Reactivity of Organozinc Compounds

The high covalent degree of the carbon-zinc bond and the small polarity of this bond lead to a moderate 

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The reactivity of organozinc compounds with various reagents such as water, amines, alcohols, and CO2 has already... 

Zinc. The salts used for the Zn reactions are the halides (chlorides, bromides and iodides) both alkyl and aryl compounds are used. Owing to the reactivity of organozinc compounds, air and moisture must be excluded from the reaction. Use of ether or dioxane, on evaporation of the solvent, leads to solvates RZnX-D (X = halide, D = Et20 or dioxane). The exchange between Et2Zn and Znl2 occurs slowly in ether at RT. 

REACTIVITY OF ORGANOZINC COMPOUNDS (See Organic Synthesis Using Metal-mediated Coupling Reaction ... 

Water as the Co-)Solvent. In view of the reactivity of organozinc compounds towards acidic hydrogens, the use of water as the (co-)solvent in Zn-Barbier reactions is even more surprising than the use of alcohols. 

Examples of the reaction with aldehydes are few since the reactivity of organozinc compounds is low. Recently, the articles about these reactions have increased by the improvement of the reactivity by means of the use of coordination... 

One of the important new directions in the study of addition reactions of organozinc compounds to aldehydes is the use of ionic liquids. Usually, application of these compounds in reactions with common organometallic reagents has a serious problem ionic solvents are usually reactive toward them, particularly Grignard and organolithium derivatives. It has been recently reported that carbonyl compounds react with allylzinc bromide formed in situ from allyl bromide and zinc in the ionic liquid 3-butyl-l-methylimidazolium tetrafluoroborate, [bmim][BF4].285 Another important finding is that the more reactive ZnEt2 alkylates aldehydes in a number of ionic liquids at room temperature.286 The best yields (up to 96%) were obtained in A-butylpyridinium tetrafluoroborate, [bpy][BF4] (Scheme 107). 

The synthesis of functionalized zinc organometallics can be accomplished with a variety of methods that have been developed in recent years. The intrinsic moderate reactivity of organozinc reagents can be dramatically increased by the use of the appropriate transition metal catalyst or Lewis acid. Furthermore, the low ionic character of the carbon-zinc bond allows the preparation of a variety of chiral zinc organometallics with synthetically useful configurational stability. These properties make organozinc compounds ideal inteimediates for the synthesis of complex and polyfunctionalized organic molecules. 

The synthesis of organozinc compounds by electrochemical processes from either low reactive halogenated substrates (alkyl chlorides) or pseudo-halogenated substrates (phenol derivatives, mesylates, triflates etc.) remains an important challenge. Indeed, as mentioned above, the use of electrolytic zinc prepared from the reduction of a metal halide or from zinc(II) ions does not appear to be a convenient method. However, recent work reported by Tokuda and coworkers would suggest that the electroreduction of a zinc(II) species in the presence of naphthalene leads to the formation of a very active zinc capable of reacting even with low reactive substrates (equation 23)11. 

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 palladium-catalyzed coupling of organozinc compounds with thioesters to form ketones. This reaction tolerates a variety of functional groups due to the low reactivity of the organozinc reagents. 

The past decade has seen extensive development of cross-coupling reactions of organozinc compounds and organic halides catalyzed by nickel or palladium catalysts. Although nickel-based catalysts are more reactive with respect to the organic halide partner, the number of failures with these catalysts and the greater selectivity realized with palladium-based catalysts have resulted in the almost exclusive use of the latter group of catalysts for these reactions. 

Whereas uncatalyzed substitution reactions of organozinc compounds are limited to very reactive electrophiles, metal-transmetallated organozinc compounds are able to perform substitution reactions on various electrophiles. In the case of conjugated electrophiles, these zinc copper reagents can follow a Sn2 or Sn2 mechanism. 

Compared to Grignard reagents, lithium organometallics have been used less frequently for the preparation of organozinc compounds. Their high reactivity or their easy availability can, however, be... 

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