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Biomimetic Iron‐Catalyzed Asymmetric Epoxidation of Aromatic Alkenes by Using Hydrogen Peroxide
Author(s) -
Gelalcha Feyissa Gadissa,
Anilkumar Gopinathan,
Tse Man Kin,
Brückner Angelika,
Beller Matthias
Publication year - 2008
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.200800595
Subject(s) - chemistry , hydrogen peroxide , catalysis , steric effects , medicinal chemistry , enantiomeric excess , alkene , pyridine , enantiomer , ethylenediamine , organic chemistry , enantioselective synthesis , photochemistry , polymer chemistry
A novel and general biomimetic non‐heme Fe‐catalyzed asymmetric epoxidation of aromatic alkenes by using hydrogen peroxide is reported herein. The catalyst consists of ferric chloride hexahydrate (FeCl 3 ⋅ 6 H 2 O), pyridine‐2,6‐dicarboxylic acid (H 2 (pydic)), and readily accessible chiral N ‐arenesulfonyl‐ N′‐ benzyl‐substituted ethylenediamine ligands. The asymmetric epoxidation of styrenes with this system gave high conversions but poor enantiomeric excesses ( ee ), whereas larger alkenes gave high conversions and ee values. For the epoxidation of trans ‐stilbene ( 1 a ), the ligands ( S , S )‐ N ‐(4‐toluenesulfonyl)‐1,2‐diphenylethylenediamine (( S , S )‐ 4 a ) and its N′ ‐benzylated derivative (( S , S )‐ 5 a ) gave opposite enantiomers of trans ‐stilbene oxide, that is, ( S , S )‐ 2 a and ( R , R )‐ 2 a , respectively. The enantioselectivity of alkene epoxidation is controlled by steric and electronic factors, although steric effects are more dominant. Preliminary mechanistic studies suggest the in situ formation of several chiral Fe‐complexes, such as [FeCl(L*) 2 (pydic)] ⋅ HCl (L*=( S , S )‐ 4 a or ( S , S )‐ 5 a in the catalyst mixture), which were identified by ESIMS. A UV/Vis study of the catalyst mixture, which consisted of FeCl 3 ⋅ 6 H 2 O, H 2 (pydic), and ( S , S )‐ 4 a , suggested the formation of a new species with an absorbance peak at λ =465 nm upon treatment with hydrogen peroxide. With the aid of two independent spin traps, we could confirm by EPR spectroscopy that the reaction proceeds via radical intermediates. Kinetic studies with deuterated styrenes showed inverse secondary kinetic isotope effects, with values of k H / k D =0.93 for the β carbon and k H / k D =0.97 for the α carbon, which suggested an unsymmetrical transition state with stepwise O transfer. Competitive epoxidation of para ‐substituted styrenes revealed a linear dual‐parameter Hammett plot with a slope of 1.00. Under standard conditions, epoxidation of 1 a in the presence of ten equivalents of H 2 18 O resulted in an absence of the isotopic label in ( S , S )‐ 2 a . A positive nonlinear effect was observed during the epoxidation of 1 a in the presence of ( S , S )‐ 5 a and ( R , R )‐ 5 a .