Nickel(II) Acetylacetonate1

[3264-82-2]  · C10H14NiO4  · Nickel(II) Acetylacetonate  · (MW 256.93)

(catalyst for oligomerization, telomerization, hydrosilylation, hydrogenation, reduction, cross-coupling, oxidation, conjugate addition, addition to multiple bonds, and rearrangements1)

Alternate Name: bis(acetylacetonato)nickel(II).

Physical Data: pale green solid, mp 240 °C (dec); see also 2,4-Pentanedione.

Solubility: sol ethers and aromatic and halogenated hydrocarbons.

Analysis of Reagent Purity: atomic absorption is the method most commonly used.

Preparative Methods: commercially available; can be prepared from Nickel(II) Chloride.1d

Purification: recrystallize from benzene and sublime under vacuum (10-3 mmHg).1c

Handling, Storage, and Precautions: nickel is now recognized as a cancer suspect agent as well as a possible teratogen and due precautions should be taken when handling the reagent. The anhydrous solid is stable, but is an irritant and hygroscopic and should preferably be stored in a sealed container to preclude contact with air and moisture. Solutions are more susceptible to atmospheric oxidation.

Catalysts for Oligomerization, Cooligomerization, and Telomerization.

Active catalytic systems used for these reactions have been prepared from many NiII salts and Ni0 complexes. The most common are Ni(acac)2, nickel halides, and nickel-alkene complexes. The reactive species are formed from the combination of Ni0, a Lewis acid usually based on aluminum, and a suitable ligand. They are referred to as Ziegler catalysts. When NiII salts are used, a reducing agent is required to produce the active Ni0. The Lewis acid present, commonly a trialkylaluminum, is generally sufficiently reactive to reduce the nickel salt. Other reducing agents such as n-Butyllithium,2 Sodium Borohydride,3 or an electric current4 have also been used. The oligomerization reactions are usually done in hydrocarbon or halogenated solvents. The mechanism is believed to involve nickel hydride, formed in situ via b-hydride elimination. First, an alkene inserts into the Ni-H bond. Further insertion of a second alkene into the C-Ni bond and reductive elimination regenerates nickel hydride and produces the oligomer. While some very interesting carbocyclic systems can be accessed via these catalysts, the product distribution can vary depending on the nickel precatalyst/Lewis acid/ligand combination chosen. The better activity resulting from using one precatalyst over another for a specific transformation is not always well understood, and often results from careful tuning of the catalytic system. Below are those in which Ni(acac)2 has been successful. Other articles and monographs on Nickel should also be consulted.1a,1b

Oligomerization and Cooligomerization of Alkenes.

Cooligomerization of monoalkenes is generally of limited synthetic interest due to the formation of many isomers and oligomers and the difficulty in establishing conditions displaying suitable selectivity. It is, however, very important in the industrial production of lower alkenes. This topic is certainly well beyond the scope of this article. Some cyclic and bicyclic alkenes have proven more suitable (eq 1), the rigidity of which provides lesser oportunities for isomerizations by ways of insertion/migration.5,6

Cooligomerization of 1,3-Dienes.

The cyclotrimerization of butadiene is performed in the presence of a ligand-free nickel catalyst, giving cyclododecatrienes (eq 2),7 with the all-trans-1,5,9-isomer as the major product. Lesser amounts of the other double-bond isomers are also found, the quantity of which are temperature and concentration dependent.8 Substituted 1,3-dienes have not received as much attention due to the large number of isomers formed during the reaction, as well as the much lower reaction rates. The cyclooligomerization can be stopped at the stage of the dimer by introducing a phosphite or phosphine ligand.9 In the reaction of butadiene the major product is 1,5-cyclooctadiene (eq 3),9 which is accompanied by small amounts of divinylcyclopropane and 4-vinylcyclohexene. The three products are believed to originate from the same di-p-allylnickel intermediate (eq 4).10 The proportion of these intermediates and consequently the product distribution is affected by the presence of a ligand, effects of which have been related to their electronic and steric nature. The less basic and more bulky ligands favor a di-p-allyl intermediate which eventually closes by reductive elimination to the 1,5-cod via a terminal, rather than internal, di-s-allyl complex.10

Codimerizations of a variety of 1,3-dienes provide a convenient access to substituted 1,5-cyclooctadienes (eq 5)11 and cyclohexenes (eq 6).12 The presence of an activating group such as an ester can provide further stabilization of the s-allyl/p-allyl complex, increasing the selectivity of the reaction (eq 7).13

The cooligomerization of 1,3-dienes and alkenes involves two molecules of the diene and one of the alkene to form substituted cyclodecadienes (eq 8)14 and variable amounts of the linear cotrimers. In the case of symmetrical alkenes it is possible to obtain a single product, in contrast to unsymmetrical alkenes in which mixtures of isomers result, in addition to those resulting from the other modes of oligomerization (i.e. linear vs. cyclic). These reactions work best in the presence of the catalytic system Ni(acac)2/ligand/diethylethoxyaluminum. This is among the few methods giving direct access to 10-membered rings.

Disubstituted alkynes as well as allenes have also served as substrates in cooligomerizations with butadienes. With the former, cis-4,5-divinylcyclohexenes and mixtures of cyclodecadienes result. Other nickel precatalysts, such as Bis(1,5-cyclooctadiene)nickel(0), have, however, proved to be more specific with regard to the type and number of products formed.15


The oligomerization of unsubstituted (eq 9)16 and monosubstituted alkynes, also known as the Reppe reaction, produces variable mixtures of linear dimers, 1,2,4- and 1,3,5-trisubstituted benzenes, and cyclooctatetraene17 isomers. Disubstituted alkynes are known not to undergo such a process although they can be used in cooligomerizations with mono- and unsubstituted alkynes. The product distribution depends on the nature of the ligands.18 Weak ligands such as acac19 or cod favor cyclotetramerization while stronger ones such as PPh3 induce cyclotrimerization.20 When coordinating solvents such as pyridine or DMF are used, only the linear dimer is formed.21

Strained systems are also reactive and their synthetic utility has been elegantly demonstrated.22 In the synthesis of 12-nor-13-acetoxymodhephene, advantage was taken of the facile Ni0-induced transannulation that can take place in eight-membered rings, made possible by coordination of Ni0 to the two double bonds (eq 10).23

Telomerization is also an important reaction catalyzed by Ni(acac)2 and other nickel species in the presence of ligands and reducing agents.24 In this process, a diene is inserted in a 1,2- or 1,4-fashion into an activated C-H bond (eq 11)25 or into the X-H bond of an alcohol (eq 12),26 phenol, amine, or silane.27

1,7-Diynes also undergo cyclization with 1,8-insertion into a Si-H bond.28 Finally, dimethylsilane adds efficiently and with excellent regioselectivity to functionalized electron-rich or -poor alkenes in a 1,2-fashion, providing access to a wide variety of silane-containing substrates.29

A catalytic asymmetric version of the codimerization of alkenes has also appeared.30 Homochiral phosphine ligands are used to induce chirality and, in selected examples, have resulted in appreciable levels of asymmetric induction (eq 13).31,32

Nickel hydride generated in situ has been postulated as the active catalyst in the transformations described above and, not surprisingly, a large number of precatalysts have been reported. Only those dealing specifically with Ni(acac)2 have been described here. A more comprehensive account is provided in the excellent reviews by Jolly and Wilke.1a,1b

Catalyst for Oxidations.

A very efficient protocol for the air oxidation of ketones, alkenes, and aldehydes, catalyzed by nickel (1,3-diketonates) has been developed by Mukaiyama and co-workers. Baeyer-Villiger oxidation of ketones to esters and lactones (eq 14)33 is achieved in moderate to high yields with 1% Ni(acac)2 or Ni(dpm)2 (dpm = dipivaloylmethanato) and 2-3 molar equivalents of isovaleraldehyde or benzaldehyde under one atmosphere of air or oxygen, in 1,2-dichloroethane at ambient temperature. High regioselectivities were observed for unsymmetrical ketones (eq 15).33 The aldehyde, which functions as a reducing agent, is converted to the acid in high yield.34 This represents a mild, convenient, and ecologically sound oxidation protocol.

The epoxidation of alkenes35 can also be realized in moderate to high yields under the same conditions using an aldehyde, or under more forcing conditions using an alcohol,36 as the coreducing agent. Very low stereospecificity is obtained for disubstituted alkenes, which provide nearly equal mixtures of cis- and trans-epoxides, irrespective of the initial alkene geometry. This is, however, a good protocol for some 1,1-symmetrical trisubstituted alkenes.37 Little information is available on the chemoselectivity and compatibility of other functional groups under the reaction conditions.

Catalyst for Conjugate Additions.

The conjugate addition of organozinc reagents in a 1,4-fashion to a,b-unsaturated ketones in the presence of 1-10 mol% of Ni(acac)2 in ethereal or aromatic solvents proceeds in good to excellent yields (eq 16).38 The reaction is fast and the conditions are very mild. This represents a nice alternative to organocopper reagents since diorganozinc reagents are much more stable than their organocopper analogs and can be used at ambient temperatures without decomposition. Addition occurs even with the most sterically demanding substrates, as demonstrated by the synthesis of (±)-b-cuparenone by Greene and co-workers (eq 17).39 Aryl-, t-butyl-, cyclohexyl-, and alkenylzinc reagents have also been used. It should be noted that only one of the two alkyl groups is transferred and monoalkylzinc halides do not add, which results in the loss of one equivalent of the nucleophile. Articles on copper reagents should be consulted for related transformations.

In the presence of Ni(acac)2 and a coreducing agent such as Diisobutylaluminum Hydride (DIBAL), alkenylzirconium reagents40 (but not alkylzirconium41 undergo 1,4-additions to a,b-enones in 60-95% yield, which decrease with substitution at C-1 of the organometallic. Similarly, the complex formed by addition of Ni(acac)2 and DIBAL (1:1) catalyzes the 1,4-addition of dialkylaluminum acetylides to a,b-enones (eq 18).42 Again, the reaction works well even with highly hindered substrates. Trimethylsilylalkynes add efficiently but the yields are considerably lower with acetylene itself. Trialkylaluminums have been used directly on enones with in situ generation of the active nickel species.43 Other nucleophiles such as lithium thiophenoxide43 and 1,3-dicarbonyls (eq 19)44,45 can be used but generally require more forcing conditions. Although the mechanism of these nickel-catalyzed additions has not been clearly established, it is believed to involve ketyl radicals resulting from single-electron transfer (SET) from a nickel(I) species.46

Catalytic Enantioselective Conjugate Additions.

In recent years, much work has been done to achieve enantioselective addition of organozinc reagents to various enones using homochiral ligands.47,46b Respectable levels of induction have been achieved with selected aromatic substrates, but attempts with simpler enones such as cyclohexenone have been unsuccessful. The accepted model to test the activity of a particular catalytic system is the addition of Diethylzinc to chalcone. Yields are generally good and enantiomeric excesses (ee) range from 25 to 95%. A variety of bi- and tetradentate ligands have been designed and the most successful are shown in eq 20. Nonlinear relationships between the ee of the ligand and the ee of the final product have been frequently observed.48,46b

Catalyst for Cross-Coupling Reactions.

While not as popular as other NiII precatalysts, Ni0 complexes, or other metals such as Pd0 complexes, Ni(acac)2 has found many applications as a catalyst for cross-coupling reactions. It is fairly stable and sufficiently soluble to be used in a large variety of solvents. Aryl halides,49 silyl enol ethers,50 enol phosphates51 and, more recently, aryl-O-carbamates and triflates52 have been successfully coupled with alkyl, aryl, and alkenyl organometallics (eq 21).53 The yields are generally good and chemoselectivity can be achieved in some cases (eq 22).54,52

The addition of mono- and bidentate phosphine ligands in ethers or aromatic solvents often has a beneficial effect on the yield and speed of the reaction. The mechanism has been well documented and involves an oxidative addition of the aryl halide to Ni0 followed by addition of the organometallic species to the NiII complex. Reductive elimination of the cross-coupling product and regeneration of Ni0 completes the catalytic cycle. The coupling of stereodefined 1-alkenylalanes and zirconium with a variety of aryl halides proceeds in good yield and stereospecifically (eq 23).55 Vinylic sulfones can serve as the electrophilic partner, coupling efficiently with phenyl- or methylmagnesium bromide (eq 24).56 An interesting example was reported in which both electrophilic and nucleophilic components are borne by the same carbon. In this example, an a-(bromomagnesium)sulfone dimerizes head-to-head to produce a mixture of cis- and trans-alkenes (eq 25).57

Again, Ni(acac)2 is only one of the many precatalysts which have been used (often interchangeably) for these transformations. The choice of proper precatalyst has, in many instances, been the result of a more or less exhaustive screening of the five or six most common ligands. Other metal ions such as palladium, copper, or chromium are often similarly effective in promoting the transformation.

Other Additions.

Grignard reagents add regioselectivity and stereoselectively to substituted trimethysilylalkynes, in presence of Ni(acac)2 and Trimethylaluminum, to give a syn addition product (9:1). These additions occur exclusively at the carbon-substituted end of the alkyne.58 If a heteroatom, such as an ether or an amine, is present on the carbon chain, isomerization to the trans product occurs (eq 26). The intermediate organometallic species can be subsequently trapped with a variety of electrophiles.

Functionalization of an unactivated terminal alkene is possible via remote chelation of a halomagnesium alkoxide (eq 27).59 Addition of Grignard reagents to propargyl chlorides proceeds in a SN2 fashion to provide good yields of the corresponding allenes. Methyl ketones can be prepared by Ni(acac)2-catalyzed addition of trimethylaluminum to a nitrile followed by hydrolysis.60a

Ni(acac)2 has been shown to promote the intramolecular coupling of a vinyl iodide with an aldehyde in the presence of Chromium(II) Chloride, to form a 13-membered ring lactone. However, only a moderate stereoselectivity was observed, as both diastereomeric alcohols were produced (60% yield).60b

In the presence of a catalytic amount of Ni(acac)2 (in addition to other metals), Cyanotrimethylsilane reacts smoothly with acetals or orthoesters derived from a,b-unsaturated carbonyls to give the corresponding O-methylcyanohydrins under neutral conditions (eq 28).61

b-Diketonates add to Malononitrile in moderate yield to give b-amino cyanides (eq 29).62 In contrast, b-keto esters, 1,3-diesters, and b-keto amides are all poor substrates. Other electrophiles have been used such as cyanogen, benzoyl cyanide,63 and Trichloroacetonitrile.64 Reaction of acetylacetonate with cyanogen has been reported to produce highly substituted pyrimidines in moderate yields.65

Optically active trialkylsilanes react with Vinylmagnesium Bromide via a pentacoordinate intermediate to produce, after loss of a hydride, tetrasubstituted silanes with almost complete retention of configuration.66


Ni(acac)2 has seldom been used as a reduction catalyst. Hydrogenation of alkenes has been realized under photochemical conditions in the presence of a ketone as a sensitizer.67 Monohydrogenation of 1,4-cyclohexadiene was effected using a homogeneous nickel catalyst generated from Ni(acac)2, Et3Al2Cl3, and PPh3. Substituted cyclohexadienes produce mixtures of cyclohexene isomers, isomerization of which was shown to be promoted by the catalyst itself. A zeolite-supported complex formed between Ni(acac)2 and an optically active 2-(aminocarbonyl)pyrrolidine ligand catalyzes the asymmetric hydrogenation of ethyl (Z)-a-benzoylaminocinnamate with ee values up to 85%. Other metals that have been studied have resulted in even higher enantioselectivities.68

Vinylic sulfones are reduced in fair to good yield to the corresponding alkenes with retention of configuration upon treatment with n-butylmagnesium bromide and a catalytic amount of Ni(acac)2.69 2-Arenesulfonyl-1,3-dienes are also reduced stereospecifically to conjugated (Z,E)-dienes under the same conditions (eq 30).70

Miscellaneous Uses of Ni(acac)2.

The isomerization of aldoximes to amides catalyzed by Ni(acac)2 and Palladium(II) Acetylacetonate has been described.71 The determination of the absolute configuration of vicinal glycols and amino alcohols complexed with Ni(acac) in protic or aprotic organic solvents has been claimed to be feasible by examination of the induced CD.72 However, this method is not very general.

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Julien Doyon

The Ohio State University, Columbus, OH, USA

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