42.On the Mechanism of the Palladium Catalyzed Intramolecular Pauson-Khand-Type Reaction
Y. Lan, L. J. Deng, J. Liu, C. Wang, O. Wiest*, Z. Yang*, Y. D. Wu*
Density functional theory calculations and experimental studies have been carried out on the intramolecular Pauson−Khand-Type reaction mediated by a PdCl2-thiourea catalyst, which proceeds under mild reaction conditions and provides a useful alternative to traditional Pauson−Khand reactions. The classical mechanism of the Pauson−Khand reaction involving the alkyne/alkene C−C bond formation as the key step has been found to be energetically unfavorable and is not in line with the experimental observations. A novel reaction mechanism has been proposed for the reaction. The first step involves the cis-halometalation of the alkyne, followed by sequential alkene and carbonyl insertion. The rate-determining fourth step is an intramolecular C−Cl oxidative addition, leading to a PdIV intermediate. A C−C bond formation by reductive elimination completes the reaction. The mechanism is in agreement with the key experimental observations including (1) the need of a chloride for catalytic activity and the absence of catalysis with Pd(OAc)2 alone; (2) the rate acceleration by the addition of LiCl; both with PdCl2 and Pd(OAc)2 catalysts; and (3) the preferred formation of the trans diastereomer in substituted cases. The cis halometalation and the formation and stability of the PdIV intermediate is studied in detail and provides general insights into these novel steps.
