Reinvestigating FeBr3-Catalyzed Alcohol Oxidation with H2O2: Is a High-Valent Iron Species (HIS) or a Reactive Brominating Species (RBS) Responsible for Alcohol Oxidation?
Chenxi He, Foqing Ma, Wei Zhang and Rongbiao Tong*
*The Hong Kong University of Science and Technology, Clearwater Bay, Kowloon, 999077 Hong Kong, China,
Email: rtong
ust.hk
C. He, F. Ma, W. Zhang, R. Tong, Org. Lett., 2022, 24, 3499-3503.
DOI: 10.1021/acs.orglett.2c01133
Abstract
CeBr3/H2O2 is a very efficient system for the green oxidation of secondary and benzylic alcohols to carbonyls. The mechanism involves the generation of a reactive brominating species (RBS) with high oxidation selectivity of secondary over primary alcohols.
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proposed mechanism
Details
In 2003, Martín et al. reported a green method for alcohol oxidation using FeBr3 and H2O2, proposing a high-valent iron species (HIS) as the active oxidant. However, recent reinvestigations suggest that a reactive brominating species (RBS) is responsible for the oxidation. Evidence includes findings from FeBr2/H2O2 and CeBr3/H2O2 systems, controlled experiments, and precedents showing similar selectivity for secondary over primary alcohols. The study found that CeBr3/H2O2 is more efficient for oxidizing secondary and benzylic alcohols, offering a new green protocol for selective oxidation to carbonyl compounds. Mechanistic studies revealed that RBS is generated in situ through Fenton chemistry, supporting the RBS-based mechanism over the HIS hypothesis. The FeBr3/H2O2 system showed high selectivity for secondary alcohols, consistent with RBS-mediated oxidation. Further experiments demonstrated that CeBr3/H2O2 could efficiently oxidize various secondary alcohols to ketones and benzylic alcohols to benzaldehydes. The proposed mechanism involves Fenton-like chemistry generating reactive oxygen species, which oxidize bromide ions to RBS, leading to alcohol oxidation. This new understanding and the development of the CeBr3/H2O2 system provide a more efficient and green method for selective alcohol oxidation, with potential applications in organic synthesis.
Key Words
ketones, aldehydes, hydrogen peroxide
ID: J54-Y2022
