Other Metal-Catalyzed Carbonylations

Workers at Argonne Laboratory have recently discovered a catalytic method for the homologation of methanol to ethanol 122-124). The workers consider the mechanism to involve initial formation of acetaldehyde, followed by its hydrogenation. 

As in the cobalt system, the reaction generates significant quantities of CH4 as a by-product (in some cases, the methane is the major product). The selectivity of homologated alcohol to hydrocarbon appears to be independent of the partial pressures of either CO or H2 127), and the authors suggest that this could be attributable, at least in the Mn system, to the relative rates of methyl migration to homolytic bond dissociation. In the iron system, methane is generated by simple reductive elimination from the HFe(CH3)(CO)4 intermediate. 

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  

Consistent with this explanation are the observations that tertiary alcohols react much faster than primary and secondary alcohols and that even when linear alcohols are carbonylated the predominant products are the branched tertiary acids. Both of these are consistent with the formation of carbonium ions, which then rearrange to form the favored tertiary structure, for example, 

The copper salts (typically Cu20) are transformed into the cationic carbonyl derivatives under the strongly acidic conditions  

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Ruthenium is not an effective catalyst in many catalytic reactions however, it is becoming one of the most novel and promising metals with respect to organic synthesis. The recent discovery of C-H bond activation reactions [38] and alkene metathesis reactions [54] catalyzed by ruthenium complexes has had a significant impact on organic chemistry as well as other chemically related fields, such as natural product synthesis, polymer science, and material sciences. Similarly, carbonylation reactions catalyzed by ruthenium complexes have also been extensively developed. Compared with other transition-metal-catalyzed carbonylation reactions, ruthenium complexes are known to catalyze a few carbonylation reactions, such as hydroformylation or the reductive carbonylation of nitro compounds. In the last 10 years, a number of new carbonylation reactions have been discovered, as described in this chapter. We ex-... 

Few examples of stereoselective transition metal catalyzed carbonylative multicomponent cycloadditions leading to other than five-membered rings have been reported1 72. Typical is the reaction of 3,3-dimethylcyclopropene with carbon monoxide, catalyzed by a palladium(O) complex, to stereoselectively form hexamcthyltrishomotropone as a tetracyclic adduct l73. 

It is generally agreed that Roelen s discovery of the hydroformylation reactiont was the birth of the transition metal-catalyzed carbonylation. Initially, Co catalysts were most extensively used, but the Rh-based processes have since been developed as a superior methods. Although Pd may have been tested along with several other metals, such as Fe, Ru, and Ni, it has never been shown to be very useful in the hydroformylation reaction, sometimes called the oxo process. A publication in 1963 by Tsuji et al. on a related but clearly different reaction of alkenes with CO and alcohols in the presence of a Pd catalyst producing esters was one of the earliesL if not the earliesL reports describing a successful and potentially useful Pd-catalyzed carbonylation reaction. This was soon followed by the discovery of another Pd-catalyzed carbonylation reaction of allylic electrophiles with CO and alcohols ° (Scheme 7). 

What British Petroleum discovered was that Ru(CO)4l2 (as well as some other metals) catalyzed the exchange of iodide for CO in the transformation of MeIr(CO)2l3 to MeIr(CO)3l2 (equation [29]) allowing the Ir catalyst to operate without being slowed by the pesky, and previously rate limiting, carbon monoxide for iodide ligand exchange. Complete mechanistic details are available for the process. In the presence of Ru, Ir operated under the same conditions as the Rh based methanol carbonylation, was equally active as a catalyst, and had acceptable ease of operation. Advantages associated with the Ir process are that the Ir process used only half as much methyl iodide co-catalyst and the Ir and Ru catalyst components were (and continue to be) much less expensive than Rh. 

In this chapter, the transition of metal catalyzed carbonylative activation of C-H bonds has been discussed. This area is dominated by Pd, Ru and Rh catalysts, whereas the ability of other metals, such as Cu and Fe, have still not been explored. From the reaction mechanism point of view, the first step is the palladation of arene to produce an Ar-Pd bond, and then be followed by CO insertion. 

Although some other examples of the transition-metal-catalyzed carbonylation reaction of carbon-carbon unsaturated compounds with thiols and CO have been reported, it is unclear whether these reactions involve the iyn-thiometallation process. 

In this chapter, the transition metal-catalyzed carbonylative C-H functionalization toward heterocycle synthesis has been summarized. Four metal catalysts such as cobalt, ruthenium, rhodium, and palladium were explored. Among them, palladium obtained more attention than the other three metals. The main challenge for the Pd-catalyzed carbonylation is the excellent reducing ability of CO introducing... 

The cycloaddition reactions of carbonyl compounds with conjugated dienes cannot be discussed in this context without trying to understand the reaction mechanistically. This chapter will give the basic background to the reactions whereas Chapter 8 dealing with theoretical aspects of metal-catalyzed cycloaddition reactions will give a more detailed description of this class of reactions, and others discussed in this book. 

For the reactions of other 1,3-dipoles, the catalyst-induced control of the enantio-selectivity is achieved by other principles. Both for the metal-catalyzed reactions of azomethine ylides, carbonyl ylides and nitrile oxides the catalyst is crucial for the in situ formation of the 1,3-dipole from a precursor. After formation the 1,3-di-pole is coordinated to the catalyst because of a favored chelation and/or stabiliza-... 

In addition to the applications reported in detail above, a number of other transition metal-catalyzed reactions in ionic liquids have been carried out with some success in recent years, illustrating the broad versatility of the methodology. Butadiene telomerization [34], olefin metathesis [110], carbonylation [111], allylic alkylation [112] and substitution [113], and Trost-Tsuji-coupling [114] are other examples of high value for synthetic chemists. 

Transition metals such as iron can catalyze oxidation reactions in aqueous solution, which are known to cause modification of amino acid side chains and damage to polypeptide backbones (see Chapter 1, Section 1.1 Halliwell and Gutteridge, 1984 Kim et al., 1985 Tabor and Richardson, 1987). These reactions can oxidize thiols, create aldehydes and other carbonyls on certain amino acids, and even cleave peptide bonds. The purposeful use of metal-catalyzed oxidation in the study of protein interactions has been done to map interaction surfaces or identify which regions of biomolecules are in contact during specific affinity binding events. 

This chapter mainly treats transition metal-catalyzed direct functionalization of carbon-hydrogen bonds in organic compounds. This methodology is emphasized by focusing on important functionalizations for synthetic use. The contents reviewed here are as follows (i) alkylation of C-H bonds, (ii) alkenylation of C-H bonds, (iii) arylation of C-H bonds, (iv) carbonylation of C-H bonds, (v) hydroxylation and the related reactions, and (vi) other reactions and applications. 

These reactions are covered in other chapters of Volume 11 (Chapters 11.06 and 11.07). This part deals only with examples which are in connection with other sections of this chapter. Additions of metallocarbenoids to unsaturated partners have been extensively studied. Most of the initial studies have involved the transition metal-catalyzed decomposition of cr-carbonyl diazo compounds.163,164 Three main reaction modes of metallocarbenoids derived from a-carbonyl diazo precursor are (i) addition to an unsaturated C-C bond (olefin or alkyne), (ii) C-H insertion, and (iii) formation of an ylid (carbonyl or onium).1 5 These reactions have been applied to the total synthesis of natural... 

Complexes of other metals are also capable of catalyzing useful carbonylation reactions under phase transfer conditions. For example, certain palladium(o) catalysts, like Co2(C0)g, can catalyze the carbonylation of benzylic halides to carboxylic acids. When applied to vinylic dibromides, unsaturated diacids or diynes were obtained, using Pd(diphos)2[diphos l,2-bis(diphenylphosphino)ethane] as the metal catalyst, benzyltriethylammonium chloride as the phase transfer agent, and t-amyl alcohol or benzene as the organic phase(18),... 

The activity of the nickel catalyst is affected by major variations in carbon monoxide partial pressure. With very low carbon monoxide partial pressure, nickel precipitates as a metal powder and occasionally as nickel iodide. Stability of the catalyst is improved with higher CO partial pressure up to a point above which the catalyst activity drops linearly. The optimum level of carbon monoxide is different from one catalyst mixture to another. This behavior is characteristic of all the nickel catalyzed carbonylation reactions we studied. In the Li-P system, optimum carbon monoxide partial pressure is in the range of 700 to 800 psi (Table V). On the other hand, the optimum carbon monoxide partial pressure for the Li-Sn system is in the range of 220 to 250 psi, at 160 C, and 450 psi at 180 C (Table VI). It is presumed that the retarding effect of higher carbon monoxide partial pressure is associated with stabilizing an inactive carbonyl species. 

Other metal carbonyls also catalyze the formylation of amines (90). 

Many such activated acyl derivatives have been developed, and the field has been reviewed [7-9]. The most commonly used irreversible acyl donors are various types of vinyl esters. During the acylation of the enzyme, vinyl alcohols are liberated, which rapidly tautomerize to non-nucleophilic carbonyl compounds (Scheme 4.5). The acyl-enzyme then reacts with the racemic nucleophile (e.g., an alcohol or amine). Many vinyl esters and isopropenyl acetate are commercially available, and others can be made from vinyl and isopropenyl acetate by Lewis acid- or palladium-catalyzed reactions with acids [10-12] or from transition metal-catalyzed additions to acetylenes [13-15]. If ethoxyacetylene is used in such reactions, R1 in the resulting acyl donor will be OEt (Scheme 4.5), and hence the end product from the acyl donor leaving group will be the innocuous ethyl acetate [16]. Other frequently used acylation agents that act as more or less irreversible acyl donors are the easily prepared 2,2,2-trifluoro- and 2,2,2-trichloro-ethyl esters [17-23]. Less frequently used are oxime esters and cyanomethyl ester [7]. S-ethyl thioesters such as the thiooctanoate has also been used, and here the ethanethiol formed is allowed to evaporate to displace the equilibrium [24, 25]. Some anhydrides can also serve as irreversible acyl donors. 

Manufacture. Trichloromethanesulfenyl chloride is made commercially by chlorination of carbon disulfide with the careful exclusion of iron or other metals, which catalyze 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 difunctional carbonyl compounds, phosphonates, phosphonites, phosphites, phosphates, or lead acetate (61). 

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