Cyclotrimerization Co, Ni

Various Ni and Co complexes catalyze cyclotrimerization reactions of alkynes to give arenes. These reactions are formally [2 + 2 + 2] cycloadditions. The cyclotrimerization of PhC=CPh using CpCo(CO)2 as catalyst is illustrative. 

The end of the catalytic cycle can be drawn differently. The cobaltacyclopenta-diene is particularly well arranged to undergo a Diels-Alder reaction with another equivalent of PhC=CPh. A [4 + 1 ] retro-cycloaddition then gives the product CsPhe and regenerates the catalyst CpCo(I). 

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