Metal carboxylate mechanism

Concerning the mechanism of metal-catalyzed carboxylations, two basic pathways—the metal hydride and the metal carboxylate mechanisms—gained general acceptance.14 16 118 119122151 The actual mechanism depends on the catalyst used and the reaction conditions particularly in the presence of additives (acids, bases). It seems reasonable to assume that both mechanisms may be operative under appropriate conditions. 

Despite less direct evidence, the metal carboxylate mechanism has also been widely proposed 134 156-159... 

Both mechanisms are predicted to show syn addition of hydride and caiboxylate to the alkene. In the metal hydride mechanism (equation 36) alkene insertion is syn and CO insertion proceeds with retention of configuration at carbon. In the metal carboxylate mechanisms (equation 37) alkene insertion is syn and cleavage of the metal-carbon bond can take place with retention at carbon. The palladium-catalyzed hydroesterification reaction produces the erythro ester from (Z)-3-methyl-2-pentene (equation 38) and the threo ester from ( )-3-methyl-2-pentene (equation 39).w... 

The transition metal carboxylate mechanism (Scheme 10), illustrated with palladium, requires the generation of this type of complex in the initial steps. This can occur by the alcoholysis of dicobalt octacarbonyl (Scheme 6) or by, for example, the reaction of methanol with a palladium(II) carbonyl (equa-... 

By employing anionic techniques, alkyl methacrylate containing block copolymer systems have been synthesized with controlled compositions, predictable molecular weights and narrow molecular weight distributions. Subsequent hydrolysis of the ester functionality to the metal carboxylate or carboxylic acid can be achieved either by potassium superoxide or the acid catalyzed hydrolysis of t-butyl methacrylate blocks. The presence of acid and ion groups has a profound effect on the solution and bulk mechanical behavior of the derived systems. The synthesis and characterization of various substituted styrene and all-acrylic block copolymer precursors with alkyl methacrylates will be discussed. 

Libiszowki J, Kowalski A, Duda A, Penczek S (2002) Kinetics and mechanism of cyclic esters polymerization initiated with covalent metal carboxylates, 5. End-group studies in the model E-caprolactone and L,L-dilactideAin(II) and zinc octoate/butyl alcohol systems. Macromol Chem Phys 203 1694—1701... 

The metal hydride mechanism was first described for the cobalt-carbonyl-catalyzed ester formation by analogy with hydroformylation.152 It was later adapted to carboxylation processes catalyzed by palladium136 153 154 and platinum complexes.137 As in the hydroformylation mechanism, the olefin inserts itself into the... 

The metal carboxylate insertion mechanism has also been demonstrated in the dicobaltoctacarbonyl-catalyzed carbomethoxylation of butadiene to methyl 3-pentenoate.66,72 The reaction of independently synthesized cobalt-carboxylate complex (19) with butadiene (Scheme 8) produced ii3-cobalt complex (20) via the insertion reaction. Reaction of (20) with cobalt hydride gives the product. The pyridine-CO catalyst promotes the reaction of methanol with dicobalt octacarbonyl to give (19) and HCo(CO)4. 

Cobalt is the catalyst of choice for the hydrocarboxylation of butadiene to adipic esters.89 The reaction is carried out in two steps, the first of which yields methyl-3-butenoate. This product can either be isolated or carried on to dimethyl adipate at high temperatures (Scheme 11). The first hydrocarboxylation occurs by the metal carboxylate insertion mechanism (vide supra). 

The acyloxylation of A -alkylsubstituted amides has received considerable attention [70,75,118], from both the mechanistic and the preparative points of view. In electrolytes containing alkali metal carboxylates the direct mechanisms probably operates, whereas in the case where a nitrate salt is the supporting electrolyte an indirect mechanism involving hydrogen abstraction from the A -alkyl group by anodically generated NO3 is indicated. The same mechanistic problem is encountered in side-chain acetoxylation of alkylaromatic compounds in the presence of N03 [123,152,153]. The method for large-scale substitution into A -formyl derivatives, which works so well for methoxylation, fails when applied to acetoxylation, probably because of the acid sensitivity of the products [80]. 

There appears to be, as yet, no general agreement concerning the chemical parameters that control the thermal reactivities of metal carboxylates or the initial step in these reactions. The numerous kinetic studies of these salts have not led to the acceptance of any common pattern of reaction mechanisms. Groups of related reactants, selected to include diverse chemical features which may help major trends to be identified, are surveyed below. 

Many metal carboxylates are prepared in the form of hydrates and water is lost in a lower temperature range than that required for the onset of anion decomposition [86]. The removal of water from the structure must be accompanied by substantial bond redistribution within the anhydrous phase so formed, and this can sometimes lead to structural reorganization within the solid [142]. Nickel salts, prepared in the form of crystalline hydrates, however, show a low degree of lattice order after water removal [116,118,121]. The crystal structures of few dehydrated metal carboxylates are known in any detail, an omission that must be remembered in formulating reaction mechanisms. 

For the decompositions of the metal carboxylates, alternative mechanisms involving breaking of each of the different principal linkages in the R-CO-O-M group have been proposed as rate controlling (reference [40] p.210), in addition to mechanisms based on charge transfer, interface strain and catalysis by an active product. 

A significantly more active catalytic system was recently reported, with a ruthenium complex generated from carboxylic acid 75. This allowed for direct arylations to occur also in apolar solvents likely via a concerted metalation-deprotonation mechanism (CMD) and set the stage for the use of aryl bromides, chlorides, and tosylates as electrophilic substrates (Scheme 9.26) [64],... 

The base-catalysed degradation of the ring of isoxazolium salts is particularly easy, requiring only alkali metal carboxylates to achieve it. The mechanism, illustrated for the acetate-initiated degradation of 2-methyl-5-phenylisoxazolium iodide, involves initial 3-deprotonation with cleavage of the N-O bond subsequent rearrangements lead to an enol acetate which rearranges to a final keto-imide. 

Deeth RJ (2008) General molecular mechanics method for transition metal carboxylates and its applicatimi to the multiple coordination modes in mono- and dinnclear Mn(n) cmnplexes. InOTg Chem 47 6711... 

The catalytic center of RNAP includes the binding site for the 3 -end of RNA and the insertion site for the incoming rNTP. In the nucleotidyl transfer reaction, the 3 -OH group in the sugar ring of the RNA primer reacts with the a-phosphorous atom of a ribonucleoside triphosphate by nucleophilic attack, then the Pa-Oap bond is broken and pyrophosphate (PPj) is released. Thus, a nucleotidyl addition to the RNA primer is achieved. Structural and biochemical data have shown that the active centers of all polymerases share certain common features a pair of metal ions (normally divalent magnesium ions Mg " and three universally conserved carboxylates. The two-metal-ion mechanism for the nucleotidyl transfer reaction... 

The base-catalysed degradation of the ring of isoxazolium salts is particularly easy, requiring only alkali metal carboxylates to achieve it. The mechanism, ... 

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