Cobaltate , tetracarbonyl

The commercially important normal to branched aldehyde isomer ratio is critically dependent on CO partial pressure which, in propylene hydroformylation, determines the rate of interconversion of the -butyryl and isobutyryl cobalt tetracarbonyl intermediates (11). 

In contrast to triphenylphosphine-modified rhodium catalysis, a high aldehyde product isomer ratio via cobalt-catalyzed hydroformylation requires high CO partial pressures, eg, 9 MPa (1305 psi) and 110°C. Under such conditions alkyl isomerization is almost completely suppressed, and the 4.4 1 isomer ratio reflects the precursor mixture which contains principally the kinetically favored -butyryl to isobutyryl cobalt tetracarbonyl. At lower CO partial pressures, eg, 0.25 MPa (36.25 psi) and 110°C, the rate of isomerization of the -butyryl cobalt intermediate is competitive with butyryl reductive elimination to aldehyde. The product n/iso ratio of 1.6 1 obtained under these conditions reflects the equihbrium isomer ratio of the precursor butyryl cobalt tetracarbonyls (11). 

A typical example of this is the dicobalt octacarbonyl catalyzed hydroformylation of olefins to yield aldehydes. According to the classical mechanism proposed by Heck and Breslow /29/ (Equations 28-31), the cobalt carbonyl reacts with hydrogen to form hydrido cobalt tetracarbonyl, which is in equilibrium with the coordinatively unsaturated HCo(C0)2. The tricarbonyl coordinates the olefin, and rearranges to form the alkyl cobalt carbonyl. 

The first catalyst used in hydroformylation was cobalt. Under hydroformylation conditions at high pressure of carbon monoxide and hydrogen, a hydrido-cobalt-tetracarbonyl complex (HCo(CO)4) is formed from precursors like cobalt acetate (Fig. 4). This complex is commonly accepted as the catalytic active species in the cobalt-catalyzed hydroformylation entering the reaction cycle according to Heck and Breslow (1960) (Fig. 5) [20-23]. 

The hydrido-cobalt-tetracarbonyl complex (I) undergoes a CO-dissocia-tion reaction to form the 16-electron species HCo(CO)3 (II). This structure forms a 7r-complex (III) with the substrate and is a possible explanation for the formation of further (C = C)-double bond isomers of the substrate. In the... 

In the next step of the reaction cycle, the carbon monoxide is inserted into the carbon-cobalt bond. At this time, the subsequent aldehyde can be considered as preformed. This step leads to the 16 electron species (VI). Once again, carbon monoxide is associated to end up in the 18 electron species (VII). In the last step of the reaction cycle, hydrogen is added to release the catalyti-cally active hydrido-cobalt-tetracarbonyl complex (I). Likewise, the aldehyde is formed by a final reductive elimination step. 

In a similar manner, Jt-allyl complexes of manganese, iron, and molybdenum carbonyls have been obtained from the corresponding metal carbonyl halides [5], In the case of the reaction of dicarbonyl(r 5-cyclopentadienyl)molybdenum bromide with allyl bromide, the c-allyl derivative is obtained in 75% yield in dichloromethane, but the Jt-allyl complex is the sole product (95%), when the reaction is conducted in a watenbenzene two-phase system. Similar solvent effects are observed in the corresponding reaction of the iron compound. As with the cobalt tetracarbonyl anion, it is... 

The alkylidyne tricobalt nonacarbonyl complexes (2) are produced from the reaction of the cobalt tetracarbonyl anion with 1,1,1-trihaloalkanes [4], under conditions analogous to those used for the synthesis of the n-allyl complexes. Although the yields for (2) appear to be low (Table 8.3), they are better than, or comparable with, those obtained by the traditional procedures [8] and are obtained under more amenable conditions. 

Kinetics show that the reaction is pseudo-first order in the RX concentration and that there is a linear correlation in the rate of consumption of RX with the concentration of the catalyst. The need for a high rate of stirring indicates that, as discussed in Chapter 1, the base-initiated formation of the cobalt tetracarbonyl anion results from an interfacial exchange process. It is significant that, when preformed NaCo(CO)4 is used, the extractability of the anion by benzyltriethylammonium cation into diisopropyl ether is three times less efficient than it is into benzene or dichloromethane, but kinetic studies show that, in spite of the lower concentration of the anion in the ether, the rate of reaction with RX in that solvent is generally higher [3]. 

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]. 

A high carbon monoxide pressure ( 5 atmos.) favours the formation of the butane. Possible mechanisms for its formation include homolytic cleavage of the benzyl-cobalt tetracarbonyl complex and recombination of the radicals to generate 2,3-diphenylbutane and dicobalt octacarbonyl, or a base-catalysed decomposition of the benzylcobalt tetracarbonyl complex (Scheme 8.4). The ethylbenzene and styrene could arise from the phenylethyl radical, or from the n-styrene hydridocobalt tricarbonyl complex. 

Although the acylcobalt tetracarbonyls react with hydroxide ion under phase-transfer conditions, in the presence of alkenes and alkynes they form o-adducts rapidly via an initial interaction with the ir-electron system. Subsequent extrusion of the organometallic group as the cobalt tetracarbonyl anion leads to a,(J-unsaturated ketones (see Section 8.4). In contrast, the cobalt carbonyl catalysed reaction of phenylethyne in the presence of iodomethane forms the hydroxybut-2-enolide (5) in... 

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... 

Carbonvlation of Benzyl Halides. Several organometallic reactions involving anionic species in an aqueous-organic two-phase reaction system have been effectively promoted by phase transfer catalysts(34). These include reactions of cobalt and iron complexes. A favorite model reaction is the carbonylation of benzyl halides using the cobalt tetracarbonyl anion catalyst. Numerous examples have appeared in the literature(35) on the preparation of phenylacetic acid using aqueous sodium hydroxide as the base and trialkylammonium salts (Equation 1). These reactions occur at low pressures of carbon monoxide and mild reaction temperatures. Early work on the carbonylation of alkyl halides required the use of sodium amalgam to generate the cobalt tetracarbonyl anion from the cobalt dimer(36). 

Before addition of the benzyl halide, the only carbonyl adsorption peak is found at 1900 cm, indicative of the cobalt tetracarbonyl anion. After addition, this band immediately disappears and peaks at 2000 cm l are observed. These most likely represent the corresponding acyl complex. Reaction with methoxide yields the product and regenerates the cobalt anion. In the absence of sufficient methoxide, the reaction requires attack by the much... 

Synonyms dicobalt octacarbonyl cobalt carbonyl cobalt tetracarbonyl dimer... 

The alkylcobalt tetracarbonyls react completely analogously with carbon monoxide, forming acyl cobalt tetracarbonyls (43). 

Metal-Carbon Compounds. Clear examples of olefin insertions into transition metal-carbon groups are rare. The obvious reaction of olefins with alkyl- or acyl-cobalt tetracarbonyls are slow, complicated, and incomplete under the usual laboratory conditions. Under high pressure at elevated temperatures, in the... 

The 2-methylenecyclopentanone initially formed presumably rearranges into 2-methyl-2-cyclopentenone under the reaction conditions. The final step of the mechanism, elimination of the cobalt carbonyl group, is not well understood but the same kind of elimination and reduction reactions occur with known 3-ketocobalt complexes. As mentioned above, crotonaldehyde, acrolein (27), and glyddaldehyde (38) react rapidly with cobalt hydrocarbonvl under similar conditions to give reduction products, rather than forming stable alkyl- or acyl-cobalt tetracarbonyl derivatives. 

Again, cobalt hydrotricarbonyl probably adds initially. The addition could be either a 1,4-addition or a 1,2-addition in which cobalt initially adds to the second carbon atom of the diene, and then undergoes an allylic rearrangement. The 3-pentenoylcobalt tricarbonyl triphenylphosphine prepared in this manner contains considerably more of the cis isomer than the same compound prepared from trans crotyl bromide and sodium cobalt tetracarbonyl. This, indicates that the hydrocarbonyl prefers cis addition (47). 

T,he hydroformylation reaction or oxo synthesis has been used on an industrial scale for 30 years, and during this time it has developed into one of the most important homogeneously-catalyzed technical processes (I). A variety of technical processes have been developed to prepare the real catalyst cobalt tetracarbonyl hydride from its inactive precursors, e.g., a cobalt salt or metallic cobalt, to separate the dissolved cobalt carbonyl catalyst from the reaction products (decobaltation) and to recycle it to the oxo reactor. The efficiency of each step is of great economical importance to the total process. Therefore many patents and papers have been published concerning the problem of making the catalyst cycle as simple as possible. Another important problem in the oxo synthesis is the formation of undesired branched isomers. Many efforts have been made to keep the yield of these by-products at a minimum. 

Cobalt Tetracarbonyl (Dicobaltocarbonyl), Co2(CO)8, mw 342.0, mp 51°, bp-decomp at higher temp with evoln of poisonous CO d 1.78 insol in w, sol in alco or eth. Can be prepd by treating finely divided Co with CO under pressure used in high anti-knock gasoline. It can react with oxidizing materials... 

Heck and Breslow (63) obtained evidence relating to Eq. (52) for the case of cobalt. They reacted a number of alkyl and acyl halides carrying functional groups with sodium cobalt tetracarbonylate under carbon monoxide. Normal acylcobalt carbonyls were readily formed except for the case of chloroacetonitrile, where a cyanomethylcobalt carbonyl was isolated. Apparently an a-nitrile group is sufficiently electronegative to stabilize the alkylcobalt carbonyl against carbonylation. 

It was suggested that such derivatives would have unusual stability like the corresponding 7r-allyl complexes and would be further reduced by hydrocarbonyl rather than undergoing carbonylation. Heck (63) has, however, attempted the preparation of similar derivatives from chloroacetone and phenacyl bromide with sodium cobalt tetracarbonylate, Instead of finding unusually stable complexes he reported an unusual instability. It seems likely that in fact normal alkylcobalt carbonyls are formed, e.g.,... 

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