Hexynes transition metal complexes

Although stoichiometric amounts of transition metal complexes were employed, the cross-[2+2+2] cycloaddition of three different alkynes has been achieved using zirconium and nickel complexes. The reaction of 2-butyne, 4-octyne, and Cp ZrEt selectively afforded unsymmetrical zirconacyclopentadiene. Subsequently, it reacted with 3-hexyne in the presence of NiBr fPPhjlj to give the desired hexasubstituted benzene (Scheme 21.6) [9]. 

The tantalum-benzyne complex (130) is much less reactive than other early transition-metal aryne complexes. It shows no reaction with acetone, benzophenone, benzaldehyde, acetonitrile, 3-hexyne, or methanol. The lack of reactivity of 130 was attributed to nonlability of the PMe3 ligand. Indeed, no phosphine exchange was observed when 130 was mixed with an excess of PMe3-d9. Refluxing 129 in a mixture of methanol and toluene (3 10 v/v) leads to clean formation of 131. This presumably results from reaction of a 16-electron benzyne complex with the alcohol. 

The interaction of aromatic hydrocarbons with iron cations has been studied in the gas phase. The results of these studies provide information useful in the proposal that polycyclic aromatic hydrocarbons undergo efficient reactions with iron and other transition metals in the interstellar medium. Fe(r/-G2H4)2(r7-PhMe) 87 is a useful precursor for a range of arene iron complexes. Thus, reaction of 87 with the desired naphthalene derivative and three molecules of butyne or hexyne leads to the (hexaalkylbenzene)naphthalene iron complexes, as shown in Scheme 15. ... 

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