Metal carbonyl compound catalysts

Some catalysts are ha2ardous materials, or they react to form ha2ardous substances. For example, catalysts used for hydrogenation of carbon monoxide form volatile metal carbonyl compounds such as nickel carbonyl, which are highly toxic. Many catalysts contain heavy metals and other ha2ardous components, and environmentally safe disposal has become an increasing concern and expense. 

Mo and W hexacarbonyls, Mo(CO)6 and W(CO)6, alone do not induce polymerization of acetylenic compounds. However, UV irradiation toward these catalysts in the presence of halogenated compounds can form active species for polymerization of various substituted acetylenes. Carbon tetrachloride, CCI4, when used as the solvent for the polymerization, plays a very important role for the formation of active species, and thus cannot be replaced by toluene that is often used for metal chloride-based catalysts. Although these metal carbonyl-type catalysts are less active compared to the metal halide-based counterparts, they can provide high MW polymers. It is a great advantage that the metal carbonyl catalysts are very stable under air and thus handling is much easier. 

The classic Hieber-base reaction 16 is that of a hydroxide with metal carbonyls, which proceeds by nucleophilic attack of the hydroxide at a carbon atom of a carbonyl ligand to give a carboxy group or consequently carbon dioxide and a metal hydride.17 Metal carbonyls are catalysts for the water-gas shift reaction.18 Pentacarbonyl(tetrafluoroborato)rhenium reacts with alkali hydroxide in a similar way however, due to the coordinatively unsaturated nature of the [Re(CO)5]+ group polynuclear compounds are formed.15... 

The use of highly dispersed catalysts from soluble salts of molybdenum is another approach to the reduction of catalyst amount because of their excellent activity despite their higher price. Recently, metal carbonyl compounds, such as Fe(CO)5, Ru3(CO)i2, and Mo(CO)6 have been investigated as metal cluster catalysts. Preparation involved their deposition and decomposition on catalyst support surfaces (71-73). 

Our earlier work with Rh6(CO)i6 was the first example of using a transition metal carbonyl compound for catalyzed oxidations. We now find that Rh6(CO)16 is not unique in functioning as a homogeneous catalyst for oxidizing ketones. The dimer Re2(CO)10 is equally effective. The reaction mixture is homogeneous throughout the catalytic reaction. The carbonyl is not decomposed and can be recovered quantitatively at the end of the oxidation. This latter situation even prevails when the oxidations are carried out in the absence of carbon monoxide. Species characterization is done by isolation methods or by IR spectroscopy in the carbonyl region. 

A final set of catalysts that has largely historical importance consists of monohydride complexes that react by radical pathways. Insertion of the olefin into the hydride occurs through radical intermediates. Many of these catalysts have been shown to reduce dienes, acrylic acid derivatives, and fumarates. As discussed in Chapter 9, migratory insertion requires an open coordination site cis to the hydride for initial binding of the olefin. Thus, catalysts based on metal carbonyl compounds that dissociate CO slowly, or porphyrin ligands that occupy the sites cis to a hydride, react dirough radical pathways or direct transfer of a true "hydride." The basic mechanism for such direct formal insertions by radical processes is shown in Scheme 15.14. 

Aluminum oxide itself catalyzes the metathesis reaction of propylene, although its activity is low. Catalysts that are obtained from oxides are active at temperatures ca 100 K higher than catalysts prepared from metal carbonyls. Compounds deposited on silica are catalytically active at temperatures which are higher by ca 200 K than compounds supported on aluminum oxide. However, because of the Si02 support these catalysts are more resistant to poisoning by polar compounds. Heterogeneous catalysts must be activated at 390-870 K before use. Catalysts obtained from metal carbonyls and alkyl compounds require lower temperatures of activation. Supported allyl complexes of Mo, W, and Re need no activation. 

Table I. Bimetallic catalysts obtained by interaction of metal carbonyl compounds with the surfaces of supported metals...

Although stoichiometric ethynylation of carbonyl compounds with metal acetyUdes was known as early as 1899 (9), Reppe s contribution was the development of catalytic ethynylation. Heavy metal acetyUdes, particularly cuprous acetyUde, were found to cataly2e the addition of acetylene to aldehydes. Although ethynylation of many aldehydes has been described (10), only formaldehyde has been catalyticaHy ethynylated on a commercial scale. Copper acetjlide is not effective as catalyst for ethynylation of ketones. For these, and for higher aldehydes, alkaline promoters have been used. 

Manufacture. Trichloromethanesulfenyl chloride is made commercially by chlorination of carbon disulfide with the careful exclusion of iron or other metals, which cataly2e the chlorinolysis of the C—S bond to produce carbon tetrachloride. Various catalysts, notably iodine and activated carbon, are effective. The product is purified by fractional distillation to a minimum purity of 95%. Continuous processes have been described wherein carbon disulfide chlorination takes place on a granular charcoal column (59,60). A series of patents describes means for yield improvement by chlorination in the presence of dihinctional carbonyl compounds, phosphonates, phosphonites, phosphites, phosphates, or lead acetate (61). 

Tertiary stibines have been widely employed as ligands in a variety of transition metal complexes (99), and they appear to have numerous uses in synthetic organic chemistry (66), eg, for the olefination of carbonyl compounds (100). They have also been used for the formation of semiconductors by the metal—organic chemical vapor deposition process (101), as catalysts or cocatalysts for a number of polymerization reactions (102), as ingredients of light-sensitive substances (103), and for many other industrial purposes. 

The hydroformylation reaction is carried out in the Hquid phase using a metal carbonyl catalyst such as HCo(CO)4 (36), HCo(CO)2[P( -C4H2)] (37), or HRh(CO)2[P(CgH3)2]2 (38,39). The phosphine-substituted rhodium compound is the catalyst of choice for new commercial plants that can operate at 353—383 K and 0.7—2 MPa (7—20 atm) (39). The differences among the catalysts are found in their intrinsic activity, their selectivity to straight-chain product, their abiHty to isomerize the olefin feedstock and hydrogenate the product aldehyde to alcohol, and the ease with which they are separated from the reaction medium (36). 

Polymerization of olefins such as styrene is promoted by acid or base or sodium catalysts, and polyethylene is made with homogeneous peroxides. Condensation polymerization is catalyzed by acid-type catalysts such as metal oxides and sulfonic acids. Addition polymerization is used mainly for olefins, diolefins, and some carbonyl compounds. For these processes, initiators are coordination compounds such as Ziegler-type catalysts, of which halides of transition metals Ti, V, Mo, and W are important examples. 

Alkenes are reduced by addition of H2 in the presence of a catalyst such as platinum or palladium to yield alkanes, a process called catalytic hydrogenation. Alkenes are also oxidized by reaction with a peroxyacid to give epoxides, which can be converted into lTans-l,2-diols by acid-catalyzed epoxide hydrolysis. The corresponding cis-l,2-diols can be made directly from alkenes by hydroxylation with 0s04. Alkenes can also be cleaved to produce carbonyl compounds by reaction with ozone, followed by reduction with zinc metal. 

Almost all the materials which are being considered as components in automobile exhaust catalyst are somewhat toxic (74)- Most of the compounds considered are low vapor pressure solids which can only escape from the exhaust system as very fine airbone dust particles formed by catalyst attrition. A few compounds, such as the highly toxic metal carbonyls and ruthenium tetroxides, are liquid under ambient conditions and have boiling points less than 100 °C. These compounds are not present in... 

Metal-induced reductive dimerization of carbonyl compounds is a useful synthetic method for the formation of vicinally functionalized carbon-carbon bonds. For stoichiometric reductive dimerizations, low-valent metals such as aluminum amalgam, titanium, vanadium, zinc, and samarium have been employed. Alternatively, ternary systems consisting of catalytic amounts of a metal salt or metal complex, a chlorosilane, and a stoichiometric co-reductant provide a catalytic method for the formation of pinacols based on reversible redox couples.2 The homocoupling of aldehydes is effected by vanadium or titanium catalysts in the presence of Me3SiCl and Zn or A1 to give the 1,2-diol derivatives high selectivity for the /-isomer is observed in the case of secondary aliphatic or aromatic aldehydes. 

Metal carbonyls form a large and important group of compounds which are used widely in the chemical industry, particularly in the preparation of heterogeneous catalysts and as precursors in CVD and metallo-organic CVD (MOCVD). 

Although considerable progress has been made in metal-catalyzed preparations of non-racemic cyanohydrins, the HNL-catalyzed reaction is still the most important method for the synthesis of chiral cyanohydrins, especially for large-scale reactions. The usefulness of HNLs as catalysts for the stereoselective addition of HCN to carbonyl compounds has increased substantially because (7 )-PaHNL... 

This method ensures the deposition of very reactive metal nanoparticles that require no activation steps before use. We shall review here the following examples of catalytic reactions that are of interest in line chemical synthesis (a) the hydrogenation of substituted arenes, (b) the selective hydrogenation of a, 3-unsaturated carbonyl compounds, (c) the arylation of alkenes with aryl halides (Heck reaction). The efficiency and selectivity of commercial catalysts and of differently prepared nanosized metal systems will be compared. 

Allylations, allenylations, and propargylations of carbonyl compounds in aqueous media can also be carried out with preformed organic tin reagent, rather than the use of metals.86,87,88 For example, the allylation reaction of a wide variety of carbonyl compounds with tetraal-lyltin was successfully carried out in aqueous media by using scandium trifluoromethanesulfonate (scandium triflate) as a catalyst (Eq. 8.40).89 A phase-transfer catalyst (PTC) was found to help the allylation mediated by tin at room temperature without any other assistance.90... 

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