Mechanism for decarboxylation

One of the several mechanisms for decarboxylation is the reverse of the familiar carboxylation reaction of organometallic compounds or carbanions. Many of the acids RCOOH that are readily decarboxylated in basic media are compounds for which the corresponding R( ) is a comparatively stable carbanion.405 The postulated intermediate has actually been trapped or diverted in a few cases as the product of an aldol condensation.406... 

Figure 10 Proposed mechanism for decarboxylative dehydration of arogenate (41) to Phe (1) by ADT. B, base.

The chemistry of acetate on transition metal surfaces is important for a variety of selective oxidation processes. Methanol and vinyl acetate syntheses are two such important oxidation chemistries where acetate intermediates have been postulated. In VAM synthesis, acetate is a critical intermediate in both VAM formation, as well as in its decomposition to CO2. The latter unselective decarboxylation path becomes important at higher operating temperatures. Understanding the mechanism for decarboxylation and VAM synthesis may ultimately aid in the design of new catalyst formulations on new operating conditions. 

The mechanism for decarboxylation of malonic acids is similar to what we have just studied for the decarboxylation of S-ketoacids. The formation of a cyclic, six-membered transition state involving a redistribution of three electron pairs gives the enol form of a carboxylic acid, which, in turn, isomerizes to the carboxylic acid. 

Propose a mechanism for this type of decarboxylation/elimination. Compare the mechanism of these decarboxylations with the mechanism for decarboxylation of jS-ketoacids. In what way(s) are the mechanisms similar ... 

Except in the case of racemases, pyridoxal phosphate enzymes are stereospecific. This has been illustrated dramatically in experiments with glutamic decarboxylase." The mechanism for decarboxylation postulated earlier involves only electron shifts about the a-carbon, while the a-hydrogen remains fixed. This mechanism was supported when it was found that the a-hydrogen does not exchange with D2O during enzymatic decarboxylation. The resulting mono-D-7-aminobutyric acid was found to... 

The mechanism for the PLP-catalyzed racemization of an L-amino acid is shown next. Notice that the mechanism for the interconversion of the enantiomers is the same as the mechanism for decarboxylation except for the group removed from the a-carbon of the amino acid in the first step. 

Our original mechanism for decarboxylative reactions included a mmiometallic palladium catalyst related to that postulated in 2002 by Myers for his oxidative decarboxylative Heck reactions (Scheme 3) [6]. 

The dicarboxylic acids found in basin brines (i.e., oxalic, malonic, and succinic) are expected to be less stable under hydrothermal conditions than monocarboxylic acids of comparable chain lengths. The stability of these acids has been discussed previously to the extent that structural factors make a-, and y-carboxyl acids susceptible to homogeneous decarboxylation. The mechanisms for decarboxylation of jff-carboxyl acids and their derivatives in solvents of varying polarity have been especially well studied and the results are believed to be generally applicable to a- and y-carboxyl acids as well (Clark 1969). For this reason, the following detailed discussion of the mechanism for homogeneous decarboxylation of dicarboxylic acids is based primarily on malonic acid. Finally, oxidation of dicarboxylic acids may be predicted, although the process has not been well studied. 

The existence of the n-C2 to n-C4 mono- and dicarboxylic acids in hydro-thermal sedimentary environments depends upon the rates of their production and the rates of decomposition and/or oxidation. These two classes of acids exhibit very different rates and mechanisms of decarboxylation. Decarboxylation of acetic acid, and probably of other aliphatic monocar-boxylic acids, proceeds by a heterogeneous catalytic mechanism apparently very different from the homogeneous mechanism for decarboxylation of the dicarboxylic acids. Due to the limited amount of experimental information regarding the kinetics of oxidation or condensation for both classes of acids, no definitive mechanistic trends can be postulated for this process. Nevertheless, it is possible to place constraints on the kinetics and mechanism for the oxidation reaction(s) if this process were assumed to control the ultimate decomposition of acetic acid. Results from studies of mono- and dicarboxylic acid decarboxylation are summarized below. 

Juni E. Evidence for a two-site mechanism for decarboxylation of a-keto acids by a-carboxylase. J. Biol. Chem. 1961 236 2302-2308. 

The first step of the u-ketoglntarate dehydrogenase reaction involves decarboxylation of the substrate and leaves a covalent TPP intermediate. Write a reasonable mechanism for this reaction. 

The mechanism for the conversion of the A -oxide (94) to the o-methylaminophenylquinoxaline (96) involves an initial protonation of the A -oxide function. This enhances the electrophilic reactivity of the a-carbon atom which then effects an intramolecular electrophilic substitution at an ortho position of the anilide ring to give the spiro-lactam (98). Hydrolytic ring cleavage of (98) gives the acid (99), which undergoes ready dehydration and decarboxylation to (96), the availability of the cyclic transition state facilitating these processes. ... 

The amino acid leucine is biosynthesized from n-ketoisocaproate, which is itself prepared from -ketoisovalerate by a multistep route that involves (1) reaction with acetyl CoA, (2) hydrolysis, (3) dehydration, (4) hydration. (5) oxidation, and (6) decarboxylation. Show lhe steps in the transformation, and propose a mechanism for each. 

Further evidence regarding the mechanism was provided by LynnandBoums643 , who found a pH-dependent carbon-13 isotope effect in the decarboxylation of 2,4-dihydroxybenzoic acid in acetate buffers. The dependence was interpreted in favour of the A-SE2 mechanism, for an increase in acetate concentration would increase kL t and hence partitioning of the intermediate so that k 2 becomes more rate-determining. 

The mechanism of decarboxylation of acids containing an amino substituent is further complicated by the possibility of protonation of the substituent and the fact that the species NH2ArCOOH is kinetically equivalent to the zwitterion NHj ArCOO. Both of these species, as well as the anion NH2 ArCOO" and even NH3 ArCOOH must be considered. Willi and Stocker644 investigated by the spectroscopic method the kinetics of the acid-catalysed decarboxylation of 4-aminosalicyclic acid in dilute hydrochloric acid, (ionic strength 0.1, addition of potassium chloride) and also in acetate buffers at 20 °C. The ionisation constants K0 = [HA][H+][H2A+] 1 (for protonation of nitrogen) and Kx = [A"][H+] [HA]-1, were determined at /i = 0.1 and 20 °C. The kinetics followed equation (262)... 

Fig. 11 A tentative mechanism for the reagent-free decarboxylation of ring-fused 2-pyridones obtained under MAOS conditions...

Influence of ionic strength on the reaction rate constant. The influence of the ionic strength on the reaction rate constant was studied using KCl as electrolyte. The results obtained in this study are listed in Table 4, where we can see that the reaction rate constant for N-Br-alanine decomposition undergoes an increment of 40 % upon changing the ionic strength from 0.27M to IM, while in the case of N-Bromoaminoisobutyric acid the increment of the reaction rate constant is of about 12 %. This is an evidence of a non ionic mechanism in the case of the decomposition of N-Br-aminoisobutyric acid, as it is expected for a concerted decarboxylation mechanism. For the decomposition of N-Br-proline the increase on the reaction rate constant is about 23 % approximately, an intermediate value. This is due to the fact both paths (concerted decarboxylation and elimination) have an important contribution to the total decomposition process. 

Fig. 8 Irreversible photoconversion of AvGFP. (a) Modification of the absorption spectra of AvGFP under UV light (A = 254 nm, 100 s irradiation, 12.9 mW) at 293 K, pH 8.0, showing the increase in anionic B band (maximum at 483 nm). (b) Proposed Kolbe mechanism for Glu222 decarboxylation through transient formation of a CH2 radical intermediate. Reproduced with permission from [166]...

A number of possible unimolecular mechanisms can be envisaged for decarboxylation ranging from one involving a free carbanion intermediate [Eq. (4)], to one involving a free carbonium ion intermediate [Eq. (5)]. In... 

When colloidal selenium was heated with mercuric trifluoroacetate or silver trifluoroacetate, bis(trifluoromethyl)diselenide was formed (43). Later work with selenium/silver carboxylate, RC02Ag (R = CF3, C2F5, or C3F7), mixtures at 280° C in a vacuum produced a mixture of the bis(perfluoroalkyl)selenide and the bis(perfluoroalkyl)diselenide (44). Formation of a polyselenium trifluoroacetate, which decarboxylates to produce the trifluoromethylselenides, was the proposed mechanism for R = CF3 (44). However, silver trifluoroacetate is a source of trifluoromethyl radicals when heated above 260° C (21), hence the trifluoromethylselenides may be formed by reaction of trifluoromethyl radicals with selenium, as in the reaction of CF3I with selenium [Eq. (34)] (45). 

The predominance of organometallics with electron-withdrawing substituents (Sections III,A-D, and F) can partly be attributed to the promotion of some mechanisms by these substituents [e.g., Eqs. (4), (6), and (15)] (Section II,B). This imposes limitations, as a number of polyha-logenobenzoates, notably 2,3,4,5-tetrafluorobenzoates, have insufficient electron-withdrawing capacity for decarboxylation to occur (Section III,D). Although mechanisms promoted by electron-donating substituents can be formulated (Section II,B), there is little evidence yet for their operation apart from classic electrophilic aromatic decarboxylation [Eq. (12)] (see also Section III,E). 

Experiments by Kenyon and Blois, with samples of phenylalanine labelled with i4C at each of the three aliphatic carbon positions, showed that the molecule could photolyse at each of the three exocyclic carbon-carbon bonds. Decarboxylation was also thought to be an important process, but unfortunately no resulting phenylethylamine was detected during this work. Mechanisms for the production of the observed products were suggested [24],... 

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