Mechanism and Selectivity in Rhodium-Catalyzed [7 + 2] Cycloaddition and Cyclopropanation/Cyclization of Allenylcyclopentane-alkynes: Metallacycle-Directed C(sp³)‑C(sp³) vs C(sp³)‑H Activation

The rhodium-catalyzed [7 + 2] cycloaddition and cyclopropanation/cyclization of allenylcyclopentane-alkynes developed by Mukai and co-workers (J. Am. Chem. Soc. 2012, 134, 19580−19583) represents a rare example of the metallacycle-directed selective C­(sp³)-C­(sp³) vs C­(sp³)-H activation. In this article, the mechanism of this intriguing reaction is investigated by means of density functional theory calculations. The calculations show that the reaction is initiated by an oxidative cyclization to form the key rhodacycle intermediate. The subsequent competing β-carbon elimination/C­(sp³)-C­(sp²) reductive elimination and metal-assisted σ-bond metathesis/C­(sp³)-C­(sp³) reductive elimination lead to the [7 + 2] cycloaddition and cyclopropanation/cyclization, respectively. The calculations reproduce quite well the experimentally observed ligand-controlled selectivity, showing that both electronic and steric effects of the ligand have an important impact on the selectivity. In particular, the ligand can significantly affect energy barriers of the metal-assisted σ-bond metathesis and C–C reductive elimination but toward opposite directions, resulting in a selectivity switch between the [7 + 2] cycloaddition and cyclopropanation/cyclization upon ligand change