Theoretical Investigation on Rhodium(I)-Catalyzed Cycloisomerizations of 4‑Allenal Species with Linked Alkyne: Ketone vs Alcohol Products

A density
functional theory study was performed to understand the mechanisms
and the origins leading to different products in the cycloisomerization
reactions of 4-allenal species with linked alkyne when two types of
Rh­(I) catalysts were employed. With RhCl­(PPh3)3 as the catalyst, the water present in the solvent promotes the H-migration
from Rh to the allenic carbon and thus prompts formation of 5–8-fused
bicyclic ketone (P1). When [Rh­(dppe)]­ClO4 was
employed as the catalyst, the positive charge on the catalyst plays
a key role in lowering the barriers of the oxidative coupling, β-H
elimination, and O–H reductive elimination steps compared to
the corresponding processes catalyzed by RhCl­(PPh3)3. In addition, the water in solvent was found to assist the
O–H reductive elimination and thus facilitates formation of
the 6–7-fused bicyclic alcohol (P2). The predicted
mechanisms provide a successful interpretation of the experimental
observations.