Decarboxylation metal catalysis

Several examples of transition metal catalysis for the synthesis of piperidines appeared this year. Palladium catalyzed intramolecular urethane cyclization onto an unactivated allylic alcohol was described as the key step in the stereoselective synthesis of the azasugar 1-deoxymannojirimycin . A new synthetic entry into the 2-azabicyclo[3.3.1]nonane framework was accomplished through a palladium mediated intramolecular coupling of amine tethered vinyl halides and ketone enolates in moderate yields . A palladium catalyzed decarboxylative carbonylation of 5-vinyl... 

To elucidate the difference between the enzymatic and nonenzymatic participation of metal ions, it is clearly desirable to be able to compare the effect of a large number of metal ions upon the same reaction both in the presence and absence of the enzyme. For such a study to be feasible it is necessary to work with a metal-activated enzymatic reaction, which will also take place when the metal, but not the enzyme, is omitted. Such a reaction is the decarboxylation of oxaloacetic acid. The mechanism of metal catalysis of this reaction is similar to that assumed for carboxypeptidase, and can be represented as follows (44). 

In the realm of homogeneous catalysis we often encounter examples of acid- and base-catalyzed hydration-dehydration and hydrolysis, metal-catalyzed hydrolysis and autoxidation, photocatalytic oxidation and reduction, metal-catalyzed electron transfer, acid-catalyzed decarboxylation, photocatalytic decarboxylation, metal-catalyzed free-radical chain reactions, acid-catalyzed nucleophilic substitutions, and enzymatic catalysis. 

Other examples of electrophilic metal catalysis are given under section 2.3.3.1. Electrophilic reactions are also carried out by enzymes which have an a-keto acid (pyruvic acid or a-keto butyric acid) at the transforming locus of the active site. One example of such an enzyme is histidine decarboxylase in which the N-terminal amino acid residue is bound to pyruvate. Histidine decarboxylation is initiated by the formation of a Schiff base by the reaction mechanism in Fig. 2.20. 

Kukula P, Matousek V, Mallat T, Baiker A. Enantioselective decarboxylation of P-keto esters with Pd/amino alcohol systems successive metal catalysis and organocatalysis. Chem. Eur. J. 2008 14 2699-2708. 

Bcamples of metal-ion catalysed organic reactions in water where the catalyst acts exclusively as Lewis acid are the hromination of diketones" " and the decarboxylation of oxaloacetate. The latter reaction has been studied in detail. In 1941 it was demonstrated that magnesium(II) ions catalyse this reaction" Later also catalysis by other multivalent metal ions, such as Zn(II), Mn(II), Cu(II), Cd(ir), Fe(II), Pb(II), Fe(III)... 

Electrophilic catalysis is catalysis by an electrophile (Lewis acid) acting as an electron-pair acceptor. For example, metal ions catalyze the decarboxylation of dimethyloxaloacetic acid. ... 

We have exploited this base catalysis of the oxygen exchange process to effect oxygen lability in the less electrophilic carbonyl sites of neutral metal carbonyl species. Because [MCOOH] intermediates are readily decarboxylated in the presence of excess hydroxide ion, in order to observe oxygen exchange processes in neutral metal carbonyl complexes it was convenient to carry out these reactions in a biphasic system employing phase transfer catalysis () (16, 17. 18). Under conditions (eq. 7) the... 

Marcus theory, first developed for electron transfer reactions, then extended to atom transfer, is now being applied to catalytic systems. Successful applications to catalysis by labile metal ions include such reactions as decarboxylation of oxaloacetate, ketonization of enolpyru-vate, and pyruvate dimerization (444). 

Steinberger and Westheimer (56) carried out an elegant analysis of the metal ion catalysis of the decarboxylation of dimethyloxaloacetic acid and some of its derivatives. The reaction utilized by them was... 

Decarboxylation of p-oxoacids. Beta-oxoacids such as oxaloacetic acid and acetoacetic acid are unstable, their decarboxylation being catalyzed by amines, metal ions, and other substances. Catalysis by amines depends upon Schiff base formation,232 while metal ions form chelates in which the metal assists in electron withdrawal to form an enolate anion.233 235... 

Catalytic decarboxylation processes occur in aliphatic keto acids in which the keto group is in an a-position to one carboxyl group and in a P-relationship to another. Thus, the normal decarboxylation of a p-keto acid is facilitated by metal coordination to the a-keto acid moiety. The most-studied example is oxaloacetic acid and it has been shown that its decarboxylation is catalyzed by many metals following the general order Ca2+ < Mn2+ < Co2+ < Zn2+ < Ni2+ < Cu2+ < Fe3+ < Al3"1".66 67 The overall rate constants can be correlated with the stability constants of 1 1 complexes of oxalic acid rather than oxaloacetic acid, as the uncoordinated carboxylate anion is essential for the decarboxylation. The generally accepted mechanism is shown in Scheme 15. Catalysis can be increased by the introduction of x-bonding ligands, which not only increase the... 

This section has dealt with the oxidation of CO to C02, especially as it enters into the water-gas shift reaction (26a). A reasonable view of the homogeneous catalysis of this reaction, whether in basic or acidic media, is emerging in which CO formation proceeds from nucleophilic attack of water or OH" on an activated carbonyl followed by either reductive decarboxylation or hetero-atom -elimination yielding, respectively, a reduced metal or a metal hydride species. 

Base hydrolysis of an ester in presence of metal ions, metal ion catalysis - analysis in terms of kinetically equivalent mechanisms, 330-331 Acid hydrolysis of a charged ester in the presence of SC>4 (aq) anion catalysis - analysis in terms of kinetically equivalent mechanisms, 332-336 Decarboxylation of /3-ketomonocarboxylic acids - formulation of the rate expression from the mechanism, 339-341... 

Vitamins, cofactors, and metals have the potential to broaden the scope of antibody catalysis considerably. In addition to hydrolytic and redox reactions, they facilitate many complex functional group interconversions in natural enzymes.131 Pyridoxal, for example, plays a central role in amino acid metabolism. Among the reactions it makes possible are transaminations, decarboxylations, racemizations, and (3,y-eliminations. It is also essential for ethylene biosynthesis. Not surprisingly, then, several groups have sought to incorporate pyridoxal derivatives into antibody combining sites. 

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