Coordinated ligands Electrophilic reactivity

The coordination of a ligand to a metal ion results in polarization of the ligand such that its electrophilic character is enhanced. This facet of reactivity has already been mentioned in the previous section, with respect to deprotonation of a coordinated ligand to produce a good nu-... 

In contrast to electrophiles, nucleophiles are expected to react preferentially with n -bonded heteroaldehyde and -ketone ligands. The site of attack is expected to be the carbon atom of the E = C group, -coordinated ligands should be less reactive. 

Paradoxically (in view of their positive charges), metal ions also may catalyze electrophilic attack by promoting loss of a proton from a coordinated ligand or by stabilizing a reactive (e.g., enol) form of the latter. ... 

This chapter, in previous volumes, concentrated on mechanistic studies of the stoichiometric reactions of coordinated a- and tt-hydrocarbons with nucleophiles and electrophiles. In order to provide a more comprehensive overview of the reactivity of coordinated ligands in general, related ligand reactions in classical coordination complexes are now also included. The stereospecificity of such processes and their potential for asymmetric synthesis has continued to attract increasing attention, and it is therefore appropriate to collect them all together in one chapter. There are three subsequent sections. The first is concerned with cobalt(III) complexes, and the second with complexes of other metals. The last section deals with the ligand reactivity of organometallic compounds. 

Although polyenes are typically subject to electrophilic attack in the free state, they are rendered amenable to nucleophilic attack as coordinated ligands, a fascinating reversal in chemical reactivity usually referred to by the German term umpolung . It is important to note that nucleophiles (Nu ) usually add to the external face of an arene ligand (i.e., an exo- attack) and tend to reduce the hapticity of the ligands to which they add (Scheme 11). 

In contrast to the increase in reactivity of hydrocarbons with nucleophiles after coordination to electron-accepting metal centers, an increase in reactivity of unsaturated hydrocarbons with electrophiles is observed upon coordination to particularly electron-ridi metal centers. This contrasting reactivity is shown schematically in Figure 11.3. Olefin complexes of very electron-rich metal centers are best described as metallacyclopropane complexes, as noted in Chapters 1 and 2. As such, the olefin ligands in these complexes contain a large degree of M-C cr-bond character and react with electrophiles. Reactions of electrophiles with coordinated ligands are described in Chapter 12. 

As discussed in Chapter 3, olefins and dienes bind to electron-poor metal centers by a flow of electrons from the olefin iT-system to the metal and from the metal to the olefin t -system. Thus, olefins bound to electron-rich and strongly backbonding metal centers react with protons and electrophiles directly at the metal-carbon bond. However, olefins and dienes coordinated to electron-poor metal centers are less reactive toward electrophiles than those bound to electron-rich metal centers or even free olefins and dienes. However, electron-poor olefin and diene complexes do imdergo reactions with electrophiles at the coordinated ligand by an indirect pathway. This indirect pathway occurs by insertion of the olefin or diene into the bond formed by attack of the electrophile at the metal. 

The reaction of nitrosyl complexes with Na ions is not limited to coordination complexes of particular metal ions. Instead, the reaction seems to be specific for compounds containing nitrosyl ligands exhibiting electrophilic reactivity, i.e. the linear nitrosyls. As discussed above, sodium nitroprusside37 and iridium nitrosyls react with azide ion in the same manner as ruthenium(II) nitrosyls. 

Ferrocen-l,l -diylbismetallacycles are conceptually attractive for the development of bimetal-catalyzed processes for one particular reason the distance between the reactive centers in a coordinated electrophile and a coordinated nucleophile is self-adjustable for specific tasks, because the activation energy for Cp ligand rotation is very low. In 2008, Peters and Jautze reported the application of the bis-palladacycle complex 56a to the enantioselective conjugate addition of a-cyanoacetates to enones (Fig. 31) [74—76] based on the idea that a soft bimetallic complex capable of simultaneously activating both Michael donor and acceptor would not only lead to superior catalytic activity, but also to an enhanced level of stereocontrol due to a highly organized transition state [77]. An a-cyanoacetate should be activated by enolization promoted by coordination of the nitrile moiety to one Pd(II)-center, while the enone should be activated as an electrophile by coordination of the olefinic double bond to the carbophilic Lewis acid [78],... 

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