Transition metal complexes of azo compounds : VI. Cycloaddition of alkynes to the complexed cis-azo group. Stepwise cyclodimerisation of diphenylacetylene to tetraphenylcyclobutadiene tricarbonyliron

The μ-(cis-azo) complexes LFe₂(CO)₆ (L  4-phenyl-3,3-bis(methoxycarbonyl)-1-pyrazoline, 4-isopropyl-3,3-bis(methoxycarbonyl)-1-pyrazoline, 2,3-diazanorbornene, and benzoc]cinnoline) each contain an activated NN bond which reacts thermally and/or photochemically with the alkynes RC₂R (R  H, CH₃, Ph, COOCH₃) by insertion into the FeN bond to give the novel cycloaduct L(RC₂R)Fe₂(CO)₆. The pyrazoline and diazanorbornene complexes are transformed smoothly into the 1,2,3-diazaferrole derivatives L(RC₂R)Fe(CO)₄. These compounds react with a further molecule of the alkyne to give the “double-addition” products L(RC₂R)₂COFe(CO)₂, which can be converted into the cyclobutadiene derivative π-(RC₂R)₂Fe)CO)₃ if R  Ph; this complex can be synthesized very conveniently in yields of 70% by reaction of diphenylacetylene and the pyrazoline compound LFe₂(CO)₆ at 150°C. It is shown that the cyclodimerisation of the alkyne occurs stepwise.