Synthesis of Alkyl Complexes by Transmetallation

CHAPTER 3 COVALENT (X-TYPE) LIGANDS BOUND THROUGH METAL-CARBON AND METAL-HYDROGEN BONDS 

Stabilized by the ionic property of the bond between the more electropositive metal and the electronegative group X. 

The dependence of the position of this equilibrium on the main group metal was described by Coates, Green, Powell, and Wade and later re-emphasized by Negishi. Most transition elements are less electropositive than Li, Mg, Zn, and Al, but there are exceptions, particularly among the early transition elements. For example, the position of the transmetallation equilibrium between Zr and Al depends on the identity of R and the ancillary ligands (Equations 3.2 and 3.3). °  

A disadvantage of the synthesis of metal-alkyl complexes by transmetallation is the potential for nucleophilic attack to occur at ancillary ligands. Carbonyl ligands are particularly vulnerable to nucleophilic attack, as shown in Equation 3.4, and described in more detail in Chapter 11. However, the extent to which this process competes with transmetallation depends on the electronic property of the metal. Thus, some carbonyl halides can be converted into alkyl carbonyl complexes (Equation 3.5). In addition, electron transfer from electron-rich carbon nucleophiles can limit the yields of transmetallation processes. 

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