Acyl cobalt

Acyl cobalt complexes are reduced to their alkyl counterparts in good yields with Et3SiH/TFA (Eq. 248).183-310-425 Acyl ferrocene derivatives are reduced to the respective methylene compounds with Et3SiH/TFA (Eqs. 249180 and 250).179 Acylcyclopentadienylmanganese tricarbonyl is similarly reduced in good yield.351... 

Most hydroformylation investigations reported since 1960 have involved trialkyl or triarylphosphine complexes of cobalt and, more recently, of rhodium. Infrared studies of phosphine complex catalysts under reaction conditions as well as simple metal carbonyl systems have provided substantial information about the postulated mechanisms. Spectra of a cobalt 1-octene system at 250 atm pressure and 150°C (21) contained absorptions characteristic for the acyl intermediate C8H17COCo(CO)4 (2103 and 2002 cm-1) and Co2(CO)8. The amount of acyl species present under these steady-state conditions increased with a change in the CO/ H2 ratio in the order 3/1 > 1/1 > 1/3. This suggests that for this system under these conditions, hydrogenolysis of the acyl cobalt species is a rate-determining step. 

Heck has studied the reaction of triphenylphospbine22-24 and trimethylol-propane phosphite25 with the substituted cobalt carbonyls listed in Tables 1-4. The general mechanism for the reaction of the acyl cobalt carbonyls shown in Table 1 in the presence of triphenylphosphine is... 

A relatively high molecular weight copolymer can be synthesized by using an acyl-cobalt complex as an initiator such a catalyst system is not accompanied by dehydration theoretically because the terminal group of a growing species is not a free alcohol but protected by an acyl group (Scheme 13). 

For the route A, acyl radicals donors like iS are readily generated from acyl selenides (ISa) or acyl cobalt derivatives (iSb) and radicals acceptors 2S are usually multiple bonds as in methyl vinyl ketone (2Sa) -although some homolytic substitutions are possible. On the other hand, nitriles GSal are useful acceptors (3S) in radical cyclisations and 4Sa is an obvious synthon equivalent of radical donor 4S (See Table 7.2). 

The overall response to the reaction variables is very similar in the carbonylation and reductive carbonylation reactions. This may indicate similar catalysts and reaction mechanisms. In the carbonylation reaction Co(CO) " was identified by its characteristic CO stretching frequency ( v(CO) r 1890 cm" as the dominant species present in high pressure infrared experiments carried out at 170 °C and 5000 psig. Similar results were obtained in the reductive carbonylation of methanol. It is known that Co(CO) " rapidly reacts with CH I to yield CH C(0)Co(C0) (J9) however, in the carbonylation and reductive carbonylation reactions acyl-cobalt complexes are not observed by infrared under catalytic conditions. This indicates that once formed, the acyl complex rapidly reacts as outlined by Equations 7 and 8. 

As an active initiator for a co-polymerization, acyl-cobalt complexes also work well. As demonstrated by Alper and Lee, an equimolar mixture of Go2(GO)s, benzyl bromide (BnBr), and dihydro-1,10-phenanthroline 17, possibly generating BnGOGo(GO)4 under the reaction conditions, co-polymerized PO or 1,2-butene oxide with GO, and the... 

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. 

A number of metal alkyls add readily to double bonds. These include the titanium alkyls, chromium aryls and alkyls, the alkylmanganese carbonyls, acyl-cobalt carbonyls, alkali metal alkyls, the magnesium alkyls, and aluminum alkyls. 

The last step represents an intramolecular nucleophilic lysis of the acyl-cobalt carbonyl, with the regeneration of cobalt hydrocarbonyl. Similar... 

Reduction by nucleoi ilic attack of the original metal hydride has been own to occur under stoichiometric reaction conditions, for instance in the reduction of acyl cobalt complexes with CoH(CO) . Kinetic considerations, Imwever, make it unlikely that such a sequence plays a major role in the hydroformylaiion process. In fact, the rate-determining step, corresponding to the hydrogenolysis of the acyl complex, is slowed by an increase of carbon... 

Many industrially important catalytic processes use alkyl and acyl cobalt carbonyl complexes, which can be synthesized from (2 ). Acyl complexes produced after formation of... 

Synthesis of intermediates. An excellent technique for confirming or refuting a postulated pathway is to synthesize intermediates and use them as starting materials. Often, a key intermediate that is reactive enough to remain at trace level under reaction conditions is stable at very low temperatures (e.g., that of liquid nitrogen) and can be synthesized. If the reaction starting with the postulated intermediate yields the same products in the same ratios, this can be taken as evidence in favor of the presumed pathway. For example, the essential features of the Heck-Breslow mechanism of hydroformylation (see Example 6.2 in Section 6.3) with cobalt hydrocarbonyl catalysts have been verified in this way by synthesis and use of the alkyl-and acyl-cobalt species [42]. 

In a very elegant mechanistic study by Ojima et al., involving the amidocarbonylation of three structurally related cyclic amides having methallyl side chains (utilizing cobalt carbonyl catalysis) they have demonstrated [23] that coordination of the amide carbonyl to the cobalt metal is essential for amidocarbonylation, whereas lactame formation is not. A general mechanism of amidocarbonylation, featuring the very unique hydrolysis (alcoholysis) of the acyl-cobalt bond by water (or alcohol) generated in situ, is reproduced in Scheme 1 [23]. 

The catalyst, HCo(CO)4, is generated under the high pressure conditions of the reaction. The catalyst precursor added to the reactor may be Co2(CO)g, which is preferable, or a soluble cobalt salt. The new bond is formed by the transformation of the alkyl cobalt tetracarbonyl to the acyl cobalt tricarbonyl. Details of this step are not known, but reaction probably proceeds either by a three-center intermediate or by direct migration of the alkyl group to one of the ligated CO groups. 

The mechanism for the radical addition-elimination, promoted by alkyl- or acyl-cobalt reagent can be explained by Michael addition followed by dehydrocobaltation (Scheme 29). 

Scheme 29. Mechanism of radical addition-elimination protocol for alkyl or acyl cobalt reagents...

The generally accepted mechanism for the hydroformylation of olefins catalyzed by Co2(CO)g was first proposed by Heck and Breslow and is depicted in Scheme 1 for the formation of a linear aldehyde. The proposed mechanism includes the generation of HCo(CO)4 from Co2(CO)g and hydrogen as the first step, followed by the three crucial unit processes mentioned above. Instead of hydrogenolysis of the acyl-cobalt species, RCH2CH2CO-Co(CO)4, reductive cleavage of the acyl-cobalt species with HCo(CO)4 is also possible to regenerate Co2(CO)g. 

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