Using Lithium-, Sodium-, or Magnesium-Zinc Mixed-Metal Bases

7 Using Lithium-, Sodium-, or Magnesium-Zinc Mixed-Metal Bases 

Due to the preferred kinetic reactivity of the zinc-nihogen bonds in comparison with the zinc-carbon bonds, amidozincates are more reactive bases than aUcylzincates. When used in THF at room temperature, (t-Bu)2Zn(TMP)Li is able to deprotonate functionahzed substrates such as alkyl benzoates and benzonitrile the generated arylzincates can be quenched by iodine, benzalde-hyde (Table 27.12), and bromine [29,112-116]. Crystals of the base were isolated and its structure identified by X-ray diffraction as an ion-contacted zincate [117]. In addition, different reaction outcomes being obtained by changing the nature of the alkali metal (sodium vs. lithium), a reactivity as solvent-separated ion pairs was found unlikely [118]. 

TABLE 27.11 Substituted Benzenes Deprotometalation Using a Lithium-Copper Base and Subsequent Interception with Electrophiles 

SCHEME 27.26 1,4-Dimethoxybenzene deprotometalation using a lithium-copper base and subsequent interception with electrophiles. 

For the reaction performed in THF at room temperature on IVW-diisopropylbenzaniide using Li(TMP)ZnEtj, a stepwise mechanism where HTMP hberated during the first step is recycled through elimination of ethane was proposed on the basis of theory and experiment [54]. Additional structural elements in favor of such a two-step mechanism were more recently obtained [56]. 

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