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[摘要]：A novel and general biomimetic non-heme Fe-catalyzed asym. epoxidn. of arom. alkenes by using hydrogen peroxide is reported herein. The catalyst consists of ferric chloride hexahydrate (FeCl3?H2O), pyridine-2,6-dicarboxylic acid (H2(pydic)), and readily accessible chiral N-arenesulfonyl-N'-benzyl-substituted ethylenediamine ligands. The asym. epoxidn. of styrenes with this system gave high conversions but poor enantiomeric excesses (ee), whereas larger alkenes gave high conversions and ee values. For the epoxidn. of trans-stilbene (1a), the ligands (S,S)-N-(4-toluenesulfonyl)-1,2-diphenylethylenediamine ((S,S)-4a) and its N'-benzylated deriv. ((S,S)-5a) gave opposite enantiomers of trans-stilbene oxide, i.e., (S,S)-2a and (R,R)-2a, resp. The enantioselectivity of alkene epoxidn. 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)]譎Cl (L*=(S,S)-4a or (S,S)-5a in the catalyst mixt.), which were identified by ESIMS. A UV/Vis study of the catalyst mixt., which consisted of FeCl3?H2O, H2(pydic), and (S,S)-4a, suggested the formation of a new species with an absorbance peak at l = 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 kH/kD = 0.93 for the b carbon and kH/kD = 0.97 for the a carbon, which suggested an unsym. transition state with stepwise O transfer. Competitive epoxidn. of para-substituted styrenes revealed a linear dual-parameter Hammett plot with a slope of 1.00. Under std. conditions, epoxidn. of 1a in the presence of ten equiv. of H218O resulted in an absence of the isotopic label in (S,S)-2a. A pos. nonlinear effect was obsd. during the epoxidn. of 1a in the presence of (S,S)-5a and (R,R)-5a. |
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