Acid catalyzed, addition hydrocarboxylation

The third reaction was related to the hydrocarboxylation of allenes with C02, catalyzed by a tridendate silyl pincer-type palladium complex [108]. In this reaction, a a-allyl palladium species (via hydropallation of allene) was formed, and its trigonal bipyramidal geometry allowed the facile coordination of C02 (presumably in a side-on fashion) and a following nucleophilic addition to realize the carboxylation of allene (Figure 4.16). This reaction proved to be very attractive not only as a C02 fixation reaction, but also as a general method for the synthesis of Py-unsaturated carboxylic acids. 

One of the first mechanistic proposals for the hydrocarboxylation of alkenes catalyzed by nickel-carbonyl complexes came from Heck in 1963 and is shown in Scheme 24. An alternate possibility suggested by Heck was that HX could add to the alkene, producing an alkyl halide that would then undergo an oxidative addition to the metal center, analogous to the acetic acid mechanism (Scheme 19). Studies of Rh- and Ir-catalyzed hydrocarboxylation reactions have demonstrated that for these metals, the HX addition mechanism, shown in Scheme 24, dominates with ethylene or other short-chain alkene substrates. Once again, HI is the best promoter for this catalytic reaction as long as there are not any other ligands present that are susceptible to acid attack (e g. phosphines). 

In hydrocarboxylations, as in the 0x0 process, selectivity of linear versus branched products is an important issue, because (in general) mixtures of isomeric carboxylic acids are obtained, owing not only the occurrence of both Markovni-kov and anti-Markovnikov addition of the alkene to the metal hydride, but also to metal-catalyzed alkene isomerization (eq. (2)). In the case of higher olefins, Co2(CO)g as catalyst leads to a number of different carboxylic acid isomers due to the isomerization activity of the catalyst. 

Addition of carbon monoxide and water to an alkene, i.e. hydrocarboxylation, is catalyzed by a variety of transition metal complexes, including [Ni(CO)4], [Co2(CO)s] and [HaPtClg]. Unfortunately this reaction usually leads to mixtures of products due to both metal-catalyzed alkene isomerization and the occurrence of Irath Markownikov and anti-Markownikov addition of the metal hydride intermediate to the alkene. The commercially available zirconium hydride [(C5Hs)2Zr(H)Cl] can be used as a stoichiometric reagent for conversion of alkenes to carboxylic acids under mild conditions (equation 23). In this case the reaction with linear alkenes gives exclusively terminal alkyl complexes even if the alkene double bond is internal. Insertion of CO followed by oxidative hydrolysis then leads to linear carboxylic acids in very good yield. 

Hydrocarboxylation is the formal addition of hydrogen and a carboxylic group to double or triple bonds to form carboxylic acids or their derivatives. It is achieved by transition metal catalyzed conversion of unsaturated substrates with carbon monoxide in the presence of water, alcohols, or other acidic reagents. Ester formation is also called hydroesterification or hydrocarb(o)alkoxylation . The transition metal catalyst precursors are nickel, iron or cobalt carbonyls or salts of nickel, iron, cobalt, rhodium, palladium, platinum, or other metals4 5. 

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