Carbonylation of alcohols

Oxidative carbonylation of alcohols with PdCh affords the carbonate 572 and oxalate 573(512-514]. The selectivity of the mono- and dicarbonylation depends on the CO pressure and reaction conditions. In order to make the reaction catalytic, Cu(II) and Fe(III) salts are used. Under these conditions, water is formed and orthoformate is added in order to trap the water. Di-/-butyl peroxide is also used for catalytic oxidative carbonylation to give carbonates and oxalates in the presence of 2,6-dimetliylpyridine(515]. 

Carbonylation of alcohols to acids, table of examples, 46, 74 Carboxylation, by formic acid, 46, 74 of 2-methylcyclohexanol by formic acid-sulfuric acid to 1-methyl-cyclohexanecarboxylic acid, 46, 72... 

Since 1985, several thousands of publications have appeared on complexes that are active as catalysts in the addition of carbon monoxide in reactions such as carbonylation of alcohols, hydroformylation, isocyanate formation, polyketone formation, etc. It will therefore be impossible within the scope of this chapter to review all these reports. In many instances we will refer to recent review articles and discuss only the results of the last few years. Second, we will focus on those reports that have made use explicitly of coordination complexes, rather than in situ prepared catalysts. Work not containing identified complexes but related to publications discussing well-defined complexes is often mentioned by their reference only. Metal salts used as precursors on inorganic supports are often less well defined and most reports on these will not be mentioned. 

Formation of acetic acid from methanol and carbonylation of alcohols still are important industrial problems, but milder conditions are needed. Other metals, such as rhodium, have proved to be more suitable. 

Oxidative carbonylation is not necessarily associated with C - C bond formation. Indeed, heteroatom carbonylation may occur exclusively, as in the oxidative carbonylation of alcohols or phenols to carbonates, of alcohols and amines to carbamates, of aminoalcohols to cyclic carbamates, and of amines to ureas. All these reactions are of particular significance, in view of the possibility to prepare these very important classes of carbonyl compounds through a phosgene-free approach. These carbonylations are usually carried out in the presence of an appropriate oxidant under catalytic conditions (Eqs. 31-33), and in some cases can be promoted not only by transition metals but also by... 

The oxidative carbonylation of alcohols and phenols to carbonates can be catalyzed by palladium or copper species [154-213]. This reaction is of particular practical importance, since it can be developed into an industrial process for the phosgene-free synthesis of dimethyl carbonate (DMC) and diphenyl carbonate (DPC), which are important industrial intermediates for the production of polycarbonates. Moreover, DMC can be used as an eco-friendly methylation and carbonylation agent [214,215]. The industrial production of DMC by oxidative carbonylation of methanol has been achieved by Enichem [216] and Ube [217]. 

A number of ruthenium-based catalysts for syn-gas reactions have been probed by HP IR spectroscopy. For example, Braca and co-workers observed the presence of [Ru(CO)3l3]", [HRu3(CO)ii]" and [HRu(CO)4] in various relative amounts during the reactions of alkenes and alcohols with CO/H2 [90]. The hydrido ruthenium species were found to be active in alkene hydroformylation and hydrogenation of the resulting aldehydes, but were inactive for alcohol carbonylation. By contrast, [Ru(CO)3l3]" was active in the carbonylation of alcohols, glycols, ethers and esters and in the hydrogenation of alkenes and oxygenates. 

Reactions lla-e add up to Reaction 10. Reactions lla-b have been shown above to be catalyzed by Rh/CH3l. Reaction 11c, i.e. acid-catalysed pyrolysis of EDA to acetaldehyde and acetic anhydride, is well documented (9). Both reaction lid, hydrogenation of aldehyde, and Reaction lie, carbonylation of alcohols, are of course well known. The reaction sequence is in agreement with the fact that EDA and AH, especially in short-duration experiments, are detected as by-products. Acetaldehyde is also observed in small quantities, but no ethanol is found. Possibly, Reactions lid and He occur concertedly. We have separately demonstrated that both EDA and AH are suitable feeds to produce propionic acid under homologation reactions conditions. We thus demonstrated... 

Carbonylation of Alcohols - Pd(tppts)3 catalyses the carbonylation of benzylic alcohols to the corresponding phenylacetic acids, in the presence of a Bronsted acid cocatalyst such as H2S04 or p-CH3C6H4S03H in biphasic aqueous/organic media (no organic solvent).305,451 For example, benzyl alcohol was converted to phenylacetic acid (Equation 6) and l-(4-isobutylphenyl)ethanol (IBPE) to ibuprofen (Figure 9). 

On the pages which follow, general methods are illustrated for the synthesis of a wide variety of classes of organic compounds including acyl isocyanates (from amides and oxalyl chloride p. 16), epoxides (from reductive coupling of aromatic aldehydes by hexamethylphosphorous triamide p. 31), a-fluoro acids (from 1-alkenes p. 37), 0-lactams (from olefins and chlorosulfonyl isocyanate p. 51), 1 y3,5-triketones (from dianions of 1,3-diketones and esters p. 57), sulfinate esters (from disulfides, alcohols, and lead tetraacetate p. 62), carboxylic acids (from carbonylation of alcohols or olefins via carbonium-ion intermediates p. 72), sulfoxides (from sulfides and sodium periodate p. 78), carbazoles... 

Table 5.29. Carbonylation of Alcohols over Nafion-H402...

Rhodium-catalyzed carbonylation of alcohols such as ethanol, n-propanol, and f-propanol have been studied. Unlike methanol carbonylation, central to all... 

The catalyst for hydroformylation is a rhodium(I) hydride species, which is clearly distinct from the species that are active for hydrogenation. The hydrogenation catalysts are cationic Rh(I)+ or neutral Rh(I)Cl species. Carbonylation of alcohols also requires an ionic Rh(I) species, e.g. [Rl CO y-- Often rhodium(I) salts are used as the precursor for hydroformylation catalysts. Under the reaction conditions (H2, CO, ligands, temperature >50°C) these salts are converted to a rhodium hydride complex, although there are several papers that seem to invoke cationic rhodium species as the catalysts. Chlorides have a particularly deleterious effect on the activity (i.e. they are not converted into hydrides under mild conditions) and it has been reported that the addition of bases such as amines has a strong promoting effect on such systems ... 

Scheme 6. Proposed mechanism of the iodide-promoted cobalt-catalyzed carbonylation of alcohols encompassing the effects discussed in Section IV,B.

Scheme 7. Suggested mechanism for the ruthenium-catalyzed carbonylations of alcohols.

The ruthenium-iodide-catalyzed carbonylation of alcohols is greatly complicated by the facility with which the same system catalyzes the competitive water-gas shift and homologation reactions. The resulting inability to totally isolate the reaction(s) of interest necessitates that conclusions are based on observations which are less direct than in other systems discussed in this article. Further work, aimed at determining the nature of the proposed transformations, perhaps through the use of model compounds, would appear to be required to unravel the finer mechanistic details of the system. 

The carbonylation of alcohols can proceed with formation of carboxylic acid by catalytic insertion of CO into the carbon-oxygen bond. An alternative reaction gives rise to oxalate or formate esters, when the CO is inserted into the oxygen-hydrogen bond. The members of the nickel triad carbonylate alcohols to give each of these products, and they will be discussed separately. 

The carbonylation of alcohols to give the next higher carboxylic acid is catalyzed by nickel. Catalytic systems are based on Ni(CO)4, NiX2, or metallic nickel in the presence of a halogen (86,87). In all of these cases, though, the active species can be derived from nickel(O) species, for example... 

Scheme 9. Suggested mechanism for the carbonylation of alcohols by nickel.

Copper(I) carbonyls in the presence of H2S04 (>85%) catalyze the carbonylation of alcohols under ambient conditions (128). In this case, yields of up to 80% have been reported. The necessity of such high acid concentrations suggest that the chemistry involved may be described as a modified Koch reaction ... 

The action of transition metals in the carbonylations of alcohols is treated by Dekleva and Forster. This is a field of important chemistry, from the point of view of coordination chemistry as well as catalysis. It is also another example of cross-stimulation between basic science and technological utility. 

This chapter focuses on the application of transition metal catalysts and enzymes for the formation of carbon-carbon bonds. Transition metal-catalyzed carbon-carbon bond formations are not always very green but they often replace even less favorable conventional approaches. The key to making them really green is that they have to be easily separable and reusable. Several of these reactions, such as the hydroformylation, oligomerisation, carbonylation of alcohols and the metathesis, are therefore also treated in Chapter 1, Section 1.8 and Chapter 7, since their greener variations are performed in novel reaction media. 

Depending on the catalyst system and the chosen reaction conditions, aliphatic and aromatic alcohols can in general act as substrates for oxidative carbonylations. In principle this reaction type can occur in the presence of metal ions which are able to oxidize CO in the presence of an alcohol function. As already mentioned above, it is also here necessary to carry out the reaction in the presence of a suitable reoxidant in order to establish a catalytic cycle process. Preferably that may be another metal salt, for example CuCU- Typical products and side products which are observed in the oxidative carbonylation of alcohols are alkyl and aryl carbonates, oxalates, formates, haloformates, acetals, and carbon dioxide. 

The preparation of dialkyl oxalates by oxidative carbonylation of alcohols was first described by Fenton et al. in the early 1970s [72-74]. For example, the reaction can be carried out at a temperature around 125 °C and a pressure of about 70 bar in the presence of PdCl2 and iron or copper salts. Water is formed as a by-product and has to be removed from the reaction mixture by the addition of water-binding agents such as trialkyl orthoformates. Instead of oxygen benzo-quinone can also be used for the reoxidation of the catalyst system. Ammonia or amines seem to have a positive influence on selectivity and efficiency of the reae-tion. For some more examples, cf. [77-80, 117]. Mechanistic studies give some indication that alkoxycarbonylpalladium species occur as intermediates [52, 75, 76] (eq. (12)). 

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