[26305-75-9]  · C54H45ClCoP3  · Chlorotris(triphenylphosphine)cobalt  · (MW 881.28)

(monovalent cobalt complex; homogeneous catalyst for hydrogenation and hydrodimerization of alkenes; strong nucleophile, reacting with alkyl acyl halides to afford coupling products; useful for the preparation of organocobalt complexes)

Physical Data: mp 177 °C (N2, sealed tube).

Solubility: sol benzene and CH2Cl2; insol EtOH.

Preparative Methods: a solution of Cobalt(II) Chloride hexahydrate (0.6 g, 2.5 mmol) and Triphenylphosphine (2.0 g, 7.6 mmol) in EtOH (70 mL) was treated under N2 and stirring with a solution of Sodium Borohydride (0.08 g, 2.1 mmol) in EtOH at 30-40 °C; the resulting brown-green precipitate was washed several times with EtOH and water and dried (yield 92%);1 reductions with powdered Zn, and electrolysis, are equally effective methods; corresponding Br and I complexes are prepared similarly from CoBr2 and CoI2.

Handling, Storage, and Precautions: the solid reagent is fairly stable in air; it has been stored under N2 atmosphere over several months at rt.

Hydrogenation Catalyst.

Like Chlorotris(triphenylphosphine)rhodium(I), CoCl(Ph3P)3 is a homogeneous hydrogenation catalyst that effects the reduction of alkenes and dienes to saturated alkanes; details are available in the patent literature. The Co complex is a less effective hydrogenation catalyst than the corresponding Rh complex, and vigorous reaction conditions are required. However, under mild reaction conditions the complex is a suitable catalyst for the selective reduction of dienes to alkenes, because of its low catalytic activity (eq 1).

Dimerization and Hydrodimerization of Alkenes.

CoCl(Ph3P)3 selectively catalyzes the dimerization of ethylene to butenes in the presence of Boron Trifluoride Etherate via formation of one-to-one complexes (eq 2).2

Terminal alkenes (1-butene, 1-pentene) neither dimerize nor isomerize under these conditions, demonstrating the highly selective behavior of the catalyst. At a higher temperature (60 °C), CoCl(Ph3P)3 acts as an effective catalyst for the cyclic dimerization of butadiene and mainly affords 1,5-cyclooctadiene (eq 3).3 A hydridocobalt species is presumed to be an active catalyst in these reactions.

Hydrodimerization of methyl acrylate is effectively catalyzed by this complex, giving mainly b-to-b coupled dimethyl adipate (eq 4),4 in contrast to HCo(dmg)2P(n-Bu)3, which affords the a-substituted cobaloxime derivative (eq 5).5

Reaction with Alkyl Halides.

CoCl(Ph3P)3 is highly nucleophilic and reacts with alkyl halides to give trivalent alkyl cobalt complexes, followed by cleavage of the resulting Co-C bond which leads to the formation of a new C-C bond. The reaction of benzylic mono-, di-, and trihalides with CoCl(Ph3P)3 gives coupling products with a single, double, and triple bond, respectively (eq 6).6

The coupling of allylic halides affords regioselectively a 1,5-diene with retention of the geometry of the double bond; squalene was synthesized by this method in moderate yields (eq 7).7

Biomimetic conversion of bromohydrins to ketones is achieved by CoCl(Ph3P)3 in good yield (eq 8).8 This process is analogous to the biological transformation of 1,2-diols to ketones catalyzed by diol dehydrogenase and vitamin B12.

Organocobalt Reagent.

Air-stable, crystalline phthaloylcobalt and malonylcobalt complexes are conveniently synthesized in high yield by the reaction of cyclobutenediones with CoCl(Ph3P)3 (eq 9).9

The addition-elimination reaction of the complex with alkynes provides an efficient and regioselective synthetic method for the preparation of substituted benzoquinones (eq 10).10

Related Reagents.

Phthaloylbis(triphenylphosphine)cobalt Chloride; Chlorotris(triphenylphosphine)rhodium(I).

1. (a) Aresta, M.; Rossi, M.; Sacco, A. ICA 1969, 3, 227. (b) Holah, D. G.; Hughes, A. N.; Hui, B. C.; Kan, C. T. CJC 1978, 56, 814.
2. (a) Kawakami, K.; Mizoroki, T.; Ozaki, A. CL 1975, 903. (b) Kawakami, K.; Mizoroki, T.; Ozaki, A. BCJ 1978, 51, 21.
3. Cairns, M. A.; Nixon, J. F. JOM 1974, 64, c19.
4. (a) Kanai, H.; Okada, M. CL 1975, 167. (b) Kanai, H.; Ishii, K. BCJ 1981, 54, 1015.
5. Schrauzer, G. N.; Holland, R. J. JACS 1971, 93, 1505.
6. (a) Yamada, Y.; Momose, D. CL 1981, 1277. (b) Momose, D.; Iguchi, K.; Sugiyama, T.; Yamada, Y. CPB 1984, 32, 1840.
7. Momose, D.; Iguchi, K.; Sugiyama, T.; Yamada, Y. TL 1983, 24, 921.
8. Momose, D.; Yamada, Y. TL 1983, 24, 2669.
9. Liebeskind, L. S.; Baysdon, S. L.; South, M. S. JOM 1980, 202, c73. (b) Baysdon, S. L.; Liebeskind, L. S. OM 1982, 1, 771.
10. Lyer, S.; Liebeskind, L. S. JACS 1987, 109, 2759.

D. Momose

Kissei Pharmaceutical Co., Minamiazumi, Japan

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