On the Mechanism of the Palladium Catalyzed Intramolecular Pauson−Khand-Type Reaction

Abstract

Density functional theory calculations and experimental studies have been carried out on the intramolecular Pauson-Khand-Type reaction mediated by a PdCl(2)-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 Pd(IV) 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 PdCl(2) 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 Pd(IV) intermediate is studied in detail and provides general insights into these novel steps.

Oxime‐Derived Palladacycles: Applications in Catalysis

“…Allyl propargyl ethers and amines as well as malonates are transformed efficiently into the corresponding fused cyclopentenones (Scheme ). The presence of LiCl and pyridine play a critical role in the catalytic cycles of these PKRs, the chloride being essential for the chloropalladation of the alkyne moiety, and therefore a Pd II /Pd IV mechanism has been proposed.…”

ChemInform Abstract: Oxime‐Derived Palladacycles: Applications in Catalysis

“…ChemCatChem 2016, 8,1865 -1881 www.chemcatchem.org ciently into the corresponding fused cyclopentenones (Scheme 37). The presence of LiCl and pyridine play ac ritical role in the catalytic cycles of these PKRs, the chloride being essential for the chloropalladation of the alkyne moiety, [110] and therefore aP d II /Pd IV mechanism has been proposed.…”

Facile Synthesis of Novel Chiral Bicyclic Thioureas and Their Crystal Structures

“…On one hand, chiral bifunctional thioureas are excellent organocatalysts [1][2][3][4][5] due to their unique dual activations of both of electrophile and nucleophile, and now it is known as the term of hydrogenbond-donor organocatalysts [6][7][8]. On the other hand, chiral thioureas were used as efficient ligands in Pdcatalyzed C-C bond formations [9][10][11][12] and Rh-catalyzed asymmetric reactions [13,14] and show moderate to good enantioselectiveties. Until now, most of these chiral thioureas are derived from cinchona alkaloids [15][16][17], 1, 2-cyclohexyldiamine [18,19], binaphthol [20,21], glucose [22,23] and L-proline [24][25][26], therefore, the exploration of preparation and application of novel chiral thioureas based on the other backbones is a challenging and interesting objective.…”