Hydroesterification mechanism

In presence of a mild base such as Na2C03 or NaOAc, a hydroesterification to furanones is possible (equation 89).437 RhCl3-3H20 or a binary carbonyl was used as catalyst. Indanont derivatives were obtained as by-products. The mechanism probably involves the formation of ar alkoxycarbonyl intermediate (Scheme 29). 

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... 

There are two mechanisms that have been proposed for hydroesterification the hydride mechanism and the alkoxy mechanism. Here, the alkoxy mechanism will be shown in detail (Scheme 22). 

Especially noteworthy is the field of asymmetric catalysis. Asymmetric catalytic reactions with transition metal complexes are used advantageously for hydrogenation, cyclization, codimerization, alkylation, epoxidation, hydroformylation, hydroesterification, hydrosilylation, hydrocyanation, and isomerization. In many cases, even higher regio- and stereoselectivities are required. Fundamental investigations of the mechanism of chirality transfer are also of interest. New chiral ligands that are suitable for catalytic processes are needed. 

One can also envision that the quinone could trap the hydridopalladium species resulting from the B-hydrogen elimination that releases the oxidized organic product (Equation 16.129). Insertion of quinone into the palladium hydride would form an enolate that would tautomerize to the phenoxide complex. Protonation with the reagent containing an 0-H or N-H bond would generate the free hydroquinone and Pd(II). As noted in Chapter 17 on carbonylation, quinone has been used as an additive with this mechanism in mind to prevent tire Pd(II) hydroesterification catalysts from undergoing reduction to palladium(O). ... 

Like many carbonylation processes, the hydrocarboxylation and hydroesterification reactions were first reported by Reppe. These first reactions involved the hydrocarboxylation of alkynes. These reactions were conducted with nickel carbonyl as catalyst and occurred with very low turnover numbers. Hydrocarboxylation and hydroesterification have now been studied extensively in both academic and industrial laboratories. As a result of these investigations, commercialization of this chemistry as part of new industrial processes has occurred, and the mechanism of these processes is now generally accepted. This section of Chapter 17 presents the scope and industrial applications of hydrocarboxylation and hydroesterification, the types of catalysts that have been used for these processes, and the elementary steps that constitute the catalytic cycle for olefin and alkyne hydroesterification. 

Two mechanisms for the hydroesterification of alkenes have been considered. One pathway—the "alkoxide cycle"— begins with the insertion of CO into a metal alkoxide (Scheme 17.18) and the other—"the hydride cycle"— begins with the insertion of an alkene into a metal hydride (Scheme 17.19). The relative importance of the different pathways depends on the identity of the dative ligand. However, hydroesterification of ethylene with the bis(di-ferf-butylphosphinomethyl)benzene ligand is now generally accepted to occur through a palladium hydride. The mechanism of the hydroesterification of alkynes is less established, but is likely to occur by a sequence that shares some steps with the mechanism for the hydroesterifcation of alkenes. 

Replacing hydrogen by water yields carboxylic adds and the modified readion is named hydrocarboxylation (Equation 7.2). Another example of synthesis taking place with very similar mechanism is hydroesterification (also termed hydroalkoxy-carbonylation) that uses alcohol as hydrogen source, yielding esters as produds (Equation 7.3). 

Seayad A, Jayasree S, Damodaran K, Toniolo L, Chaudhari RV. On the mechanism of hydroesterification of styrene using an in situ-formed cationic palladium complex. J. Org. Chem. 2000 601 100-107. 

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