Phosphine ligands, ethylene

The reaction of methyl propionate and formaldehyde in the gas phase proceeds with reasonable selectivity to MMA and MAA (ca 90%), but with conversions of only 30%. A variety of catalysts such as V—Sb on siUca-alumina (109), P—Zr, Al, boron oxide (110), and supported Fe—P (111) have been used. Methjial (dimethoxymethane) or methanol itself may be used in place of formaldehyde and often result in improved yields. Methyl propionate may be prepared in excellent yield by the reaction of ethylene and carbon monoxide in methanol over a mthenium acetylacetonate catalyst or by utilizing a palladium—phosphine ligand catalyst (112,113). 

Coordination-catalyzed ethylene oligomerization into n-a-olefins. The synthesis of homologous, even-numbered, linear a-olefins can also be performed by oligomerization of ethylene with the aid of homogeneous transition metal complex catalysts [26]. Such a soluble complex catalyst is formed by reaction of, say, a zero-valent nickel compound with a tertiary phosphine ligand. A typical Ni catalyst for the ethylene oligomerization is manufactured from cyclo-octadienyl nickel(O) and diphenylphosphinoacetic ester ... 

Table 4.7. Poly(ethylene oxide)-substituted phosphine ligands in rhodium-catalyzed...

Propionic Acid. The Reppe reaction is used to transform ethylene, carbon monoxide, and water to propionic acid.82 [Ni(CO)4] is formed in situ from nickel salts under reaction conditions (270-320°C, 200-240 atm). The addition of halogens and phosphine ligands allows milder reaction conditions (Halcon process, 170-225°C, 10-35 atm). Propionic acid yields are around 95%. 

Another simple oligomerization is the dimerization of propylene. Because of the formation of a relatively less stable branched alkylaluminum intermediate, displacement reaction is more efficient than in the case of ethylene, resulting in almost exclusive formation of dimers. All possible C6 alkene isomers are formed with 2-methyl-1-pentene as the main product and only minor amounts of hexenes. Dimerization at lower temperature can be achieved with a number of transition-metal complexes, although selectivity to 2-methyl-1-pentene is lower. Nickel complexes, for example, when applied with aluminum alkyls and a Lewis acid (usually EtAlCl2), form catalysts that are active at slightly above room temperature. Selectivity can be affected by catalyst composition addition of phosphine ligands brings about an increase in the yield of 2,3-dimethylbutenes (mainly 2,3-dimethyl-1-butene). 

Palladium catalysts with simple monodentate phosphine ligands (e.g. PPh3) can catalyze the methoxycarbonylation of ethylene. However, the Lucite process employs a bulky diphosphine, 1,2-( Bu2PCH2)2CgH4, and is highly active and selective under quite mild conditions (10 bar/80°C). Two alternative catalytic cycles are possible, based either upon a palladium hydride or a palladium methoxide complex (Figure 9), and mechanistic and spectroscopic studies indicate that the hydride cycle is dominant. The alkene and CO insertion steps are the same as those in the Pd-catalyzed co-polymerisation of CO and alkenes to polyketones (Section 4.4). 

Other means of immobilizing ferrocenyl phosphine ligands inclnde covalent see Covalent Bonds) attachment to either sihca gel see Silica) or solid snpport poly(ethylene glycol) (TantaGel), or confinement within mesoporons MCM-41. " ... 

Fig. 2. Comparison of the Pt coordination spheres in [(C6H5)3P]2Pt(ir, -C2H4) (upper) (JO) and [(C6Hs)jPl2Pt( n -C6o)-(C4HgO) (lower). The atoms PI, P2, Ptl, Cl, and C2 are coplanar to within 0.02 A. The phosphine ligands in the Cgo complex are bent back more than in the ethylene complex, which may reflect the greater steric bulk of versus ethylene.

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