Metal decarbonylation

Manufacture. Furan is produced commercially by decarbonylation of furfural in the presence of a noble metal catalyst (97—100). Nickel or cobalt catalysts have also been reported (101—103) as weU as noncatalytic pyrolysis at high temperature. Furan can also be prepared by decarboxylation of 2-furoic acid this method is usually considered a laboratory procedure. 

Pure cis-71 could be isolated in 65% yield and was characterized by means of x-ray structure analysis (Fe-Si 2.270(1)/2.272(1) A). This ds-conformer is exclusively formed. The formation of 71 requires a photoinduced silyl exchange followed by a decarbonylation reaction and further steps of an oxidative addition to the metal [176]. 

The insertion of a carbonyl group into a metal-alkyl or metal-aryl bond, and the reverse reaction involving decarbonylation of an acyl complex, have been studied from both the synthetic and mechanistic points of view. The mechanism proposed for this type of reaction seems well established and is... 

One of the properties of transition metal acyl complexes is their ability to lose CO, usually on heating or photolysis. This so-called decarbonylation often represents a special case of the reverse of the CO insertion in Eq. (8), where L = CO. 

The decarbonylation can be effected (1) thermally, (2) photolytically, and (3) chemically, by using a metal complex such as Rh(PPh3)3Cl. The three types of processes will be considered in turn. 

This is the most extensively, as well as intensively, studied type of the decarbonylation. Nevertheless, kinetic data are scarce. Conversion of metal acyls to their respective alkyls has been investigated for two systems RCOMn(CO), 37, 50, 51) and XCH2COIr(PPh3)2Cl2 (X = Ph or a substituted Ph group) 160). 

Solvent effects on the rate of the decarbonylation of MeCOMn(CO)5 were examined by Calderazzo and Cotton (50) and are presented in part in Table IV. In general they are very small, and no regular trends can be discerned. This virtual lack of dependence of the rate on the nature of the solvent and very little correlation between the rate and the dielectric constant of the solvent are typical of substitution reactions of metal carbonyls (J). In the light of the foregoing, a qualitative observation that CpFe(CO)2-COMe decarbonylates much more readily on treatment at reflux in nonpolar heptane or cyclohexane than in polar dioxane is somewhat intriguing 219). 

The ultraviolet light-induced decarbonylation has been successfully effected in several types of transition metal acyl. It is particularly important when applied to those systems which do not eliminate CO under thermal conditions, e.g., CpFe(CO)2COR (141). Most of the work has been of a synthetic rather than mechanistic variety, however. 

In some instances the decarbonylation can be effected, under mild conditions, by using a stoichiometric amount of a CO-abstracting metal complex. This method has proved to be fruitful for several CpFe(CO)2COR compounds (7, 2, 2a) and for CpMo(CO)2(PPh3)COPh 189) in conjunction with Rh(PPh3)jCl. 

Although Rh(PPh3)3Cl has been the only decarbonylating reagent employed to date, other platinum metal complexes may prove to be equally, if not more effective. Still to be tested are Ru(II), Pt(0), Pd(0), and other Rh(I) compounds, to mention a few. 

A complete description of stereochemistry of the carbon monoxide insertion and decarbonylation requires knowledge of configurational changes at the metal and a-carbon. Calderazzo and Noack (54) showed that the optical activity of the equilibrium mixture... 

There has been only one study concerned with stereochemical changes at an asymmetric metal. It dealt with the photochemical decarbonylation 208) ... 

Organomercurial carbonylation. Use of Co2(CO)g as a stoichiometric and as a catalytic reagent Organic synthesis reactions using palladium compounds Decarbonylation reactions using transition metal compounds... 

Synthesis of Metal Carbonyl Clusters and Decarbonylated Clusters... 

Although decarbonylation of supported metal carbonyl clusters sometimes occurs almost without changes in the metal frames, the chemistry is complex and only partially understood. When decarbonylation takes place at elevated temperatures (depending on the support), migration and aggregation of the metal inevitably occur, and these processes are less well understood than the decarbonylation with near retention of the metal frame. 

The decarbonylation of oxide-supported metal carbonyls yields gaseous products including not just CO, but also CO2, H2, and hydrocarbons [20]. The chemistry evidently involves the support surface and breaking of C - O bonds and has been thought to possibly leave C on the clusters [21]. The chemistry has been compared with that occurring in Fischer-Tropsch catalysis on metal surfaces [20] support hydroxyl groups are probably involved in the chemistry. 

Good images indicating nearly uniform clusters of other metals are lacking, but evidence from EXAFS spectroscopy, combined with IR spectroscopy and extraction of clusters into solution, has provided a basis for structure determination of a number of small metal carbonyl clusters and clusters formed by their decarbonylation. Compilations of these are reported elsewhere [6,12,26]. 

These comparisons prompted the Rosch group [32,33] to conclude that some Ugands remained on the supported clusters after decarbonylation this conclusion may be quite general—supported metal clusters are highly reactive, and typical oxide and zeoUte supports are not unreactive. Thus, a representation of supported clusters such as tetrairidium on 7-AI2O3 as 4/7-AI2O3 is a simplification that fails to account for the ligands on the cluster. 

Synthesis methods such as those described earlier for monometallics have been applied with metal carbonyls incorporating two metals. The resultant supported species may be small supported metal clusters [41,42], and, as for monometallics, the usual products are supported species that are nonuniform in both composition and structure [42]. There are several examples of well-defined metal carbonyl clusters in this category but hardly any examples of well-defined decarbonylated bimetalhcs on supports. 

Decarbonylation of the acyl is likely to be metal-assisted (Ru11) giving rise to a Ru carbonyl, which is subsequently decarbony-lated by nucleophilic attack by nBu3P. This phosphine can displace coordinated carbonyl, as exemplified by reaction 3 ... 

However, formation of an R species, either free or within a radical-pair cage with the metal (14), is strongly favored in view of the methylcyclopentane noted in the cyclohexen-4-al decarbonylation, since the rearrangement shown in eq. 5, metal-assisted if necessary, seems plausible (15) ... 

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