Gas phase decarbonylation

On a commercial scale, furan is obtained from 2-formylfuran (furfural, furan-2-carbaldehyde) (see Section 6.2.7) by gas-phase decarbonylation, but in the laboratory, furans can be formed by the cyclodehydration of 1,4-dicarbonyl compounds. Heating in boiling benzene with a trace of /7-toluenesulfonic acid as a catalyst in a Dean-Stark apparatus is often effective (Scheme 6.30a). 

In contrast to earlier interpretations gas phase decarbonylation of ionized 2,3,4,5-tetraphenyl-2,4-cyclopentadienone (147) does not result in the formation of an ionized tetraphenyl-substituted tetrahedrane (148), but yields exclusively the isomeric cyclo butadiene derivative (149) (Scheme 24). 

Furans are volatile, fairly stable compounds with pleasant odours. Furan itself is slightly soluble in water. It is readily available, and its commercial importance is mainly due to its role as the precursor of the very widely used solvent tetrahydrofuran (THF). Furan is produced by the gas-phase decarbonylation of furfural (2-formylfuran, furan-2-carboxaldehyde), which in turn is prepared in very large quantities by the action of acids on vegetable residues mainly from the manufacture of porridge oats and cornflakes. Furfural was first prepared in this way as far back as 1831 and its name is derived from furfur which is the latin word for bran in due course, in 1870, the word furan was coined from the same root. 

Irradiation of cyclic ketones having perfluoroalkyl groups causes cleavage of a ring to yield acyclic products [I74 (equation 44) Similarly, perfluonnated ketones undergo decarbonylation when irradiated [775]. Gas-phase photolysis of perfluorodiazoketones, in the presence of a trapping agent, yields fluorinated furan as a major product [176] (equation 45)... 

In the gas phase, this decarbonylation reaction is very efficient. Radical rearrangements, which are also possible if the change leads to a more stable product ... 

In solution, diffusion apart of the radicals is inhibited by the solvent, resulting in the radicals recombining to form propanone. In the gas phase, however, the radicals do not combine and the acetyl radical breaks down to form carbon monoxide and another methyl radical. This elimination is known as decarbonylation. The methyl radicals then combine and the overall products are ethane and carbon monoxide (Scheme 9.1). 

R = alkyl or H depends on literature Arrhenius parameters for decarbonylation of the propanoyl radical in the gas phase. 

Deposition of Co2(CO)g from the gas phase under a CO or N2 atmosphere on mesoporous high surface-area MCM-41 material has been reported [148]. Under CO, a Co2(CO)g monolayer coverage of up to 21 wt% cobalt was obtained. Although treatment at circa 150°C under N2 produced total decarbonylation, the surface area and pore size of the sample did not change and the presence of metalUc cobalt species could not be determined from the XRD patterns of decarbonylated samples these facts could indicate a good metal dispersion and capabilities for catalytic uses in hydrogenation reactions [148]. 

Tetracarbonylfoctahydrotriborato(l-)] manganese is an air-sensitive liquid that decomposes slowly (about 5% in 4 days) at room temperature as a neat liquid in a vacuum. It is soluble in benzene, toluene, dichloromethane, diethyl ether, and tetrahydrofuran (THE), but decomposes upon heating at reflux in these solvents (especially in THF). Gas-phase thermal decarbonylation or solution photodecarbonylation of (CO)4Mn(B3H8) yields the novel and reactive compound (CO)3Mn(B3H8), in which the octahydrotriborate(l-) ligand is tridentate.3... 

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). 

Thus, it is clear that when the Cr(CO)5 species reaches the ground-state (S0) it retains excess vibrational energy. This is used to expel a further CO loss producing Cr(CO)4 in L4 (930 100 fs). It is important to note that the timescale at which Cr(CO)4 is produced is slow compared to solvent-induced vibrational relaxation processes. Thus, Cr(CO)4 is formed only in the gas phase while photoinduced decarbonylation of Cr(CO)6 stops at Cr(CO)5 in the condensed phase. 

First, following the results of the 1,6-dioxa-spiro[2.5]octane rearrangement (5,19), continuous gas phase conditions were applied in a fixed bed reactor and secondly under liquid phase conditions in a slurry reactor. The catalytic experiments carried out showed that two main reactions took place rearrangement of 18 to the aldehyde 19 and a oxidative decarbonylation reaction to the olefine 1,3,3,4-tetramethyl-cyclohex-l-ene 20, which is assumed to be caused by a formaldehyde elimination reaction. Also observed was a deoxygenation reaction to the alkane 1,1,2,5-tetra-methylcyclohexane 21 (Eq. 15.2.7), explained by elimination of CO. There are several other side-products such as 2,2,3,6-tetramethylcyclohex-l-enyl-methanol, ringcontracting compounds and double bond isomers of dimethyl-isopropylene-cyclopentene. 

Using zeolites with the same structure but in different ratios of Si02/Al203, we found an increased tendency towards selectivity to 24 with higher Si02/Al203 ratios, as was previously observed in liquid phase isomerization. All tested zeolites yielded high conversions, except for Na-ZSM-5 and silicalite. A shorter contact time seems to have a positive effect on the aldehyde formation, possibly due to reduced decarbonylation. The influence of solvents in the gas phase is similar to that of the liquid phase. Best yields were obtained with toluene, anisole and o-xylene. 

Furfural 69 has been used as a chemical feedstock for the production of furan via two production methods involving the decarbonylation of furfural <2005MI7>. Processes in both the liquid and gas phases were described for the preparation of furan through the decarbonylation of furfural using noble metal and metal oxide catalysts. The results of the study led the authors to state that the research trends for preparing furan based on the decarbonylation of furfural should mainly be concentrated on more effective catalysts and environmentally friendly processes. 

In each of the aryl esters discussed above, the acyl radical formed upon lysis of the excited singlet state of the ester loses CO very slowly at the temperatures of the irradiations. At 296K, the rates of loss of CO by acetyl and propanoyl radicals in the gas phase are 4.0 and 2.1 x 10 s respectively." As aresult, no products from decarbonylation and rearrangement are expected" (or have been found) when either of the NA or NM isomers is irradiated in liquid solvents or bulk polymers, and kinetic information from photoproducts alone is limited to relative rates of radical pair processes (Scheme 13.3). For example, if no Fries products from 1-NA or 1-NM emanate from reencounter of radicals that have escaped from their initial cages, [2-AN]/[4-AN]/[l-NOL] = /c2a/ 4a/ nol-... 

To date, the photochemistry of cyclobutanone has been most extensively studied in the gas phase (14), where the two processes, 3-cleavage (or cycloelimination) and decarbonylation, are the only observable reactions. The solution-phase photochemistry of this parent ketone, on the other hand, has been less extensively studied. Turro and Southam (15) investigated the photochemical transformations of cyclobutanone [21] in methanol and observed products attributable to the three expected reactions. 

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