An improved cobalt catalyst for homo Diels–Alder reactions of acyclic 1,3-dienes with alkynes

An efficient cobalt(I)-catalysed homo Diels–Alder reaction of acyclic 1,3-dienes and acetylene derivatives is described. The catalyst system consists of a mixture of a readily available CoBr2(dppe) complex, zinc iodide as a cocatalyst, and tetrabutylammonium borohydride as reducing agent. This system increases the reactivity of the cobalt(I) catalyst, so that acyclic dienes and acetylenes can be transformed to the 1,4-dihydroaromatic products in good yield and excellent purity. The regioselectivity of the homo Diels–Alder reaction greatly favours the 1,4-substitution pattern of the formed product.