Preparation of Organozincs

Preparation of organozinc halides using transmetalation reactions. . . 296... 

There are some other methods of preparation of organozincs that are discrete from any of the above-described methods, as shown in Scheme 21. 

For reviews of the preparation of organozincs other than References 8a and 8b, see ... 

B. Electrochemical Preparation of Organozinc Compounds using Nickel... 

In addition, the organozinc compound obtained from ethyl 4-bromobutanoate reacts under similar conditions to those described previously, with 4-bromoacetophenone, to give the corresponding product in good yield15. The preparation of organozinc compounds and their cross-coupling with aryl halides can be carried out in one step (equation 7). 

III. ELECTROCHEMICAL PREPARATION OF ORGANOZINC REAGENTS USING NICKEL OR COBALT CATALYSIS... 

This activation has also been used to successfully develop an electrochemical process for the preparation of organozinc compounds. Results, mechanisms and reactivity of organoz-inc compounds are reported in the next part (see Section . ). 

The electrochemical preparation of organozinc compounds obtained from the corresponding aromatic halides and with the use of a nickel complex as catalyst is only efficient in dimethylformamide as solvent. Moreover, in most cases and as described previously, the reaction requires the presence of excess 2,2 -bipyridine (five molar equivalents with respect to nickel) to achieve the transmetallation reaction leading to the organozinc compound and to avoid the formation of biaryl, Ar-Ar (equation 53). 

The electrochemical preparation of organozinc compounds in the presence of C0X2 as catalyst is thus feasible from the corresponding iodides or bromides, provided ZnX2 (X = Cl, Br) or Zn2+ (Zn(BF4)2) is present before running the electrosynthesis. Under these conditions, Scheme 16 can be proposed. 

More importantly, the association of synthesis and molecular electrochemistry has allowed one to discover new and original chemical approaches in the preparation of organozinc compounds as well as to develop original chemical reactions concerning the selective activation of functionalized substrates54-56. 

The most significant obstacle to be overcome in the preparation of organozinc halides by direct insertion of zinc metal into an organic halide is the intrinsic lack of reactivity of commercially available zinc metal. This is primarily due to a surface coating, for example of zinc oxide, which prevents direct reaction with an organic substrate. The success of attempts to insert zinc metal into an organic halide is entirely dependent upon the care with which the zinc metal is... 

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