Rieke-zinc

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... 

Ni-Catalyzed preparation of regioregular HT poly(3-alkylthiophenes) (HT-388) from 2,5-dibromoalkylthiophenes (389) and Rieke zinc (Zn ) (Chart 2.164). 

T.-A. Chen, X. Wu, and R.D. Rieke, Regiocontrolled synthesis of poly(3-alkylthiophenes) mediated by Rieke zinc their characterization and solid-state properties, J. Am. Chem. Soc., 117 233— 244, 1995. 

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]. 

Transition metal-catalyzed polymerizations have been reported to give high-purity, regioregular poly-3-alkylthio-phenes suitable for use in electronic devices. Thus, the Negishi protocol using a nickel catalyst and Rieke zinc was employed for the polymerization of 63 to produce the product as a low molecular weight polymer <2003CC2548>. 

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). 

SCHEME 4. Preparation and uses of tertiary alky I zinc reagents using Rieke zinc... 

Very limited are applications of classical Reformatsky protocols in macrocyclisation reactions, where samarium(II) is the most widely used metal, as shown in Table 1. Vedejs succeeded in cyclizing 45 by using Rieke zinc unfortunately, the intermediate adduct underwent spontaneous dehydration and the isolated product was 46 (equation 31)107. 

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... 

As a good compromise between preformed zinc/copper couple which is very easy to prepare on large scale, but not especially reactive, and very reactive Rieke zinc, it is possible to activate commercially available zinc dust (Aldrich -325 mesh) by treatment first with 1,2-dibromoethane and then with chlorotrimethylsilane.7 This process, which is routinely carried out in situ, is a reliable and quick method for the preparation of zinc which is sufficiently reactive for many purposes. 

Organometallics by transmetallation pyridylzinc chlorides can be accessed either by transmetallation of a pyridyllithium with anhydrous zinc chloride <2001EJ04207> or by direct insertion of zinc into carbonhalogen bonds using Rieke zinc in THF under reflux <2003SL852>. 

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 highly activated Rieke zinc, prepared in situ from ZnBr2 and selectively... 

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