Tris(acetoacetonyl)cobalt

[21679-46-9]  · C15H21CoO6  · (MW 356.26)

(catalyst for the formation of C-C bonds and oxidation reactions)

Alternate Name: tris(acetylacetonyl)cobalt; tris(2,4-pentanedionato)cobalt(III); cobaltic acetylacetonate, tris(2,4-pentanedionato-O,O)-cobalt, cobalt(III)-2,4-pentanedionate.

Physical Data: 1 mp 213 °C; d 1.43 g cm-3.

Solubility: soluble in most organic solvents; insoluble in H2O.1

Form Supplied in: dark green crystals, widely available.

Analysis of Reagent Purity: mp,1 1H NMR,2 IR,2 VIS.2

Preparative Methods: reaction of CoCO3, H2O2, and acetylacetone.1

Purification: recrystallize from toluene and heptane.2

Handling, Storage, and Precautions: wear appropriate NIOSH/MSHA-approved respirator in non-ventilated areas and/or for exposure above the TLV. Chemical-safety goggles and long rubber or neoprene gauntlet gloves are also recommended. Use only in a chemical fume hood with a safety shower and eye bath available. Do not breathe dust. Avoid contact with eyes, skin, and clothing. Wash thoroughly after handling. Severe eye irritant, skin irritant, and irritating to mucous membranes and upper respiratory tract. Storage: avoid heat, sunlight, and strong oxidizing agents. Protect from metal. Store in a cool and dry place. Disposal: the material should either (i) be dissolved in water, (ii) be dissolved in acid solution, or (iii) oxidized to a water-soluble state. Precipitate the material as a sulfide, adjusting the pH of the solution to pH 7 to complete precipitation. Filter the insoluble material and dispose them off in a hazardous-waste site. Destroy any excess sulfide with sodium hypochlorite.

Cycloadditions

Cobalt(III) acetylacetonate [Co(acac)3] acts as a catalyst for both [2 + 2 + 2] and [4 + 2 + 2] cycloadditions. A catalyst mixture of 1-5% [1:1:4 Co(acac)3/dppe/Et2AlCl] readily promotes [2 + 2 + 2] cycloaddition reactions between a 1:1 mixture of norbornadiene and an acetylenic hydrocarbon (1),3 where R1, R2 = H, alkyl, Ph, TMS, in yields of up to 100%.3-7 [2 + 2 + 2] Cycloadditions are conducted intramolecularly with the acetylene tethered to the norbornadiene (2),3,8 asymmetrically (up to 98% ee) using chiral phosphine ligands,3,4,9,10 with substituted norbornadienes,3,4 and with olefins instead of acetylenes.6,11

[4 + 2 + 2] Cycloadditions between a 1:2 mixture of norbornadiene and an alkyl-substituted diene give yields of up to 94% using a catalyst mixture comprised of 1:3:30 Co(acac)3/dppe/Et2AlCl.3,6 An asymmetric version of the [2 + 2 + 4] cycloaddition with chiral phosphine ligands gives enantiomeric excess of up to 79% (3).12

Cyclizations

Catalytic Co(acac)3-mediated reductive carbocyclizations of acyclic enediene substrates give up to 86% yield, 25:1 trans/cis, and >20:1 E/Z when a catalyst mixture comprised of 1:2:3 Co(acac)3/PPh3/Et2AlCl is used (4).13

Codimerization and Olefin Coupling Reactions

Ethene codimerizes with norbornadiene to give vinylnortricyclene,6 and the coupling of butadiene with norbornadiene gives 5-butadienylnorborn-2-ene in 52% yield, the latter case requiring 4 equiv of 1:3.5 Co(acac)3/Et2AlCl (5).14

Catalysis of Organometallic Coupling and Addition Reactions

Cross-coupling of zinc organometallics R2Zn or RZnX with alkenyl halides with retention of the configuration of the double bond (6) proceeds in yields of up to 75% when vinyl iodide/BuZnBr 1:2 and 10 mol % of Co(acac)3 is used.15 OctZnI reacts with (E)-5-chloro-1-iodopent-1-ene to give Cl(CH2)3 CH=CHOct in 74% yield and >99% E, and with (Z)-BuCH=CHI to give BuCH=CHOct in 71% yield and >99% Z.15

Methylation of 2-cyclohexen-1-one with 1.2 equiv of Me3ZnLi gives a quantitative GC yield of the 1,4-addition product when 2 mol % of Co(acac)3 is used as a catalyst (7).16

The addition of 1.5 equiv of MeMgBr to one equiv of 2-methylbenzophenone can be controlled by transition metal catalysts (8); when 0.5 mol % of Co(acac)3 mediates the reaction, the 1,2-addition product and pinacol are obtained in a ratio of 49:51.17

Radical Addition to Olefins

Bis-Dihydrofurans are synthesized using a Co(acac)3-promoted annulation.18 When the diene in 9 and Co(acac)3 react in a 1:6 molar ratio, the reaction gives 92% yield (9).18 Similarly, dihydrofuran derivatives are prepared by the photoreaction of amines or enamines with Co(acac)3 in yields of up to 78%.19 a-Hydroxy-g-lactones are prepared from alcohols, a,b-unsaturated esters and dioxygen by radical generation and addition using a N-hydroxyphthalimide (NHPI)/Co(OAc)2/Co(acac)3 system in yields as high as 83%.20

Oxidation Reactions

Oxidative carbonylation of phenol to diphenyl carbonate is catalyzed by the system Pd(OAc)2/benzoquinone/Co(acac)3/tetrabutylammonium bromide in a 1:30:3:60 molar ratio.21 Co(acac)3 catalyzes carbonylation of alkyl sulfonates with alcohols; a mixture of n-octyl sulfonate, EtOH, Co(acac)3, NaI in a 1:10:0.1:0.5 molar ratio gives the ester in 53% yield (10).22

Co(acac)3 catalyzes the epoxidation of olefins.23,24 exo-Norbornene oxide is prepared in 98% yield by the reaction of norbornene in THF saturated with O2 at 70 °C in the presence of 2% Co(acac)3 (11).24 Vinyl-substituted aromatics undergo oxidative C=C cleavage to form acids and aldehydes; PhCH=CH2 gives PhCO2H and PhCHO (86:14, 99% overall) when treated with O2 and 2 mol % of Co(acac)3.24 THF and other cyclic ethers are transformed into lactones under similar conditions (12).24 Cyclic alkanes are oxidized to cyclic ketones and dicarboxylic acids by Co(acac)3; cyclohexane reacts with dioxygen (1 atm), 10 mol % of N-hydroxyphthalimide (NHPI), 0.5 mol % of Co(acac)3 in CHCOOH at 75 °C to give cyclohexanone and hexanedioic acid (49:51) in 48% overall yield.25 The NHPI/Co(acac)3 system also catalyzes the oxidation of alcohols to carbonyl compounds; borneol was oxidized to camphor under the same conditions as above with MeCN as solvent (13).26

Related Reagents.

Ni(CN)2(PPh3)2; Ni(CO)2(PPh3)2; Fe (acac)3, Co(acac)2; Cu(acac)2; Mn(acac)3; Co(OAc)3; Mn(OAc)3; Co2(CO)8.


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Peter D. Mayo & William Tam

Guelph-Waterloo Centre for Graduate Work in Chemistry and Biochemistry, University of Guelph, Ontario, Canada



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