Origin of enantioselection in chiral alcohol oxidation catalyzed by Pd[(-)-sparteine]Cl2.
A kinetic investigation into the origin of enantioselectivity for the Pd[(-)-sparteine]Cl(2)-catalyzed aerobic oxidative kinetic resolution (OKR) is reported. A mechanism to account for a newly discovered chloride dissociation from Pd[(-)-sparteine]Cl(2) prior to alcohol binding is proposed. The mechanism includes (1) chloride dissociation from Pd[(-)-sparteine]Cl(2) to form cationic Pd(-)-sparteine]Cl, (2) alcohol binding, (3) deprotonation of Pd-bound alcohol to form a Pd-alkoxide, and (4) beta-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 k(rel) values when using the extracted constants. Besides a factor of 10 difference between the relative rates of beta-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.
Mueller
Jaime A
JA
Department of Chemistry, University of Utah, Salt Lake City, Utah 84112, USA.
Cowell
Anne
A
Chandler
Bert D
BD
Sigman
Matthew S
MS
D000438:Alcohols / Q000138:chemical synthesis* / Q000737:chemistry; D002384:Catalysis; D002712:Chlorides / Q000737:chemistry*; D008968:Molecular Conformation; D010084:Oxidation-Reduction; D010165:Palladium / Q000737:chemistry*; D013034:Sparteine / Q000737:chemistry*; D013237:Stereoisomerism
D000438:Alcohols; D002712:Chlorides; D013034:Sparteine; D010165:Palladium