Reinvestigating FeBr<sub>3</sub>-Catalyzed Alcohol Oxidation with H<sub>2</sub>O<sub>2</sub>: Is a High-Valent Iron Species (HIS) or a Reactive Brominating Species (RBS) Responsible for Alcohol Oxidation?
Chenxi He, Foqing Ma, Wei Zhang, Rongbiao Tong
Abstract
In 2003, Martı́n et al. reported a green alcohol oxidation with FeBr3(cat.)/H2O2 and proposed a high-valent iron species (HIS) responsible for the alcohol oxidation. Reinvestigating this FeBr3(cat.)/H2O2 method led us to propose a different mechanism that involves a reactive brominating species (RBS) for the oxidation of alcohols. The evidence to support this RBS-based mechanism includes (1) our recent findings of in situ-generated RBS from the related FeBr2/H2O2 or CeBr3/H2O2 systems, (2) our results of a series of controlled experiments, and (3) some related RBS-based precedents (NBS, NBA, or Br2) showing similar high oxidation selectivity of secondary over primary alcohols. These studies enable us to discover that a RBS from CeBr3/H2O2 is much more efficient for the oxidation of secondary and benzylic alcohols, which represents a new green protocol for selective oxidation of alcohols to carbonyls.