Ligand and solvent effects on cobalt(I)-catalysed reactions: Alkyne dimerisation versus [2+2+2]-cyclotrimerisation versus Diels-Alder reaction versus [4+2+2]-cycloaddition

The cobalt catalysed conversion of phenyl acetylene led to linear enyne dimerisation products when CoBr2(dppe) was activated with magnesium in the absence of a Lewis acid. In contrast, in the presence of a Lewis acid the cyclotrimerisation process is favoured. Among several ligand systems and solvents tested the best results were obtained using a catalyst system consisting of a diimine cobalt bromide complex, zinc and zinc iodide in acetonitrile. With 2-5 mol% of the cobalt catalyst at ambient temperatures 1,2,4-triphenylbenzene could be obtained in 99% yield and in excellent regioselectivity (95:5) in 10 min reaction time. Competition experiments of phenylacetylene and isoprene were performed. A preference for the cyclotrimerisation reaction was found for the diimine cobalt complex in acetonitrile, while the Diels-Alder reaction is favoured with the cobalt(dppe) complex in dichloromethane. Also a regioselectively substituted cyclooctatriene product was formed in a [4+2+2]-cycloaddition process and isolated which allows assumptions on the reaction mechanism. (c) 2005 Elsevier B.V. All rights reserved.