Structural Features and Reactivity of (Sparteine)PdCl2: A Model for Selectivity in the Oxidative Kinetic Resolution of Secondary Alcohols

Abstract

The chiral ligand (−)-sparteine and PdCl₂ catalyze the enantioselective oxidation of secondary alcohols to ketones and thus effect a kinetic resolution. The structural features of sparteine that led to the selectivity observed in the reaction were not clear. Substitution experiments with pyridine derivatives and structural studies of the complexes generated were carried out on (sparteine)PdCl₂ and indicated that the C₁ symmetry of (−)-sparteine is essential to the location of substitution at the metal center. Palladium alkoxides were synthesized from secondary alcohols that are relevant steric models for the kinetic resolution. The solid-state structures of the alkoxides also confirmed the results from the pyridine derivative substitution studies. A model for enantioinduction was developed with C₁ symmetry and Cl⁻ as key features. Further studies of the diastereomers of (−)-sparteine, (−)-α-iso- and (+)-β-isosparteine, in the kinetic resolution showed that these C₂-symmetric counterparts are inferior ligands in this stereoablative reaction [Mohr, J. T., Ebner, D. C., and Stoltz, B. M. Org. Biomol. Chem. 2007, 5, 3571−3576].