[23295-32-1]  · C13H19ClCoN5O4  · Chlorobis(dimethylglyoximato)(pyridine)cobalt(III)  · (MW 403.75)

(starting material for alkyl(pyridine)cobaloxime(III) complexes,1 precursor for cobaloximes(II) and cobaloximes(I),2 which have found widespread applications as catalysts for carbon-carbon,3 carbon-oxygen,4 carbon-nitrogen,5 and carbon-hydrogen6 bond formations)

Alternate Name: chloro(pyridine)cobaloxime(III).

Physical Data: mp 235 °C; polarographic half-wave potentials (acetonitrile solution, vs. Ag/AgNO3 electrode): -0.65, -1.45, -2.42, -2.91 V.

Solubility: sol CH2Cl2; limited sol ethyl acetate, hot ethanol; sparingly sol most other organic solvents and water. Chloro(pyridine)cobaloxime(III) can be replaced without any change in reactivity by its 4-t-butylpyridine7 derivative which has good solubility in toluene, carbon tetrachloride, alcohols and ethyl acetate.

Handling, Storage, and Precautions: forms brown, air stable crystals. The axial pyridine ligand can be removed by dilute acid extraction from ethyl acetate solutions of chloro(pyridine)cobaloxime(III). Due to their low solubility in common organic solvents, chloro(pyridine)cobaloxime(III) and -cobaloxime(II) (often the cobalt remainder in cobaloxime mediated transformations) are easily removed by adsorptive filtration of the reaction mixture on a short silica gel column. Acidic aqueous ethanol solutions of chloro(pyridine)cobaloxime(III) are photodecomposed when irradiated at wavelengths below 500 nm, yielding cobalt(II) salts, dimethylglyoxime, and pyridinium salts. For some applications the use of either iodo- or bromocobaloxime(III)7 is advantageous, as activation of the halogen-carbon bond is essential for chlorocobaloxime(III) to be employed in organic synthesis. However, reduction potentials, bond strengths, and photostabilities increase on moving from the iodo to the chloro ligand,8 so iodo(pyridine)cobaloxime(III) must be stored in tinted bottles.9

Inhalation of chlorocobaloxime(III) dust and contact with eyes should be avoided; wearing of protecting gloves for handling this compound is recommended. This compound should be handled in a fume hood.

Ligand Exchange Reactions.

Chlorocobaloxime(III) is a convenient source of the cobaloxime residue. It reacts with organomagnesium or -lithium compounds to yield the corresponding alkylcobaloximes(III).1 This reaction is an efficient approach to arylcobaloximes which are otherwise difficult to obtain (eq 1).

The chloro ligand can be substituted in the presence of metal ions (Ag+, Hg2+, Tl3+) which form precipitating metal chlorides.10 The entering ligand can be a solvent molecule that binds reversibly in a dynamic equilibrium, such as DMF, or an alkyl ligand that binds irreversibly. An example of the latter reaction is the formation of nitromethylcobaloxime which is formed from nitromethane, presumably via the nitronic acid (eq 2).

Upon reduction of chlorocobaloxime(III) by iron(II) salts,11 Zinc in protic solvents,12 Sodium Borohydride in alcohols,1 Sodium Amalgam in THF, or on the surface of a cathode,3a the chloro ligand is removed from the complex and versatile catalytic properties of the cobaloxime moiety are unmasked. Hence immobilized derivatives of chlorocobaloxime(III) have been prepared by reaction of chloroaquacobaloxime with the copolymers of 4-vinylpyridine/styrene and 4-vinylpyridine/acrylamide (eq 3).13

Carbon-Carbon Bond Formation.

In situ electrochemical reduction of chloro(pyridine)cobaloxime(III) to nucleophilic cobaloxime(I) allows carbon-carbon bond formation. The example given in eq 4 illustrates this catalytic transformation for the conversion of a bromoacetal to hexahydrobenzofuran.3 The key step of the reaction is a stereoselective cyclization of the intermediate alkoxyalkyl radical which is generated from an intermediate alkylcobaloxime. The alkene function which was present in the starting material is regenerated in the product via hydridocobaloxime elimination.

A combination of intramolecular 7-endo cyclization and intermolecular, cobaloxime-mediated, stereoselective alkyl-alkenyl cross coupling allows formation of the 5,7-fused bicyclic lactone from the linalool derivative in eq 5.

Alkyl-alkenyl cross coupling is a major area in alkylcobaloxime research.3a,b In contrast to the alternative carbon-carbon bond forming reactions via alkyl radicals under reductive conditions, application of the cobaloxime methodology allows the regeneration of the carbon-carbon double bond in the adduct. In particular photochemical alkyl-alkenyl cross couplings have been investigated under either stoichiometric or catalytic conditions. The stoichiometric reaction starts from an alkylcobaloxime(III) which is photolyzed in the presence of an alkene such as acrylonitrile or styrene. However, it should be noted that the solvent and the alkene used have a marked influence upon the yields and the product distribution of the reaction. The catalytic version requires in situ regeneration of cobaloxime(I). The adduct radical which is formed in the first step of the reaction can also be reduced to yield a carbanion which is then protonated. Aryl substituted alkenes are best suited for this transformation. Several usefully functionalized alkenes are simply reduced under these reaction conditions (eq 6).3b Application of this methodology to perfluoroiodides is straightforward and allows selective introduction of the perfluoroalkyl substituent, but bromocobaloxime(III) has been proven to be more efficient in this process than chlorocobaloxime (eq 7).3c

Carbon-Heteroatom Bond Formation.

Photolysis of solutions of alkylcobaloxime(III) in the presence of O2 or NO yields alkylperoxycobaloximes(III) or oximes.15,16 The former can then be transformed into products such as alcohols or substituted tetrahydrofurans, the latter then into amino sugars.17 Catalytic versions of these processes require Sodium Borohydride as reducing agent. The scope of these reactions is still limited to aryl substituted alkenes (eqs 8-10).4,5

Carbon-Hydrogen Bond Formation.

Sodium borohydride reduction of chlorocobaloxime(III) under a hydrogen atmosphere leads to an efficient homogeneous hydrogenation catalyst which selectively reduces carbon-carbon bonds of substituted pyridines or acrylates.6 Reduction of pyridinium salts to dihydropyridines requires the presence of a base such as Sodium Carbonate, whereas sodium bicarbonate leads to significant amounts of tetrahydropyridines (eq 11).

1. Schrauzer, G. N.; Deutsch, E. JACS 1969, 91, 3341.
2. Schrauzer, G. N.; Kohnle, J. CB 1964, 97, 3056.
3. (a) Bhandal, H.; Pattenden, G.; Russell, J. J. TL 1986, 27, 2299. (b) Branchaud, B. P.; Detlefsen, W. D. TL 1991, 32, 6273. (c) Hu, C.-M.; Qiu, Y.-L. TL 1991, 32, 4001. (d) Ali, A.; Harrowven, D. C.; Pattenden, G. TL 1992, 33, 2851.
4. Okamoto, T.; Oka, S. JOC 1984, 49, 1589.
5. Okamoto, T.; Kobayashi, K.; Oka, S.; Tanimoto, S. JOC 1987, 52, 5089.
6. (a) Okamoto, T.; Yamamoto, S.; Oka, S. J. Mol. Catal. 1987, 39, 219. (b) Ricroch, M. N.; Gaudemer, A. JOM 1974, 67, 119.
7. Bulkowski, J.; Cutler, A.; Dolphin, D.; Silverman, R. B. Inorg. Synth. 1980, 20, 127.
8. (a) Balasubramanian, P. N.; Vijayaraghavan, V. R. ICA 1980, 42, 179. (b) Costa, G.; Puxeddu, A.; Tavagnacco, C. JOM 1985, 296, 161.
9. Toscano, P. J.; Seligson, A. L.; Curran, M. T.; Skrobutt, A. T.; Sonnenberger, D. C. IC 1989, 28, 166.
10. (a) Randaccio, L.; Bresciani-Pahor, N.; Toscano, P. J.; Marzilli, L. G. IC 1981, 20, 2722. (b) Samus, N. M.; Luk'yanets, T. S.; Ablov, A. V. Russ. J. Inorg. Chem. (Engl. Transl.) 1974, 19, 1353.
11. Dayalan, A.; Viljayaraghavan, V. R. JCS(D) 1992, 2491.
12. Roussi, P. F.; Widdowson, D. A. CC 1979, 810.
13. Nishikawa, H.; Kasai, M.; Terada, E.; Tsuchida, E. BCJ 1977, 50, 3419.
14. Gridnev, A. A. Polym. J. (Tokyo) 1992, 24, 613.
15. Ghosez, A.; Göbel, T.; Giese, B. CB 1988, 121, 1807.
16. (a) Howell, A. R.; Pattenden, G. CC 1990, 103. (b) Hartung, J.; Giese, B. CB 1991, 124, 387. (c) Gupta, B. D.; Roy, M.; Das, I. JOM 1990, 397, 219.
17. Veit, A.; Giese, B. SL 1990, 166.

Bernd Giese

University of Basel, Switzerland

Jens Hartung

University of Würzburg, Germany

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