Bispidin‐Ligandeneffekte in der Eisen‐Wasserstoffperoxid‐Chemie

Abstract

. ‥und doch so verschieden: Nicht‐Häm‐Eisenkomplexe können die stereoselektive Oxidation von C‐H‐ und CC‐Bindungen katalysieren. Die isomeren FeIII‐η1‐Hydroperoxo‐ und FeIII‐η2‐Peroxokomplexe sind dabei wichtige Zwischenprodukte (siehe Bild). Über die spektroskopischen Eigenschaften der FeIII‐Hydroperoxo‐ und Peroxokomplexe wird berichtet.

Catalytic Epoxidation and 1,2‐Dihydroxylation of Olefins with Bispidine–Iron(II)/H2O2 Systems

“…Homolytic cleavage of the OÀO bond also yields oxygencentered radicals, which may give rise to typical radical reactions. For a number of synthetic, low-molecular-weight model systems with tetra-and pentadentate amine/pyridinecontaining ligands there is unambiguous spectroscopic and/or structural evidence for end-on {Fe III -OOH}, side-on {Fe III -(O 2 )}, and {Fe IV =O} intermediates, [4,[12][13][14][15][16][17][18][19] [12][13][14] while oxygen transfer from oxo transfer agents such as peracids or iodosylbenzene is used to produce the ferryl products. [15][16][17][18][19] Direct formation of {Fe IV =O} complexes from the Fe II precursor, using H 2 O 2 , without formation of the usual Fe III intermediates, is another possible pathway.…”

“…[22] In 1 the coligand X is coordinated trans to N3 (equatorial, E) through a short and strong Fe II ÀX bond, while in 2 the X ligand is positioned trans to N7 (axial, A) through a long and weak Fe II ÀX bond. [12,22] The structural difference between the two isomers is manifested in their reactions with H 2 O 2 in MeOH at À40 8C, which afford spectroscopically distinct iron(iii) complexes. [12] The low-spin end-on hydroperoxo-and high-spin side-on peroxo complexes differ in terms of their thermal stability, with that of L 4 being among the most stable species of this class of compounds.…”

“…[12,22] The structural difference between the two isomers is manifested in their reactions with H 2 O 2 in MeOH at À40 8C, which afford spectroscopically distinct iron(iii) complexes. [12] The low-spin end-on hydroperoxo-and high-spin side-on peroxo complexes differ in terms of their thermal stability, with that of L 4 being among the most stable species of this class of compounds. [12] Herein we compare the catalytic capabilities of the two [(L , whereas most of the products are formed in the experiments with 2 within the first half hour, the reactions with 1 take about seven times longer to achieve a comparable product yield (Table 1, entries 4 a-d and 5 a-d).…”

“…[12] The low-spin end-on hydroperoxo-and high-spin side-on peroxo complexes differ in terms of their thermal stability, with that of L 4 being among the most stable species of this class of compounds. [12] Herein we compare the catalytic capabilities of the two [(L , whereas most of the products are formed in the experiments with 2 within the first half hour, the reactions with 1 take about seven times longer to achieve a comparable product yield (Table 1, entries 4 a-d and 5 a-d). Thus, by this criterion, 1 is almost as effective a catalyst as 2, but is much slower in activating the H 2 O 2 oxidant.…”

Formation of an Aqueous Oxoiron(IV) Complex at pH 2–6 from a Nonheme Iron(II) Complex and H2O2

“…[22][23][24] The reaction of 1 with H 2 O 2 in water clearly exhibits an outcome different from the analogous reaction in MeOH, in which a purple Fe III À OOH intermediate is observed. [25] The latter requires 1.5 equivalents of H 2 O 2 in a two-step process that involves an Fe III À OH intermediate (Scheme 1, top). [26] For the reaction of 1 with H 2 O 2 in aqueous media, there is no evidence for the formation of an Fe III ÀOOH chromophore that in turn decays to form 2.…”