Decarbonylation Decarboxylation

EPR spectroscopy is usually used to calibrate the clock (i.e., to determine kc). The method described here uses EPR to detect the two radicals. These are the parent (R1 ) and the product (R2 ) of its reaction, be it cyclization, decarbonylation, decarboxylation, rearrangement, or whatever. The radical R1 is produced photochemi-cally in the desired inert solvent by steady and usually quite intense light irradiation of the EPR cavity. Typically, R1 and R2 attain steady-state concentrations of 10-8 to 10 6 M. 

Processes such as decarbonylation, decarboxylation, elimination of water, and several other reactions may also occur prior to ionization, i.e., as non-mass spectral reactions, typically as a result of thermal degradation upon heating of the sample to enforce evaporation. In such a case, the mass spectrum obtained is not that of the analyte itself, but of its decomposition product(s). Sometimes, those thermal reactions are difficult to recognize, because the same neutral loss may also occur by a true mass spectral fragmentation of the corresponding molecular ion. 

The linear polyester portion undergoes scission similar to thermoplastic polyesters, undergoing decarbonylation, decarboxylation, or splitting off of methylacetylene. 

The oxidation of coke molecules begins by their hydrogen atoms with formation of oxygenated compounds which can undergo various reactions decarbonylation, decarboxylation, condensation. The greater the density of the acid sites the faster the oxidation of coke. Radical cations formed through reaction of molecular oxygen with coke molecules adsorbed on protonic sites would be intermediates in coke oxidation on acid zeolites. 

Decarbonylation decarboxylation. At 110-170° this complex in catalytic amounts effects decarbonylation of simple aldehydes in quantitative yield.- Decarbonylation of typical indole-2-carboxaldehydes with in j/ M-generated catalyst proceeds in 82-95% yield. In fact, decarboxylation of some indole-2-carboxylic acids can be effected in higher overall yield by conversion to the aldehyde (LiAlH4 MnO,) followed by decarbonylation than by copp)er-catalyzed decarboxylation. ... 

The FTIR data indicate that two types of oxidised hydrocarbon species in coke on ZSM-5 exposed to air at high temperatures. This correlates with a loss of methyl groups and some aromatic rings in coke. Also pyrolysis of coke is likely to be a competitive process in coke removal. TPO demonstrates that decarbonylation-decarboxylation is the final step in regeneration. 

Matsubara and coworkers have recently developed a nickel-catalyzed cycloaddition of isatoic anhydrides with alkynes to afford 2,3-disubstituted indoles in good yields (Scheme 22.16). The cycloaddition reaction proceeds via a decarbonylation, decarboxylation, and alkyne insertion sequence [24]. 

A number of other carbonyl compounds can also be decarboxylated, including cyclic anhydrides. Catalysed by nickel, this reaction is a decarbonylation-decarboxylation leading to an alkene (Scheme 4.99). ... 

Under milder conditions, and with the right choice of ligands, nickelacycles 4,288 can be isolated that may correspond to intermediates in the decarbonylation-decarboxylation reaction, and are related to carboxylation products (see Scheme 4.93). The alkyl carbon-nickel bond in these complexes can be intercepted by both alkyl halides (Scheme 4.100)," and organometallic species (Scheme 4.101). ... 

Stoner, C.E., Jr., and Brill, T.B. (1989) Thermal Decomposition of Energetic Materials 34. Decarbonylation, Decarboxylation and Coupling Reactions of Metal Propiolate Salts, M[02CC=CH] Inorganic Chemistry, in press. 

Nevertheless, the two methods developed for formation of ferrilactone complexes show wider substrate tolerance than either the thermal or photo-lytic reactions and avoid problems that can occur at higher temperature, such as decarbonylation, decarboxylation or metal-catalysed hydrogen shifts [265]. They both give comparable yields of ferrilactone under the conditions described (see Table 8). The one exception is the case of vinyl epoxide (59), when four equivalents of Fe2(CO)9 were required [206]. This is probably due to coordination of the reactive species to the oxygens of the cw-dibenzoyl system reducing the effective concentration of tetracarbonyliron in solution. Conversely, the ultrasound reaction presumably takes place at the surface of the solid Fe2(CO)9 where the local concentration of Fe(CO)4 is much higher and this is reflected in the yields obtained. 

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. 

Reaction 21 is the decarbonylation of the intermediate acyl radical and is especially important at higher temperatures it is the source of much of the carbon monoxide produced in hydrocarbon oxidations. Reaction 22 is a bimolecular radical reaction analogous to reaction 13. In this case, acyloxy radicals are generated they are unstable and decarboxylate readily, providing much of the carbon dioxide produced in hydrocarbon oxidations. An in-depth article on aldehyde oxidation has been pubHshed (43). 

Acyl radicals can fragment with toss of carbon monoxide. Decarbonylation is slower than decarboxylation, but the rate also depends on the stability of the radical that is formed. For example, when reaction of isobutyraldehyde with carbon tetrachloride is initiated by t-butyl peroxide, both isopropyl chloride and isobutyroyl chloride are formed. Decarbonylation is competitive with the chlorine-atom abstraction. 

Decarbonylation, photochemical 885 Decarboxylation 605 Dehydrohalogenation 417 n-Delocalization 392 Demeton sulphones, mass spectra of 155... 

At elevated temperatures, methylene carbons cleave from aromatic rings to form radicals (Fig. 7.44). Further fragmentation decomposes xylenol to cresols and methane (Fig. 7.44a). Alternatively, auto-oxidation occurs (Fig. 1.44b ). Aldehydes and ketones are intermediates before decarboxylation or decarbonylation takes place to generate cresols and carbon dioxide. These oxidative reactions are possible even in inert atmospheres due to the presence of hydroxyl radicals and water.5... 

Reductive decarbonylation and decarboxylation can be carried out by (TMSlsSiH using acyl chlorides, phenylseleno esters, or N-hydroxypyridine-2-thione esters. Examples are shown in Reactions (17)-(19). Hydrolysis of the methyl ester followed by decarbonylation at the C2 position of hexahydropyrro-loindole (+)-17 afforded the desired tricycle (+)-18 in 84% yield and >99% ee. ... 

The decarboxylation of carboxylic acid in the presence of a nucleophile is a classical reaction known as the Hunsdiecker reaction. Such reactions can be carried out sometimes in aqueous conditions. Man-ganese(II) acetate catalyzed the reaction of a, 3-unsaturated aromatic carboxylic acids with NBS (1 and 2 equiv) in MeCN/water to afford haloalkenes and a-(dibromomethyl)benzenemethanols, respectively (Eq. 9.15).32 Decarboxylation of free carboxylic acids catalyzed by Pd/C under hydrothermal water (250° C/4 MPa) gave the corresponding hydrocarbons (Eq. 9.16).33 Under the hydrothermal conditions of deuterium oxide, decarbonylative deuteration was observed to give fully deuterated hydrocarbons from carboxylic acids or aldehydes. 

Hydrocarbon formation involves the removal of one carbon from an acyl-CoA to produce a one carbon shorter hydrocarbon. The mechanism behind this transformation is controversial. It has been suggested that it is either a decarbonylation or a decarboxylation reaction. The decarbonylation reaction involves reduction to an aldehyde intermediate and then decarbonylation to the hydrocarbon and releasing carbon monoxide without the requirement of oxygen or other cofactors [88,89]. In contrast, other work has shown that acyl-CoA is reduced to an aldehyde intermediate and then decarboxylated to the hydrocarbon, releasing carbon dioxide [90]. This reaction requires oxygen and NADPH and is apparently catalyzed by a cytochrome P450 [91]. Whether or not a decarbonylation reaction or a decarboxylation reaction produces hydrocarbons in insects awaits further research on the specific enzymes involved. 

The next task was removal of the C3,C3 -esters. Although the palladium-catalyzed decarboxylation protocol performed well in previous systems, a competing C-H insertion reaction was discovered with the methylidene bridge needed for cercosporin (see below). Since reexamination of alternate decarboxylation methods [48] led to no success, a decarbonylation strategy was explored [49]. Formation of the requisite dialdehyde was best accomplished by overreduction using DIB AL and... 

---