Cyclobutadiene complexes thermal decomposition

The reaction of cyclooctyne (14) with NiBr2 and Nil2 gave the corresponding cyclobutadiene-complexes in low yield, while with Ni(CO)4 the nickel(O) cyclopentadienone complex (76) was formed, whose thermal decomposition yielded the cyclopentadienone (77). In this case again a cyclooctyne Ni(0) complex (78) was postulated as an intermediate, but could not be isolated due to its instability l93). 

Criegee and his co-workers 28,29,32) were the first to study the thermal decomposition of a cyclobutadiene complex, tetramethylcyclobutadiene-nickel chloride (XIV). The most important reactions and products obtained are summarized in Fig. 4. 

Cyclobutadiene-metal complexes have also been suggested as intermediates in a number of other reactions, notably in the formation of benzenes by trimerization of acetylenes 2, 18, 57, 90) and also in other reactions. Conclusive evidence is still lacking, but the inertness of the known cyclobutadiene complexes towards acetylenes in Diels-Alder type reactions makes this rather unlikely. Intermediates with open-chain structures such as (XCVI) appear more attractive. Arnett and Bollinger (2) have isolated by-products from the dicobalt-octacarbonyl-catalyzed trimerization of diisopropylacetylene which are very similar to some, e.g., (LI), obtained in the thermal decomposition of tetramethylcyclobutadienenickel chloride complexes (29). Again, here, however, the evidence is by no means conclusive and a variety of intermediates other than a cyclobutadiene-metal complex can be postulated to explain the observed products however, see also the Appendix. 

Thermal decomposition of cyclobutadiene complexes leads to various organic compounds. Heating [Ni2Cl4(C4Me4)2] forms, among others, the products shown in scheme (8.65). 

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