Complexes homogeneous catalysis

Keywords H-P addition Unsaturated carbon linkage Transition metal complex Homogeneous catalysis... 

Our understanding of organic reactions catalyzed by soluble metal complexes ( homogeneous catalysis ) is based on the properties and stoichiometric reactions of organometallic complexes, defined as molecules containing metal-carbon bonds. Significant aspects are summarized below, but for details the reader is recommended to one of the excellent texts cited at the end of this Appendix. 

Dr. Creutz s research interests include kinetics and mechanisms of ground and excited-state reactions of transition metal complexes, homogeneous catalysis in water, and charge transfer processes in nanoscale clusters. 

A concise summary of chemistry of technologically important reactions catalysed by organometallic complexes in solution. Cornils B and Herrmann W A (eds) 1996 Applied Homogeneous Catalysis with Organometallio Compounds (Weinheim VCH) A two-volume, multiauthored account with emphasis on industrial applications. 

Khan, M. M. T. 1974, Homogeneous Catalysis by Metal Complexes, Vol. II, Activation of Alkenes and Alkynes, Academic Press New York - London... 

G. W. ParshaH, Homogeneous Catalysis The applications and Chemistry of Catalysis by Soluble Transition Metal Complexes,Johm. Wiley Sons, Inc., New York, 1980, 240 pp. An excellent treatment of catalysis by coordination compounds. 

Hydrido complexes of all three elements, and covering a range of formal oxidation states, are important because of their roles in homogeneous catalysis either as the catalysts themselves or as intermediates in the catalytic cycles. 

Applied Homogeneous Catalysis with Organometallic Complexes, (B. Cornils and W. A. Herrmann eds.), VCH, Weinheim, 1996. 

Many transition metal complexes dissolve readily in ionic liquids, which enables their use as solvents for transition metal catalysis. Sufficient solubility for a wide range of catalyst complexes is an obvious, but not trivial, prerequisite for a versatile solvent for homogenous catalysis. Some of the other approaches to the replacement of traditional volatile organic solvents by greener alternatives in transition metal catalysis, namely the use of supercritical CO2 or perfluorinated solvents, very often suffer from low catalyst solubility. This limitation is usually overcome by use of special ligand systems, which have to be synthesized prior to the catalytic reaction. 

Since no special ligand design is usually required to dissolve transition metal complexes in ionic liquids, the application of ionic ligands can be an extremely useful tool with which to immobilize the catalyst in the ionic medium. In applications in which the ionic catalyst layer is intensively extracted with a non-miscible solvent (i.e., under the conditions of biphasic catalysis or during product recovery by extraction) it is important to ensure that the amount of catalyst washed from the ionic liquid is extremely low. Full immobilization of the (often quite expensive) transition metal catalyst, combined with the possibility of recycling it, is usually a crucial criterion for the large-scale use of homogeneous catalysis (for more details see Section 5.3.5). 

In comparison to heterogeneous catalyzed reactions, homogeneous catalysis offers several important advantages. The catalyst complex is usually well defined and can be rationally optimized by ligand modification. Every metal center can be active in the reaction. The reaction conditions are usually much milder (T usually < 200 °C), and selectivities are often much higher than with heterogeneous catalysts. 

These advantages notwithstanding, the proportion of homogeneous catalyzed reactions in industrial chemistry is still quite low. The main reason for this is the difficulty in separating the homogeneously dissolved catalyst from the products and by-products after the reaction. Since the transition metal complexes used in homogeneous catalysis are usually quite expensive, complete catalyst recovery is crucial in a commercial situation. 

The past fifteen years have seen evidence of great interest in homogeneous catalysis, particularly by transition metal complexes in solution predictions were made that many heterogeneous processes would be replaced by more efficient homogeneous ones. There are two motives in these changes—first, intellectual curiosity and the belief that we can define the active center with... 

A discussion of the different types of solute-solute and solute-solvent interactions acting in homogeneous catalysis by transition metal complexes. E. Cesarotti, R. Ugo and L. Kapan, Coord. Chem. Rev., 1982,43, 275-298 (47). 

Trimethylphosphane complexes of nickel, cobalt and iron — model compounds for homogeneous catalysis. H. F. Klein, Angew. Chem., Int. Ed. Engl., 1980,19, 362-375 (108). 

For a general account of transition-metal-NHC complexes in homogeneous catalysis, see Yong BS, Nolan SP (2003) Chemtracts 16 205... 

Brown JM, Chalnoer PA (1983) In Pignolet NH (ed) Homogeneous catalysis with metal phosphine complexes. Plenum, New York, chap 4... 

Pignolel. 1.. H. Homogeneous Catalysis with Metal Phosphine Complexes Plenum New York. 

J. Halpem Developments in homogeneous catalysis [using ruthenium complexes], pp. 146-162 (65). 

As explained in Chapter 1, catalytic reactions occur when the reacting species are associated with the catalyst. In heterogeneous catalysis this happens at a surface, in homogeneous catalysis in a complex formed with the catalyst molecule. In terms of kinetics, the catalyst must be included as a participating species that leaves the reaction unaltered, as indicated schematically in Fig. 2.7, which shows the simplest conceivable catalytic cycle. We will investigate the kinetics of this simple two-step mech-... 

Such a broad range of classical elementary reactions of homogeneous catalysis with metal complexes, that can be facilitated by photons, make illumination of reaction solution a very useful instrument for substantial increase of the possibilities of homogeneous metal complex catalysis in organic synthesis. Particular examples of light-assisted syntheses will be given in section 3. 

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