Chiral Cobalt-Catalyzed Enantiomer-Differentiating Oxidation of Racemic Benzoins by Using Molecular Oxygen as Stoichiometric Oxidant

A study was conducted to demonstrate chiral cobalt-catalyzed enantiomer-differentiating oxidation of racemic benzoins using molecular oxygen as stoichiometric oxidant. The cobalt-catalyzed asymmetric oxidation was performed with racemic benzoins due to their importance in pharmaceutical industry and difficulty in synthesizing them in enantiomerically pure form. The investigation was conducted with (±)-4-methoxy benzoin as model substrate for oxidative kinetic resolution. The oxidation took 7 d for 44% conversion and provided 41% of 4-methoxy benzil when (±)-4-methoxy benzoin was reacted with 5 mol% of TEMPO and 5 mol% of enantiopure (L)-proline-Co(OAc)2 complex in the presence of molecular oxygen at room temperature. It was observed that 52% of 4-methoxy benzoin was recovered with 4% enantiomeric excess and selectivity in the asymmetric oxidation reaction.