Intramolecular Pauson-Khand Reaction of Alkyne-Ketenimines and Related [4+2] and [2+2] Cycloadditions

The first examples of intramolecular Pauson-Khand reactions involving ketenimines are reported. The alkyne-ketenimines participate through the C=C bond of the ketenimine fragment in the [2+2+1] cyclization process giving rise to cyclopenta[b]quinolin-2-ones. In the absence of metal carbonyl complexes, two competitive periselective cycloadditions occur when the alkyne-ketenimines are heated in benzene solution: a [4+2] process leading to benz[b]acridines as the major products, and a [2+2] route giving minor amounts of cyclobuta[b]quinolines.