Ligand in hydrocarboxylations

Ligand in hydrocarboxylation catalysis 14.6.4.2., 14.6.4.3. Cx2H2gAILi03 LiAlfO-t-butyOjH In hydrogenation catalysis 14.3.4.5. 

Bridging ligand 14, 9.2.1 P(Ph>2C5H4N (2-pyridyldiphenyl-phosphine) Ligand in hydrocarboxylation catalysis 16, 14.6.4.3 C 7H 4N2Ni... 

Platinum also catalyzes hydrocarboxylation, under comparable reaction conditions (240 bar, 80 C). The best ligand in this case is triphenylarsine and a 10-fold excess of tin(II) chloride provides even higher selectivity to the linear ester (entry 4, Table 5).78... 

Asymmetric hydrocarboxylation of styrenes.1 Use of (S)- or (R)-l as a chiral ligand in the palladium-catalyzed hydrocarboxylation of p-isobutylstyrene (2) results in (S)- or (R)-2 (ibuprofen) in 83-84% ee. Similar enantioselectivity obtains in hydrocarboxylation of a 2-vinylnaphthalene to form naproxen. 

Hydrocarboxylation. The cyclic phosphate resolved according to eq 2 can be used as the chiral ligand in the palladium(II) catalyzed asymmetric hydrocarboxylation of arylethylenes. The 1-arylpropanoic acid is obtained regiospecifically with high enan-tioselectivity (91% ee) (eq 4). 

Since the discovery and development of highly efficient Rh catalysts with chiral diphosphites and phosphine-phosphites in the 1990s, the enantioselectivity of asymmetric hydroformylation has reached the equivalent level to that of asymmetric hydrogenation for several substrates. Nevertheless, there still exist substrates that require even further development of more efficient chiral ligands, catalyst systems, and reaction conditions. Diastereoselective hydroformylation is expected to find many applications in the total synthesis of complex natural products as well as the syntheses of biologically active compounds of medicinal and agrochemical interests in the near future. Advances in asymmetric hydrocarboxylation has been much slower than that of asymmetric hydroformylation in spite of its high potential in the syntheses of fine chemicals. 

Styrene characteristically yields the branched acid in the presence of palladium and monodentate phosphine ligands,132 142 and in the [Fe(CO)5]-promoted process.143 Palladium with certain bidentate phosphines, in turn, produces more linear acid.142 Asymmetric hydrocarboxylations with palladium complexes and chiral ligands with enantiomeric excesses up to 84% have been reported.144 145... 

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). 

In the presence of 3-8 mol equiv. of pyridine as ligand (compared with Co) the phenomenon of ligand-accelerated catalysis [8] is observed with higher activity and improved selectivity of the catalyst system [9]. The cobaltcarbonyl/ pyridine catalyst system is applied industrially for the synthesis of higher alkanoic acids, e.g., the hydrocarboxylation of isomers of undecene yields dodecanoic acid with approximately 80% selectivity [10],... 

A variant of this process, studied by DuPont and DSM [32c], includes the hydrocarboxylation (hydroxycarbonylation) of butadiene with carbon monoxide and water this technology offers potential savings in raw material costs. The reaction primarily yields 3-pentenoic acid using a palladium/crotyl chloride catalyst system, with a selectivity of 92%. Further conversion of pentenoic acids by reaction with carbon monoxide and methanol and a palladium/ferrocene/phosphorous ligand catalyst has demonstrated a selectivity to dimethyl adipate of 85% the latter is finally hydrolyzed to AA. The main problem in this reaction is the propensity of pentenoic acid to undergo acid-catalyzed cyclization to y-valerolactone one way to circumvent the problem is to carry out the hydrocarboxylation of pentenoic acid using the y-valerolactone as the solvent. 

Structure 2. Examples of chiral ligands used in Pd-catalyzed asymmetric hydrocarboxylations... 

While early results of asymmetric hydrocarboxylation were considered as promising developments l3, for a long time only a few examples of low-to-medium asymmetric induction were reported. This failure to achieve successful asymmetric hydrocarboxylation is attributed to the fact that high carbon monoxide pressures (359.1 —452.2 bar) are necessary8. Usually palladium ) chloride is used together with Diop-type ligands. This catalyst system, prepared in situ, requires milder reaction conditions (50 °C) than cobalt (140 °C) or other metal catalysts. 

Many other chiral phosphane ligands are used in asymmetric hydrocarboxylation. In the presence of NMDPP and trifluoroacetic acid in methanol carbomethoxylation of vinyl aromatics. in particular styrene, with palladium(0)bis(dibenzylidcncacetone) takes place with marked asymmetric induction (up to 52% cc) and high selectivities towards the branched product (94%)20. With other chiral ligands and other acids only smaller inductions are observed using the same catalytic system. With an increase in carbon monoxide pressure the asymmetric induction decreases. Involvement of the complex PdH(PR3),OCOCF3 is presumed20. 

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