[12184-35-9]  · C13H17Co  · (1,5-Cyclooctadiene)(cyclopentadienyl)cobalt(I)  · (MW 232.11)

(catalyst for cyclotrimerization of alkynes and cotrimerization with nitriles to yield pyridine derivatives;1,2 source of other cobalt complexes and of substituted cyclooctadienes15-17)

Physical Data: mp 102-104 °C; subl 70 °C/0.1 mmHg.

Solubility: readily sol common org solvents; insol H2O.

Form Supplied in: golden-yellow cryst.

Preparative Methods: of several effective synthetic routes the most efficient is the direct reaction of cobalt salts [conveniently Co(acac)3] with cyclopentadiene + cycloocta-1,5-diene + anthracene-activated magnesium metal.1

Handling, Storage, and Precautions: air sensitive; best stored at low temp (<0 °C) under inert gas (N2, Ar). Use in a fume hood.

General Remarks.

This compound is representative and is the most commonly used of the class of (dialkene)CoCp complexes which share similar reactivity and can be used more or less interchangeably in reactions which involve replacement of the (di)alkene moiety. The entry therefore also makes reference to derivatives with substituted cyclopentadienyl ring and to analogs, notably CpCo(cyclopentadiene) and CpCo(C2H4)2 of which the latter is probably the most reactive, while the C5H6 complex is arguably the most readily accessible. (C5H5)Co(C5H6) [3302-03-0], MW 190.13, mp 98-99 °C, is the product of hydride addition to Bis(cyclopentadienyl)cobalt Hexafluorophosphate; it forms air-sensitive, wine-red crystals, sol in hydrocarbons and sublimable in vacuo. (C5H5)Co(C2H4)2 [69393-67-5], MW 180.14, is efficiently synthesized from Bis(cyclopentadienyl)cobalt and forms hydrocarbon soluble, air-sensitive, red-brown needles.

Catalysis of Alkyne Cyclotrimerization and Related Reactions.

Cyclotrimerization of alkynes and cotrimerization of two molecules of alkyne with one of a nitrile to give 2-substituted pyridines and related reactions (see below) are efficiently catalyzed by cobaltocene and by a range of cobalt complexes of the type CpCoL2. Of the latter, the carbonyl (L = CO), Dicarbonyl(cyclopentadienyl)cobalt(I), and the cyclooctadiene (cod, L2 = C8H12) complex are the most commonly used, but many derivatives and analogs are comparable in efficiency, consistent with the evidence suggesting that the key steps are eqs 1 and 2. Ring closure to a cobaltacyclopentadiene system is probably induced by addition of the third alkyne (eq 3) or nitrile molecule (eq 4). Further reaction with alkyne then continues a chain in which the ligands L or L2 play no further part. They therefore influence only the conditions necessary to start the process.

The complex CpCo(cod) has been most widely used for the pyridine synthesis and the effect of varying catalyst structure on the reaction temperature and selectivity has been extensively studied.1,2 The choice of propyne reacting with propionitrile (eq 5) allowed the effect on chemo- and regioselectivity to be evaluated at the same time.

In a flow system, the temperature required for 65% conversion of propyne was 147-150 °C independent of L, although the initiation temperature varied widely, being <30 °C for L = C2H4, 75-85 °C for L2 = C5H6, and 120-125 °C for L2 = cod, but CpCo(CO)2 was unsuitable as catalyst under these conditions, giving only 5% conversion at 150 °C. The reaction temperature was dropped to 125 °C for both the indenyl analog, (C9H7)Co(cod), and for the tetraphenyl-substituted complex, (C5Ph4H)Co(cod), and the latter gave the highest chemoselectivity (ratio of pyridines to arenes) of 6.9, compared to 1.8 for CpCo(cod). Moreover, the tetraphenyl derivative also gave one of the highest regioselectivities (2,4,6-:2,3,6-trisubstituted pyridine) of 2.70, compared to 1.71 for CpCo(cod). In ethylpyridine synthesis (from acetylene + propionitrile), the complex (C5Ph4H)Co(cod) was again the outstanding catalyst with a turnover number of 6946 and 91.8% yield. It also gives relatively good results (TON 1625, 73.7% yield) in the vinylpyridine synthesis (eq 6), which must be conducted at moderate temperature to avoid polymerization.

High temperatures can be avoided by employing photolysis3a to effect the initiation step (eq 1). Thus an alternative route to 2-vinylpyridine3b employs this method to obtain intermediate 2-ethoxy- (or 2-methoxy)ethylpyridine (eq 7) with 99% selectivity (50% conversion); irradiation of acetylene and propionitrile with CpCo(cod) at 15 °C for 1.5 h gave 43% conversion to 2-ethylpyridine.3a

The reaction has been used in approaches to natural products, as illustrated by syntheses of (N-benzyloxycarbonylpyrrolidinyl)pyridine (eq 8)4 and by a route to vitamin B6.5 One version of this utilizes the regioselective CpCo(CO)2, Cp2Co, or CpCo(cod) catalyzed step (eq 9).5a The possibility of using homochiral substrates in such reactions without loss of optical purity has been demonstrated in the examples shown in eq 106a and eq 11.6b

The ethylene complex was used as catalyst7 in a study of the dependence of regioselectivity on substrate structure; the same catalyst was employed in a study of the cyclooligomerization of propargyl alcohol, which yielded approximately equal amounts of the aromatic trimerization products and the cyclooctatetraene derivatives (eq 12).8

The use of isocyanates or carbodiimides in cotrimerization with alkynes leads to N-substituted pyridones (eq 13) or the corresponding imines (eq 14).9

Other Catalytic Reactions.

The ethylene complex CpCo(C2H4)2 catalyzes the ring enlargement and hydrogen shifts which convert octamethyl[4]radialene to an arene (eq 15).10 Ethylene is also added to 2,5-dimethyl-2,3,4-hexatriene, initially yielding a complexed conjugated triene; under ethylene pressure this is displaced and its formation (eq 16)10 becomes catalytic.

Stoichiometric Reactions.

Reaction of carbodiimides with stoichiometric amounts of the ethylene complex CpCo(C2H4)2 yields a dinuclear and a tetranuclear complex with incorporation of an ethylene ligand into the latter (eq 17).11 Further use of these products has not been explored. An ethylene fragment is also incorporated in the reaction with diphenylketene (eq 18),12 whereas reaction with thiocyanates involves coupling of two molecules thereof and both cleavage and formation of C-S bonds (eq 19).13

That complexation of alkenes or dienes as CpCo(dialkene) can serve for their protection appears to have been demonstrated only for cyclooctatetraene; its 1,2,5,6-h complex has been converted to the bis(cyclopropane) derivative, leaving the metal-bound double bonds intact, by the reaction shown in eq 20;14 the free organic ligand was liberated by cerium(IV) oxidation in unspecified yield.

Reaction of CpCo(cod) with trityl cations leads to hydride abstraction from the 3-position of the cod ligand.15 The resultant cationic complex readily adds nucleophiles to give complexed 3-substituted cycloocta-1,5-dienes (eq 21).16 A more profound change of the cod ligand involving ring closure and alkylation occurred when the complex was treated with a cationic allyliron complex (eq 22); (C5Me5)Co(cod) reacted similarly.17

1. Bönnemann, H. AG(E) 1985, 24, 248.
2. Bönnemann, H.; Brijoux, W. NJC 1987, 11, 549.
3. (a) Schulz, W.; Pracejus, H.; Oehme, G. TL 1989, 30, 1229. (b) Schulz, W.; Pracejus, H.; Oehme, G. J. Mol. Catal. 1991, 66, 29.
4. Chelucci, G.; Falorni, M.; Giacomelli, G. S 1990, 1121.
5. (a) Geiger, R. E.; Lalonde, M.; Stoller, H.; Schleich, K. HCA 1984, 67, 1274. (b) Parnell, C. A.; Vollhardt, K. P. C. T 1985, 41, 5791.
6. (a) Botteghi, C.; Schionato, A.; Chelucci, G.; Brunner, H.; Kürzinger, A.; Obermann, U. JOM 1989, 370, 17. (b) Chelucci, G.; Falorni, M.; Giacomelli, G. G 1990, 120, 731.
7. Diversi, P.; Ingrosso, G.; Lucherini, A.; Vanacore, D. J. Mol. Catal. 1987, 41, 261.
8. Walther, D.; Braun, D.; Schulz, W.; Rosenthal, U. Z. Anorg. Allg. Chem. 1989, 577, 270.
9. Diversi, P.; Ingrosso, G.; Lucherini, A.; Malquori, S. J. Mol. Catal. 1987, 40, 267.
10. Stehling, L.; Wilke, G. AG 1988, 100, 575; AG(E) 1988, 27, 571.
11. Stella, S.; Floriani, C.; Chiesi-Villa, A.; Guastini, C. AG 1987, 99, 84; AG(E) 1987, 26, 68.
12. Beaumont, I. R.; Begley, M. J.; Harrison, S.; Wright, A. H. CC 1990, 1713.
13. Gambarotta, S.; Stella, S.; Floriani, C.; Chiesi-Villa, A.; Guastini, C. JCS(D) 1987, 1789.
14. (a) Reger, D. L.; Gabrielli, A. JACS 1975, 97, 4421; (b) Reger, D. L.; Gabrielli, A. JOM 1980, 187, 243.
15. Lewis, J.; Parkins, A. W. JCS(A) 1967, 1150.
16. (a) Kane-Maguire, L. A. P.; Mouncher, P. A.; Salzer, A. JOM 1979, 168, C42. (b) Kane-Maguire, L. A. P.; Mouncher, P. A.; Salzer, A. JOM 1988, 347, 383.
17. Connelly, N. G.; Gilbert, M.; Orpen, A. G.; White, J. M. JCS(D) 1988, 1631.

Peter L. Pauson

University of Strathclyde, Glasgow, UK

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