Cobalt metal acyls

I.3.4.2.5.2. Other Transition-Metal-Acyl Complexes 1.3.42.5.2.1. Chiral Cobalt-Acyl Complexes... 

Methyl acetate probably originates from the reaction of methanol with the intermediate cobalt-acyl complex. The reaction leading to the formation of acetaldehyde is not well understood. In Equation 8, is shown as the reducing agent however, metal carbonyl hydrides are known to react with metal acyl complexes (20-22). For example, Marko et al. has recently reported on the reaction of ri-butyryl- and isobutyrylcobalt tetracarbonyl complexes with HCo(CO) and ( ). They found that at 25 °C rate constants for the reactions with HCo(CO) are about 30 times larger than those with however, they observed that under hydroformylation conditions, reaction with H is the predominant pathway because of the greater concentration of H and the stronger temperature dependence of its rate constant. The same considerations apply in the case of reductive carbonylation. Additionally, we have found that CH C(0)Co(C0) L (L r PBu, ... 

The active species is formed under normal operating conditions (70-I50 C, 50 100 atm of total pressure of H2 + CO 1 I). In the absence of tertiary phosphine, or with little tertiary phosphine, the catalyst is HRh(CO)4 . which is formed in situ. The catalytic cycle is completely analogous to that of IICo(CO)4 (.Scheme 3). as found with cobalt, and the rate-determining step is the hydro genolysis of the metal- acyl species. 

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]. 

Accelerators. To allow the degradation of the peroxides at room temperature, the activation energy must be reduced. This is done by adding accelerators. Hydroperoxides are cleaved by heavy metal salts and acyl peroxides by tertiary aromatic amines. Up to 2% of the latter are added by the manufacturers to resins for fillers. These are known as amine-accelerated UP resins. It is common to add 0.02-0.05% cobalt (calculated as cobalt metal) in the form of cobalt naphthenate or cobalt octoate dissolved in aromatics (not white spirit) to the systems hardened with hydroperoxides. The accelerator should only be added shortly before application for reasons of storage stability and drift. 

N-Acyl-a-amino acids are important compounds in both chemistry and biology. They are easily obtained in a transition metal-catalyzed, three-component domino reaction of an aldehyde, an amide, and CO. Whereas cobalt was mainly used for this process, Beller and coworkers [159] have recently shown that palladium has a... 

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. 

The amidocarbonylation of aldehydes provides highly efficient access to N-acyl a-amino acid derivatives by the reaction of the ubiquitous and cheap starting materials aldehyde, amide, and carbon monoxide under transition metal-catalysis [1,2]. Wakamatsu serendipitously discovered this reaction when observing the formation of amino acid derivatives as by-products in the cobalt-catalyzed oxo reaction of acrylonitrile [3-5]. The reaction was further elaborated to an efficient cobalt- or palladium-catalyzed one-step synthesis of racemic N-acyl a-amino acids [6-8] (Scheme 1). Besides the range of direct applications, such as pharmaceuticals and detergents, racemic N-acetyl a-amino acids are important intermediates in the synthesis of enantiomeri-cally pure a-amino acids via enzymatic hydrolysis [9]. 

Recently proof has been reported for a heterometallic bimolecular formation of aldehyde from a manganese hydride and acylrhodium species [2], Phosphine free, rhodium carbonyl species show the same kinetics as the cobalt system, i.e. the hydrogenolysis of the acyl-metal bond is rate-determining. Addition of hydridomanganese pentacarbonyl led to an increase of the rate of the hydroformylation reaction. The second termination reaction that takes place according to the kinetics under the reaction conditions (10-60 bar, 25 °C) is reaction (3). The direct reaction with H2 takes place as well, but it is slower on a molar basis than the manganese hydride reaction. 

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... 

Many transition metal alkyls react with carbon monoxide to give acyl compounds. In all these cases the acyl derivatives can be detected at least by infrared methods and in most cases isolated. Molybdenum, manganese, rhenium, iron, cobalt, rhodium, nickel, palladium, and platinum alkyls, Grignard reagents, and boranes, all react with carbon monoxide, and one can explain the products from these on the basis of carbon monoxide inserting into the metal alkyl. 

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. 

Friedel-Crafts acylations of the metal acetylacetonate rings are much slower than the electrophilic substitutions described above, probably because of the considerable steric bulk at the reaction site. Furthermore, the strongly acidic conditions during the reaction and subsequent hydrolysis step give rise to considerable degradation, particularly in the case of the more sensitive chromium and cobalt chelates. This consideration places severe limitations on the reaction conditions that can be employed. 

The key features of both catalytic cycles are similar. Alkene coordination to the metal followed by insertion to yield an alkyl-metal complex and CO insertion to yield an acyl-metal complex are common to both catalytic cycles. The oxidative addition of hydrogen followed by reductive elimination of the aldehyde regenerates the catalyst (Scheme 2 and middle section of Scheme 1). The most distinct departure in the catalytic cycle for cobalt is the alternate possibility of a dinuclear elimination occurring by the in-termolecular reaction of the acylcobalt intermediate with hydridotetracarbonylcobalt to generate the aldehyde and the cobalt(0) dimer.11,12 In the cobalt catalytic cycle, therefore, the valence charges can be from +1 to 0 or +1 to +3, while the valence charges in the rhodium cycles are from +1 to +3. 

The isomerizations have been studied by Takegami et al. by reacting the isomeric alkyl or acyl halides with an alkali metal cobalt carbonylate (142, 148,149). 

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