(4+1) vs (4+2): Catalytic Intramolecular Coupling between Cyclobutanones and Trisubstituted Allenes via C-C Activation.

Herein we describe a rhodium-catalyzed (4+1) cyclization between cyclobutanones and allenes, which provides a distinct [4.2.1]-bicyclic skeleton containing two quaternary carbon centers. The reaction involves C-C activation of cyclobutanones and employs allenes as a one-carbon unit. A variety of functional groups can be tolerated, and a diverse range of polycyclic scaffolds can be accessed. Excellent enantioselectivity can be obtained, which is enabled by a TADDOL-derived phosphoramidite ligand. The bridged bicyclic products can be further functionalized or derivatized though simple transformations.