Organozinc reactivity

Organozinc reagents aie not nearly as reactive towaid aldehydes and ketones as Grig-nai d reagents and organolithium compounds but are intermediates in certain reactions of alkyl halides. 

The organozinc compound 2 is less reactive than an organomagnesium compound the addition to an ester carbonyl group is much slower than the addition to an aldehyde or ketone. Nevertheless the addition of 2 to the carbonyl group of unreacted a-halo ester 1 is the most frequently observed side-reaction ... 

Ketones are less reactive than aldehydes toward organozinc reagents, and they are inherently less stereoselective because the differentiation is between two carbon substituents, rather than between a carbon substituent and hydrogen. Recently, a diol incorporating both franr-cyclohexanediamine and camphorsulfonic acid has proven effective in conjunction with titanium tetraisopropoxide.159... 

Immonium salts are sufficiently reactive to add organozinc halides in the absence of a catalyst.161 Diallylamines were used because of the ease of subsequent deallylation (see Section 3.5.2). 

The reactivity of these reagents is similar to the corresponding organozinc compounds. 

Organozinc compounds are more reactive nucleophiles requiring no activation for the trans-metalation step, while allowing for much wider functionality to be present in the molecule than in organomagnesium compounds. However, organozinc compounds cannot be stored without special precautions, and therefore should be generated immediately prior to use, which makes... 

The general utility of the oxidative addition of functionalized organic halides to zinc was demonstrated by the formation of organozinc iodides 28 from protected (3- and 7-amino acids (Scheme 26).73 The organozinc iodides prepared in this manner were neither sufficiently stable nor sufficiently reactive in THF, but excellent yields were obtained in more polar aprotic solvents, such as DMF and DMSO. 

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 presence of Cu(i) or Cu(n) salts in the aforementioned reactions is critical. It is believed that organozinc reagents undergo transmetallation with copper species to yield more reactive complexes.301 A proposed301 catalytic cycle (Scheme 118) suggests that the alkyl group transferred to the enone from the copper metal in a bimetallic intermediate 207. 

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