Origin of Enantioselection in Chiral Alcohol Oxidation Catalyzed by Pd[(−)-sparteine]Cl2

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Abstract
A kinetic investigation into the origin of enantioselectivity for the Pd[(−)-sparteine]Cl2-catalyzed aerobic oxidative kinetic resolution (OKR) is reported. A mechanism to account for a newly discovered chloride dissociation from Pd[(−)-sparteine]Cl2 prior to alcohol binding is proposed. The mechanism includes (1) chloride dissociation from Pd[(−)-sparteine]Cl2 to form cationic Pd(−)-sparteine]Cl, (2) alcohol binding, (3) deprotonation of Pd-bound alcohol to form a Pd−alkoxide, and (4) β-hydride elimination of Pd−alkoxide to form ketone product and a Pd−hydride. Utilizing the addition of (−)-sparteine HCl to control the [Cl-] and [H+] and the resulting derived rate law, the key microscopic kinetic and thermodynamic constants were extracted for each enantiomer of sec-phenethyl alcohol. These constants allow for the successful simulation of the oxidation rate in the presence of exogenous (−)-sparteine HCl. A rate law for oxidation of the racemic alcohol was derived that allows for the successful prediction of the experimentally measured krel values when using the extracted constants. Besides a factor of 10 difference between the relative rates of β-hydride elimination for the enantiomers, the main enhancement in enantiodetermination results from a concentration effect of (−)-sparteine HCl and the relative rates of reprotonation of the diastereomeric Pd−alkoxides.

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