Homogeneous transition metal complex

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 ... 

A large number of homogeneous transition-metal complexes have been reported as catalysts for the stereoselective hydrogenation of alkynes, although the... 

Catalysis in liquid-liquid biphasic systems has developed recently into a subject of great practical interest because it provides an attractive solution to the problems of separation of catalysts from products and of catalyst recycle in homogeneous transition metal complex catalysis. Two-phase systems consist of two immiscible solvents, e.g., an aqueous phase or another polar phase containing the catalyst and an organic phase containing the products. The reaction is homogeneous, and the recovery of the catalyst is facilitated by simple phase separation. 

Several reports of catalysis of the decomposition of formic acid involving homogeneous transition metal complexes and proceeding by means of metalloformate intermediates have recently appeared in the literature. For example, Rh(C6H4PPh2)(PPh3)2 (8) catalyzes the decomposition of formic acid to C02 and H2 via the intermediacy of the product of oxidative-addition of HCOOH, Rh(HC02)(PPh3)3 (56). -Elimination of the hydride from the... 

This section will review some recent work in the areas of catalytic and stoichiometric reduction ofC02, as well as incorporation of carbon dioxide into organic compounds, promoted by homogeneous transition metal complexes. 

Many heterogeneous catalysts have been reported in the past to be prepared by anchoring or grafting, processes whereby stable, covalent bonds are formed between an homogeneous transition metal complex and an inert polymer or inorganic support [1-4] The aim was to combine the potential versatility and selectivity of homogeneous catalysts with the practical advantages of a solid material [5]... 

Ionic liquids can be used as replacements for many volatile conventional solvents in chemical processes see Table A-14 in the Appendix. Because of their extraordinary properties, room temperature ionic liquids have already found application as solvents for many synthetic and catalytic reactions, for example nucleophilic substitution reactions [899], Diels-Alder cycloaddition reactions [900, 901], Friedel-Crafts alkylation and acylation reactions [902, 903], as well as palladium-catalyzed Heck vinylations of haloarenes [904]. They are also solvents of choice for homogeneous transition metal complex catalyzed hydrogenation, isomerization, and hydroformylation [905], as well as dimerization and oligomerization reactions of alkenes [906, 907]. The ions of liquid salts are often poorly coordinating, which prevents deactivation of the catalysts. 

Many of the complexes discussed in the previous sections are catalysts for alkyne oligomerization. In fact, alkyne dimerization and trimerization (see Cyclodimerization -tri-merization Reactions) at a cobalt center is recognized as one of the most synthetically useful catalytic reactions mediated by a homogeneous transition metal complex. The cobalt complexes most useful and extensively studied are CpCoL2, where L is CO, alkene, diene, or phosphine. The complex types... 

On the other hand, several homogeneous transition metal complexes such as Fe(CO)5, FeH(CO)4, Ru3(CO)i2, [Ru(bipy)2(CO)a]-, FeH2Ru3(CO)i3, K[Ru(H-EDTA)-(CO)[, [Rh(CO)2l2], and Pt[P(i-Pr)3]3, have been shown to catalyze the reaction at low temperature [108a[. Among them, ruthenium complexes are very efficient catalysts, and this reaction is used to reduce organic compounds without using molecular hydrogen. 

In addition to the advantages associated with immobilizing homogeneous transition metal complexes on polymers for catalytic reactions, there are certain disadvantages and unresolved problems. One problem characteristic of all the chemistry associated with surface modified materials is the difficulty in determining what the actual chemical structures present on the heterogeneous... 

Direct use of the carbon-hydrogen bond in organic synthesis with the aid of the homogeneous transition metal complexes has been the subject of recent interest. This review surveys some of the recent advances in the field of the transition metal-catalyzed functionalization of carbon-hydrogen bonds. 

Metallacyclic complexes play an important role as reactive intermediates in catalytic cycles initiated by homogeneous transition-metal complexes. Thus, metallacyclobutanes are discussed as intermediates in alkene metathesis, isomerization of strained cyclopropane compounds and many other reactions. On the other hand, numerous examples of isolable me-tallacyclobutane complexes have been reported. These can be formed by different routes such as carbon-carbon bond cleavage of cyclopropane compounds (A), cyclometallation via C — H bond cleavage (B), nucleophilic addition to allyl complexes (C), rearrangement of metallacyc-lopentanes (D) or transmetalation of 1,3-dimetallalated carbon chains (E). ... 

The topic of this chapter is enantioselective hydrogenation over chiral or chirally modified solid catalysts. Diastereoselective hydrogenation of chiral compounds and asymmetric hydrogenation with heterogenized (supported, embedded) homogeneous transition metal complexes will not be discussed. 

An enolate formed from an unsymmetric ketone can be of two types the less electronegative (usually more substituted) side is deprotonated preferentially by many homogeneous transition metal complexes with N-donor or halide ligands, " in agreement with local hard-soft acid-base concepts. But steric hindrance plays a greater role with heterogeneous catalysts, to such an extent that the less substituted side is often deprotonated preferentially. ... 

Attaching homogeneous transition metal complexes became an important discipline in the early 1970s. In addition to the use of phosphine-derived polymers, there are published accounts using virtually every conceivable type of attached ligand. 

A second interesting feature of the first wave of research in this area was that very few oxidation systems were investigated. This was partly because fewer well characterised homogeneous transition metal complex catalysts were known and certainly their chemistry was not well understood and secondly because their was an implicit belief that polymers would be unsuitable as supports in oxidation reactions because of their own intrinsic thermo-oxidative instability. The situation was reviewed in 1988 [133]. 

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