Quantum Mechanical Models for Screening New Ligands for the Enantioselective Oxidation of Secondary Alcohols
The key interactions responsible for the unique reactivity of (--)-sparteine-PdX2 complexes (X = chloride, acetate) in the enantioselective oxidation of secondary alcohols have been elucidated using quantum mechanics (B3LYP DFT with the PBF polarizable continuum solvent model.) From examining many possible pathways, we find the mechanism involves:
1) substitution of the alcohol in place of an X-group
2) deprotonation of the bound alcohol by the deposed anion and free sparteine
3) beta-hydride elimination through a four-coordinate transition state in which the second anion is displaced but tightly associated
4) replacement of the ketone product with the associated anion.
The enantioselectivities observed under base-rich reaction conditions follow directly from calculated energies of diastereomeric beta-hydride elimination transition states incorporating (R) and (S) substrates. This relationship reveals an important role of the anion, namely to communicate the steric interaction of the ligand on one side of the Pd(II) square plane and the substrate on the other side. When no anion is included, no enantioselectivity is predicted. Locating these transition states in different solvents shows that higher dielectrics stabilize the charge separation between the anion and metal and draw the anion farther into solution. Thus the solvent influences the barrier height (rate) and selectivity of the oxidation.
With the ability to predict selectivities and estimate reaction rates for catalyst/substrate combinations, we have screened new ligands and catalyst compositions to determine what structural components will work together to yield stable, selective catalysts.
Personnel: Robert (Smith) Nielsen and Jason Keith
This project is in collaboration with Dr. Brian Stoltz (Caltech)
- Nielsen, R. J.; Keith, J. M.; Stoltz, B. M.; Goddard, W. A. III "A Computational Model Relating Structure and Reactivity in Enantioselective Oxidations of Secondary Alcohols by (-)-Sparteine-Pd(II) Complexes" J. Am. Chem. Soc. 2004, 126, 7967.
- Nielsen, R. J.; Goddard, W. A. III "Mechanism of the Aerobic Oxidation of Alcohols by Palladium Complexes of N-Heterocyclic Carbenes" J. Am. Chem. Soc. accepted.