Organozinc halide

According to molecular weight determinations, rmsolvated organozinc halides, e.g. EtZnCl and EtZnBr, are associated in organic solution, with the formation of tet-rameric species [1]. Little is knovra about the nature of these associates. 

In the solid state, apparently only one structure has been elucidated by X-ray diffraction. Thus, ethylzinc iodide, EtZnI, was found to contain polymeric chains, 1, in which each zinc atom forms two long (2.91 A) and one short (2.64 A) zinc-iodine bonds [2]. This leads to the formation of Zn h quasi-rings, which are interconnected in long chains. 

A dodecameric macrocyclic compound [ClZnCHMePEt2N(SiMe3)]i2 consisting of six units, shown in 2, is formed by zinc-halogen bridging (Zn-Cl 2.395 A) [3]. 

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The study of the Schlenk equilibrium for organozinc compounds represents a major chapter in the understanding of these reagents in general [26]. Before elaborating the studies on zinc carbenoids, it is appropriate to briefly review the definitive investigations on organozinc halides themselves. 

Eor ethylzinc chloride, CH3CH2ZnCl, and ethylzinc bromide, CH3CH2ZnBr, there is a linear relationship between the observed chemical shift and the ratio of ethylzinc halide to diethylzinc. Extrapolation of these lines to x=l (mol fraction of CH3CH2Z11X) gives predicted values for the average chemical shift that closely match those measured for these species. This indicates that for these two organozinc halides, the Schlenk equilibrium lies heavily on the side of the ethylzinc halide in toluene. However, in the case of ethylzinc iodide, CH3CH2ZnI, there is a... 

Fig. 3.2 H NMR investigation of some organozinc halides. [Boersma, ). Noltes, J.G.J. Organo-met. Chem. 1267, S, 551. Reprinted with permission from Elsevier, Ltd.]...

Recently, the required heteroaromatic organozinc halides for the Negishi reaction have also been prepared using microwave irradiation [23]. Suna reported that a Zn - Cu couple (activated Zn), prepared using a slightly modified LeGoff procedure from Zn dust and cupric acetate monohydrate, allowed the smooth preparation of (3-pyridinyl)zinc iodide and (2-thienyl)zinc iodide... 

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). 

Organozinc Halides by the Oxidative Addition of Organohalides to Zinc 329... 

Organozinc Halides by Transition Metal-catalyzed Reactions 330... 

Most organic halides, with the exception of organic fluorides, can be converted to the corresponding organozinc halides by one of the following methods ... 

TM-catalyzed reaction between a diorganozinc or organozinc halide and an organic halide. 

Functionalized organozinc halides are best prepared by direct insertion of zinc dust into alkyl iodides. The insertion reaction is usually performed by addition of a concentrated solution (approx. 3 M) of the alkyl iodide in THF to a suspension of zinc dust activated with a few mol% of 1,2-dibromoethane and MeaSiCl [7]. Primary alkyl iodides react at 40 °C under these conditions, whereas secondary alkyl iodides undergo the zinc insertion process even at room temperature, while allylic bromides and benzylic bromides react under still milder conditions (0 °C to 10 °C). The amount of Wurtz homocoupling products is usually limited, but increases with increased electron density in benzylic or allylic moieties [45]. A range of poly-functional organozinc compounds, such as 69-72, can be prepared under these conditions (Scheme 2.23) [41]. 

Evans and Uraguchi also examined the rhodium-catalyzed allylic alkylation with hard nucleophiles [31]. Aryl organozinc halides proved optimal nucleophiles for the regio- and stereospecific allylic alkylation of enantiomerically enriched unsymmetrical allylic alcohol derivatives (Tab. 10.4). The reaction occurs with net inversion of absolute... 

The third reaction pathway involves the redistribntion reaction of a diaUcyl- or diarylzinc compound with a zinc salt. An example of this ronte is the formation of the nnsolvated organozinc halides EtZnCl, EtZnBr and EtZnI from the reaction of pure EtiZn with the corresponding zinc halide in a 1 1 ratio ... 

Although the simple alkylzinc halides RZnX (R = Me or Et, X = Cl, Br, I) were among the first known organozinc compounds, for a long time nothing was known about the actual structures of such compounds in solution or in the solid state. The constitution of these compounds in solution seemed to depend on the particular solvent employed " . It should be noted that in ethereal solutions, the possibility of the presence of a Schlenk equilibrium should always be considered. In these solvents the organozinc halides most... 

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