Acetylacetone is an organic compound of molecular formula C5H8O2. This diketone is officially named 2.4-pentanedione, although, as explained below, this name does not correctly describe the predominant structure, as it is a vinyl carboxylic acid. It is a precursor of the common bidentate ligand and a basic element for the synthesis of heterocyclic compounds.
Characteristics of acetylacetone:
The keto and enol forms of acetylacetone coexist as a solution; these forms are tautomer. The C2v symmetry for the enol shape displayed on the right in diagram 1 has been verified by many methods, including microwave spectroscopy. The hydrogen bond in the enol reduces the steric repulsion between the carbonyl groups. In the gas phase, K is 11.7. The constant tends to be high in non-polar solvents: cyclohexane is 42, toluene is 10, THF 7.2, dimethyl sulfoxide (K = 2) and water (K = 0.23).
The preparation process of acetylacetone:
Two common procedures are used to synthesize acetylacetone. Acetone and acetic anhydride react when the BF3 catalyst is added.
(CH3CO)2O + CH3C(O)CH3 CH3C(O)CH2C(O)CH3
The second synthesis involves condensation catalyzed by a base of acetone and ethyl acetate, followed by acidification
NaOEt + EtO2CCH3 + CH3C(O)CH3 NaCH3C(O)CHC(O)CH3 + 2EtOH
NaCH3C(O)CHC(O)CH3 + HCl CH3C(O)CH2C(O)CH3 + NaCl
Due to the ease of these synthesizes, many analogs of acetylacetonates are known. Some examples include C6H5C(O)CH2C(O)C6H5(dbaH) and (CH3)3CC(O)CH2C(O)CC(CH3)3. Hexafluoro acetylacetonate is additionally widely want to generate volatile metal complexes.
Acetylacetone as anion:
C5H7O2– is the combined base of the 2.4-pentanedione. In reality, the free ion does not exist as a solution but is linked to the corresponding cation, such as Na+. In practice, the existence of free anion, commonly abbreviated acac–, is a useful model.
Acetylacetonate anion forms complexes with numerous transition metal ions in which the two oxygen atoms bind to the metal to form a six-chain chelate cycle. Some examples: Mn(acac)3, VO(acac)2, Fe(acac)3 and Co(acac)3. Any complex shape M(acac)3 is chiral (has a mirror image not superimposed). In addition, M(acac)3 complexes can be reduced electrochemically, with the reduction rate dependent on the solvent and the metal center. The Bis- and sorting complexes of type M(acac)2 and M(acac)3 are generally soluble in organic solvents, unlike related metal halide. Because of these properties, these complexes are mostly used as catalyst precursors and reagents. Important applications include their use as “shift reagents” RMN and as catalysts for organic synthesis and precursors of industrial hydroformylation catalysts.
C5H7O2– in some cases also binds to metals via the central carbon atom; this binding mode is more common for third-row transition metals such as platinum (II) and iridium (III).
Chromium (III) acetylacetonate
Cr(acac)3 is used as a spin relaxation agent to improve sensitivity in quantitative carbon 13 NMR spectroscopy.
Copper (II) acetylacetonate
Cu(acac)2, prepared by treating acetylacetone with commercially available, watery Cu(NH3), catalyzes carbene coupling and transfer reactions.
Copper (I) acetylacetonate
Unlike copper chelate (II), copper acetylacetonate (I) is an air-sensitive oligomer species. It is used to catalyze Michael’s additions
Manganese (III) acetylacetonate
Mn(acac)3, an electron oxidizer, is used to pair phenols. It is prepared by the direct reaction of potassium acetylacetone and permanganate. In terms of electronic structure, Mn(acac)3is high spin. Its distorted octahedral structure reflects geometric distortions due to the Jahn-Teller effect. The two most common structures for this complex include a tetrahedral elongation and one with tetragonal compression. For the lengthening, two Mn-O links are 2.12 Å, while the other four are 1.93 Å. For compression, two Mn-O links are 1.95 and the other four are 2.00 Å. The effects of tetrahedral elongation are significantly greater than the effects of tetragonal compression.
Nickel (II) acetylacetonate
“Nickel acac” is not Ni(acac)2 but the trimer [Ni (acac)2]3. This emerald green solid, which is soluble in benzene, is widely used in the preparation of Ni(O) complexes. During exposure to the atmosphere, [Ni (acac)2]3 transforms into a chalky green monomer hydrate.
Vanadyl acetylacetonate is a blue V(O)(acac)2 complex. It is useful in the epoxidation of allylic alcohols.
The Zn (acac)2H2O (m.p. 138-140 oC) complex is pentacoordinate, adopting a square pyramidal structure. Dehydration of this species gives the hygroscopic derivative anhydride (m.p. 127 oC). This more volatile derivative was used as a precursor to ZnO films.
C5H7O2– in some cases also binds to metals via the central carbon atom (C3); this binding mode is more common for third-row transition metals such as platinum (II) and iridium (III). The Ir (acac)3 complexes and the corresponding Lewis Ir (acac)3L (L = amine) addition sets contain a carbon-related acac ligand. The IR spectra of O-binding acetylacetonate are characterized by relatively low-energy vCO bands of 1535 cm-1, while in carbon-related acetylacetonate, carbonyl vibration occurs closer to the traditional range for C=O ketone, 1655 cm-1.
Other Chemical reactions of acetylacetone:
Deprotonations: very strong bases will doubly deprotonate acetylacetone, from C3 but also from C1. The resulting species can then be alkylated in C-1.
Forerunner of heterocycles: acetylacetone is a versatile precursor to heterocycles. Hydrazine reacts to produce pyrazoles. Urea gives pyrimidines.
Forerunner of the related imino ligands: acetylacetone condenses with amines to give successively the mono- and di-diketimines in which the O atoms in the acetylacetone are replaced by NR (R = aryl, alkyl).
Enzymatic decomposition: The acetylacetone enzyme dioxygenase clive the carbon-carbon bond of diketones, producing acetate and 2-oxopropanal. The enzyme is dependent on Fe (II), but it has been proven to bind to zinc as well. Acetylacetone degradation has been characterized in the bacterium Acinetobacter johnsonii.
C5H8O2 + O2 C2H4O2 + C3H4O2
Arylation: Acetylacetonate displaced halide of certain halo-substituted benzoic acids. This reaction is catalyzed by copper.
2-BrC6H4CO2H + NaC5H7O2 2-((CH3CO)2HC)-C6H4CO2H + NaBr