Diels−Alder Approach to Biaryls: Elucidation of Competing Tandem [2+2] Cycloaddition/[1,3] Sigmatropic Shift Pathway

Reaction of 2-halo-6-nitrophenylacetylene with an electron deficient diene gives rise to a [4+2] cycloaddition/cycloreversion biaryl product and a bicyclo[4.2.0]octadiene resulting from a competing [2+2] cycloaddition pathway. The cyclobutene can be opened to give a mixture of cyclooctatriene and biaryl in varying amounts depending on heat and light exposure. The conversion of the cyclobutene into biaryl occurs through a [1,3] sigmatropic carbon shift followed by [4+2] cycloextrusion of ethylene gas.