Oxo species

Epoxidation systems based on molybdenum and tungsten catalysts have been extensively studied for more than 40 years. The typical catalysts - MoVI-oxo or WVI-oxo species - do, however, behave rather differently, depending on whether anionic or neutral complexes are employed. Whereas the anionic catalysts, especially the use of tungstates under phase-transfer conditions, are able to activate aqueous hydrogen peroxide efficiently for the formation of epoxides, neutral molybdenum or tungsten complexes do react with hydrogen peroxide, but better selectivities are often achieved with organic hydroperoxides (e.g., TBHP) as terminal oxidants [44, 45],... 

Scheme 10.5 Tentative mechanism for cytochrome P450-cata-lyzed epoxidation of a double bond. The reactive iron-oxo species VII (see Scheme 10.4) reacts with the olefin to give a charge transfer (CT) complex. This complex then resolves into the epoxide either through a radical or through a cationic intermediate.

Scheme 10.27 Catalytic cycle of HppE. Dashed arrows indicate electron transport. In this scheme HPP binds to iron1". After a one-electron reduction, dioxygen binds and reoxidizes the iron center. The peroxide radical is capable of stereospecifically abstracting the (pro-R) hydrogen. Another one-electron reduction is required to reduce one peroxide oxygen to water. Epoxide formation is mediated by the resulting ironlv-oxo species.

Nothing is known about the identity of the iron species responsible for dehydrogenation of the substrate. Iron-oxo species such as FeIV=0 or Fem-OOH are postulated as the oxidants in most heme or non-heme iron oxygenases. It has to be considered that any mechanistic model proposed must account not only for the observed stereochemistry but also for the lack of hydroxylation activity and its inability to convert the olefinic substrate. Furthermore, no HppE sequence homo-logue is to be found in protein databases. Further studies should shed more light on the mechanism with which this unique enzyme operates. 

Even more exciting was the discovery of bands at higher frequency (430-1600 cm ). Exchanging the sample in H2 0 resulted in a shift to lower frequency. A shift to lower frequency was also observed when the protein was enriched in Fe or Fe (Fig. 6). These data indicated the presence of an iron-oxo species. It was argued that this functionality may be related to the physiological function. 

Abstract In this review, recent developments of iron-catalyzed oxidations of olefins (epoxidation), alkanes, arenes, and alcohols are summarized. Special focus is given on the ligand systems and the catalytic performance of the iron complexes. In addition, the mechanistic involvement of high-valent iron-oxo species is discussed. 

The formation of ROO and RO radicals, and M" =0 species is expected when phthalocyanines and porphyrins are used as catalysts (21, 22). The formation of the epoxide, Fignre 49.1, may be associated with the attack of metal oxo species (Fe = O) to the double bond (23). For a-pinene this attack is possibly favored by its rigid structure that causes an orbital overlapping, making the allylic hydrogen abstraction difficult (24). 

Fig. 3. Proposed routes for conversion of the peroxo intermediate to intermediate Q, one involving loss of water (left-hand side) and one not (right-hand side). In the former case the resulting diiron(IV) oxo species could bind an oxygen atom with one iron, or the oxygen could be bound symmetrically by both iron atoms. Although written as an iron(IV) oxo species, Q can also be formulated as an iron(III) oxyl radical complex (35,51).

Yoshizawa, K., Shiota, Y., Yamabe, T., 1999, Intrinsic Reaction Coordinate Analysis of the Conversion of Methane to Methanol by an Iron-Oxo Species A Study of Crossing Seams of Potential Energy Surfaces , J. Chem. Phys., Ill, 538. 

Catalytic turn-over [59,60] in McMurry couplings [61], Nozaki-Hiyama reactions [62,63], and pinacol couplings [64,65] has been reported by Fiirst-ner and by Hirao by in situ silylation of titanium, chromium and vanadium oxo species with McaSiCl. In the epoxide-opening reactions, protonation can be employed for mediating catalytic turn-over instead of silylation because the intermediate radicals are stable toward protic conditions. The amount of Cp2TiCl needed for achieving isolated yields similar to the stoichiometric process can be reduced to 1-10 mol% by using 2,4,6-collidine hydrochloride or 2,6-lutidine hydrochloride as the acid and Zn or Mn dust as the reduc-tant (Scheme 9) [66,67]. 

Figure 2.26. Energy landscape (BP/DNP) for the reaction of the copper-oxo species Cu—OJZSM-5 with N20, N02, and NO molecules, including associated spin isomers calculated for the M5 site (the values are given in kcal x mol-1).

H. B. Gray Work on the excited states of soluble metal oxo species that in a sense are the homogeneous-solution analogues of Ti02-type materials is a promising direction to take, in my view. 

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