Abstract
Density functional theory (DFT) has been used to study the mechanism of three transition-metal catalyzed reactions: Suzuki cross-coupling, Heck olefination, and zirconocene-mediated olefin polymerization. The DFT calculations generally employed the BP86 functional, basis sets of medium size, and a small-core pseudopotential for the metal. Full catalytic cycles were computed, with complete optimization of all intermediates and transition states.
The standard Suzuki cross-coupling reaction starts with an oxidative addition of an aryl halide to a palladium(0) catalyst. The calculations confirm the presence of three-coordinate anionic palladium(0) species as proposed by Amatore and Jutand, but do not provide any evidence for the existence of the postulated five-coordinate palladium(II) complexes. Instead the decisive intermediate is a four-coordinate structure, with linear coordination of the aryl halide via a hypervalent halogen atom, which can then rearrange without significant barriers to enter a catalytic cycle dominated by cis-configured palladium(II) complexes.
For the Suzuki cross-coupling between phenyl boronic acid and acetic anhydride, multiple interconnected catalytic cycles have been studied that start from the neutral Pd(PMe3)2 molecule, the two-coordinate anionic [Pd(PMe3)OAc]– complex, and the three-coordinate anionic [Pd(PMe3)2OAc]– complex. The initial oxidative addition is easier on the anionic pathways because of the higher propensity to coordinate to carbon electrophiles. There are two competing pathways for the subsequent transmetalation step, both of which involve anionic palladium(II) monophosphine complexes, with cis or trans arrangement of the acetate ligands. The final reductive elimination step is rather facile in each case. Overall, the anionic pathways are favored over the neutral pathways in the chosen model system.
Palladium(0) complexes of Staab-type proton sponges derived from quino[7,8-h]quinolines have recently been identified as excellent catalysts for Heck olefination. These proton sponges have been characterized computationally with regard to structure, basicity, electronic properties, and complexation by palladium(II) and palladium(0). The catalytic activity of the palladium(0) complexes in Heck olefination reactions is consistent with the computed relative energies of the intermediates in a plausible catalytic cycle.
A previously proposed single-center, two-state kinetic model for zirconocene-catalyzed ethylene polymerization has been explored computationally by considering different conformers and isomers of the propyl group in the cations [L2Zr-Pr]+ (L=Cp, Cp*; Pr = n-propyl) corresponding to two catalysts with different observed rate orders. The calculations suggest that equilibria involving such conformers and isomers cannot account for the requirements of this kinetic model, implying that interactions with counterions would need to be considered.
For all three investigated types of reaction, the DFT calculations provide detailed mechanistic insight. The computational results for the Suzuki cross-coupling reactions call for a reevaluation of some commonly accepted mechanistic notions.
