Reinvestigating FeBr 3 -Catalyzed Alcohol Oxidation with H 2 O 2 : Is a High-Valent Iron Species (HIS) or a Reactive Brominating Species (RBS) Responsible for Alcohol Oxidation?

In 2003, Martı́n et al. reported a green alcohol oxidation with FeBr 3 (cat.)/H 2 O 2 and proposed a high-valent iron species (HIS) responsible for the alcohol oxidation. Reinvestigating this FeBr 3 (cat.)/H 2 O 2 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 FeBr 2 /H 2 O 2 or CeBr 3 /H 2 O 2 systems, (2) our results of a series of controlled experiments, and (3) some related RBS-based precedents (NBS, NBA, or Br 2 ) showing similar high oxidation selectivity of secondary over primary alcohols. These studies enable us to discover that a RBS from CeBr 3 /H 2 O 2 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.