Reaction carboxylation

Novel aromatic carboxylation reactions have been observed in the anaerobic transformation of phenols to ben2oates (82). A mixed anaerobic microbial consortium apparentiy transforms phenol (33) through an intermediate to ben2oic acid (34) via dehydroxylation. This reaction has not yet been widely exploited for its obvious synthetic value. 

Alkylphenols undergo a carboxylation reaction known as the Kolbe Schmidt reaction. In the following example, the phenolate anion of /)-nonylphenol (15) reacts with carbon dioxide under pressure. Neutralization generates a sahcyhc acid (16) (10). 

Work in the mid-1970s demonstrated that the vitamin K-dependent step in prothrombin synthesis was the conversion of glutamyl residues to y-carboxyglutamyl residues. Subsequent studies more cleady defined the role of vitamin K in this conversion and have led to the current theory that the vitamin K-dependent carboxylation reaction is essentially a two-step process which first involves generation of a carbanion at the y-position of the glutamyl (Gla) residue. This event is coupled with the epoxidation of the reduced form of vitamin K and in a subsequent step, the carbanion is carboxylated (77—80). Studies have provided thermochemical confirmation for the mechanism of vitamin K and have shown the oxidation of vitamin KH2 (15) can produce a base of sufficient strength to deprotonate the y-position of the glutamate (81—83). 

Garboxylation Reaction. The carboxylation reaction represents the conversion of acetylene and olefins into carboxyHc acids (qv) or their derivatives. The industrially important Reppe process is used in the synthesis of P-unsaturated esters from acetylene. Nickel carbonyl is the catalyst of choice (134). 

FIGURE 25.2 (a) The acetyl-CoA carboxylase reaction produces malonyl-CoA for fatty acid synthesis, (b) A mechanism for the acetyl-CoA carboxylase reaction. Bicarbonate is activated for carboxylation reactions by formation of N-carboxybiotin. ATP drives the reaction forward, with transient formation of a carbonylphosphate intermediate (Step 1). In a typical biotin-dependent reaction, nncleophilic attack by the acetyl-CoA carbanion on the carboxyl carbon of N-carboxybiotin—a transcarboxylation—yields the carboxylated product (Step 2). 

H Biotin Coenzyme in carboxylation reactions in gluco-neogenesis and fatty acid synthesis Impaired fat and carbohydrate metabolism, dermatitis... 

Biotin functions to transfer carbon dioxide in a small number of carboxylation reactions. A holocarboxylase synthetase acts on a lysine residue of the apoenzymes of acetyl-CoA carboxylase, pymvate carboxylase, propi-onyl-CoA carboxylase, or methylcrotonyl-CoA carboxylase to react with free biotin to form the biocytin residue of the holoenzyme. The reactive intermediate is 1-7V-carboxybiocytin, formed from bicarbonate in an ATP-dependent reaction. The carboxyl group is then transferred to the substrate for carboxylation (Figure 21—1). 

Phenol is an important intermediate in the anaerobic degradation of many complex and simple aromatic compounds. Tschech and Fuchs proposed that the carboxylation of phenol to 4-hydroxybenzoate is the first step in the degradation of phenol under denitrifying conditions. However, 4-hydroxybenzoate is not detected in the cultures or cell extracts of the denitrifying Pseudomonas species in the presence of CO2 and phenol, but it is detected if phenol is replaced by phenolphosphate. In contrast, 4-hydroxybenzoate is readily detected as an intermediate of phenol degradation in the iron-reducing bacterium GS-15, and 4-hydroxybenzoate may prove to be a common intermediate in the anaerobic transformation. Thus, in anaerobic degradation of phenolic compounds, it has been postulated that carboxylation reactions may play important roles. 

Figure 1 Time course of carboxylation reaction of phenol. Closed circles, 4-hydroxybenzoate open squares, phenol.

The reaction product of the reserve carboxylation reaction was isolated and identified to be 3,4-dihydroxybenzoic acid by NMR and NMR with the authentic 3,4-dihydroxybenzoic acid as a reference. The carboxylation reaction of catechol to 3,4-dihydroxybenzoate was affected by the concentration of KHCO3. The carboxylation activity of E. cloacae P241 3,4-dihydroxybenzoate decarboxylase in the presence of 0.1 M KHCO3 was only 15% of that in the presence of 3 M KHC03. In the case of C. hydroxybenzoicum 3,4-dihydroxybenzote decarboxylase, only 0.01 mM 3,4-dihydroxybenzoate was formed from 6mM catechol in the presence of 50 mM NaHC03 by 40 min incubation. The difference in molar conversion ratios might be caused by the concentration of bicarbonate added to the reaction mixture. 

The enzymes catalyzing the Kolbe-Schmitt carboxylation seem to occur ubiquitously. Some of them, such as 2,6-dihydroxybenzoate decarboxylase and pyrrole-2-carboxylate decarboxylase, catalyze efficiently the reverse carboxylation reaction and accumulate high concentration of 2,6-dihydroxybenzoate from 1,3-dihydroxybenzene and pyrrole-2-carboxylate from pyrrole, respectively, in the... 

However, if we can design some sophisticated routes to generate carbanion equivalents in the active site of the enzyme, carboxylation reaction might be possible. In fact, acetyl-CoA is carboxylated with the aid of biotin in the biosynthetic pathway of long-chain fatty acids. 

The carboxylation reaction is extremely important not only from the synthetic standpoint but also from the enviromnental point of view. Accordingly, studies toward this end are highly expected to develop in near future. 

Houwen FP, C Dijkema, AIM Stams, AJB Zehnder (1991) Propionate metabolism in anaerobic bacteria determination of carboxylation reactions with C-NMR spectroscopy. Biochim Biophys Acta 1056 126-132. 

Ramanand K, JM Suflita (1991) Anaerobic degradation of m-cresol in anoxic aquifer slurries carboxylation reactions in a sulfate-reducing bacterial enrichment. Appl. Environ. Microbiol. 57 1689-1695. 

Sander and co-workers have also previously examined the effects of carbene spin state and philicity on the carboxylation reaction in low-temperature matrices. ... 

OPh, or Ome x = 1, 2, or 3] (47). The carboxylation reaction was favored when the ratio of tertiary phosphine to copper salt was increased to 3 1. This suggests that an intermediate Cu—(C02) complex is not formed. At this ratio the copper complexes are considered coordinately saturated. Carboxylation was also favored when phosphines of high o- donor ability were used. 

This process (106— 107) required the incubation of an aqueous mixture of AIR (106), potassium carbonate, and enzyme (AIR carboxylase). The equilibrium of the carboxylation reaction was shown to lie to the left (59JBC1799), but in the presence of 0.3 M aqueous potassium bicarbonate solution, yields of C-AIR (107) approaching 50% were obtained. 

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