Cobalt complexes octacarbonyl, reactions

A combination of Co-mediated amino-carbonylation and a Pauson-Khand reaction was described by Pericas and colleagues [286], with the formation of five new bonds in a single operation. Reaction of l-chloro-2-phenylacetylene 6/4-34 and dicobalt octacarbonyl gave the two cobalt complexes 6/4-36 and 6/4-37 via 6/4-35, which were treated with an amine 6/4-38. The final products of this domino process are azadi- and azatriquinanes 6/4-40 with 6/4-39 as an intermediate, which can also be isolated and separately transformed into 6/4-40 (Scheme 6/4.11). 

Aryl methyl ketones have been obtained [4, 5] by a modification of the cobalt-catalysed procedure for the synthesis of aryl carboxylic acids (8.3.1). The cobalt tetracarbonyl anion is converted initially by iodomethane into the methyltetra-carbonyl cobalt complex, which reacts with the haloarene (Scheme 8.13). Carboxylic acids are generally obtained as by-products of the reaction and, in several cases, it is the carboxylic acid which predominates. Unlike the carbonylation of haloarenes to produce exclusively the carboxylic acids [6, 7], the reaction does not need photoinitiation. Replacement of the iodomethane with benzyl bromide leads to aryl benzyl ketones in low yield, e.g. 1-bromonaphthalene produces the benzyl ketone (15%), together with the 1-naphthoic acid (5%), phenylacetic acid (15%), 1,2-diphenylethane (15%), dibenzyl ketone (1%), and 56% unchanged starting material [4,5]. a-Bromomethyl ketones dimerize in the presence of cobalt octacarbonyl and... 

The metal carboxylate insertion mechanism has also been demonstrated in the dicobaltoctacarbonyl-catalyzed carbomethoxylation of butadiene to methyl 3-pentenoate.66,72 The reaction of independently synthesized cobalt-carboxylate complex (19) with butadiene (Scheme 8) produced ii3-cobalt complex (20) via the insertion reaction. Reaction of (20) with cobalt hydride gives the product. The pyridine-CO catalyst promotes the reaction of methanol with dicobalt octacarbonyl to give (19) and HCo(CO)4. 

This reaction was first reported in the early 1970s as an unexpected result in search for the synthesis of new organometallic cobalt complexes [13-18]. Dicobalt octacarbonyl was the only cluster used in its beginnings, although now, many cobalt species and other metal complexes are able to mediate or catalyze this reaction. Thus, the stoichiometric reaction has been performed with Zr, Ni, Fe, Ti, W and Mo derivatives. In addition, heterobimetallic Co - W and Co - Mo complexes (1), are suitable precursors for the PKR and impart a high degree of selectivity in the process giving exclusively endo adducts 2 (Scheme 2) [19,20]. 

In general, the acetylenic triple bond is highly reactive toward hydrogenation, hydroboration, and hydration in the presence of acid catalyst. Protection of a triple bond in disubstituted acetylenic compounds is possible by complex formation with octacarbonyl dicobalt [Co2(CO)g Eq. (64) 163]. The cobalt complex that forms at ordinary temperatures is stable to reduction reactions (diborane, diimides, Grignards) and to high-temperature catalytic reactions with carbon dioxide. Regeneration of the triple bond is accomplished with ferric nitrate [164], ammonium ceric nitrate [165] or trimethylamine oxide [166]. 

A particularly useful synthesis of cyclopentanones involves the coupling of an alkene, an alkyne and carbon monoxide in the presence of dicobalt octacarbonyl (equation 17). The reaction proceeds via an al-kyne-cobalt complex (7) and with relatively unreactive alkenes such as cyclopentene it is preferable to synthesize the complex in a separate step. With highly strained alkenes such as norbomadiene, how-... 

Although there is a striking similarity between the iron complexes, V and VI, and the corresponding cobalt complexes (dicobalt octacarbonyl and IV), there is one important difference the iron complexes are anions, while the cobalt complexes are imcharged. With this in mind, it is possible to explain the course of the hydroxymethylation reaction in a manner analogous to that postulated for the hydroformylation reaction. 

Decomposition of the complex by reaction with hydrogen or cobalt hydrocarbonyl, HCo(CO)4, which also may be present, is then suggested to yield tile aldehyde and a precursor of dicobalt octacarbonyl. Dicobalt octacarbonyl is formed again by reaction of this precursor with carbon monoxide. 

The reaction of propargyl alcohols with dicobalt octacarbonyl to give the complex salts 148 (X = BF4 or PF6) and synthetic uses of the latter have been reviewed. The salts react with electron-rich aromatic compounds ArH, such as anisole, phenol or N,N-dimethylaniline, to yield substitution products 149 after oxidative demetallation with an iron(III) or cerium(I V) salt with j5-diketones or j -keto esters the corresponding propargyl-substituted compounds 150 are obtained k Acetone reacts in an analogous fashion to give 151. The action of the cobalt complexes 148 on allylsilanes 152 leads to enynes 153. Indole reacts with the complex 148 (R = H R = R = Me) in the presence of boron trifluoride etherate to give 154, which was converted into 155 by the action of iron(III) nitrate " ... 

The Pauson-Khand reaction is the formation of cyclopentenones 486 by the action of dicobalt octacarbonyl on an alkyne, followed by an alkene. The process involves the intermediacy of cobalt complexes 485 (equation 53). (For reviews, see References 302 and 303.) Terminal alkynes, including acetylene and arylacetylenes, give better yields than internal acetylenes. An example of an intramolecular Pauson-Khand reaction is the conversion of the enyne 487 into the cyclopentanocyclopentenone 488 ". ... 

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). 

The Nicholas reaction enables efficient substitution reactions of propargyl alcohols, ethers, and acetates. Prior to the substitution step, dicobalt octacarbonyl reacts with the alkyne to yield cobalt-alkyne complex 1. The resulting organometallic complex reacts with inter- or intramolecular nucleophiles in the presence of a Lewis or protic acid to furnish desired substitution products 2. The cobalt-complexed alkyne can be oxidatively removed after this step or used to further fiinctionalize the Nicholas reaction products. The stereoselective synthesis of chiral products using the title reaction is also possible. ... 

A second interfacial exchange reaction of the o-acylcobalt complex with hydroxide ion leads to the production of the alkanecarboxylate anion, which migrates into the aqueous phase, leaving the cobalt tetracarbonyl anion in the organic phase for subsequent reaction (Scheme 8.2). Optimum yields of the carboxylic acids are obtained with ca. 40 1 ratio of the alkyl halide to dicobalt octacarbonyl. Co(Ph,P)2Cl2 can also be used and has the advantage that the cobalt can be recycled easily [5]. 

One complex with a metal—metal bond that has been added to an olefin is cobalt octacarbonyl. It reacts with tetrafluoroethylene and it seems reasonable that this is an insertion reaction but again it has not been proved. 

The species shown in the system 8.13 are those present under reaction conditions. At lower temperatures and pressures, a wealth of different complexes are present, including dicobalt octacarbonyl and cationic complexes of cobalt that differ depending on what cobalt compound was initially charged [17,23-25]. 

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. ° ... 

M. Orchin University of Cincinnati) Some very recent work done in our laboratory by Lawrence Kirch strongly suggests that an olefin-hydrocarbonyl complex is the important intermediate in the oxo synthesis. This new evidence was made possible by the experimental technique of quenching the hot, pressured autoclave in dry ice and releasing the gases below — 50°. The results of this work (1) show that (a) dicobalt octacarbonyl is rapidly converted to cobalt hydrocarbonyl (6) the hydrocarbonyl is rapidly complexed by olefin (c) when the olefin is consumed by normal oxo reaction, the cobalt again appears as the hydrocarbonyl (d) the extent of conversion of dicobalt octacarbonyl to cobalt hydrocarbonyl is dependent on the hydrogen partial pressure. 

Apart from cobalt carbonyl catylyzed hydroformylation, Pauson-Khand (PK) reaction is another type of reaction catalyzed with bimetallic carbonyl complex. Formally Pauson-Khand (PK) is a [2 -i- 2 -i- 1] cycloaddition of an alkyne, an alkene, and a CO group into cyclopentenone [128-130]. This process was initially discovered in 1973 [131], and early studies focused on using dicobalt octacarbonyl as both reaction mediator and the source of the carbonyl functional group. Since several variants of the original thermal protocol were introduced, PK reaction has received more and more fundamental and organic synthesis interests [132, 133]. 

From the above discussion it is clear that there are a large number of potential reaction conditions. In practice the most common conditions use a slight excess of cobalt octacarbonyl, in acetonitrile or dichloromethane to form the alkyne complex under an atmosphere of nitrogen or argon. The formation of the alkyne complex is easily monitored by thin layer chromatography on silica gel. The alkene and TMANO or NMO are then added and the reaction was allowed to proceed at room temperature until completion. If the results are not satisfactory, the reaction can be optimized with additives or by changing the temperature. 

Functional moieties for click reaction can either be incorporated in the polymer chain directly with the monomer, by quenching or via post-functionalization approaches. If the functional moiety is introduced with the monomer or the quencher, one has to consider the cross-reactivities between the metathesis catalyst and the functional moiety. In this case, the specific functional group can either be introduced by using protective groups, for example, complexing alkyne with Co2(CO)g, di-cobalt octacarbonyl or has to be introduced after the actual polymerization reaction. Post-functionalization approaches thereby often... 

Monomers with unprotected alkyne groups in the side chain cannot be polymerized without side reactions, as the alkylidene catalyst will engage in a competing alkyne metathesis. This side reaction can be avoided by using monomers with alkyne groups either complexed with di-cobalt octacarbonyl 1 or TMS-protected 2 (Scheme 9.3a,b) [14, 35, 36]. Alkyne incorporation at the chain end was achieved in a post-functionalization approach, reacting hydroxyl-terminated polynorbornene (PNBE) 5 with propargylic acid 6 (Scheme 9.3c) [37]. 

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