Phthaloylbis(triphenylphosphine)cobalt Chloride1

[75895-97-5]  · C44H34ClCoO2P2  · Phthaloylbis(triphenylphosphine)cobalt Chloride  · (MW 751.10)

(complex which reacts with alkynes to give naphthoquinones;1 -3 precursor to other phthaloyl complexes which react with alkynes more readily to give naphthoquinones4,5)

Physical Data: mp 245-246 °C.

Solubility: sol CH2Cl2; slightly sol MeCN.

Form Supplied in: brick red solid; not commercially available.

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

Preparative Method: reaction of benzocyclobutenedione with Chlorotris(triphenylphosphine)cobalt in chlorobenzene for 24 h at 40 °C.3,6

Purification: crystallization from dichloroethane-hexane.

Handling, Storage, and Precautions: the pure complex should be stored in solid form in a sealed container at reasonable ambient temperatures. Quality may deteriorate over time when exposed to air.

Direct Synthesis of Naphthoquinones.

Phthaloyl transition metal complexes can be used in the synthesis of naphthoquinones according to the following scheme: first, an alkyne must coordinate to a free site on a coordinatively unsaturated metal center; secondly, the alkyne must insert into the metal-carbonyl bond; and thirdly, the metal must reductively eliminate to give the naphthoquinone (eq 1).

This particular complex will not react with alkynes unless activated by &egt;2 equiv of Silver(I) Tetrafluoroborate; then it will react with a wide variety of alkynes to give naphthoquinones (eq 2).

Complexes of this type are prepared by the insertion of CoCl(PPh3)3 into cyclobutenediones,1,3,6 a preparation which is found to occur quite generally (eq 3).

The overall potential of this synthetic strategy was demonstrated in the synthesis of the natural product (±)-nanaomycin A (eq 4).7 In this scheme, the alkyne was linked via a tether to control the regiochemistry of the alkyne addition.

Precursor to More Reactive Phthaloyl Complexes.

With the method described above, terminal alkynes generally show reduced yields, silyl alkynes are sensitive to desilylation, some alkynes containing heteroatoms require more than 2 equiv of AgBF4, and alkynes containing propargyl ethers or acetals give no naphthoquinone product.2,3 AgBF4 aids in the ionization of the Co-Cl bond and it complexes with free phosphine ligands to keep an open axial coordination site on cobalt, which seems to be necessary for the quinone-forming reaction (see eq 1).4,5 By reacting this complex with only one equiv of AgBF4, in the mildly coordinating solvent Acetonitrile (AN), a somewhat stable cationic cobalt complex is obtained.4 Although this species cannot be crystallized, it reacts with alkynes to give naphthoquinones. Reaction of this cation with 1,2-Bis(diphenylphosphino)ethane (diphos) affords a very stable diphos cation complex (eq 5).4 This diphos cation (BF4- salt) is crystalline and can be stored in a closed container indefinitely.

The diphos cation complex is quite reactive with alkynes, giving naphthoquinones (eq 6). This method is simpler than the AgBF4-mediated reaction of phthaloylbis(triphenylphosphine)cobalt chloride with alkynes because it does not have the complications associated with the use of AgBF4 (use of a hygroscopic reagent and sensitivity of alkynes to Ag+).

The fact that the diphos ligand cannot occupy both axial sites on cobalt allows the labile MeCN ligand to occupy an axial site, which is necessary for coordination of the incoming alkyne.3-5 In further studies it was found that phthaloylbis(triphenylphosphine)cobalt chloride reacts with 1.1 equiv of dimethylglyoxime (dmg) in pyridine to give phthaloyl(dmg)(pyridine)cobalt chloride. The dmg ligand in cobalt complexes is known to prefer chelating in an equatorial sense; this allows both axial sites to be occupied by more labile ligands (pyridine and chloride) (eq 7).3-5

This dmg complex is the most useful of the cobalt complexes derived from phthaloylbis(triphenylphosphine)cobalt chloride since it reacts with the widest variety of alkynes, both electron rich and electron deficient. In contrast to phthaloylbis(triphenylphosphine)cobalt chloride, phthaloyl(dmg)(pyridine)cobalt chloride reacts with alkynes under relatively mild thermal conditions with no need for AgBF4. For example, 1.5 equiv of 3-hexyne in 1,2-dichloroethane heated at 80 °C for 18 h gives a 77% yield of 2,3-diethyl-1,4-naphthoquinone. Lewis acidic additives enhance the rate of the reaction of the dmg complex. AgBF4 works well, but the most generally useful Lewis acid catalysts are Cobalt(II) Chloride, which significantly enhances the rate of quinone formation at 80 °C, and Tin(IV) Chloride, which actually gives good quinone yields from many alkynes at rt (eq 8).

General Considerations.

Naphthoquinones can be prepared from a wide variety of alkynes with the methods outlined above. This approach also works for a variety of cyclobutenediones, giving rise to many substituted benzoquinones1,3,8-10 as well as substituted naphthoquinones.3,5,7 Much work has been done investigating the regioselectivity of the alkyne additions in cases where the phthaloyl and maleoyl ligands are unsymmetrical.3,7 -9 In particular, the dmg -cobalt complexes show good regioselectivity in the alkyne additions.8,9 In using this approach to prepare quinone-containing molecules, the key considerations are: (1) is the necessary cyclobutenedione available? and (2) can the regiospecificity of the alkyne addition be controlled sufficiently? Recent advances in the preparation of cyclobutenediones,11-18 coupled with the information from reported regiochemical studies, will provide strong guidance in the preparation of some complicated structures containing a quinone nucleus.

Related Reagents.


1. Liebeskind, L. S. T 1989, 45, 3053.
2. Liebeskind, L. S.; Baysdon, S. L.; South, M. S. JACS 1980, 102, 7397.
3. Liebeskind, L. S.; Baysdon, S. L.; South, M. S.; Iyer, S.; Leeds, J. P. T 1985, 41, 5839.
4. Baysdon, S. L.; Liebeskind, L. S. OM 1982, 1, 771.
5. Liebeskind, L. S.; Baysdon, S. L.; Goedken, V.; Chidambaram, R. OM 1986, 5, 1086.
6. Liebeskind, L. S.; Baysdon, S. L.; South, M. S. JOM 1980, 202, C73.
7. South, M. S.; Liebeskind, L. S. JACS 1984, 106, 4181.
8. Liebeskind, L. S.; Leeds, J. P.; Baysdon, S. L.; Iyer, S. JACS 1984, 106, 6451.
9. Iyer, S.; Liebeskind, L. S. JACS 1987, 109, 2759.
10. Liebeskind, L. S.; Chidambaram, R.; Nimkar, S.; Liotta, D. TL 1990, 31, 3723.
11. Liebeskind, L. S.; Baysdon, S. L. TL 1984, 1747.
12. Liebeskind, L. S.; Fengl, R. W.; Wirtz, K. R.; Shawe, T. T. JOC 1988, 53, 2482.
13. Reed, M. W.; Pollart, D. J.; Perri, S. T.; Foland, L. D.; Moore, H. W. JOC 1988, 53, 2477.
14. Liebeskind, L. S.; Wang, J. TL 1990, 31, 4293.
15. Liebeskind, L. S.; Wirtz, K. R. JOC 1990, 55, 5350.
16. Liebeskind, L. S.; Lescosky, L. J.; McSwain, Jr., C. M. JOC 1989, 54, 1435.
17. Liebeskind, L. S.; Fengl, R. W. JOC 1990, 55, 5359.
18. Xu, S.; Yerxa, B R.; Sullivan, R. W.; Moore, H. W. TL 1991, 32, 1129.

Charles F. Jewell Jr.

Sandoz Research Institute, East Hanover, NJ, USA

Copyright 1995-2000 by John Wiley & Sons, Ltd. All rights reserved.