Acetylacetone

Acac redirects here. For other uses, see ACAC.

Acetylacetone
IUPAC name	Pentane-2,4-dione
Other names	Hacac
Identifiers
CAS number	123-54-6 Y
SMILES	CC(=O)CC(=O)C
Properties
Molecular formula	C5H8O2
Molar mass	100.13 g/mol
Density	0.98 g/mL
Melting point	−23 °C
Boiling point	140 °C
Solubility in water	16 g/100 mL
Hazards
EU Index	606-029-00-0
EU classification	Harmful (Xn)
R-phrases	R10, R22
S-phrases	(S2), S21, S23, S24/25
NFPA 704	2 2 0
Flash point	34 °C
Autoignition temperature	340 °C
Explosive limits	2.4–11.6%
Y (what is this?)  (verify) Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
Infobox references

Acetylacetone is an organic compound with molecular formula C₅H₈O₂. This diketone is formally named 2,4-pentanedione. It is a precursor to acetylacetonate (acac), a common bidentate ligand. It is also a building block for the synthesis of heterocyclic compounds.

Properties

The keto and enol forms of acetylacetone coexist in solution; these forms are tautomers. The C_2v symmetry for the enol form displayed on the left in scheme 1 has been verified by many methods including microwave spectroscopy.^[1] Hydrogen bonding in the enol reduces the steric repulsion between the carbonyl groups. In the gas phase K is 11.7. The equilibrium constant tends to be high in nonpolar solvents: cyclohexane is 42, toluene is 10, THF 7.2, dimethyl sulfoxide (K=2), and water (K=0.23).^[2] The enol form is a vinylogous analogue of a carboxylic acid.

Preparation

Acetylacetone is prepared industrially by the thermal rearrangement of isopropenylacetate.^[3]

CH₂(CH₃)COC(O)Me → MeC(O)CH₂C(O)Me

Laboratory routes to acetylacetone begin also with acetone. Acetone and acetic anhydride upon the addition of BF₃ catalyst:^[4]

(CH₃CO)₂O + CH₃C(O)CH₃ → CH₃C(O)CH₂C(O)CH₃

A second synthesis involves the base-catalyzed condensation of acetone and ethyl acetate, followed by acidification:^[4]

NaOEt + EtO₂CCH₃ + CH₃C(O)CH₃ → NaCH₃C(O)CHC(O)CH₃ + 2 EtOH

NaCH₃C(O)CHC(O)CH₃ + HCl → CH₃C(O)CH₂C(O)CH₃ + NaCl

Because of the ease of these syntheses, many analogues of acetylacetonates are known. Some examples include C₆H₅C(O)CH₂C(O)C₆H₅ (dbaH) and (CH₃)₃CC(O)CH₂C(O)CC(CH₃)₃. Hexafluoroacetylacetonate is also widely used to generate volatile metal complexes.

Acetylacetonate anion

The acetylacetonate anion, C₅H₇O₂⁻, is the conjugate base of 2,4-pentanedione. It does not exist as a free ion in solution, but is bound to the corresponding cation, such as Na⁺. In practice, the existence of the free anion, commonly abbreviated acac⁻, is a useful model.

Sodium acetylacetonate may be prepared by deprotonating acetylacetone with sodium hydroxide in a mixture of water-methanol.^[5]

Coordination chemistry

The acetylacetonate anion forms complexes with many transition metal ions wherein both oxygen atoms bind to the metal to form a six-membered chelate ring. Some examples include: Mn(acac)₃,^[6] VO(acac)₂, Fe(acac)₃, and Co(acac)₃. Any complex of the form M(acac)₃ is chiral (has a non-superimposable mirror image). Additionally, M(acac)₃ complexes can be reduced electrochemically, with the reduction rate being dependent on the solvent and the metal center.^[7] Bis and tris complexes of the type M(acac)₂ and M(acac)₃ are typically soluble in organic solvents, in contrast to the related metal halides. Because of these properties, these complexes are widely used as catalyst precursors and reagents. Important applications include their use as NMR "shift reagents" and as catalysts for organic synthesis, and precursors to industrial hydroformylation catalysts.

C₅H₇O₂⁻ in some cases also binds to metals through the central carbon atom; this bonding mode is more common for the third-row transition metals such as platinum(II) and iridium(III).

Metal acetylacetonates

Chromium(III) acetylacetonate

Cr(acac)₃ is used as a spin relaxation agent to improve the sensitivity in quantitative Carbon-13 NMR spectroscopy.^[8]

Copper(II) acetylacetonate

Cu(acac)₂, prepared by treating acetylacetone with aqueous Cu(NH₃)₄²⁺ and is available commercially, catalyzes coupling and carbene transfer reactions.

Copper(I) acetylacetonate

Unlike the copper(II) derivative, copper(I) acetylacetonate is an air sensitive oligomeric species. It is employed to catalyze Michael additions.^[9]

Manganese(III) acetylacetonate

Ball-and-stick model of ∆-Mn(acac)₃, with Jahn-Teller tetragonal elongation

Mn(acac)₃, a one-electron oxidant, is used for coupling phenols.^[6] It is prepared by the direct reaction of acetylacetone and potassium permanganate. In terms of electronic structure, Mn(acac)₃ is high spin. Its distorted octahedral structure reflects geometric distortions due to the Jahn-Teller effect. The two most common structures for this complex include one with tetrahedral elongation and one with tetragonal compression. For the elongation, two Mn-O bonds are 2.12 Å while the other four are 1.93 Å. For the compression, two Mn-O bonds are 1.95 and the other four are 2.00 Å. The effects of the tetrahedral elongation are noticeably more significant than the effects of the tetragonal compression.^[10]

Nickel(II) acetylacetonate

Stick model of [Ni(acac)₂]₃

"Nickel acac" is not Ni(acac)₂ but the trimer [Ni(acac)₂]₃. This emerald green solid, which is benzene soluble, is widely employed in the preparation of Ni(O) complexes. Upon exposure to the atmosphere, [Ni(acac)₂]₃ converts to the chalky green monomeric hydrate.

Vanadyl acetylacetonate

A ball-and-stick model of VO(acac)₂

Vanadyl acetylacetonate is a blue complex with the formula V(O)(acac)₂. It is useful in epoxidation of allylic alcohols.

Zinc acetylacetonate

The monoaquo complex Zn(acac)₂H₂O (m.p. 138-140 °C) is pentacoordinate, adopting a square pyramidal structure.^[11] Dehydration of this species gives the hygroscopic anhydrous derivative (m.p. 127 °C). ^[12] This more volatile derivative has been used as a precursor to films of ZnO.

Iridium acetylacetonates

Both iridium(I) and Ir(III) form stable acetylacetonato complexes. The Ir(III) derivatives include trans-Ir(acac)₂(CH(COMe)₂)(H₂O) and the more conventional D₃-symmetric Ir(acac)₃. The C-bonded derivative is a precursor to homogeneous catalysts for C-H activation and related chemistries.^[13][14] Iridium(I) derivatives include square-planar Ir(acac)(CO)₂ (C_2v-symmetry).

Aluminium(III) acetylacetonate

Al(C₅H₇O₂)_n, or shortened to Al(acac)₃

C-bonded acetylacetonates

C₅H₇O₂⁻ in some cases also binds to metals through the central carbon atom (C3); this bonding mode is more common for the third-row transition metals such as platinum(II) and iridium(III). The complexes Ir(acac)₃ and corresponding Lewis-base adducts Ir(acac)₃L (L = an amine) contain one carbon-bonded acac ligand. The IR spectra of O-bonded acetylacetonates are characterized by relatively low-energy νCO bands of 1535 cm⁻¹, whereas in carbon-bonded acetylacetonates, the carbonyl vibration occurs closer to the normal range for ketonic C=O, i.e. 1655 cm⁻¹.

Other reactions of acetylacetone

• Deprotonations: Very strong bases will doubly deprotonate acetylacetone, starting at C3 but also at C1. The resulting species can then be alkylated at C-1.
• Precursor to heterocycles: Acetylacetone is a versatile precursor to heterocycles. Hydrazine reacts to produce pyrazoles. Urea gives pyrimidines.
• Precursor to related imino ligands: Acetylacetone condenses with amines to give, successively, the mono- and the di-diketimines wherein the O atoms in acetylacetone are replaced by NR (R = aryl, alkyl).
• Enzymatic breakdown: The enzyme acetylacetone dioxygenase cleaves the carbon-carbon bond of acetyacetone, producing acetate and 2-oxopropanal. The enzyme is Fe(II)-dependent, but it has been proven to bind to zinc as well. Acetylacetone degradation has been characterized in the bacterium Acinetobacter johnsonii.^[15]

C₅H₈O₂ + O₂ → C₂H₄O₂ + C₃H₄O₂

• Arylation: Acetylacetonate displaced halides from certain halo-substituted benzoic acid. This reaction is copper-catalyzed.

2-BrC₆H₄CO₂H + NaC₅H₇O₂ → 2-(CH₃CO)₂HC)-C₆H₄CO₂H + NaBr

References

1. ^ W. Caminati, J.-U. Grabow (2006). "The C2v Structure of Enolic Acetylacetone". Journal of the American Chemical Society 128 (3): 854–857. doi:10.1021/ja055333g. 
2. ^ Solvents and Solvent Effects in Organic Chemistry, Christian Reichardt Wiley-VCH; 3 edition 2003 ISBN 3-527-30618-8
3. ^ Hardo Siegel, Manfred Eggersdorfer “Ketones” in Ullmann’s Encyclopedia of Industrial Chemistry, Wiley-VCH, 2002, Wienheim. doi:10.1002/14356007.a15_077
4. ^ ^{a b} C. E. Denoon, Jr., "Acetylacetone", Org. Synth., http://www.orgsyn.org/orgsyn/orgsyn/prepContent.asp?prep=cv3p016 ; Coll. Vol. 3: 16 
5. ^ Robert G. Charles (1963), "Tetraacetylethane", Org. Synth., http://www.orgsyn.org/orgsyn/orgsyn/prepContent.asp?prep=cv4p0869 ; Coll. Vol. 4: 869 
6. ^ ^{a b} B. B. Snider, "Manganese(III) Acetylacetonate" in Encyclopedia of Reagents for Organic Synthesis (Ed: L. Paquette) 2004, J. Wiley & Sons, New York. doi:10.1002/047084289
7. ^ W. Fawcett, M. Opallo (1992). "Kinetic parameters for heterogeneous electron transfer to tris(acetylacetonato)manganese(III) and tris(acetylacetonato)iron(III) in aproptic solvents". Journal of Electroanalytical Chemistry 331: 815–830. doi:10.1016/0022-0728(92)85008-Q. 
8. ^ Caytan, Elsa; Remaud, Gerald S.; Tenailleau, Eve; Akoka, Sergehjhk,j (2007). "Precise and accurate quantitative ¹³C NMR with reduced experimental time". Talanta 71 (3): 1016–1021. doi:10.1016/j.talanta.2006.05.075. 
9. ^ E. J. Parish, S. Li "Copper(I) Acetylacetonate" in Encyclopedia of Reagents for Organic Synthesis (Ed: L. Paquette) 2004, J. Wiley & Sons, New York. doi:10.1002/047084289X.rc203
10. ^ Cotton, F. Albert; Wilkinson, Geoffrey; Murillo, Carlos A.; Bochmann, Manfred (1999), Advanced Inorganic Chemistry (6th ed.), New York: Wiley-Interscience, ISBN 0-471-19957-5 
11. ^ H. Montgomery and E. C. Lingafelter "The crystal structure of monoaquobisacetylacetonatozinc" Acta Crystallographica (1963), volume 16, pp. 748-752. doi:10.1107/S0365110X6300195X.
12. ^ G. Rudolph and M. C. Henry "Bis(2,4-Pentanedionato)zinc (Zinc Acetylacetonate)" Inorganic Syntheses, 1967, volume X, pp. 74-77.
13. ^ Bennett, M. A.; Mitchell, T. R. B. "γ-Carbon-bonded 2,4-pentanedionato complexes of trivalent iridium" Inorganic Chemistry 1976, volume 15, pp. 2936-8. doi:10.1021/ic50165a079
14. ^ Bhalla, G.; Oxgaard, J.; Goddard, W. A., II, and Periana, R. A., "Hydrovinylation of Olefins Catalyzed by an Iridium Complex via CH Activation", Organometallics, 2005, 24, 5499-5502.doi:10.1021/om050614i
15. ^ Straganz, G.D., Glieder, A., Brecker, L., Ribbons, D.W. and Steiner, W. "Acetylacetone-Cleaving Enzyme Dke1: A Novel C-C-Bond-Cleaving Enzyme." Biochem. J. 369 (2003) 573-581 doi:10.1042/BJ20021047

Further reading

• Bennett, M. A.; Heath, G. A.; Hockless, D. C. R.; Kovacik, I.; Willis, A. C. "Alkene Complexes of Divalent and Trivalent Ruthenium Stabilized by Chelation. Dependence of Coordinated Alkene Orientation on Metal Oxidation State" Journal of the American Chemical Society 1998: 120 (5) 932-941. doi:10.1021/ja973282k
• Albrecht, M. Schmid, S.; deGroot, M.; Weis, P.; Fröhlich, R. "Self-assembly of an Unpolar Enantiomerically Pure Helicate-type Metalla-cryptand" Chemical Communications 2003: 2526–2527. doi:10.1039/b309026d
• Charles, R. G., "Acetylacetonate manganese (III)" Inorganic Synthesis, 1963, 7, 183-184.
• Richert, S. A., Tsang, P. K. S., Sawyer, D. T., "Ligand-centered redox processes for manganese, iron and cobalt, MnL₃, FeL₃, and CoL₃, complexes (L = acetylacetonate, 8-quinolinate, picolinate, 2,2'-bipyridyl, 1,10-phenanthroline) and for their tetrakis(2,6-dichlorophenyl)porphinato complexes [M(Por)] "Inorganic Chemistry, 1989, 28, 2471-2475. doi:10.1021/ic00311a044
• Wong-Foy, A. G.; Bhalla, G.; Liu, X. Y.; Periana, R. A.. "Alkane C-H Activation and Catalysis by an O-Donor Ligated Iridium Complex." Journal of the American Chemical Society, 2003: 125 (47) 14292-14293. doi:10.1021/ja037849a
• Tenn, W. J., III; Young, K. J. H.; Bhalla, G.; Oxgaard. J.; Goddard, W. A., III; Periana, R. A. "CH Activation with an O-Donor Iridium-Methoxo Complex." Journal of the American Chemical Society, 2005: 127 (41) 14172-14173. doi:10.1021/ja051497l
• N. Barta, "Bis(acetylacetonato)zinc(II)" in Encyclopedia of Reagents for Organic Synthesis (Ed: L. Paquette) 2004, J. Wiley & Sons, New York. doi:10.1002/047084289X.rb097

External links

• International Chemical Safety Card 0533