Facile [2+2] Cycloaddition of DDQ to an Alkyne: Synthesis of Pyrrolyl- and Indolylbicyclo[4.2.0]octadienes from C-Ethynylpyrroles or C-Ethynylindoles

Abstract

C-Ethynylpyrroles or -indoles, which can be prepared by cross-coupling of pyrroles or indoles with haloalkynes on active surfaces, undergo [2+2]-cycloaddition reactions with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone to give 2-pyrrolyl- or 3-indolylbicyclooctadienes in almost quantitative yields. The adducts are charge-transfer complexes that are paramagnetic in the solid state, and therefore represent a new family of densely functionalized pyrrole and indole derivatives potentially useful as pharmaceutical candidates, highly potent building blocks, or precursors of advanced materials. The reaction contributes to the practical and basic chemistry of pyrroles, indoles, alkynes, and quinones.