Dramatic Effect of Carboxylic Acid on the Electronic Structure of the Active Species in Fe(PDP)-Catalyzed Asymmetric Epoxidation

Abstract

The electronic structure of the iron–oxygen intermediates responsible for catalytic transformations in the biomimetic catalyst systems [((S,S)-PDP)FeII(OTf)2]/H2O2/RCOOH has been found to be strongly dependent on the structure of the carboxylic acid RCOOH. For carboxylic acids with primary and secondary α-carbon atoms (acetic acid, butyric acid, caproic acid), the active species exhibit electron paramagnetic resonance (EPR) spectra with large g-factor anisotropy (g1 = 2.7, g2 = 2.4, g3 = 1.7), whereas for those with tertiary α-carbon atoms (2-ethylhexanoic acid, valproic acid, 2-ethylbutyric acid), the active species display EPR spectra with small g-factor anisotropy (g1 = 2.07, g2 = 2.01, g3 = 1.96). The EPR spectra of the latter intermediates are very similar to those of the intermediates previously assigned to oxoiron(V) species. The systems featuring intermediates of the second type ensure higher enantioselection in the epoxidation of electron-deficient olefins.