Decarboxylative metal-catalysts

Manufacture. Furan is produced commercially by decarbonylation of furfural in the presence of a noble metal catalyst (97—100). Nickel or cobalt catalysts have also been reported (101—103) as weU as noncatalytic pyrolysis at high temperature. Furan can also be prepared by decarboxylation of 2-furoic acid this method is usually considered a laboratory procedure. 

By 1984, the palladium-catalyzed aUyhc alkylation reaction had been extensively studied as a method for carbon-carbon bond formation, whereas the synthetic utility of other metal catalysts was largely unexplored [1, 2]. Hence, prior to this period rhodium s abihty to catalyze this transformation was cited in only a single reference, which described it as being poor by comparison with the analogous palladium-catalyzed version [6]. Nonetheless, Yamamoto and Tsuji independently described the first rhodium-catalyzed decarboxylation of allylic phenyl carbonates and the intramolecular decarboxylative aUylation of aUyl y9-keto carboxylates respectively [7, 8]. These findings undoubtedly laid the groundwork for Tsuji s seminal work on the regiospecific rho-... 

A survey for natural benzoic acid was carried out by Nagayama et al [28] and further surveys for benzene in fruits, retail fruit juices, fruit drinks and soft drinks were carried out by the Canadian Health Protection Branch [29] and by the FDA in foods [30]. Decarboxylation of benzoic acid in the presence of ascorbic acid and a transition metal catalyst has been reported to yield benzene [31]. Studies on benzene formation in beverages at the National Laboratory of Food Drugs in China [32] showed that ascorbic, sodium benzoate and hydrogen peroxide increase benzene formation initially, but when a certain concentration was reached, the effect was reversed ethanol and Fe " ions inhibited benzene formation. 

Gardner, L.K. and Lawrence, G.D., Benzene production from decarboxylation of benzoic acid in the presence of ascorbic acid and a transition-metal catalyst, J. Agile. Food Chem., 41 (5), 693-695, 1993. 

The benzyloxycarbonyl group can also be removed by treatment with H2 in the presence of a transition metal catalyst (hydrogenolysis. Section 21.5C). In hydro-genolysis of a Z-protecting group, one product is toluene. The other is a carbamic acid, which undergoes spontaneous decarboxylation to give carbon dioxide and the unprotected peptide. 

In Chapter 3, the authors present a general overview on decarboxylative coupling reactions catalyzed by transition-metal catalysts. This is actually a very short chapter. 

Methodologies for the selective arylation of indoles at the C3 position are largely limited to couplings of free indole with bromoarenes. Larrosa et al. employed a decarboxylative functionalization approach to selectively C3-arylate Af-pivaloylindole (94) with electron-poor benzoic acids to afford 95A-C in good yields one of the reasons that this method is attractive is because CO2 is the sole waste by-product (Scheme 10.32). The authors proposed a mechanism based on two catalytic cycles linked by the transmetallation of an arylsilver species to palladium in which the metal catalyst is responsible for the C—H activation and reductive elimination steps and the silver salts perform the decarboxylative activation step. 

Le N6tre, J., Witte-van Dijk, S.C.M., van Haveren, J., Scott, E.L. et al (2014) Synthesis of bio-based methacrylic acid by decarboxylation of itaconic acid and citric acid catalyzed by solid transition-metal catalysts. ChemSusChem, 7, 2712-2720. 

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)... 

Despite its synthetic importance, the mechanism of the copper-quinoline method has been studied very little, but it has been shown that the actual catalyst is cuprous ion. In fact, the reaction proceeds much faster if the acid is heated in quinoline with cuprous oxide instead of copper, provided that atmospheric oxygen is rigorously excluded. A mechanism has been suggested in which it is the cuprous salt of the acid that actually undergoes the decarboxylation. It has been shown that cuprous salts of aromatic acids are easily decarboxylated by heating in quinoline and that arylcopper compounds are intermediates that can be isolated in some cases. Metallic silver has been used in place of copper, with higher yields. ... 

A reaction mechanism with Fe304 as catalyst has been proposed [68], in agreement with previous work concerning decarboxylation of acids in the presence of a metal oxide [83]. After the transient formation of iron(II) and iron(III) carboxylates from the diacid and Fe304 (with elimination of water), the thermal decarboxylation of these salts should give the cyclic ketone and regeneration of the catalyst. 

In this context we postulated that the shift reaction might proceed catalytically according to a hypothetical cycle such as Scheme I. There are four key steps in Scheme I a) nucleophilic attack of hydroxide or water on coordinated CO to give a hydroxycarbonyl complex, b) decarboxylation to give the metal hydride, c) reductive elimination of H2 from the hydride and d) coordination of new CO. In addition, there are several potentially crucial protonation/deprotonation equilibria involving metal hydrides or the hydroxycarbonyl. The mechanistic details have been worked out (but only incompletely) for a couple of the alkaline solution WGSR homogeneous catalysts. In these cases,... 

Ruthenium(III) catalyses the oxidative decarboxylation of butanoic and 2-methylpropanoic acid in aqueous sulfuric acid. ° Studies of alkaline earth (Ba, Sr) metal alkoxides in amide ethanolysis and of alkali metal alkoxide clusters as highly effective transesterification catalysts were covered earlier. Kinetic studies of the ethanolysis of 5-nitroquinol-8-yl benzoate (228) in the presence of lithium, sodium, or potassium ethoxide revealed that the highest catalytic activity is observed with Na+.iio... 

In the case of oxide catalysts or alkali metal-doped oxide catalysts, basic surface sites can be generated by decarboxylation of a surface metal carbonate exchange of hydroxyl hydrogen ions by electropositive cations thermal dehydroxylation of the catalyst surface condensation of alkali metal particles on the surface and reaction of an alkali metal with an anion vacancy (AV) to give centers (e.g., Na + AV — Na + e ). 

The most important apphcation of this metal is as control rod material for shielding in nuclear power reactors. Its thermal neutron absorption cross section is 46,000 bams. Other uses are in thermoelectric generating devices, as a thermoionic emitter, in yttrium-iron garnets in microwave filters to detect low intensity signals, as an activator in many phosphors, for deoxidation of molten titanium, and as a catalyst. Catalytic apphcations include decarboxylation of oxaloacetic acid conversion of ortho- to para-hydrogen and polymerization of ethylene. 

Since aluminum ion is a strong catalyst, no valence change of the metal ion is involved in the reaction. The following observations of the decarboxylations catalyzed by ferric ion are strongly indicative of the above process. 

Based on the conventional analysis of the mechanism of decarboxylation of thiamin-derived intermediates, there is no role for a catalyst in the carbon-carbon bond-breaking step of this reaction. The thiazolium nitrogen is at its maximum electron deficiency with no available coordination sites. Ultimately, there is no place for a proton or other cation to position itself in order to promote the reaction by stabilizing a transition state that resembles the product of the reaction. Since there is no role for an acid, base, or metal to accelerate the decarboxylation of these intermediates by stabilizing the transition state for C-C bond-breaking, the means by which this could be achieved became a source of interest and speculation. 

The thermal decarboxylation of acids over a metal oxide catalyst (Expts 5.92 and 5.93). 

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