Rhodium-Catalyzed [2+2+2] Cycloaddition Reactions of Linear Allene-Ene-Ynes to afford Fused Tricyclic Scaffolds: Insights into the Mechanism.

Allene-(E)-ene-yne N-tosyl-tethered substrates 3 a-3 h were efficiently prepared and their rhodium-catalyzed [2+2+2] cycloaddition reactions were evaluated. The cycloadditions are chemoselective as only the proximal double bond of the allene reacted to afford exocyclic double bonds in the fused-tricyclic scaffolds. The stereoselectivity depended on the catalytic system used. Reactivity between allene, alkene, and alkyne was studied for the first time by density functional theory calculations. This mechanistic study determines the order in which the unsaturated groups take part in the catalytic cycle.