Heteroaryl zinc halide

In conclusion, facile synthetic routes for the preparation of a wide range of 5-substituted 2-furaldehydes are possible. They were accomplished through either Pd-catalyzed cross-coupling reaction of various aryl- and heteroaryl-zinc halides with 5-bromo-2-furaldehyde (route A) or utilization of a new organozinc reagent, 5-(l,3-dioxolan-2-yl)-2-furanylzinc bromide 1, which was... 

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]. 

TABLE 7. Preparation of aryl-zinc halides from heteroaryl halides... 

Heteroaryl zinc compounds are particularly useful in palladium-catalysed coupling, being compatible with many functional groups. They are often prepared in situ via lithiation, followed by reaction with zinc halides, but direct zincation of halides can be carried out, using either Riecke zinc, as in the example below, or, more conveniently, ordinary zinc dust, with various means of activation. 

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. 

Zinc Reagents. Coupling of heteroaryl iodides or bromides with ethynylzinc halides yields alkynylated heterocycles (Scheme 31). The ethynylzinc reagent can be prepared in situ by addition of a zinc halide to the ethynylmagnesium halide, which is either... 

Electrosynthesis of Aryl or Heteroaryl Zinc Species from the Corresponding Halide via a Nickel Catalysis p4]... 

Alkynylzinc chlorides are useful imermedtates in Pd°-catalysed couplings with vinyl, heteroaryl [56) and acyl halides [89] tsee Chaps. V and X). The conversion of lithium alkyn-vlides into alkynylzinc halides with anhydrous zinc chloride in THF proceeds smoothly and quantitatively under mild conditions. 

The reaction of heterocyclic lithium derivatives with organic halides to form a C-C bond has been discussed in Section 3.3.3.8.2. This cannot, however, be extended to aryl, alkenyl or heteroaryl halides in which the halogen is attached to an sp2 carbon. Such cross-coupling can be successfully achieved by nickel or palladium-catalyzed reaction of the unsaturated organohalide with a suitable heterocyclic metal derivative. The metal is usually zinc, magnesium, boron or tin occasionally lithium, mercury, copper, and silicon derivatives of thiophene have also found application in such reactions. In addition to this type, the Pd-catalyzed reaction of halogenated heterocycles with suitable alkenes and alkynes, usually referred to as the Heck reaction, is also discussed in this section. 

Activation with cobalt chloride and allyl chloride has been used for chlorothiophenes (2-Cl is more reactive than 3-Cl) and activated aryl chlorides. Commercial zinc dust and the heteroaryl halide can be used to make the heteroarylzinc in both electron-rich and electron-poor systems. ... 

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