Metastable Intermediates (Isoindenes, Isofluorenes, and Isobenzofurans) from Alkyne Precursors via Free Carbenes

We report here reactions between a free carbene center and a suitably appended π-bond (C═X) that give rise to reactive intermediates that, in turn, undergo various types of additional transformations (cycloadditions, rearrangements, and ring openings). Participation of the following C═X functional groups is demonstrated: alkene, arene, heteroarene, aldehyde, ester, ketone, amide, and carbamate. The overall transformations are thermally driven and often produce no byproducts. Each is initiated by the generation of a carbene that is fueled by the potential energy provided by two complementary alkyne coreactants.