Decarbonylation dimerization

At temperature below the decarbonylation level cyclopropenones are preferentially transformed to stable dimers, which do not eliminate CO at higher temperatures. Thus, thermolysis of diphenyl cyclopropenone at temperatures above 160 °C gives mainly diphenyl acetylene, whilst heating in the molten state to 145— -... 

Thermolysis also produced the decarbonylation product plus a dimer, structure C. 

Such pentacarbonyl species can be further decarbonylated when the sample is heated to 373 K under an inert gas stream and under reduced pressure. This slow decarbonylation process provides the surface Mo(CO)3 species depicted in Figure 9.4, which is stable up to 473 K [14]. In contrast with the relevant chemical behavior in solution (9.1 and 9.2), in the solid state, where the species are somewhat diluted and present low mobility, no dimeric species have been identified as resulting from penta- or tricarbonyl species. Heating to 673 K gives rise to the evolution of H2, CO, CO2 and CH4, due to redox reactions between the metal center and the OH surface groups. The resulting oxidation states, as determined by XPS measurements, are mainly II and IV, besides some Mo(0) species ]20]. It is worth underHn-... 

When heated, 5-arylfuran-2,3-diones (346) readily evolve carbon monoxide to give 3-aroyl-6-aryl-4-hydroxypyran-2-ones (75KGS1468). Based on a kinetic study of this thermal decarbonylation, it is proposed that the formation of pyranones involves an initial cheletropic cycloreversion. The benzoylketene so formed dimerizes through a [4 + 2]-cycloaddition to the pyranone (Scheme 107) (78JOU2245). 

Pyridinealdehyde yields pyridine and some dipyridyl, a reaction which proceeds without destruction of the heterocyclic ring 19). Ketones decarbon-ylate equally well. Benzophenone and dibenzoyl form diphenyl in high yields 2°). Several interesting eliminations have been found for cyclic and bi-cyclic ketones. In the case of cyclohexanone the decarbonylation accounts for only a small proportion of the reaction products. Benzoquinone decarbon-ylates to cyclopentadienone in good yields 20K Under the reaction conditions this compound dimerizes and decarbonylates again. 

Photolysis of Cp Mn(CO)3 in THF leads to the solvent complex Cp Mn-(CO)2(THF). Removal of solvent at -20°C followed by warming to room temperature while maintaining reduced pressure results in dimerization of solvent complex, decarbonylation, and solvent loss to form the air-sensitive 8 (17,51,88). While not isolated, the related Cp complex 8 has been observed in the gas phase. It is seen, in the electron-impact mass spectrum of the THF complex CpMn(CO)2(THF), which shows a molecular ion and cracking pattern assignable to 8 rather than the THF complex itself (51). The rhenium complex 9 is formed on photolysis of Cp Re-(CO)3 in THF (18) and in the carbonylation (15-20 atm, THF or toluene) of Cp 2Re2(0)2(ju.-0)2 (89,90). 

The fraction of the acyl/naphthoxy radical pairs that escape from their initial cages (/tesc) should be known. That fraction can be approximated from the yields of (Bz)2, the dimerization product of the decarbonylated radical. In practice, the fractions of NOL and (Bz)2 arising from the /tesc and /tesc pathways cannot be determined easily. As long as the relative yield of (Bz)2 is very small, the values for /t2A and calculated from Equations 13.10 and 13.11 are overestimated by amounts that are usually less than the experimental error in determining the relative yields of the 2-AN, 4-AN, 2-BN, 4-BN, BzON, and... 

Irradiation of tetramethyl-1,3-cyclobutanedione in nonhydroxylic solvents results in decarbonylation as the major course of reaction with g-cleavage being observed as a minor reaction path. A low yield of the lactone [115] has also been reported (73) however, since this material might result from dimerization of dimethyl-ketene, it may not be a primary photoproduct (i.e., ketene dimer is isolated from irradiation of [69]. 

Decarbonylation of (14), either thermally in boiling hexane (X = Cl, Br) or photochemically (X = 1), gives the halide-bridged dimers [ReX(CO)4]2 (15) that catalyze Friedel-Crafts alkylation and acylation, as well as benzyl ester deprotection. 

When cyclopropenone, or its dichloro or diphenyl analogues are heated to ca. 100°C dimerization, rather than decarbonylation, takes place and spirocyclic compounds (316) are obtained. These most likely result from nucleophilic attack on the carbonyl carbon of one molecule by the oxygen atom of the second followed by 1,2-cleavage and cyclization ... 

At temperatures low the decarbonylation barrier, stable dimers are observed. Thus, thermolysis of diphenylcyclopropenone at 150°C or in refluxing toluene leads to the dimer 48 . The formation of 48 can be rationalized as a dipolar addition of the ring-opened species 49 to the C = O group of a second molecule (equation 40). The codimerization of... 

Mechanistic studies strongly suggest the intermediacy of a samarium acyl anion. For example, the phe-nylacetyl radical (PhCHiCO ) is known to rapidly decarbonylate k = 5.2 X 10 s" ), providing a benzyl radical which dimerizes to bibenzyl. However, addition of phenylacetyl chloride to a solution of Sml2 in THF leads to a 75% yield of the expected diketone, and neither toluene nor bibenzyl is detected. Apparently, intermolecular reduction of this acyl radical to the corresponding anion proceeds at a rate which is greater than 5.2 x 10 s . ... 

The reactor used was a 10 mm diameter stainless sted tube filled with 0.1 g or 0.25 g catalyst diluted with 1.0 mm glass beads. The reactions considcied were the consecutive reaction 2-ethyl-hexenal (A) —> 2-Etbyl-hexanal (B) —> 2-ethyI-hexanol (C) dimerization of the aldehydes, and the fomiation of heptane by decarbonylization. AU experiments were performed at atmospheric pressure and at a reaction temperature of 120°C. The hydrogen pressure was 1250 Pa (1.23% H ) and 300 Pa. The space velocity was adjusted to get similar conversion... 

Figure 1 shows the product distribution as a function of time on stream. The rate of the main leactioOp A —> B, decreases quickly during the first 10 hours. After this initial deactivation of the catalyst the deactivation rate is lower. The selectivity for production of B in the consecutive reaction A —> B —> C increases with lime on stream. The rate of dimerization and decarbonylization reactions follow the same pattern as the main reaction. A used catalyst that had been regenerated by oxidation in air (380"C. L2h) and reduced in hydrogen (50% H2, 380, 16h) showed a selectivity that was very similar to that of a fresh catalyst. 

Bernard, K. A., Atwood, J. D. Evidence for carbon-oxygen bond formation, aldehyde decarbonylation, and dimerization by reaction of formaldehyde and acetaldehyde with trans-ROIr(CO)(PPh3)2. Organometallics 1988, 7, 235-236. 

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