Metal-Complex-Catalyzed Reactions

1 Potential Benefits of Using Supercriticai Fluids in Metai-Compiex-Catalyzed Reactions 

Before discussing individual examples of metal catalysis in SCFs in detail (Sections 4.7.2-4.7.6), it is appropriate to outline briefly some experimental approaches to carrying out a metal-complex-catalyzed SI.  

In the second approach, there is again only one supercritical phase consisting of solvents and reactants, but the metal complex is now insoluble in this medium and the reaction occurs, at the solid-SCF interface (Type II). The insolubility may be an intrinsic property of the metal complex or may arise from anchoring the metal species to an insoluble support. The arguments that make the use of SCFs attractive in these cases are similar to those discussed for classical heterogeneous catalysis (see Chapter 4.8). 

The third possibility comprises a two-phase system made up from a liquid phase and a supercritical fluid. The liquid phase may be considerably swollen by dissolved gas, as is for example observed with mixtures of organic liquids and compressed CO2 [13,14]. This can alter some physical properties, such as the solubility of gases in the liquid phase, to some extent. Type III can be further differentiated into two extremes, depending on the location of the catalyst. In Type Ilia, the catalyst is dissolved in the liquid phase only, whereas the substrates and/or products are mainly in the SCF. Type mb refers to a system with the catalyst separating exclusively in the supercritical phase. Both systems are very similar to those used in conventional biphasic homogeneous catalysis [8]. 

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Jessop PG, Leitner W (1999) Metal-Complex Catalyzed Reactions. In Jessop PG, Leitner W (eds) Chemical Synthesis Using Supercritical Fluids. Wiley, Weinheim,... 

Another example of a bis(oxazoline)-metal complex catalyzed reaction in total synthesis was illustrated by Murai and co-workers in the construction of a precursor... 

Ethers, sulfides, amines, carbonyl compounds, and imines are among the frequently encountered Lewis bases in the ylide formation from such metal carbene complex. The metal carbene in the ylide formation can be divided into stable Fisher carbene complex and unstable reactive metal carbene intermediates. The reaction of the former is thus stoichiometric and the latter is usually a transition metal complex-catalyzed reaction of a-diazocarbonyl compounds. The decomposition of a-diazocarbonyl compounds with catalytic transition metal complex has been the most widely used approach to generate reactive metal carbenes. For compressive reviews, see Refs 1,1a. 

Transition metal complex catalyzed reactions exhibit different outcomes depending on whether they are conducted in water or not. Chao-Jim Li highlighted the greening of chemical reactions, focusing on the use of water as an alternative solvent for reactions involving metals, R = CH2Ph (Li, 1996) (see fig. 6.8). 

Organic reactions are extensively catalyzed by metal complexes. The catalytic cycle of a metal complex-catalyzed reaction is illustrated in Scheme 13. 

The oxo synthesis (hydroformylation), which was discovered by Otto Roelen in 1938 at Ruhrchemie AG, is one of the most important metal-complex-catalyzed reactions of alkenes. Today, the worldwide capacity for oxo products is about 6 million tons/year. 

The reactions of nitric oxide involve either oxidation or reduction (or both simultaneously in disproportionation and decomposition). Except for oxidation, these reactions of nitric oxide require catalysts for them to proceed at significant rates. An important stimulus to studying these catalyzed reactions lies in the environmental hazards posed by oxides of nitrogen, of which NO is the parent member, as discussed at the beginning of this section. Particular attention in the area of metal complex catalyzed reactions has focused in the last five years on the reduction of nitric oxide by carbon monoxide, (113). 

The idea that transition-metal complexes can act as PT agents was presented in some early studies of metal-complex-catalyzed reactions under PTC conditions. Lately, several approaches to the design of bifunctional catalysts have been made. Organometallic complexes capable of effecting phase transfer reactions were prepared for the first time in 1982 [192, 193]. Tertiary phosphines containing polyether substituents react with Pd(PhCN)2Cl2 to afford complexes capable of catalyzing the reduction of bromobenzene with NaH suspended in toluene. 

Such metal complex-catalyzed reactions, notably those involving the dissociation or formation of H—C—C, and C—bonds, proceed through step-wise mechanisms involving discrete organometallic intermediates [i.e., through pathways analogous to Eq. 

These homogeneous metal complex-catalyzed reaction processes have several disadvantages a low reaction rate, required use of high pressure, and involvement of an energy intensive step for catalyst recovery. Immobilization of... 

Figure 4.7-2 The three different types of metal-complex-catalyzed reactions in SCFs. Type I fully homogeneous single-phase reaction. Type II single-phase reaction medium with an insoluble metal complex as the catalyst. Type III two-phase catalysis with...

Metal-Complex-Catalyzed Reactions - HC02H/NEt3 (2 1)... 

Metal-Complex-Catalyzed Reactions 383 Ring opening metathesis polymerization in SCCO2... 

Collectively, these studies illustrate the broad range of possible applications of SCFs in metal-mediated organic synthesis, and the challenge is now to make efficient use of these methodologies. Investigations towards the understanding of coordination chemistry in SCFs (Chapter 4.2) will stimulate the elucidation of metal-complex-catalyzed reactions. However, it should be evident from this chapter that Ae field of complex-catalyzed SFRs is far from being complete, and much remains to be done. The potential of the technique has been hinted at, but many new ways to exploit the special properties of SCFs in metal complex catalysis are still to be discovered. 

The studies of details of transmetalation for the last few decades seem to have reinforced importance of transmetalation in orgnometallic chemistry and its contribution to this and related fields. Chemical Abstracts has added transmetalation to the keyword list in 1995. In this chapter, the author describes the transmetalation of organometallic compounds, both of main group metals and of transition metals, as well as its mechanism and relevance to transition metal complex-catalyzed reactions. 

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