Cobalt hexacarbonyl dicobalt

Two possible explanations for the observed enantioselectivity were suggested. It was proposed that the A-oxide is able to selectively decarbonylate the hexacarbonyl dicobalt complex, or alternatively (or additionally), that the amine (brucine) resulting from the attack of the A-oxide is able to stabilise the coordinatively unsaturated cobalt atom that results from the loss of a CO ligand, presumably through coordination to the vacant site. In... 

The novel cobalt complex came about as a result of the intramolecular coordination of a double bond, present in one of the R groups on the acetylene, to one of the cobalt atoms - taking the place of a CO ligand (67 - 68). They found that the new pentacarbonyl complex could be readily formed in CDC13 at room temperature from the hexacarbonyl dicobalt precursor. Attempts to use the pentacarbonyl complex as a substrate in the PK reaction led to no formation of cyclopentenone product. It was proposed that this is due to the alkene occupying a pseudo-equatorial site -alkene insertion is thought to occur from the axial position (see section 2.4). 

Because of the instability of hexacarbonyl dicobalt complexes at elevated temperatures, other cobalt complexes have been used for the PKR. For example, Chimg and Jeong reported the reaction of norbomadiene with phenylacetylene at 100 °C under 15 atm of CO in 93% yield in the presence of (indenyl)Co(COD) (Equation 17.76). Chung also reported the combination of Co(acac),and NaBH as catalyst for the same transformation. ... 

Metal Hydrides. Metal hydrides generally react readily with acetylenes, often by an insertion mechanism. Cobalt hydrocarbonyl gives complicated mixtures of compounds with acetylenes. The only products which have been identified so far are dicobalt hexacarbonyl acetylene complexes (34). Greenfield reports that, under conditions of the hydroformy lation reaction, acetylenes give only small yields of saturated monoaldehydes (30), probably formed by first hydrogenating the acetylene and then reacting with the olefin. Other workers have identified a variety of products from acetylene, carbon monoxide, and an alcohol with a cobalt catalyst, probably cobalt hydrocarbonyl. The major products observed were succinate esters (74,19) and succinate half ester acetals (19). 

Dicyclopentadienyldinickeldiphenylbutadiyne-dicobalt hexacarbonyl has been prepared from diphenyldiacetylene in which one triple bond acts as a bridging group between two nickel atoms and the other between two cobalt atoms (203). Reduction of the diphenylacetylene complex (R = R = Ph) with sodium and alcohol in liquid ammonia yields dibenzyl, showing that the diphenylacetylene grouping is bonded only to the nickel atoms. The corresponding complex of acetylene (R = R = H) has also been prepared from nickeloccne and acetylene (69) ... 

The molecule of diphenylacetylene dicobalt hexacarbonyl has been shown by an x-ray investigation.57 to have the structure shown in Figure 11-19. Each cobalt atom forms six bonds, directed toward the corners... 

Pig. 11-19.—The structure of dicobalt hexacarbonyl diphenylacetylene, Coa(CO)fCi (C H )j. Large circles represent cobalt atoms, small circles carbon atoms, and circles of intermediate size oxygen atoms. 

Hexacarbonyl(1-methyl-2-propynylium)dicobalt tetrafluoroborate Cobalt(1+), hexacarbonyl[y-[2,3,-n 2,3-n)-l-methyl-2-propynylium]]di-, (Co-Co), tetrafluoroborate(l-) (10) (62866-98-2)... 

Like the double bond, the carbon-carbon triple bond is susceptible to many of the common addition reactions. In some cases, such as reduction, hydroboration and acid-catalyzed hydration, it is even more reactive. A very efficient method for the protection of the triple bond is found in the alkynedicobalt hexacarbonyl complexes (.e.g. 117 and 118), readily formed by the reaction of the respective alkyne with dicobalt octacarbonyl. In eneynes this complexation is specific for the triple bond. The remaining alkenes can be reduced with diimide or borane as is illustrated for the ethynylation product (116) of 5-dehydro androsterone in Scheme 107. Alkynic alkenes and alcohols complexed in this way show an increased structural stability. This has been used for the construction of a variety of substituted alkynic compounds uncontaminated by allenic isomers (Scheme 107) and in syntheses of insect pheromones. From the protecting cobalt clusters, the parent alkynes can easily be regenerated by treatment with iron(III) nitrate, ammonium cerium nitrate or trimethylamine A -oxide. ° ... 

Alkyne-dicobalt hexacarbonyl complexes, formed in situ from alkynes RC=CH (R = octyl or Ph), cobalt(II) bromide, zinc dust and carbon monoxide in THF, give mixtures of 489 and 490 by the action of norbornene. ... 

Mixed phosphite ester-phosphine coordinated acylcobalt carbonyl derivatives have also been prepared. Methyl(triphenylphosphine)cobalt tricarbonyl, which cannot be produced by a simple ligand replacement reaction, can be obtained by another method. Hieber and Lindner (21) found that bis(triphenylphosphine)dicobalt hexacarbonyl reacts with sodium amalgam to form sodium(triphenylphosphine)cobalt tricarbonyl and that this compound reacted with methyl iodide to form methyl-(triphenylphosphine)cobalt tricarbonyl. 

Nicholas, K. M. Chemistry and synthetic ntility of cobalt-complexed propargyl cations. (1987). Acc. Chem. Res., 20, 207-214. Teobald, B. J. (2002). The Nicholas reaction the use of dicobalt hexacarbonyl-stabilised propargylic cations in synthesis. Tetrahedron, 58, 4133 170. 

FIGURE 6.5 Carbon-cobalt spheres from the pyrolysis of dicobalt-hexacarbonyl-decorated PPEs. (From Scholz, S., et al., Adv. Mater, 17. 1052, 2005, in press. With permission.)... 

The intermolecular Pauson-Khand annulation using dicobalt hexacarbonyl-alkyne complexes met only with limited success, possibly because of the higher energy of the cobalt-carbonyl bond. In contrast, chromium carbonyls dissociate more easily, and the reactions of metallocarbenes of chromium, e.g., the Dotz annulation of alkynes, are improved by sonication. 

Cobalt.- A number of publications are concerned with the preparation of dicobalt alkyne complexes of the type [Co2(CO)e(p-alkyne)]. These include reports on the selective introduction of the dicobalt hexacarbonyl unit into methotrexate , the preparation of derivatives of cortisol, testosterone and dihydrotestosterone for potential use as tracers in immunoassays , and the... 

Dicobalt-hexacarbonyl-alkyne complexes are another class of organometallic compounds with good stability imder physiological conditions. Complexation of the alkyne proceeds smoothly under mild conditions by reaction with Co2(CO)g imder loss of two molecules of CO [79]. The applicability of this reaction to peptides was shown by Jaouen and coworkers by the reaction of Co2(CO)g with protected 2-amino-4-hexynoic acid (Aha) and dipeptides thereof (Boc-Phe-Aha-OMe and Ac-Aha-Phe-OMe) [80]. Similarly, Cp2Mo2(CO)4 complexes of these alkynes were obtained. It has been shown that the C-terminal Met" in SP can be replaced by isostere analogs without appreciable loss of physiological activity. The same is true for the C-terminal Met in neurokinin A (NKA), another tachykinin peptide hormone (Scheme 5.16). Alkyne analogs of SP and NKA were obtained by replacement of these methionines with norleucine acetylene residues. Alternatively, Lys in NKA may be replaced by an alkyne derivative which can also be complexed to Co2(CO)g as shown in Scheme 5.16. Complexation with Co2(CO)g proceeds smoothly in about 50% yield for all derivatives [81]. After HPLC purification, these cobalt alkyne peptides were comprehensively characterized spectroscopically. Most notably, they exhibit typical IR absorptions for the metal carbonyl moieties between 2000-2100 cm [3]. Recently, there is renewed interest in Co2(CO)5(alkyne) complexes because of their cytotoxicity [82-84]. 

The structure of dicobalt hexacarbonyl diphenylacetylene is shown in Figure 17.6. Two 7i-orbitals of the carbon-carbon triple bond bond to two cobalt atoms. The bond length of C-C is slightly longer (1.46 A) than that of an ordinary carbon-carbon double bond. The cobalt atom forms a distorted octahedral configuration [37]. 

Scheme 3.47 Asymmetric allylborations of 3-decynal dicobalt hexacarbonyl cobalt-induced reversal of enantioselectivity.

Reaction of [S(CCPh)2] with [Co2(CO)g] afforded 2 jhe crystallographically characterised adduct [(PhCC)S Co2(CO)6(Ti2-CCPh) ] which, on treatment with further [Co2(CO)g] afforded [S Co2(CO)6(Tl2-CCPh) 2l. The reaction of pentacarbonyl allyloxy((4-methylphenyl)ethynyl)carbene) chromium and tungsten complexes with dicobalt octacarbonyl afforded 63 tpe alkyne complexes (62) (M = Cr, W), which failed to undergo intramolecular Pauson-Khand reactions instead, at room temperature, a 1,2-elimination occurs, regenerating the metal hexacarbonyl with predominant formation of dinuclear cobalt complexes (63) - (65). 

Tyrrell demonstrated a three step tandem sequence involving an intermolecular Nicholas reaction, intramolecular Nicholas reaction, and a cationic cyclization. Treatment of silyl enol ether 55 with hexacarbonyl(propiolaldehyde diethyl acetal) dicobalt and boron trifluoride provides cobalt-alkyne complex 56. Exposure of this material to tetrafluoroboric acid promotes an intramolecular Nicholas reaction to form the second six-membered ring. Alkyne decomplexation with ceric ammonium nitrate enables the final cyclization step to yield the target tricycle 57." ... 

Nicholas published an Organic Syntheses paper highlighting the preparation of 2-(l-methyl-2-propynyl)cyclohexanone (65) from the reaction of 1-trimethylsiloxycyclohexene (62) with hexacarbonyl (l-methyI-2-propy-nylium)dicobalt tetrafluoroborate (63) and subsequent decomplexation of cobalt complexed alkyne 64. This report also includes procedures for the synthesis of the two starting materials (62 and 63). ... 

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