[99807-97-3]  · C14H9CoO3  · Carbonyl(cyclopentadienyl)(phthaloyl)cobalt  · (MW 284.16)

(precursor to other cyclopentadienyl(phthaloyl)cobalt complexes which react with alkynes readily to give quinones or cyclopentadienylcobalt complexed quinones1)

Physical Data: orange-yellow solid; mp 132-133 °C.

Solubility: very sol most organic solvents.

Form Supplied in: not commercially available.

Analysis of Reagent Purity: 1H NMR, IR, microanalysis.

Preparative Method: prepared in 98% yield by reaction of benzocyclobutenedione with commercially available dicarbonyl cyclopentadienylcobalt in xylene for 3.25 h at 160 °C.

Purification: flash chromatography on silica gel with petroleum ether.

Preparation of Phthaloyl- or Maleoyl(carbonyl)(cyclopentadienyl)cobalt Complexes.

Benzocyclobutenedione and 3,4-dimethylcyclobutenedione each react with Dicarbonyl(cyclopentadienyl)cobalt(I) in xylene to give carbonyl(cyclopentadienyl)cobalt complexes (1) and (2).1 Analogously, they react with a mixture of Octacarbonyldicobalt and pentamethylcyclopentadiene to give carbonyl(pentamethylcyclopentadienyl)cobalt complexes (3) and (4) (eqs 1-4).2-4

Synthesis of Quinones.

In general, phthaloyl and maleoyl transition metal complexes can be used in the synthesis of quinones5,6 according to the following scheme: 1) an alkyne must coordinate to a free site on a coordinatively unsaturated metal center; 2) the alkyne must insert into the metal-carbonyl bond; and 3) the metal must reductively eliminate to give the quinone (eq 5).

Complexes (1-4) do not react under thermal conditions (xylene, 120 °C) with alkynes. However, photolysis leads to dissociation of carbon monoxide, so when phthaloyl complex (1) is photolyzed in the presence of 3-hexyne it gives 2,3-diethyl-1,4-naphthoquinone (eq 6). However, when maleoyl complex (2) is treated under identical conditions, the cyclopentadienylcobalt complex of the corresponding benzoquinone is obtained (eq 7).1 The 1,4-benzoquinones form more stable complexes with cyclopentadienylcobalt than the 1,4-naphthoquinones.

Activation of complex (1) with Trimethylamine N-Oxide, with acetonitrile as solvent, allows the isolation of the acetonitrile complex (5). This reacts with 3-hexyne to give the cyclopentadienyl(2,3-diethylnaphthoquinone)cobalt complex (7). Complex (7) can be formed in better yield under milder conditions by using the more labile benzonitrile complex (6) (eq 8).1

Naphthoquinone complexes such as (7) are very unstable. The analogous complex formed by reaction with the terminal alkyne, 1-hexyne, is not isolated under the conditions which give (7).1 However, 1,4-naphthoquinone complexes are formed generally by reaction of the pentamethylcyclopentadienyl complex (3) with alkynes upon activation of the CO ligand with N-Methylmorpholine N-Oxide (NMMO) (Table 1).3 The extra steric bulk of the pentamethylcyclopentadienyl ligand protects the 1,4-naphthoquinone complexes from decomplexation relative to the corresponding cyclopentadienyl complexes.

The cyclopentadienylcobalt complexes of 1,4-benzoquinones are quite easy to form from reactions of cyclopentadienyl(maleoyl)cobalt complexes.1 Photolysis of cyclopentadienyl(maleoyl)cobalt complex (2) in the presence of diethyl sulfide gives complex (8). This reacts with a wide variety of alkynes to give cyclopentadienyl(1,4-benzoquinone)cobalt complexes (Table 2).

Photolysis of complex (2) in acetonitrile forms a red complex which on trituration with hexane is converted to the yellow cyclopentadienyl(dimethylbisketene)cobalt complex (9) (eq 9).2 Complex (9) was reactive with alkynes to give cyclopentadienyl(1,4-benzoquinone)cobalt complexes at room temperature (Table 3).

General Considerations.

The phthaloyl- and maleoyl(cyclopentadienyl)cobalt complexes should be compared to other phthaloyl- and maleoylcobalt complexes when being considered for use in the synthesis of quinones.5,6 The main advantage of the cyclopentadienylcobalt complexes is the fact that quinone complexes can be obtained. This is currently the best method for making cyclopentadienyl(quinone)cobalt complexes.3 Very limited work has been done on the application of these complexes in synthesis. Preliminary deprotonation/alkylation studies of maleoyl complexes (2) and (4) have been carried out but are only reported in dissertation form at this time.4 This dissertation also contains some very preliminary work on the use of cyclopentadienyl(quinone)cobalt complexes in synthesis and the results are promising. The cyclopentadienyl ligand in these complexes makes them amenable to standard chromatographic purification, thus making the process of developing synthetic modifications of these complexes very feasible.

Related Reagents.

Phthaloylbis(triphenylphosphine)cobalt Chloride.

1. Liebeskind, L. S.; Jewell, Jr., C. F. JOM 1985, 285, 305.
2. Jewell, Jr., C. F.; Liebeskind, L. S.; Williamson, M. JACS 1985, 107, 6715.
3. Cho, S. H.; Wirtz, K. R., Liebeskind, L. S. OM 1990, 9, 3067.
4. Jewell, C. F., Jr. Ph.D. Dissertation, Emory University, 1986.
5. Liebeskind, L. S. T 1989, 45, 3053.
6. Liebeskind, L. S.; Baysdon, S. L., South, M.; Iyer, S.; Leeds, J. P. T 1985, 41, 5839.

Charles F. Jewell, Jr.

Sandoz Research Institute, East Hanover, NJ, USA

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