Carboxylate complexes decarboxylation

Mercuric o-carborane-l-carboxylate complexes decarboxylated when heated above their melting points or when refluxed in decane or benzene solution [Eq. (102), L = phen, bpy, or (py)2] (109,110). Asymmetric organomercury carboranes and their 1,10-phenanthroline complexes were also formed by thermal decarboxylation [e.g., Eq. (103), R = Me or Ph] (111). 

Ketoacids126,127 form the same intermediates as the allyl 3-ketoesters by nucleophilic addition of the carboxylate to a n-allylpalladium complex. Decarboxylation generates the allylpalladium enolate, which again yields Pd° and allylated ketone. Enol silyl ethers have also been employed with allyl arsenites93 to provide allylated ketones. 

We have presented evidence that pyrrole-2-carboxylic acid decarboxylates in acid via the addition of water to the carboxyl group, rather than by direct formation of C02.73 This leads to the formation of the conjugate acid of carbonic acid, C(OH)3+, which rapidly dissociates into protonated water and carbon dioxide (Scheme 9). The pKA for protonation of the a-carbon acid of pyrrole is —3.8.74 Although this mechanism of decarboxylation is more complex than the typical dissociative mechanism generating carbon dioxide, the weak carbanion formed will be a poor nucleophile and will not be subject to internal return. However, this leads to a point of interest, in that an enzyme catalyzes the decarboxylation and carboxylation of pyrrole-2-carboxylic acid and pyrrole respectively.75 In the decarboxylation reaction, unlike the case of 2-ketoacids, the enzyme cannot access the potential catalysis available from preventing the internal return from a highly basic carbanion, which could be the reason that the rates of decarboxylation are more comparable to those in solution. Therefore, the enzyme cannot achieve further acceleration of decarboxylation. In the carboxylation of pyrrole, the absence of a reactive carbanion will also make the reaction more difficult however, in this case it occurs more readily than with other aromatic acid decarboxylases. 

Fig. 5.4. Two types of energy metabolism in cestodes. (a) Type 1 homolactate fermentation, (b) Type 2 Malate dismutation. Reaction 3 involves a carboxylation step decarboxylation occurs at 6, 7 and 10. Reducing equivalents are generated at reactions 6 and 7 one reducing equivalent is used at reaction 9. Thus, when the mitochondrial compartment is in redox balance and malate is the sole substrate, twice as much propionate as acetate is produced. Key 1, pyruvate kinase 2, lactate dehydrogenase 3, phosphoenolpyruvate carboxykinase 4, malate dehydrogenase 5, mitochondrial membrane 6 malic enzyme 7, pyruvate dehydrogenase complex 8, fumarase 9, fumarate reductase 10, succinate decarboxylase complex. indicates reactions at which ATP is synthesised from ADP cyt, cytosol mit, mitochondrion. (After Bryant Flockhart, 1986.)...

The chemistry of aqueous solutions of COj in the presence of aquated transition metal complexes has been extensively studied by a group of researchers led by Harris, Palmer and vanEldik, and they have presented extensive kinetic studies on the carboxylation and decarboxylation of a variety of bis(ethylenediamine)carbonato complexes of The decarboxylation of [Rh(en)2(C03)] ... 

A classic paper by Steinberger and Westheimer [16] provided much information on the metal-ion-catalysed process. They established that the decarboxylation of the monoester of aa-dimethyloxaloacetic acid EtOOCCOC(CHj)2C02H was not catalysed by metal ions. This result suggested that the catalytically active species formed by aa-dimethyloxaloacetic acid (which was catalysed) was the five-membered a-oxo carboxylate complex (7.1), Scheme 7.2. 

How could this complex bicycllc compound be synthesized efficiently Using the positioning of the nitrogen atom reiative to the ketone, Robinson believed that the entire molecule could potentially arise from a dialdehyde, methylamine, and acetone dicarboxyiic acid in a single, one-pot transformation, as color-coded below. The key reactions in the actual union would be a series of carefully orchestrated iminium ion formations and Mannich reactions to make the new C—C bonds (colored in green), followed by carboxylic acid decarboxylations to complete the target. 

L = [14]ane N4) haAre been used to make RS thiyl radicals, which in turn unexpectedly react with [EtCo(L)(H2P)]2+ to foim C2H4 and RSH.239-241 Addition of base to [(tacn)Co(L)] (tacn = 1,4,7-tiiazacyclononane L = l,6-diamino-3-thiahexane) causes deprotonation of L and formation of an N,C,N-bonded ligand, in equilibrium with its N,S,N-bonded fonn. > 3 other Co(III) alkyls have been produced by photochemical decarboxylation of chelate carboxylate complexes. The synthesis and structure of the nl-C 02 polymer [ Co(en)2(C02)) QO4 H20 ]n has been reported.246 The structure of [Co(salen)(Pr )(py)] has been determined,247 and tandem cyclisation reactions involving organocobalt salen and salophen complexes have been described.248.249... 

Electrode modification can be carried out by methods that vary greatly. A reaction can be affected simply by addition to the electrolysis solution of a substance that is readily adsorbed onto the electrode surface. Thus, additimi of a thiocyanate salt to the medium diverts the anodic oxidatimi of carboxylates frran decarboxylative dimerization (Kolbe reaction) to peracid formation [1]. Often, a polymer solutimi containing an electrocatalyst is placed on a surface, and the solvent evaporated or a monomer is electrochemicaUy polymerized in situ from solution mito the surface. Electrocatalysts deposited in this manner include organometallic electrocatalyst complexes such as vitamin B12 [2], oxidizable heterocycles such as pyrrole or thiophene, or metal ions [3]. Successive layers of complementary materials may be laid down on an electrode to achieve the desired immobilization effect. Thus, a polymer (PDAA polydimethyldiallyl ammonium chloride) bearing... 

We reasoned that such a decarboxylation step could also be employed in a redox-neutral cross-coupling reaction with carbon electrophiles. On this basis, we drew up a catalytic cycle that starts with an oxidative addition of aryl halides or pseudohalides to a coordinatively unsaturated palladium(O) species f (Scheme 5). The more weakly coordinating the leaving group X, the easier should be its subsequent replacement by a carboxylate. At least for X = OTf, the palladium(ll) carboxylate h should form quantitatively, whereas for X = halide, it should be possible to enforce this step by employing silver or thallium salts as species g. The ensuing thermal decarboxylation of the palladium(ll) intermediate i represents the most critical step. Myers results indicated that certain palladium(ll) carboxylates liberate carbon dioxide on heating. However, it remained unclear whether arylpalladium (II) carboxylate complexes such as i would display a similar reactivity. If this were to be the case, they would form Ar-Pd-Ar intermediates k, which in turn are... 

Metals, especially copper compounds (Wiley and Smith, 1963), are usually required and such reactions proceed via the intermediates composed of metal carboxylate complexes. Alkylcarboxylic acids and their salts, however, do not always undergo decarboxylation readily (March, 1985). 

Alkyl radicals produced by oxidative decarboxylation of carboxylic acids are nucleophilic and attack protonated azoles at the most electron-deficient sites. Thus imidazole and 1-alkylimidazoles are alkylated exclusively at the 2-position (80AHC(27)241). Similarly, thiazoles are attacked in acidic media by methyl and propyl radicals to give 2-substituted derivatives in moderate yields, with smaller amounts of 5-substitution. These reactions have been reviewed (74AHC(i6)123) the mechanism involves an intermediate cr-complex. 

The chiral center would be installed from either Unear carbamate 15 or branched carbamate 16 via the asymmetric addition of malonate anion to the 7i-allyl Mo complex reported by Trost et al. [11] to afford the branched chiral malonate derivative 17. Decarboxylation of 17 should provide the mono-carboxylic acid 18. Masa-mune homologation with 18 affords our common precursor 14. Linear carbamate 15 was obtained from the corresponding cinnamic acid, and branched 16 was prepared in one pot from the corresponding aldehyde. 

The reversible complexing of carbon dioxide by bis[bis(l,2-diphe-nylphosphino)ethane]iridium(I) chloride, [Ir(dpe)2]Cl, in acetonitrile [Eq. (36)] (48) appears not to involve carboxylation of a cyanomethylir-idium(III) complex or its formation by decarboxylation of the cyanoacetate... 

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