Hydrocarboxylation of alkenes

As a unique reaction of Pd(II), the oxidative carbonylation of alkenes is possible with Pd(ll) salts. Oxidative carbonylation is mechanistically different from the hydrocarboxylation of alkenes catalyzed by Pd(0), which is treated in Chapter 4, Section 7.1. The oxidative carbonylation in alcohol can be understood in the following way. The reaction starts by the formation of the alkoxy-carbonylpalladium 218. Carbopalladation of alkene (alkene insertion) with 218 gives 219. Then elimination of /3-hydrogen of this intermediate 219 proceeds to... 

The related field involving the hydrocarboxylation of alkenes is also under investigation11481, not least because of its potential importance in the synthesis of NSAI drugs. An indirect way to the latter compounds involves the hydro-vinylation of alkenes. For example catalysis of the reaction of ethylene with 2-methoxy-6-vinylnaphthalene at 70°C using (allylNiBr)2 and binaphthyl (63)... 

Non-oxidative hydrocarboxylation of alkenes to carboxylic acids with CO and H20 is catalyzed by palladium complexes such as PdCl2(PhCN)2 or PdCl2(PPh3)2, and a-methyl acids predominate in the presence of HC1.374,443 A recent improvement of this reaction consisted of the use of a PdCl2/CuCl2/HCl catalyst under oxidative conditions.377 Almost quantitative yields of a-methyl carboxylic acids and dicarboxylic acids were obtained from terminal alkenes and terminal dialkenes respectively, at room temperature and atmospheric pressure (equation 174).377... 

The final step in the catalytic cycle is the cleavage of the metal-alkyl bond with acid, which must take place faster in the hydrocarboxylation of alkenes than -elimination. 

The HCo(CO)4-catalyzed hydrocarboxylation of alkenes has also been known for a long time. The mechanism is analogous to that presented for hydroformyla-tion (Scheme 1), except that H2O is used instead of H2. Hydrocarboxylation is generally slower than hydro-formylation, and it is believed that the concentrations of the intermediate species are quite low relative to those seen for hydroformylation. Pyridine has a rateenhancing effect that is believed to be due to the facile cleavage of the (acyl)Co(CO)4 intermediate. This reaction forms [pyridine-acyl] + [Co(CO)4] , which is more rapidly hydrolyzed by water to form the product carboxylic acid and HCo(CO)4. 

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). 

Chiral Ligand for Asymmetric Catalysts. (5)-(+)- and (R)-(—)-BNPPA are efficient chiral ligands for the Pd-catalyzed hydrocarboxylation of alkenes. Naproxen can be obtained re-gioselectively in 91% ee (eq 2). 

The acid-catalyzed hydrocarboxylation of alkenes (the Koch reaction) can be performed in a number of ways. In one method, the alkene is treated with carbon monoxide and water at 100-350°C and 500-1000-atm pressure with a mineral acid catalyst. However, the reaction can also be performed under milder conditions. If the alkene is first treated with CO and catalyst and then water added, the reaction can be accomplished at 0-50°C and 1-100 atm. If formic acid is used as the source of both the CO and the water, the reaction can be carried out at room temperature and atmospheric pressure.The formic acid procedure is called the Koch-Haaf reaction (the Koch-Haaf reaction can also be applied to alcohols, see 10-77). Nearly all alkenes can be hydrocarboxylated by one or more of these procedures. However, conjugated dienes are polymerized instead. Hydrocarboxylation can also be accomplished under mild conditions (160°C and 50 atm) by the use of nickel carbonyl as catalyst. Acid catalysts are used along with the nickel carbonyl, but basic catalysts can also be employed. Other metallic salts and complexes can be used, sometimes with variations in the reaction procedure, including palladium, platinum, and rhodium catalysts. The Ni(CO)4-catalyzed oxidative carbonylation with CO and water as a nucleophile is often called Reppe carbonylationP The toxic nature of nickel... 

More effective regio- and enantiosclcctivc asymmetric synthesis of branched optically active free carboxylic acids via palladium-catalyzed hydrocarboxylation of alkenes is achieved in the presence of (-)-(/ )- or ( + )-(5)-2,2/-(l,r-binaphthyl)phosphoric acid23. 

Additions. Hydrocarboxylation of alkenes" and alkynes with HCOOH under CO is accomplished by Pd catalysis. A similar reaction undergone by a propar-gylic carbonate after an S 2 process leads to an itaconic diester segment.The intermediate is converted into a cyclopentenone when the propargyl carbonate... 

Diphasic hydrocarboxylations of alkenes yield carboxylic acids with a typical linear to branched (n/i) ratio which ranges from 1 to 1.4 (Scheme 5). 

The most widely used catalysts for the carbonylation of alcohols are rhodium or cobalt, the former possessing the advantage that the reaction can ensue under milder conditions. In the Rh catalyzed carbonylation of methanol, the acetic acid selectivity is 99 % at low CO pressure—as low as 1 bar. Several companies have been working on the Rh catalyzed hydrocarboxylation of alkenes, RhCls or Rh(CO)2Cl2 often used as a catalyst. 

Hydrocarboxylation of alkenes or alkynes involves the formal addition of a carboxylic acid O—H bond across a C=C or C=C bond (Scheme 2.25) [63]. In particular, intramolecular hydrocarboxylation provides an atom economical strategy for lactone synthesis. The aforementioned reaction is thermodynamically favorable but there is a large intrinsic kinetic barrier for this type of cydization, thus requiring the addition of a catalyst. Catalysts for hydrocarboxylation typically facilitate addition by alkene or alkyne binding. This process increases the inherent electrophilicity of the C=C and C=C bonds, respectively. Subsequent protonolysis (or 3-H elimination under Pd catalysis) regenerates the catalytic spedes. 

The regioselective hydrocarboxylation of alkenes and methylenecycloalkanes to linear carboxylic acids was also achieved in good yields and selectivity by the use of oxalic acid (in place of formic add) as the source of hydrogen and carbon monoxide, with the catalytic system Pd(OAc)2-dppb-PPh3 at 150 C and 20 atm.t ... 

Nonoxidative hydrocarboxylation of alkenes to carboxylic acids with CO and water catalyzed by Pd catalysts is greatly improved in the presence of oxygen (Scheme 16). Almost quantitative yields of mono- and dicarboxylic adds were obtained from the corresponding terminal alkenes and terminal dialkenes, respectively, under atmospheric pressure at room temperature. ... 

Fatty acids are usually produced industrially by the hydrolysis of triglycerides), with the removal of glycerol. Phospholipids represent another source. Some fatty acids are produced synthetically by hydrocarboxylation of alkenes. 

Catalytic carboxylation of internal alkynes with CO2 has been achieved in the presence of Ni [50] or Cu [51] catalysts, and through a hydrocarboxylative pathway, not involving the intermediacy of a metaUaoxacyclopentenone, but, as previously noted for the hydrocarboxylation of alkenes (see Sect. 5.3), implying the intermediate formation of an M—H catalyticaUy active species, which may easily be generated in the reactimi mixture using diethylzinc [50] or hydrosilanes [51] as hydride transfer agent. 

Figure 2 State-of-the art hydrocarboxylation of alkenes/alkynes with CO2 (A), an early mechanistic proposal for the reductive carboxylation of ethylene (B), and recent kinetic considerations in a putative catalytic cycle (C).

Highlighted in Zhang Y, Riduan SN. Catalytic hydrocarboxylation of alkenes and alkynes with CO2. Angew Chem. 2011 123 6334-6336, Angew Chem Int Ed Engl. 2011 50 6210-6212. 

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