Benzene formation from three different alkynes

3 Benzene formation from three different alkynes 

Transition metal mediated or catalyzed benzene formation reactions have been reported using various metals. However, the use of three different alkynes is difficult [38], In many cases, a mixture of several benzene derivatives is obtained. In 1974, Wakatsuki and Yamazaki used three different alkynes with Co complexes [27b], but isomers were formed and a tedious chromatographic separation was necessary. The first selective coupling of three different alkynes in high yields was reported in 1995 using a combination of unsymmetrical zirconacydopentadienes and CuCl, as shown in Eq. 2.52 [7k]. 

Benzene formation from three different alkynes using CuCl via a zirconacyclopentadiene. 

The zirconacyclopentadiene prepared from unsymmetrical trimethylsilylpropyne and diphenylacetylene reacts with DMAD in the presence of 2 equivalents of CuCl and 3 equivalents of DMPU to give the corresponding benzene derivative 72c in 56% yield as a single product in a one-pot reaction. Some other examples are shown in Table 2.3. 

Unsymmetrical zirconacyclopentadiene lm, prepared from 2-butyne and 4-octyne, reacts with 3-hexyne in the presence of NiCl2(PPh3)2 to give a single product (72d), as shown in Eq. 2.53 [8a]. 

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Takahashi T, Xi Z, YamazaM A et al (1998) Cycloaddition reaction of zirconacyclopentadienes to alkynes highly selective formation of benzene derivatives from three different alkynes. J Am Chem Soc 120 1672-1680... 

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