Cobalt catalysed processes

The hydroformylation of alkenes was accidentally discovered by Roelen while he was studying the Fischer-Tropsch reaction (syn-gas conversion to liquid fuels) with a heterogeneous cobalt catalyst in the late thirties. In a mechanistic experiment Roelen studied whether alkenes were intermediates in the Aufbau process of syn-gas (from coal, Germany 1938) to fuel. He found that alkenes were converted to aldehydes or alcohols containing one more carbon atom. It took more than a decade before the reaction was taken further, but now it was the conversion of petrochemical hydrocarbons into oxygenates that was desired. It was discovered that the reaction was not catalysed by the supported cobalt but in fact by HCo(CO)4 which was formed in the liquid state. 

The last reaction of the catalytic cycle has been the subject of much controversy. Two reactions may lead to the product  

The latter reaction might involve an oxidative addition, rapidly followed by a reductive elimination, or alternatively it might involve a o-bond metathesis 

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. 

This reaction includes loss of CO prior to reaction of the rhodium fragment and the manganese fragment to form more reactive, electron deficient species, which we have omitted from the reaction equation. 

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Since Shell s report on the use of phosphines in the cobalt catalysed process, which included preliminary data for the use of rhodium as well [1], many industries started to apply phosphine ligands in rhodium catalysed processes [2], While alkylphosphines are the ligands of choice for cobalt, they lead to slow reactions when applied in rhodium catalysis. In the mid-sixties the work of Wilkinson showed that arylphosphines should be used for rhodium and that even at mild conditions active catalysts can be obtained [3], The publications were soon followed by those of Pruett, in which phosphites were introduced (Figure 8.1) [4],... 

It is now nearly 40 years since the introduction by Monsanto of a rhodium-catalysed process for the production of acetic acid by carbonylation of methanol [1]. The so-called Monsanto process became the dominant method for manufacture of acetic acid and is one of the most successful examples of the commercial application of homogeneous catalysis. The rhodium-catalysed process was preceded by a cobalt-based system developed by BASF [2,3], which suffered from significantly lower selectivity and the necessity for much harsher conditions of temperature and pressure. Although the rhodium-catalysed system has much better activity and selectivity, the search has continued in recent years for new catalysts which improve efficiency even further. The strategies employed have involved either modifications to the rhodium-based system or the replacement of rhodium by another metal, in particular iridium. This chapter will describe some of the important recent advances in both rhodium- and iridium-catalysed methanol carbonylation. Particular emphasis will be placed on the fundamental organometallic chemistry and mechanistic understanding of these processes. 

Development of a heterogeneously catalysed processes based on cobalt. 

A cobalt catalysed carbonylation reaction converts A-substituted 1-aza-1,3-dienes into A-allylacetamides by a reductive acylation process [31]. Acetamides are byproducts of the reaction. In contrast, Schiff bases undergo a double A,C-acetylation under the same conditions producing a-acetamido ketones and A,A-disubstituted acetamides [32],... 

In the following sections a few typical processes will be described. An example of a cobalt catalysed hydroformylation reaction for higher alkenes is the Kuhlmann process (now Exxon process), for which the flow-scheme -a liquid/liquid separation- is shown in Figure 7.4. In this process the hydroformylation is done in one, organic phase consisting of alkene and aldehyde. The reactor is often a loop reactor or a reactor with an external loop to facilitate heat transfer. 

The hydroformylation of olefins is the most widely used homogeneous catalytic process using CO gas. It involves the addition of one molecule of CO and H2 to an olefin in the presence of a transition metal catalyst, most frequently based on cobalt or rhodium, resulting in the formation of an aldehyde. Generally, it is believed that the activation of H2 in cobalt-catalysed hydroformylation occurs on the unsaturated species Co2(CO)7 or Co(acylXCO)3 formed by the following reactions ... 

Unligated rhodium has the ability to hydroformylate a wide range of different olefins, both branched internal forms as well as linear. Virtually no water is used or created in the process (unlike a cobalt-catalysed system which needs water for catalyst recovery). Other key advantages of a rhodium (I) system are ... 

Many alkylpyridines are manufactured commercially by chemically complex processes which often produce them as mixtures. A good example is the extraordinary Chichibabin synthesis, in which paraldehyde and ammonium hydroxide react together at 230 °C under pressure to afford 52% of 5-ethyl-2-methylpyridine so here, four mol equivalents of acetaldehyde and one of ammonia combine. Also of commercial significance is the cobalt-catalysed interaction of a nitrile and acetylene. ... 

Amidocarbonylation is the only transition metal-catal) ed multi-component reaction, by which the amino acid framework is constructed directly from simple building blocks. [73] In the early 1970s, Hachiro Wakamatsu at Ajinomoto discovered by chance the cobalt-catalysed amidocarbonylation during investigation of the Oxo process with acrylonitrile, where he also found traces of -aminobutyric add. To determine the mechanism of this side-reaction, he... 

In subsequent processes, now predominating, benzene is hydrogenated to cyclohexane for aerial oxidation (cobalt catalysed) to KA oil (or Bashkirov oxidation to cyclohexanol). One or two companies appear to have substituted aerial oxidation of KA oil for the nitric acid oxidation process, while Celanese produce their 1,6-diaminohexane via 1,6-hexanediol, rather than adiponitrile. 

The catalytic system proved not only applicable to alkyl hahdes, but also allowed for the intramolecular conversion of aryl halides. Interestingly, the corresponding Mizoroki-Heck-type cyclization products were formed selectively, without traces of reduced side-products (Scheme 10.27) [55]. Therefore, a radical reaction via a single electron-transfer process was generally disregarded for cobalt-catalysed Mizoroki-Heck-type reactions of aromatic hahdes. Instead, a mechanism based on oxidative addition to yield an aryl-cobalt complex was suggested [51]. 

There are several problems with cobalt catalysed hydroformylation. The pressures required are high, leading to high capital costs. The selectivity (n iso) is rather low and there are side reactions. Catalyst losses arise through its volatility and also by decomposition to metallic cobalt. The metal has to be removed with acids from time to time, causing corrosion. Fortunately, cobalt is cheap. As better processes are now available it is unlikely that new plants of this type will be built. It is economic, however, to keep plants running which are already in operation. This is still the major industrial route to butanal. 

Reactions with alkynes may lead to the formation of cyclized products. The reaction of iodobenzenes with two equivalents of an alkyne has been shown to give naphthalene derivatives in the presence of cobalt catalyst with a manganese reduc-tant. The process, shown in Scheme 15, is thought to involve oxidative addition of the aryliodide to cobalt followed by double alkyne insertion. The cobalt-catalysed annulation step probably involves an pathway. The cyclopentadienyl-rhodium-catalysed annulation of benzoic acids with alkynes has been used to form isocoumarin derivatives, such as (126). The process is thought to involve cyclorhodation at the ortho-position of a rhodium benzoate intermediate, followed by alkyne insertion to form a seven-membered rhodacycle and reductive elimination The silver-catalysed annulations of diphenylphosphine oxides with alkynes proceed in the absence of rhodium. Benzophosphole oxides such as (127), formed with diphenylethyne, are produced. Here, the proposed mechanism involves homolytic cleavage of the phosphorus-hydrogen bond to give a radical which can add to the alkyne and subsequently cyclize. ... 

Rajabi P, Balu AM, Toreinia P, Luque R. A versatile supported cobalt(II) complex for heterogeneously catalysed processes conventional vs. microwave irradiation protocob. 

A new one-stage synthesis of annelated pyridines, involving the cobalt-catalysed co-oligomerization of dialkynes HC=C(CH2)nC=CH (n = 3, 4, or 5) with nitriles RCN (R = alkyl or aryl), has been announced. Under high dilution and in the presence of cyclopentadienylcobalt dicarbonyl, co-oligomerization to pyridines of type (12) takes place in a remarkably selective manner, yields of 43—81% being realised. The process is particularly useful for the preparation of the otherwise not easily accessible 5,6,7,8-tetrahydroquinolines. An interesting extension of the reaction uses ethyl cyanoacetate in place of the nitrile RCN,... 

Cobalt-catalysed cyclotrimerization of alkynes R C=CR with isocyanates R NCO and the carbodi-imide PhN=C=NPh constitutes a new synthesis of the 2-0X0- and 2-imino-l,2-dihydropyridines (20 X = O) and (20 X = NPh) respectively. Attempts to extend the process to the preparation of pyridine-2-thiones by using an isothiocyanate-alkyne mixture failed in that preferential desulphurization of the isothiocyanate took place. Chloro-cyanines, e.g. (21), prepared... 

Fischer-Tropsch (FT) synthesis is a catalysed chemical reaction in which carbon monoxide (CO) and hydrogen (H2) are converted into liquid hydrocarbons of various forms. Typical catalysts used are based on iron (Fe) and cobalt (Co). The production of liquid hydrocarbons using FT synthesis is a well known process. It was invented by Franz Fischer and Hans Tropsch in the 1920s in Germany. It follows the reaction ... 

A second process that came on stream in the fifties is the oligomerisation of ethene using cobalt complexes, but the number of homogeneously catalysed... 

Cobalamin catalysed electrochemical reduction of the 2-chloroethanol ester 68 at negative potentials, without photochemical assistance, leads to a 1,2-elimination process (see p. 115) [228]. This contrasts with the lack of 1,2-elimination during reaction of 66 and 67, Thus in the purely electrochemical carbon-cobalt bond... 

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