Decarbonylation iridium catalysts

Decarbonylation iridium catalysts, 1160 Dcspujolsite, 104 Dienes reactions... 

Table 8. Catalytic Decarbonylation of Benzaldehyde Using Iridium Catalysts ...

Tsuji s group utilized an iridium catalyst for the decarbonylation of saturated aldehydes in boiling dioxane (Scheme 8.12) [12]. The same catalytic system was also highly efficient for the decarbonylation of naturally occurring enals such as citral. 

At temperatures above ca. 200°C, the decarbonylation reaction can be driven catalytically (1,4,14, 20). Scheme I illustrates the proposed catalytic reaction scheme (15,16). This catalytic reaction is slow (activity for benzaldehyde decarbonylation at 178°C is 10 turnovers hr-1) presumably because the oxidative addition of RCOX to RhCl(CO)(PPh3)2 is difficult (7, 21, 22). Consistent with this, the rate is significantly greater when IrCl(CO)(PPh3)2 is used as the catalyst (benzaldehyde, 178°C, activity is 66 turnovers hr-1) (23). Oxidative addition to iridium complexes is well known to be more facile than with their rhodium analogues. 

Catalytic activities of the zeolite-supported clusters (Table 4) are reported as turnover frequencies these are rates per total iridium atom for such small clusters. Rates were also reported for conventional (structurally nonuniform) supported catalysts consisting of aggregates of metallic iridium on supports, these rates, per unit of metal surface area, are markedly greater than those observed for the supported clusters [15]. Changing the support from zeolite NaY to MgO had little effect on the activities of the decarbonylated clusters. 

Catalytic decarbonylation of benzaldehyde using several iridium complexes has also been examined. Results of these experiments are shown in Table 8. The main points to be made here are (i) [Ir(P-P)2] catalysts have activities that are ca. twenty times lower than their Rh analogs (ii) the iridium mono-diphosphine catalysts are better than the 6w-diphosphine Ir catalysts (opposite trend noted using rhodium, see Table 5) and (iii) IrCl(CO)(PPh3)2 is a much better catalyst than RhCl(CO)(PPh3)2 and is also better than most of the iridium diphosphine catalysts. The results for the [M(P-P)2] catalysts may be explained in terms of the proposed mechanistic scheme in Figure 11.3. Since Ir-P bonds should be stronger than Rh-P bonds, the value of ki will be smaller for the Ir catalysts, thus... 

Hydroformylation with formaldehyde derives benefit from the general property of some transition catalysts based on rhodium, iridium, ruthenium, or cobalt to decarbonylate aromatic or aliphatic aldehydes (see also Chapter 8) [11]. In the reaction with formaldehyde, decomposition leads to CO or H2 (Scheme 3.2). 

Chiral aldehydes can be decarbonylated under full retention of the configuration, but occasionally partial racemization may take place [10]. In general, decarbonylation with stoichiometric rhodium-arylphosphine complexes can be achieved at ambient temperature, but usually the catalytic version requires more severe conditions. Goldman et al. [11] discovered that trialkylphosphines form significantly more active catalysts, as exemplified with the binuclear complex [Rh(PMe3)(CO)Cl]2. The complex operates even at room temperature. A similar effect was also noted with iridium-phosphine catalysts [12]. 

Also alcohols can be submitted as substrates to the decarbonylation with rhodium catalysts, provided an Oppenhauer oxidation precedes the reaction (Scheme 8.7) [9]. The tandem reaction with an iridium-based Oppenhauer catalyst was exemplified with 1-nonanol in benzophenone as solvent and gave M-octane in 63% yield. 

Wilkinson s catalyst and chlorocarbonylbis(triphenylphosphine)rhodium [or iridium] can catalyze the thermal decarbonylation of aromatic acid halides to aryl halides. An intermediate Rh(III) hydride is involved in the reaction (Suggs, 1978). For aliphatic acid halides, subsequent elimination of HX frequently occurs from the generated alkyl halide (Ohno and Tsuji 1968 Blum et al., 1%7, 1971 Strohmeier and Pfohler, 1976). 

Exchange of formyl hydrogens for tritium is observed to occur in both aryl aldehydes (with concurrent ortho labeling in appropriate cases) and aliphatic aldehydes using [(cod)Ir(PCy3)(py)]PF6. Partial reduction of aldehydes to alcohols may occur in some cases. Labeling by other iridium phosphine catalysts has not been reported but is likely to occur. This type of catalytic activity, which likely involves reversible oxidative addition of the iridium center into the formyl C—H bond, is different in outcome from that of organorhodium complexes, whose insertion into formyl C—H bonds proceeds instead to decarbonylation. 

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