Ethynylzinc bromide

Similar observations have been made with the addition of alkynylzinc reagents to aldehydes in the presence of the lithium salt of Wmethylephedrine (26)28. Whereas addition of 2-phenyl-ethynylzinc bromide to an aromatic aldehyde led to the chiral alcohol in 88% ee, addition of ethyl(2-phenylcthynyl)zinc afforded the same alcohol with only 42% ee. 

Monosubstitution of acetylene itself to prepare terminal alkynes is not easy. Therefore, trimethylsilylacetylene (134) is used as a protected acetylene. After the coupling, the silyl group is removed by the treatment with fluoride anion. The hexasubstitution of hexabromobenzene (135) with 134 afforded hexaethynylbenzene (136) after desilylation in total yield of 28%. The product was converted to tris(benzocyclobutadieno)benzene (137) using a Co catalyst (see Section 7.2.1). Hexabutadiynylbenzene was prepared similarly [60], As another method, terminal alkynes 139 are prepared in excellent yields by the coupling of commercially available ethynyl Grignard (138) or ethynylzinc bromide with halides, without protection and deprotection [61]. 

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... 

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