Dual Role of (−)-Sparteine in the Palladium-Catalyzed Aerobic Oxidative Kinetic Resolution of Secondary Alcohols

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Abstract
The mechanistic details of the palladium-catalyzed aerobic oxidative kinetic resolution of secondary alcohols have been elucidated. (−)-Sparteine was found to have a dual role as a chiral ligand and an exogenous base. Saturation kinetics were observed for the dependence on (−)-sparteine concentration. A first-order dependence on [alcohol] and [catalyst] as well as inhibition by addition of (−)-sparteine HCl were observed. These results are consistent with rate-limiting deprotonation under low (−)-sparteine concentrations and rate-limiting β-hydride elimination using saturating (−)-sparteine concentrations. This conclusion is further supported by a kinetic isotope effect of 1.31 ± 0.04 under saturation. The enantioselectivity events are also controlled by addition of (−)-sparteine in which high concentrations afford a more selective kinetic resolution.
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