Organorhenium oxides

Apart from the wealth of monoalkyl- and monoarylrhenium complexes Re03R (type I), other mono- and dinuclear organorhenium oxides in the oxidation states seven (types II and III) and six (types IV to VII) have been reported (see Scheme 9.6) [119-121]. The first representative of the type II family was Re02(CH2SiMe3)3, which was obtained by Wilkinson s group in 1975, albeit in low yield [123]. Mertis and Wilkinson prepared also the... 

Scheme 9.6 The series of the known mono- and dinuclear organorhenium oxides in the oxidation states seven (types I III) and six (types IV-VII)...

It is a precursor for organorhenium oxides which catalyse olefin oxidatiort, metathesis and other trarrsformations. It is a Lewis-acdic metal oxide which forms adducts of the formtrla Re2O7.nL with a variety of O, N and S ligands such as 1,2-dimetiioxyethane, 4,4 -di-ieri-butyl-2,2 -bipyridine, and 1,4,7-trithiacyclononane respectively [Herrmarm et al. Inorg Chem 34 4701 1995]. 

Today, there is a whole range of organorhenium oxides available, and these can be considered as one of the best examined classes of organometallic compounds [63,64]. From a catalytic point of view, though, MTO is one of few organorhenium oxides that... 

Adam, W, Herrmann, W., Lin, J., et al. (1994). Homogeneous-catalytic Oxidation of Arenes and a New Synthesis of Vitamin K3, Angew. Chem. Int. Ed. Engl, 33, pp. 2475-2477 Herrmann, W, Correia, G., Fischer, R., et al. (1995). Oxidation of Electron-rich Aromatic Componnds Using Organorhenium Oxide Catalysts, European Patent EP 665209 Fischer, R., Haider, J., Herrmann, W., et al. (1998). Rhenium Catalysts for Selective Oxidation of... 

In order to circumvent this problem, there has been significant activity directed toward the search for a less environmentally toxic and more selective oxidizing agent than chromium. For example, Hoechst has patented a process which uses organorhenium compounds. At a 75% conversion, a mixture of 86% of 2-methyl-l,4-naphthoquinone and 14% 6-methyl-l,4-naphthoquinone was obtained (60). Ceric sulfate (61) and electrochemistry (62,63) have also been used. 

Another route to immobilising an organorhenium (Vll) oxide derived from MTO on the surface of an iodosilane-modified MCM-41 was applied (Scheme 16). 

Considerable improvements have been made in the field of organorhenium and organomolybdenum oxide catalyst heterogenisation over the last decade. These include, in particular, the application of zeolites and mesoporous materials as supporting systems. Such heterogeneous systems have proven to be, in most cases, efficient and selective catalysts in various reactions, especially olefin epoxidation and (to a lesser extent) silane oxidation, other oxidation... 

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