Rieke zinc halides

With special techniques for the activation of the metal—e.g. for removal of the oxide layer, and the preparation of finely dispersed metal—the scope of the Refor-matsky reaction has been broadened, and yields have been markedly improved." The attempted activation of zinc by treatment with iodine or dibromomethane, or washing with dilute hydrochloric acid prior to use, often is only moderately successful. Much more effective is the use of special alloys—e.g. zinc-copper couple, or the reduction of zinc halides using potassium (the so-called Rieke procedure ) or potassium graphite. The application of ultrasound has also been reported. ... 

Zinc/copper couple (Zn/Cu), produced by the treatment of zinc with HC1, followed by the addition of copper(n) sulfate, has been known since the seminal work of Simmons and Smith.67 A less reactive form of activated zinc, but one that is sufficiently active for most applications, is produced by the treatment of the metal with 1,2-dibro-moethane, followed by Me3SiCl.68 The most reactive form of activated zinc, the so-called Rieke zinc (Zn ), is finely divided metallic zinc produced by the homogenous reduction of zinc halides in THF.69... 

Insertion of zinc dust into aryl or heteroaryl iodides is also possible, but polar cosolvents are required in some cases [48, 49]. The use of highly activated zinc (Rieke zinc) prepared by reduction of zinc halides with lithium results in faster insertion (Scheme 2.24) [50-52]. 

Under these conditions, a broad range of polyfunctional alkyl iodides are converted to the corresponding organozinc halides in high yields . In the case of primary alkyl iodides, the insertion occurs at 40-50 °C whereas secondary alkyl iodides already react at 25-30°C. Secondary alkyl bromides also react under these conditions , but primary alkyl bromides are usually inert with this type of activation and much better results are obtained by using Rieke zinc L Thus, the reduction of zinc chloride with finely cut lithium and naphthalene produces within 1.5 h highly reactive zinc (Rieke zinc). 

This activated zinc would be an aggregation of very fine zinc particles dispersed in the DMF solution. The size of these particles is smaller than that obtained in the previous process, which was performed in the absence of naphthalene12. This electrochemical method is comparable to the chemical Rieke procedure in which the activated zinc is prepared by reduction of zinc halide with alkali metal naphthalenide in THF13. 

At this time the applicability of organozinc chemistry had been limited to those zinc reagents that could be prepared by insertion of zinc powder into the corresponding alkyl iodide.20 Further developments, notable the use of highly reactive zinc (Rieke zinc), obtained by the reduction of zinc halides with lithium naphthalenide, allowed the preparation of zinc reagents from otherwise unreactive organic substrates, such as aryl iodides and aryl bromides (Scheme 1.7).21... 

Alternatively, more activated zinc prepared by the reduction of zinc halides (Rieke zinc) can be used when starting with tlie less reactive aryl iodides or bromides, but also with secondary and tertiary alkyl bromides [16,17,18]. 

The Reformatsky reaction involves condensation of ester-derived zinc enolates with aldehydes or ketones to furnish the con-esponding [3-hydroxy esters. The zinc enolates are generated by addition of an a-haloester in THF, DME, Et20, benzene, or toluene to an activated zinc, such as a Zn-Cu couple or zinc obtained by reduction of zinc halides with potassium (Rieke zinc). An example of a Reformatsky condensation using Rieke zinc is shown below. ... 

Zhu, L., Wehmeyer, R. M., Rieke, R. D. The direct formation of functionalized alkyl(aryl)zinc halides by oxidative addition of highly reactive zinc with organic halides and their reactions with acid chlorides, -unsaturated ketones, and allylic, aryl, and vinyl halides. J. Org. Chem. 1991,56, 1445-1453. 

The selectivity of Rieke zinc in its reaction with organic halides is demonstrated in coupling of a benzylic bromide. ... 

A useful technique for the preparation of n and Cu powders) is by electrolysis with puls< efficiency in allylation achieved by using s involves reduction of ZnCl with Na in liquid 1 Ketones. Rieke zinc produced by the r reacts with alkyl halides, and the organozin treatment with CuCN and acid chlorides. S readily prepared in this direct manner. a-Chkj method using chloroacetyl chloride in the couf 3-Thienylzinc halides. These reagenii Rieke zinc. They undergo Ni-catalyzed coupln... 

Thienylzinc haUdes. These reagents are obtained from 3-thienyl halides and Rieke zinc. They undergo Ni-catalyzed coupling with aryl iodides. 

Heteroarylzinc compounds are of particular use in palladium-catalysed couplings, being compatible with many functional groups. They have usually been prepared by exchange reactions in situ) of zinc halides with heteroaryllithiums but this method limits their usefulness. Efficient methods are now available for their direct preparation from either Rieke zinc " or commercial zinc dust and the heteroaryl halide, in both electron-rich and electron-poor systems. 

Compatible with a wide variety of functional groups The rate of insertion is dependent upon the nature of the substitution pattern benzylic and allylic (0 °C), primary halides require more forcing conditions Rieke Zinc can be paired up with less reactive organohalides. 

I A new method was introduced for the preparation of highly reactive, finely dispersed metals by reduction of their halides with the aid of alkali metals. These metals are named Rieke metals (such as Rieke magnesium. Mg, Rieke zinc, Zn, etc.) after the inventor of the process [53-56]. Their reactivity is extremely high and reactions have been made possible which were simply unthinkable before. 

It is clear from the preceding chapter that the oxidative addition of Rieke zinc is general and can be used with most any organic halide. The reaction proceeds with alkyl iodides, bromides and even most chlorides. Significantly, the reaction proceeds with aryl iodides, bromides, and even some chlorides. Most unusual of all, the reaction proceeds with vinyl iodides and bromides. Moreover, the yields are always in the range of90-95%. In all cases, the molecules can contain... 

Rieke-zinc proved also to be very useful reagent for the preparation of aryl- and heteroarylzinc halides. For example, the reaction of Rieke-zinc with / -bromobenzonitrile (220) in refluxing THF provides after 3 h the corresponding zinc reagent 221, which can be benzoylated leading to the ketone 222 in 73% yield (Scheme 2-88). 

Another approach, recently described, consists in the electrochemical preparation of a very active zinc with the use of a mediator3. This zinc is able to react with low reactive organic halides. This electrochemical process is analogous to the chemical reaction developed by Rieke in which the naphthalene anion radical is used to activate metals such as zinc4. 

Lithium halides (bromide or Iodide) may well modify the Lewis character of the zinc atom, probably via a zincate species [53], and prevent the efficient coordination of the zinc atom to the double bond, coordination which is required for the carbocyclization. Thus, in the Rieke method, it is essential to wash the active zinc thoroughly since the lithium naphthalenide reduction of zinc bromide also generates lithium bromide, which is detrimental to the success of the reaction. Indeed, the insertion of Rieke s zinc in the presence of LiBr leads to the linear organozinc iodide but not to the cyclic product [52]. 

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