aluminium sec-propanolate
Aluminium triisopropoxide
2-Propanol aluminium salt
AIP
Section1 = Chembox Identifiers
SMILES = CC(C)OAl(OC(C)C)OC(C)C
CASNo = 555-31-7
RTECS = BD0975000
Section2 = Chembox Properties
Formula = C₉H₂₁O₃Al
MolarMass = 204.25 g/mol
Appearance = white solid
Density = 1.035 g/cm³, solid
Solubility = Decomposes
Solvent = isopropanol
SolubleOther = Soluble
MeltingPt = Sensitive to purity:
138–142 °C (99.99+%)
118 °C (98+%) [Ishihara, K.; Yamamoto, H. "Aluminum Isopropoxide" in Encyclopedia of Reagents for Organic Synthesis, 2001 John Wiley. DOI: 10.1002/047084289X.ra084]
BoilingPt = @10 torr 135 °C (408 K)
Section3 = Chembox Structure
CrystalStruct = monoclinic
Section7 = Chembox Hazards
MainHazards = Flammable (F)
NFPA-H = 2
NFPA-R = 2
NFPA-F = 1
FlashPt = 16 °C
RPhrases = R11
SPhrases = S8, S16

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.

tructure

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. For aluminium isopropoxide this phenomenon is mainly due to the trimer-tetramer transformation described in detail in the early works by Turova et al. The tetrameric structure of the solid crystalline material 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)₂₃. [cite journal | author = Folting, K.; Streib, W. E.; Caulton, K. G.; Poncelet, O.; Hubert-Pfalzgraf, L. G. | title = Characterization of aluminum isopropoxide and aluminosiloxanes | journal = Polyhedron | year = 1991 | volume = 10 | pages = 1639–46 | doi = 10.1016/S0277-5387(00)83775-4] [Turova, N. Y.; Kozunov, 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.] 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 dimer with the formula Al₂(μ-O-t-Bu)₂(O-t-Bu)₄ [cite book | author = Holleman, A. F.; Wiberg, E. | title = Inorganic Chemistry | publisher = Academic Press | location = San Diego | year = 2001 | isbn = 0-12-352651-5] It is prepared analogously to the isopropoxide. [OrgSynth | author = Wayne, W.; Adkins. H. | title = Aluminum tert-Butoxide | collvol = 3 | collvolpages = 48 | year = 1955 | prep = cv3p0048]

Preparation

A widely accepted method for preparing aluminium isopropoxide was published in 1936 by Young, Hartung, and Crossley. [cite journal | author = Young, W.; Hartung, W.; Crossley, F. | title = Reduction of Aldehydes with Aluminum Isopropoxide | journal = J. Am. Chem. Soc. | volume = 58 | pages = 100–2 | year = 1936 | doi = 10.1021/ja01292a033] 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, and homoallylic alcohols are converted to α,β-unsaturated carbonyls.OrgSynth | author = Eastham, J. F.; Teranishi, R. | title = Δ4-Cholesten-3-one | collvol = 4 | pages = 192 | year = 1963 | prep = cv4p0192] 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. [cite journal | author = Tian, D.; Dubois, Ph.; Jérôme, R. | title = Macromolecular Engineering of Polylactones and Polylactides. 22. Copolymerization of ε-Caprolactone and 1,4,8-Trioxaspiro [4.6] -9-undecanone Initiated by Aluminum Isopropoxide | journal = Macromolecules | year = 1997 | volume = 30 | pages = 2575–2581 | doi = 10.1021/ma961567w]

History

Aluminium isopropoxide was first reported in the Dissertation of Alexandre Tischenko in the Annals of the Russian Physico-Chemical society in 1898. This contribution included detailed description of its synthesis, peculiar physico-chemical behavior and catalytic activity in Tischenko reaction (catalytic transformation of aldehydes into esters). It was later found also to display catalytic activity as a reducing agent by Meerwein and Schmidt in the Meerwein-Ponndorf-Verley reduction ("MPV") in 1925. [cite journal | author = Meerwein, H.; Schmidt, R. | title = Ein neues Verfahren zur Reduktion von Aldehyden und Ketonen | journal = Justus Liebig's Annalen der Chemie | year = 1925 | volume = 444 | pages = 221–238 | doi = 10.1002/jlac.19254440112] 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. [cite journal | author = Oppenauer, R. V. | title = Dehydration of secondary alcohols to ketones. I. Preparation of sterol ketones and sex hormones | journal = Recueil des Travaux Chimiques des Pays-Bas et de la Belgique | year = 1937 | volume = 56 | pages = 137–44]

References