Pyrrolo[2,3-b]quinolines via a Rhodium-Catalyzed Pauson-Khand-Type Reaction

Significance

Previous work on Pauson-Khand-type reactions by this group has focused on the synthesis of fused [5,5]-ring systems, such as thieno[2,3-b]indol-2-ones (e.g., T. Saito et al. Chem. Commun. 2008, 172). The present incarnation of this reaction involves extension to the formation of [6,5]-ring systems in the form of ­pyrrolo[2,3-b]quinolines. Thus, the formal [2+2+1] reaction of an alkyne, a carbodiimide, and carbon monoxide under rhodium catalysis forms the tri­cycles shown. In one case a [6,6,4] system was formed due to the inability of the rhodium intermediate to absorb carbon monoxide, presumably due to steric reasons. No reason for the inter­mittent use of dppp [1,3-bis(diphenylphosphino)-propane] was given.