Bis(acetylacetonato)zinc(II)

Zn(MeCOCHCOMe)2(H2O)

[14024-63-6]  · C10H16O5Zn  · Bis(acetylacetonato)zinc(II)  · (MW 263.60)

(catalyst for the coupling of aryl substituted allylic alcohols with b-dicarbonyl compounds,1 and of organoboranes with organohalides;2 selective C-alkylation of b-diketones;3 preparation of amino acid derivatives, aminomaleimides,4 and b-trichloromethyleneamino diones5)

Alternate Name: Zn(acac)2.

Physical Data: mp: anhydrous: 127 °C; monohydrate: 135-138 °C.

Solubility: dissolves readily in most organic solvents.

Form Supplied in: crystalline; widely available.

Preparative Method: from the reaction of ZnSO4.7H2O with acetylacetone (2,4-pentanedione) and NaOH.6

Purification: anhydrous: sublimation, 110 °C (0.1 mmHg), recrystallized from n-hexane;6 monohydrate: recrystallized from ethyl acetate.6

Handling, Storage, and Precautions: hygroscopic; irritating to eyes, respiratory system and skin. The monohydrate is unstable toward heat, and forms byproducts on dissolution of the hydrate. The anhydrous zinc complex is stable toward heat.

Introduction.

Zn(acac)2 is a widely available catalyst which is prepared through the reaction of ZnSO4, acetylacetone and NaOH, to give either the hydrate or the anhydrous zinc complex.5 Zn(acac)2 can perform two general types of reactions: (1) stoichiometric reaction at the nucleophilic carbon of the zinc acetylacetonate, and (2) catalytic reaction at the nucleophilic carbon of other b-dicarbonyl substrates.

Stoichiometric Coupling Reactions.

Zn(acac)2 efficiently forms a carbon-carbon bond through the coupling of aryl substituted allylic alcohols with the nucleophilic central carbon of the acetoacetonate ligand (eq 1).1 Starting with either (E) or (Z) allylic alcohols, these reactions give (E)-g,d-unsaturated ketones through syn-anti isomerization of the intermediate p-allyl palladium species. The Titanium Tetraisopropoxide serves to (1) activate the allylic alcohol toward p-allyl complex formation via an allyl titanate, and (2) activate the coupled dicarbonyl species toward deacylation. An intractable mixture of products occurs when alkyl rather than aryl substituted allylic alcohols are used.1 Of the various palladium catalysts investigated for this reaction, Tetrakis(triphenylphosphine)palladium(0) is effective while PdCl2, Pd(OAc)2, and PdCl2(PhCN)2 are not. Zn(acac)2 can also be prepared in situ by mixing Zinc Chloride and acetylacetone in the presence of Triethylamine, and can then be used to perform transformations similar to those previously described. This in situ preparation can also be employed to couple allylic alcohols with the zinc enolate of b-keto esters and b-diesters as well as other b-diketones (eq 2).1

Unsymmetrical ketones are produced through the carbonylative cross-coupling reaction of aryl iodides or benzyl halides with trialkylboranes mediated by Zn(acac)2 and PdCl2(PPh3)2. Typically, yields for this coupling process range from 43-82%.2 The use of either Et3B or B-alkyl-9BBN derivatives proved to be efficient in the transfer of the desired alkyl group (eq 3).2 For activation of these coupling reactions, Zn(acac)2 is superior to the acetoacetonate complexes of various other metals (i.e. Li, Mg, Al, Ca, Ti, Mn, Fe, Co, Cu, Mo, and Sn).

Selective C-alkylation occurs through the addition of various SN1-type organohalides to the nucleophilic carbon of the Zn(acac)2 (eq 4).3 This transformation provides yields comparable to those obtained with Co(acac)2, but overall these reagents are less effective than Co(acac)3.3

Catalytic Reactions with b-Dicarbonyl Compounds.

Unsaturated a-amino acid derivatives are obtained through the Zn(acac)2 catalyzed reaction of ethyl cyanoformate with b-diketones, b-keto esters, and b-diesters (eq 5).4 Similar reactions with b-keto amides result in the formation of cyclic aminomaleimides (eq 6).4

Zn(acac)2 also effectively promotes the addition of b-diketones or b-keto esters to trichloroacetonitrile, giving b-trichloromethyleneamino diones in 40-90% yields.5 The addition of b-dicarbonyl compounds to cyanogen can be achieved with the use of either Cu(acac)2 or Zn(acac)2 (eq 7). These metal catalyzed reactions of cyanogen with b-keto amides give different regioselectivity when compared to catalysis by the ethoxide ion.7


1. Itoh, K.; Hamaguchi, N.; Miura, M.; Nomura, M. JCS(P1) 1992, 2833.
2. Wakita, Y.; Yasunaga, T.; Akita, M.; Kojima, M. JOM 1986, 301, C17.
3. González, A.; Güell, F.; Marquet, J.; Moreno-Mañas, M. TL 1985, 26, 3735.
4. Veronese, A. C.; Gandolfi, V.; Longato, B.; Corain, B.; Basato, M. J. Mol. Catal. 1989, 54, 73.
5. Veronese, A. C.; Talmelli, C.; Gandolfi, V.; Corain, B.; Dasato, M. J. Mol. Catal. 1986, 34, 195.
6. Rudolph, G.; Henry, M. C. Inorg. Synth. 1967, 10, 74.
7. (a) Basato, M.; Campostrini, R.; Corain, B.; D'Angeli, F.; Veronese, A. C.; Valle, G. ICA 1985, 98, L17. (b) Corain, B.; Basato, M.; Marcomini, A.; Klein, H.-F. ICA 1983, 74, 1. (c) Corain, B.; Basato, M.; Mori, E.; Valle, G. ICA 1983, 76, L277.

Nancy S. Barta & John R. Stille

Michigan State University, East Lansing, MI, USA



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