Decarboxylation of various carboxylic acids

A number of carboxylic acids are found in nature and also present in metabolic pathways. Accordingly, if monobasic acids are smoothly decarboxylated, they are expected to provide us with new routes to supply useful materials for chemical industry without depending on petroleum. Actually, there are some already known examples. The representative examples are the decarboxylation of cinnamic acid derivatives (Table 8).7879

Although the reaction mechanism of this type of reactions is not fully elucidated, it is easily anticipated that no intramolecular special stabilization effect for the carbanion generated from decarboxylation is expected, different from the case of malonic acid-type compounds. Moreover, cinnamic acid derivatives that have both the electron-donating and withdrawing substituents have been reported to undergo this reaction. This fact suggests that the enzyme itself stabilizes the transition state without the aid of mesomeric and inductive effects of the other part of the substrate molecule itself. If such unknown mechanism also works for other

Table 8

Microbial decarboxylation of styrene derivatives x-

Substrate

Biocatalyst

Yield (%) Reference

F. solani C. roseus N. tabacum

H. capslata C. roseus

C. intermedia C. cinensis

Quant Quant

97 30

79 79

compounds without special functional groups to stabilize the transition state, there is a possibility that some microorganisms may be found to decarboxylate aliphatic carboxylic acids and the scope of decarboxylation reactions will be extended further.

Another interesting example of the fission of non-activated C—C bond with the liberation of carbon dioxide is the decarboxylation of oxalate. The enzymes related to degradation of oxalate have a number of potential applications especially in relation to diagnosis and human health.80 Also, the reaction mechanism of this enzyme is interesting. It requires metal ions to activate the substrate and this might give some hints to develop decarboxylation reactions of other types of compounds. In this way, the future extension is expected in this area.

Formate dehydrogenase can be said to catalyze a kind of decarboxylation reaction and is the most widely used in NADH regeneration. However, as the reaction does not include C—C bond fission, the studies on this enzyme are not described in this chapter.

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