Nucleophilic Attack on a Coordinated Ligand

Upon coordination to a metal center the electronic environment of the ligand obviously undergoes a change. Depending on the extent and nature of this change, the ligand may become susceptible to electrophilic or nucleophilic 

Nucleophilic attack by water on coordinated ethylene, as shown by Reaction 2.12, is the key step in the manufacture of acetaldehyde by the Wacker process (see Chapter 8). In Reaction 2.13 the high oxidation state of titanium makes the coordinated oxygen atom sufficiently electrophilic for it to be attacked by an alkene. As we will see in Chapter 8, this reaction is the basis for the homogeneous catalytic epoxidation of alkenes, using organic hydroperoxides as the oxygen atom donors. 

The last reaction (2.14) has relevance as a model in the base-promoted water gas shift reaction, and is similar to Reaction 2.12. Instead of palladium-coordinated ethylene it is iron-coordinated carbon monoxide that undergoes attack by HO. The extent to which the reactivity of the ligand may be affected 

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Identify reaction steps in Fig. 8.2 that illustrate the concepts of (a) the trans effect, (b) nucleophilic attack on a coordinated ligand, (c) insertion of alkene into an M-H bond, and (d) intramolecular electron transfer. 

These reactions are steps in the synthesis of organometallic complexes, in transformations for organic synthesis, and in the catalytic chemistry of CO. For example, the first synthesis of a carbene complex was achieved by a two-step sequence initiated by nucleophilic attack on a coordinated CO ligand. In addition, the production of CO and fi om... 

Although many more examples of attack on coordinated CO have been published than examples of attack on isonitriles, the attack on isonitrile ligands is well established. The reaction in Equation 11.7 illustrates one example of such a nucleophilic attack on a coordinated isonitrile. The kinetics of this reaction are consistent with the direct nucleophilic attack of the amine on the coordinated isonitrile to form a stable intermediate, which undergoes a subsequent proton transfer to form the observed carbene complex. The resulting electron-rich carbene complex resists further nucleophilic attack. 

Examples of palladium- and rhodium-catalyzed hydroaminations of alkynes are shown in Equations 16.90-16.92 and Table 16.9. The reaction in Equation 16.90 is one of many examples of intramolecular hydroaminations to form indoles that are catalyzed by palladium complexes. The reaction in Equation 16.91 shows earlier versions of this transformation to form pyrroles by the intramolecular hydroamination of amino-substituted propargyl alcohols. More recently, intramolecular hydroaminations of alkynes catalyzed by complexes of rhodium and iridium containing nitrogen donor ligands have been reported, and intermolecular hydroaminations of terminal alkynes at room temperature catalyzed by the combination of a cationic rhodium precursor and tricyclohexylphosphine are known. The latter reaction forms the Markovnikov addition product, as shown in Equation 16.92 and Table 16.9. These reactions catalyzed by rhodium and iridium complexes are presumed to occur by nucleophilic attack on a coordinated alkyne. 

In contrast, spectroscopic and crystal structure analysis indicates that nucleophilic attack of hydride on 72 occurs on the face of the ligand which is coordinated to the metal (Scheme 17). No intermediate species could be detected for this latter reaction. Monitoring of the reduction of the rhenium analog 74 with sodium borohydride indicated the intermediacy of a rhenium formyl complex 75, presumably formed by attack on a coordinated carbon monoxide. Signals for 75 eventually disappear and are replaced by those of the (diene)rhenium product 76 (Scheme 18)95. 

Just like the isoelectronic carbon monoxide, an isocyanide is an excellent ligand to metal ions. The chemistry of metal isocyanide complexes has been reviewed by Singleton and Oosthuizen. Only a few examples will be given here. Insertion of an isocyanide into a metal-carbon bond frequently occurs. It is not always clear whether the key step is electrophilic or nucleophilic attack on the coordinated isocyanide or whether the reaction is concerted. Insertion into metal-carbene and metal-carbyne complexes have been reviewed by Aumann. Coordination to the metal considerably affects the chemistry of the isocyanide. If the metal is electron-donating, as in nitrogenase-like centres, the coordinated isocyanide is apt to electrophilic attack at nitrogen cf. Section III. 

The C-0 and C-N bond in the product of the oxidations with oxygen and nitrogen donors forms by either nucleophilic attack on the coordinated olefin or by insertion of the olefin into a palladium-oxygen or palladium-nitrogen bond. More detailed descriptions of the nucleophilic attack on coordinated ligands were provided in Chapter 11 and a more detailed description of migratory insertions was provided in Chapter 9. These reactions are discussed in the context of the effect of additives on the stereochemistry of the catalytic processes in several earlier sections on the Wacker process. Henry conducted the same stereochemical study for reactions of alcohols with the resolved allylic alcohol in Scheme 16.24 as was conducted for reactions of water. The results of these experiments were similar to those on the reactions of water. ... 

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