Homogeneous catalysis insertion

Much of the recent interest in insertion reactions undeniably stems from the emphasis placed on development of homogeneous catalysis as a rational discipline. One or more insertion is involved in such catalytic processes as the hydroformylation (31) or the polymerization of olefins 26, 75) and isocyanides 244). In addition, many insertion reactions have been successfully employed in organic and organometallic synthesis. The research in this general area has helped systematize a large body of previously unrelated facts and opened new areas of chemistry for investigation. Heck 114) and Lappert and Prokai 161) provide a comprehensive compilation and a systematic discussion of a wide variety of insertion reactions in two relatively recent (1965 and 1967) reviews. 

Insertion of carbon monoxide and alkenes into metal-carbon bonds is one of the most important reaction steps in homogeneous catalysis. It has been found for insertion processes of platinum [16] that the relative positions of the hydrocarbyl group and the unsaturated fragment must be cis in the reacting complex [17], The second issue concerns the stereochemical course of the reaction, insertion versus migration as discussed in Chapter 2.2. 

Electrochemical reductions of CO2 at a number of metal electrodes have been reported [12, 65, 66]. CO has been identified as the principal product for Ag and Au electrodes in aqueous bicarbonate solutions at current densities of 5.5 mA cm [67]. Different mechanisms for the formation of CO on metal electrodes have been proposed. It has been demonstrated for Au electrodes that the rate of CO production is proportional to the partial pressure of CO2. This is similar to the results observed for the formation of CO2 adducts of homogeneous catalysts discussed earlier. There are also a number of spectroscopic studies of CO2 bound to metal surfaces [68-70], and the formation of strongly bound CO from CO2 on Pt electrodes [71]. These results are consistent with the mechanism proposed for the reduction of CO2 to CO by homogeneous complexes described earlier and shown in Sch. 2. Alternative mechanistic pathways for the formation of CO on metal electrodes have proposed the formation of M—COOH species by (1) insertion of CO2 into M—H bonds on the surface or (2) by outer-sphere electron transfer to CO2 followed by protonation to form a COOH radical and then adsorption of the neutral radical [12]. Certainly, protonation of adsorbed CO2 by a proton on the surface or in solution would be reasonable. However, insertion of CO2 into a surface hydride would seem unlikely based on precedents in homogeneous catalysis. CO2 insertion into transition metal hydrides complexes invariably leads to formation of formate complexes in which C—H bonds rather than O—H bonds have been formed, as discussed in the next section. 

As already indicated in the discussion on formyl formation, the transition state on a metal surface may be closely related to that found in the organometallic complexes used in homogeneous catalysis. The p orbital stabilizing interactions have also been shown to play an important role in insertion reactions occurring in organic-metallic complexes [84,85]. It explains, for instance, the higher activa-... 

We then focus on the Pichler-Schulz CO insertion mechanism (39). This reaction has been much less investigated than the carbide mechanism. We recognize that in homogeneous catalysis, alkene hydroformyla-tion has been investigated extensively it appears that hydroformylation is much more difficult on metallic surfaces than in the presence of mononuclear cationic metal complexes (40). 

Whereas the CO insertion reaction has been investigated extensively in organometallic chemistry and homogeneous catalysis, there are only few first-principles investigations available for this reaction on transition metal surfaces (32,33,60). 

The CO insertion mechanisms of the Pichlcr-SchuU type have been largely influenced by principles of homogeneous catalysis and are related to the mechanism proposed for the hydroformylation reaction (sec Mlydroformylation this volume. 

Insertion of CO and Isonitriles into Pd C Bonds (See Carbonylation Processes by Homogeneous Catalysis)... 

Homogeneous catalysis by transition metals is one of the fastest growing fields of chemistry. No doubt much of the interest stems from the potential commercial importance of these reactions and for that reason most of the earlier work was concerned with the preparative aspects of homogeneous catalysis. However more recently a considerable amount of mechanistic work has been carried out on systems which are amenable to this type of Study and we now have a fair understanding of two of the basic reactions of homogeneous catalysis, the oxidative addition (31, 38, 39, 80, 276, 277) and insertion reactions (168, 293, 294). 

The great importance of group VIII complexes in homogeneous catalysis has focused more attention on their insertion chemistry, but the number of authenticated insertions of monoalkenes into M-H bonds is small. Insertions of dienes and allenes are more numerous because of the greater thermodynamic stability of the product xt-allyl complexes. Some examples of insertions of complexes of group VIII are listed in Table... 

Homogeneous catalysis by redox metals is also known for nonelectro-chemical processes. Thus, ethylene is oxidized to acetaldehyde in the Wacker process in aqueous solutions containing Pd " (504). Apart from complex formation and insertion (505), ionic oxidation and reduction may take place. It is noteworthy that palladium oxidation to form ions that act as homogeneous catalysts has been suggested as an important step in ethylene electrooxidation on solid palladium electrocatalysts 28, 29). 

Reactions (98) and (100) could be considered analogous to insertion reactions in homogeneous catalysis. Palladium participates in insertion reactions... 

One important class of cluster catalysts are dimetallic catalysts such as Rh2(carboxylate)4, which effect additions and insertions of carbenoids derived from diazoalkanes. This class is not included below because the strict definition of a cluster as containing three or more metal atoms has been adopted. This area of homogeneous catalysis has been reviewed recently " and is treated in the current edition concerning transition metals in organic synthesis. 

In the following, we turn our attention to reactivities which involve cleavage of the L M-H bond, inducing hydrogen or hydride transfer reactions. These processes can normally be viewed as insertion reactions into the metal-hydride bond and represent valuable elementary steps of homogeneous catalysis. It is very important to note at this point that such insertions do not require precoordination of the inserting substrate. 

Homogeneous catalysis with defined soluble transition metal complexes as catalysts has become one of the most effective means of transforming simple olefins into more valuable materials. The technically important hydroformylation of olefins to aldehydes or alcohols the Wacker process the dimerization of propylene to linear hexenes the oligomerization of ethylene to linear a-olefins are only a few examples. A feature common to all these processes is the insertion of a substrate olefin molecule, which is coordinatively bonded to the transition metal center M, into a metal-carbon or metal-hydrogen bond present at the same center ... 

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