Trioxorhenium

The Re0 7HZ catalyst was prepared by the following procedure. Methyl trioxorhenium (MTO) was sublimed under vacuum at 333 K and the vapor was allowed to enter the chamber, where the zeolites were pretreated in situ at 673 K under vacuum. After the chemical vapor deposition (CVD) into zeolite pores, undeposited MTO was removed by evacuation at RT. The catalyst was treated at 673 K in He before using. 

Stable trioxorhenium(VII) complexes have been isolated with various ligands. A dominating motif is a tripodal donor atom arrangement which stabilizes [Re03L] + complexes perfectly. 

In the earlier volume of this book, the chapter dedicated to transition metal peroxides, written by Mimoun , gave a detailed description of the features of the identified peroxo species and a survey of their reactivity toward hydrocarbons. Here we begin from the point where Mimoun ended, thus we shall analyze the achievements made in the field in the last 20 years. In the first part of our chapter we shall review the newest species identified and characterized as an example we shall discuss in detail an important breakthrough, made more than ten years ago by Herrmann and coworkers who identified mono- and di-peroxo derivatives of methyl-trioxorhenium. With this catalyst, as we shall see in detail later on in the chapter, several remarkable oxidative processes have been developed. Attention will be paid to peroxy and hydroperoxide derivatives, very nnconunon species in 1982. Interesting aspects of the speciation of peroxo and peroxy complexes in solntion, made with the aid of spectroscopic and spectrometric techniqnes, will be also considered. The mechanistic aspects of the metal catalyzed oxidations with peroxides will be only shortly reviewed, with particular attention to some achievements obtained mainly with theoretical calculations. Indeed, for quite a long time there was an active debate in the literature regarding the possible mechanisms operating in particular with nucleophilic substrates. This central theme has been already very well described and discussed, so interested readers are referred to published reviews and book chapters . 

Scheme 16 Immobilisation of trioxorhenium(VII) derived from MTO on an iodosilane-modified MCM...

Scheme 17 Benzaldehyde olefination with ethyl diazoacetate/triphenylphosphine/MCM-supported trioxorhenium(Vll) catalyst...

Unfortunately, many of the organometallic trioxorhenium complexes are light or temperature sensitive, rendering them poor catalysts. Of all the organotri-oxorhenium complexes, only Cp Re03 and MTO have been found to be stable enough to perform well as catalysts and especially the latter is well known as an excellent olefin oxidation catalyst. 

The fact that phenyltrioxorhenium shows some activity in the oxidation reaction gave rise to the synthesis of a N,C,N-pincer trioxorhenium and a C,N-half-pincer... 

Epoxides are a very versatile class of compounds and the interest in catalytic epoxidation reactions is very high.70,71 They are the key raw materials in the syntheses of a wide variety of chemicals. A number of compounds have been shown to be catalytically active, but the regular laboratory reagents for epoxidations are generally methyl trioxorhenium(VII)72-81 and the Jacobsen-Katsuki-catalysts82-94 which can even introduce chirality. They are also theoretically well investigated95-106 and are described below. 

Methyl trioxorhenium (MTO) has proven to be an efficient catalyst in the presence of hydrogen peroxide, which leads to the formation of mono- and bisperoxo compounds an additional aquo ligand has been found to stabilize the latter complex. MTO as well as the corresponding monoperoxo- and bisperoxo complexes have been studied, in both their free and monohydrated forms.96... 

In the early 1990s Herrmann and coworkers [56] reported the use of methyl-trioxorhenium (MTO) as a catalyst for epoxidation with anhydrous H202 in tert-butanol. In the initial publication cyclohexene oxide was obtained in 90% yield using 1 mol% MTO at 10 °C for 5 h. At elevated temperatures (82 °C) the corre-... 

A large number of catalysts have been shown to be active in the oxidation of cycloalkanones to lactones using only hydrogen peroxide as the oxidant. Methyl-trioxorhenium (MTO) is moderately active in the oxidation of linear ketones [242] or higher cycloalkanones [243] but it is particularly active in the oxidation of cyclobutanone derivatives (Fig. 4.82), which are oxidized faster with MTO than with other existing methods [244]. 

The conversion of thioketones to sulfines (R2C=S=0) is difficult to categorize into the sections available, and it placed after oxidation of ketones and aldehydes. The reaction of a thioketone with hydrogen peroxide and a catalytic amount of MTO (methyl trioxorhenium) gives the sulfine. ... 

Therefore, the method described allows a novel economic as well as ecologically sound synthesis of quinone derivatives. Higher condensed arenes, e.g., anthracene, are converted to the quinones or cleaved to dicarboxylic acids, as in the case of phenanthrene (yield ca. 50 %). Besides MTO, in principle all alkyl- and to some extent also aryl-substituted trioxorhenium compounds, e.g., cyclopropylrhenium trioxide or cyclopentadienylrhenium trioxide, can be used as active catalysts. However, until now MTO apparently constitutes the most active and easy-to-handle catalyst (see Figure 1, p. 439). The solvent of choice for the reaction and the workup procedure, is concentrated acetic acid, also used to dilute the H2O2 (85 wt.%), yielding a water-poor reaction medium which is advantageous for the catalyst lifetime. 

D. V. Deubel, The surprising nitrogen-analogue chemistry of the methyl trioxorhenium-catalyzed olefin epoxidation, ]. Am. Chem. Soc. 125 (2003) 15308. 

Boehlow, T.R., and Spilling, C.D., The regio- and stereo-selective epoxidation of alkenes with methyl trioxorhenium and urea-hydrogen peroxide adduct. Tetrahedron Lett., 37, 2717, 1996. 

Table 17 Reaction Energies (kcal/mol) for the Trioxorhenium-Catalyzed Oxidation of Olefins...

FIGURE 3.22 OAT with trioxorhenium complexes such as MTO and CpRe(0)3. 

An important class of reactions is the remote functionalization of alkanes. An efficient C-H insertion reaction of H2O2 into hydrocarbons by homogeneous methyltrioxorhenium(VII) (MTO), heterogeneous poly(4-vinylpyridine)/methyl trioxorhenium (PVP/MTO) and microencapsulated polystyrene/methyltrioxor henium (PS/MTO) systems in ionic liquids, has been developed (Scheme 5.2-130). In some cases higher activity was observed when compared with the same reaction in molecular solvents. The heterogeneous catalysts are stable systems under the reaction conditions and can be recycled for more transformations [282]. 

The oxidation of pyridines to their corresponding N-oxides catalyzed by methyl-trioxorhenium (MTO) was reported to occur with various substituted pyridines [123,124]. The oxidant employed is either H2O2 [123] or MeaSiOOSiMes [124]. The reaction gives high yields with both electron-rich and electron-deficient pyridines (Eq. (8.29)). 

A. Veldkamp and G..Frenking, OrganometalUcs, submitted for publication. Structure and Bonding of Trioxorhenium Compounds LReO, (L = H, F, CHj, C3H5). 

Another variant of homogeneous catalysis towards TMQ makes use of methyl-trioxorhenium (MTO), which efficiently activates H202. Dimethylcarbonate has been shown to be an efficient medium for this transformation, as MTO dissolves very well in this solvent. Photo-oxidation of 3 has been described using porphyrins or metallophthalocyanins. ... 

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