Decarbonylation reductive decarboxylation

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. ... 

It has been referred to as a "reductive decarboxylation" in both plants (28) and insects (2). However, Kolattukudy and co-workers have presented evidence that in plant (29,30) and vertebrate tissues (31) the reaction js a decarbonylation and that the carboxyl carbon is reduced to the aldehyde before being removed in the form of carbon monoxide. Clearly more work is needed to determine the nature of this reaction in insects. 

Overall reductive decarboxylation of a carboxylic acid may be achieved by the reaction of the derived acyl chloride with triisopropyisilane (equation 12). Relatively high temperatures are required to bring about efficient decarbonylation of die intermediate acyl radical. A related mediod involves the reaction of acyl phenyl selenides with tri-it-butyltin hydride. Here again relatively high temperatures are required for primary and secondary, although not for tertiary, acids (equation 13). 

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. 

Aromatic acid chlorides are decarbonylated to aryl chlorides when they are heated to 300-360 C with palladium on carbon. The reaction proceeds by way of an aroylpalladium chloride, then to an arylpalla-dium chloride and finally through a reductive elimination to the aryl chloride. If the reaction is conducted in the presence of a reactive alkene under mild conditions the aroylpalladium chloride intermediate will sometimes acylate the alkene, as noted in Section 4.3.5.3.I. More usually, however, decarboxylation is more rapid than acylation, especially at higher temperatures (>100 C), and decarbonylation occurs. The... 

Several acyl radical clocks have been calibrated, and these are collected in a recent excellent review of the general subject [44]. Examples of the two types of unim-olecular clock reactions, decarbonylations and cyclizations, are shown in Fig. 7, with rate constants for reactions at ambient temperature. Decarbonylations of acyl radicals, as shown for radical 16 [45], and the related decarboxylations of alkox-ycarbonyl radicals such as 17 [2] have log A terms of about 13 for cases where alkyl radical products are formed [46, 47]. The decarbonylation reactions involve a reduction in charge separation in the transition states, and the kinetics are sensitive to solvent polarity with decreases in rates as polarity increases [45]. Cyclization reactions, such as that shown for radical 18, are complicated. The 5-exo products shown are the predominant first-formed products, but they further rearrange to the thermodynamically favored 6-endo products by addition of the radical center to the carbonyl group to give a cyclopropyloxyl radical followed by ring opening [48]. 

In the palladium-catalyzed carbonylation process, allyl formate, prepared by the reaction of allyl alcohol with formic acid, oxidatively adds to Pd(0) species with the C-0 bond cleavage to give allyl palladium formate. The CO insertion into the allylpalladium bond produces butenoyl palladium formate, which reductively ehminates butenoic formic anhydride with regeneration of the catalytically active Pd(0) species. Spontaneous decarbonylation of the mixed anhydride yields 3-butenoic acid, which isomerizes to 2-butenoic acid [61]. The process to give the butenoic acid proceeds only under CO pressure, suggesting that the CO insertion into the allyl-Pd bond is favored under CO pressure. When the reaction is carried out under normal pressure of CO, decarboxylation of the formate to give palladium hydride takes place. Reductive elimination of the allylpalladium hydride yields hydrogenation product of the allyl moiety [62]. 

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