Metal Carbonyls as Catalysts

Metal carbonyls have a long history as catalysts and, although this section is little more than a catalogue of recent advances in the use of carbonyls in this role, the papers reported give a good picture of the current trends in this area. 

The chemistry of [Ir C0)2(acac) ] in Na.X zeolites under a CO atmosphere has been thoroughly examined . At 70 C there are significant amounts of [HIr4(CO)2 2. this is largely converted to [ Irg (CO ] at 175 C. Zeolites (NaY on this occasion) are also used to explore the growth of [Pt3 (C0)3 (p2 cO)3 species .  

Selective alkene hydroformylation is the central driving force behind a studies by Shido and co-workers . and Izumi and Iwasawa using rhodium carbonyl clusters 

The carbonylation of (dichloromethyl)benzene using both cobalt and iron carbonyls has been reported and a similar reaction on l-bromo-2,5-di(bromomethyl)benzene is also noted . Hydrodesulphurisation of dibenzothiophene is noted in two papers, using different catalysts . 

Other organic processes facilitated by metal carbonyl clusters include a palladium carbonyl catalysed Diels-Alder reaction the selective reduction of aromatic nitro compounds using rhodium and ruthenium phosphine-carbonyls aza- and oxa-carbonylations of allyl phosphates by rhodium carbonyls Michael reactions of alkoxy-alkenones using iron 

Cl and Cudo have reported the carboii catafysed akrdio rm of HSiMe2Ph. Usmg [Mn(CO)y r]2 they catalysed, for exaitqde, the reaction of HSiMc2Ph witii 2-metiiy m pan-2-ol to give (CH3)3CO-SiMe2Ph in 82% yield after 35 minutes. 

The catalytic bydrosOation of CO using cobalt carbonyls has been reported and the reaction of CO and various alkenes has been covered where a Mu2 unit is used as the cata c centre (especial the compound Mn2(C0)g(/i-( t02C)C=C(C02 t)C0). Fmalfy, the carbonylation of benzene has been catal rsed using Rh(PMe3)2(CO)Cl. 

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The logical basis for employing metal carbonyls as catalysts would be the CO activation through coordination which facilitates nucleophilic attack by water or OH" (6). The key step then may be the formation of a hydroxy-carbonyl species followed by 6-hydrogen elimination reaction (eq. 2,3). Another important elemental re-... 

The water-gas shift reaction has also been studied under high pH with metal carbonyls as catalysts. The catalytic cycle with Fe(CO)5 as the precatalyst is shown in Fig. 4.6. This reaction with low turnover is carried out at 130-180°C, under 10-40 bar of CO, with alkali metal hydroxide as a promoter. 

The synthesis of carboxylic acids by carbonylation of unsaturated hydrocarbons or alcohols was developed mainly by Reppe and his co-workers in the laboratories of BASF at Ludwigshafen. Many industrially important processes such as the synthesis of acrylic acid, propionic acid, and acetic acid were elaborated there in the period from the late 1930s to the mid-1950s [1, 2]. Reppe s introduction of metal carbonyls as catalysts for carbonylation reactions was of paramount importance and many processes, which are still industrially relevant today, were developed rapidly (eq. (1), [3]). 

Molecular oxygen has become a commonly used co-catalyst for inactive or weakly active transition metal complexes [1-5]. In addition, other oxidizing agents, mainly peroxides, have recently been used in active rhodium complexes in particular, but also in metal carbonyls, as catalysts for hydrosilylation. The catalytic activity of bis(triphenylphosphine)carbonylrhodium(I) in the hydrosilylation of C=C and C=0 bonds can be much increased by the addition of about a 50 % molar excess of tert-butyl hydroperoxide [100]. Chromium triad carbonyls M(CO)e, where M = Cr, Mo, W, have been tested to examine the effect of various organic peroxides on the hydrosilylation of 2,3-dimethyl-1,3-butadiene by triethyl-, triethoxy- and methyldiethoxysilanes [100]. The evidence for organic oxidant promotion of RhCl(cod)phosphine-catalyzed hydrosilylation of 1-hexene was demonstrated previously [101]. 

For carbonylation reactions involving metal carbonyls as catalyst precursors, the best fit to requirements 1 and 2 apparently is given by 4d transition elements. It is known that when activation of CO is required, carbonyl complexes of 4d metals are better catalyst precursors than their 3d and 5d congeners. Pertinent to this point are the following experimental observations ... 

TABLE 7.11 Systems Derived from the Mononuclear Metal Carbonyls as Catalysts for the Water Gas Shift Reaction ... 

For the activation of a substrate such as 19a via coordination of the two carbonyl oxygen atoms to the metal, one should expect that a hard Lewis acid would be more suitable, since the carbonyl oxygens are hard Lewis bases. Nevertheless, Fu-rukawa et al. succeeded in applying the relative soft metal palladium as catalyst for the 1,3-dipolar cycloaddition reaction between 1 and 19a (Scheme 6.36) [79, 80]. They applied the dicationic Pd-BINAP 54 as the catalyst, and whereas this type of catalytic reactions is often carried out at rt or at 0°C, the reactions catalyzed by 54 required heating at 40 °C in order to proceed. In most cases mixtures of endo-21 and exo-21 were obtained, however, high enantioselectivity of up to 93% were obtained for reactions of some derivatives of 1. 

On the other hand, hi- or multi-metallic supported systems have been attracting considerable interest in research into heterogeneous catalysis as a possible way to modulate the catalytic properties of the individual monometalUc counterparts [12, 13]. These catalysts usually show new catalytic properties that are ascribed to geometric and/or electronic effects between the metalUc components. Of special interest is the preparation of supported bimetallic catalysts using metal carbonyls as precursors, since the milder conditions used, when compared with conventional methods, can render catalysts with homogeneous bimetallic entities of a size and composition not usually achieved when conventional salts are employed as precursors. The use of these catalysts as models can lead to elucidation of the relationships between the structure and catalytic behavior of bimetalUc catalysts. 

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. 

Hydrocarboxylation can also be accomplished under mild conditions (160°C and 50 atm) by the use of nickel carbonyl as catalyst. This is more often applied to triple bonds to give a,P-unsaturated acids, in which cases the conditions are milder still. Acid catalysts are used along with the nickel carbonyl, but basic catalysts can also be employed.567 Other metallic... 

It was thought that the formation of inactive cobalt clusters such as Co4(CO)i2, formed by dimerisation of the remaining cobalt carbonyl species after release of the cyclopentenone product, were responsible for the shutdown of the catalytic cycle when dicobalt octacarbonyl was employed.51 Krafft and co-workers were able to show that Co4(CO)12 can actually be exploited as a catalytic species in the PK reaction and were able to obtain excellent yields if cyclohexylamine was introduced as an additive alongside the metal cluster.57,58 The use of metal clusters as catalysts for the reaction has been extended to involve mixed metal clusters.59... 

The acid-catalyzed hydrocarboxylation of alkenes (the Koch reaction) can be performed in a number of ways. In one method, the alkene is treated with carbon monoxide and water at 100-350°C and 500-1000-atm pressure with a mineral acid catalyst. However, the reaction can also be performed under milder conditions. If the alkene is first treated with CO and catalyst and then water added, the reaction can be accomplished at 0-50°C and 1-100 atm. If formic acid is used as the source of both the CO and the water, the reaction can be carried out at room temperature and atmospheric pressure.The formic acid procedure is called the Koch-Haaf reaction (the Koch-Haaf reaction can also be applied to alcohols, see 10-77). Nearly all alkenes can be hydrocarboxylated by one or more of these procedures. However, conjugated dienes are polymerized instead. Hydrocarboxylation can also be accomplished under mild conditions (160°C and 50 atm) by the use of nickel carbonyl as catalyst. Acid catalysts are used along with the nickel carbonyl, but basic catalysts can also be employed. Other metallic salts and complexes can be used, sometimes with variations in the reaction procedure, including palladium, platinum, and rhodium catalysts. The Ni(CO)4-catalyzed oxidative carbonylation with CO and water as a nucleophile is often called Reppe carbonylationP The toxic nature of nickel... 

Oxidative carbonylation of aliphatic and aromatic amines in the presence of supported platinum metals or platinum metal salts as catalysts and iodide ions gives carbamates [118, 119]. Iodide is presumed to promote the partial redox reactions... 

Nickel metal catalysts give mixtures of the corresponding silyl ether and silyl enol ether. The former is produced via hydrosilylation, while the latter is produced via de-hydrogenative silylation. The reaction catalyzed by zinc chloride proceeds under drastic conditions, and the product of aldehydes disproportionates. The reaction of a,jS-unsatu-rated carbonyl compounds with H2PtCl6 proceeds by 1,4-addition, while coupling is also observed on using metallic Ni as catalyst. ... 

In general, carbonyl forming metals such as nickel, iron or cobalt, must not be present in the formation of metallic carbonyls as their presence in one part of the apparatus may lead to the subsequent deposition uf the metal on the catalyst by the decomposition of the carbonyl in a hotter portion of the apparatus, with destruction of the catalyst activity. The catalysts may be activated by the addition of metal halides such as potassium fluoride or iodide, sodium chloride, or aluminum chloride.102... 

This use of a dimeric metal carbonyl as a catalyst for a reaction not involving carbon monoxide is not too surprising. The carbonyl supplies the simplest surface, two metal atoms, the olefin forms a bridge across these atoms, and the nitrile is formed by transfer of reactants within the complex. 

This reaction has been modified using a group VIB metal complex as catalyst in combination with trace amounts of cobalt or rhodium carbonyl complex." ... 

The addition of HCN to olefins catalyzed by complexes of transition metals has been studied since about 1950. The first hydrocyanation by a homogeneous catalyst was reported by Arthur with cobalt carbonyl as catalyst. These reactions gave the branched nitrile as the predominant product. Nickel complexes of phosphites are more active catalysts for hydrocyanation, and these catalysts give the anti-Markovnikov product with terminal alkenes. The first nickel-catalyzed hydrocyanations were disclosed by Drinkard and by Brown and Rick. The development of this nickel-catalyzed chemistry into the commercially important addition to butadiene (Equation 16.3) was conducted at DuPont. Taylor and Swift referred to hydrocyanation of butadiene, and Drinkard exploited this chemistry for the synthesis of adiponitrile. The mechanism of ftiis process was pursued in depth by Tolman. As a result of this work, butadiene hydrocyanation was commercialized in 1971. The development of hydrocyanation is one of tfie early success stories in homogeneous catalysis. Significant improvements in catalysts have been made since that time, and many reviews have now been written on this subject. ... 

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