Methylrhenium trioxide

In summary, the groups of Espenson and Loh observe catalysis of Diels-Alder reactions involving monodentate reactants by Lewis acids in water. If their observations reflect Lewis-acid catalysis, involvirg coordination and concomitant activation of the dienophile, we would conclude that Lewis-acid catalysis in water need not suffer from a limitation to chelating reactants. This conclusion contradicts our observations which have invariably stressed the importance of a chelating potential of the dienophile. Hence it was decided to investigate the effect of indium trichloride and methylrhenium trioxide under homogeneous conditions. 

Pyridyl)hydrazine (Aldrich), 4-acetylpyridine (Acros), N,N,N -trimethylethylenediamine (Aldrich), methylrhenium trioxide (Aldrich), InQj (Aldrich), Cu(N0j)2-3H20 (Merck), Ni(N03)2-6Il20 (Merck), Yb(OTf)3(Fluka), Sc(OTf)3 (Fluka), 2-(aminomethyl)pyridine (Acros), benzylideneacetone (Aldrich), and chalcone (Aldrich) were of the highest purity available. Borane dimethyl sulfide (2M solution in THE) was obtained from Aldrich. Methyl vinyl ketone was distilled prior to use. Cyclopentadiene was prepared from its dimer immediately before use. (R)-l-acetyl-5-isopropoxy-3-pyrrolin-2-one (4.15) has been kindly provided by Prof H. Hiemstra (University of Amsterdam). 

Indium trichloride [30] and methylrhenium trioxide [31] catalyze the aqueous Diels-Alder reaction of acrolein and acrylates with cyclic and open-chain dienes. Some examples of the cycloaddition of methyl vinyl ketone with 1,3-cyclohexadiene are reported in Scheme 6.18. MeReOs does not give satisfactory yields for acroleins and methyl vinyl ketones with substituents at the jS-position and favors the self-Diels-Alder reaction of diene. 

Study of the kinetics of the oxidation of asymmetric secondary hydroxylamines to nitrones with H2O2, catalyzed by methylrhenium trioxide, has led to the elucidation of the mechanism of the reaction (104). Full transformation of N,N -disubstituted hydroxylamines into nitrones upon treatment with H2O2 occurs on using polymeric heterogeneous catalysts such as polymer-supported methylrhenium trioxide systems (105). 

Diels-Alder reactions are one of the most famous examples which are accderated by a Lewis acid. Various water-stable Lewis adds such as Ln(OTf)3,1371 methylrhenium trioxide,1381 copper nitrate,1391 copper bis(dodecyl sulfate) (4b),1401 indium chloride,1411 and bismuth triflate1421 have been used for Diels-Alder and aza-Diels-Alder reactions in water. Furthermore, a catalytic asymmetric Dids-Alder reaction in water using a copper complex of an amino... 

The reverse reaction is catalysed by copper sulphate in an ethanol/water (50 50) mixture297 298. Indium(III) chloride catalysis of Diels-Alder reactions was also reported, but the effects were poor and comparison to uncatalysed reactions was made only in a few cases299,300. A very versatile Lewis acid catalyst for such reactions is methylrhenium trioxide (MTO)300. This catalyst can be used without a solvent, in pure organic solvents like chloroform and even in pure water. While the catalyst is active in the latter two solvents (Table 22), it gives the best results in water (Table 23). 

The oxidation chemistry of methylrhenium trioxide (MTO) has been reviewed.72 The oxidation of thiophenes by hydrogen peroxide has also been studied, using MTO as a catalyst.73 The latter reacts with H2O2 to generate 1 1 and 1 2 rhenium peroxides, which are able to transfer an oxygen atom to the sulfur of the substrate, to give first the sulfoxide and then the sulfone. Whilst electron-donating substituents accelerate the first oxidation, the reverse trend is observed for oxidation of the sulfoxide. 

The reaction of diazo compounds with aldehydes is catalyzed by methylrhenium trioxide and in the presence of PR3 (R = Bu, Ph) as O-acceptor leads to the formation of alkenes 100... 

Methylrhenium trioxide is an effective catalyst for the epoxidation of olefins by aqueous H202. The reaction is accelerated by pyridine and efficiently oxidizes simple and functionalized 1-alkenes.165... 

Diels-Alder reactions of cyclopentadiene and methacrolein with crotonaldehyde are also catalyzed by complexes formed in situ between NbCls or TaCls and bidentate ligands (2 equiv.) such as L-tartrate esters, or a-amino acids (e.g. tryptophan, alanine). Yields with the Ta catalysts are often somewhat better (14-78 %) than with the Nb catalysts. Good exo. endo ratios are obtained but enantioselectivities are still low (7-40 % ee) [185]. Methylrhenium trioxide is an efficient catalyst in these reactions and its best performance is in aqueous solution. Acrolein derivatives and methyl vinyl ketones react with a variety of dienes to give single diastereoisomers in very high yield with as little as 1 % catalyst loading [186]. Examples are shown in Sch. 49. The reaction is sluggish with disubstituted dienophiles and dienes. 

Methylrhenium trioxide MTO can be stored at room temperature without decomposition. The compound forms pale yellow needles, m.p. 112 °C. Direct exposure to light should be avoided. 

A. Al-Ajlouiri, H. Espenson, Epoxidation of styrenes by hydrogen peroxide as catalyzed by methylrhenium trioxide,/. Am. Chem. Soc. 117 (1995) 9243. 

O. Pestovski, R.v. Eldik, P. Huston, J. H. Espenson, Mechanistic study of the co-ordination of hydrogen peroxide to methylrhenium trioxide, /. Chem. Soc. Dalton Trans. (1995) 133. 

P. J. Hansen, J. H. Espenson, Oxidation of chloride ions by hydrogen peroxide, catalyzed by methylrhenium trioxide, Inorg. Chem. 34 (1995) 5839. 

R. Saladino, V. Neri, A. R. Pelliccia, R. Caminiti, C. Sadun, Preparation and structural characterization of polymer-supported methylrhenium trioxide systems as efficient and selective catalysts for the epoxidation of olefins, J. Org. Chem. 67 (2002) 1323. 

Methylrhenium trioxide (CH3Re03) has proved to be an excellent catalyst in organic solvents, and in water when the dienophile is an a, -unsaturated ketone (or aldehyde). Nearly exclusively one product isomer was formed, the same one that usually predominates [47]. Likewise, scandium triflate [48] and indium trichloride [49] were found to catalyze the Diels-Alder reaction in a tetrahydro-furan/water mixture and in pure water, respectively. 

One problem relating to the oxidation of hydrocarbons with hydrogen peroxide is the difficulty of having appreciable concentrations of the non-polar substrate and the polar oxidant together at the catalytic centre. One approach is based on the attachment of a mixture of polyethylene oxide) and polypropylene oxide) to a silica, followed by the physisorption of methylrhenium trioxide (Figure 4.7) allow the efficient mixing of both reaction partners.81 The material catalyses the efficient epoxidation of alkenes with hydrogen peroxide. 

In addition to those reactions described next, those promoted by the H Oj-MeReOj system are listed under methylrhenium trioxide. 

Other Catalysts. Other catalysts with metals of rhenium and platinum have shown catalytic reactivities for cyclopropanation. Methylrhenium trioxide (MTO) was the first rhenium catalyst for catalytic cyclopropanation, with yields of 57-87% obtained for the cyclopropanation of alkyl or aryl alkenes with EDA (45). As for platinum, a number of complexes have been screened for cyclopropanation catalytic activity (46). PtCl4 was the most active, giving good yield (79%) of cyclopropane from styrene and EDA. However, all reactions had to proceed at elevated temperature. Nonmetal catalysts such as tris(4-bromophenyl)-aminium hexachloroantimonate have been utilized as catalysts for mechanistic studies of cyclopropanation of a series of raras-stilbenes with EDA (47). A cation radical mechanism for this catalysis has been proposed. 

Arene oxide intermediates were postulated for the acid-catalyzed oxidation of arenes by dimeth-yldioxirane [46] and aromatic hydroxylations with hydrogen peroxide activated by methylrhenium trioxide (MTO, CH ReOj) [47,48]. Scheme 14.9 shows a proposed mechanism for pseudocumene (PC) oxidation with the MTO/H O system. 

The catalysts applied to alkene epoxidation in fluorinated alcohol solvents can be subdivided into those which are metal/chalconide-based and those which are purely organic in nature (Scheme 4.5). The former comprise arsanes/arsane oxides [27,28], arsonic acids [29, 30], seleninic acids/diselenides ]31-35], and rhenium compounds such as Re207 and MTO (methylrhenium trioxide) ]36,37]. As shown in Scheme 4.5, their catalytic activity is ascribed to the intermediate formation of, for example, perseleninic/perarsonic adds or bisperoxorhenium complexes. In other words, their catalytic effect is due to the equilibrium transformation of hydrogen peroxide to kmetically more active peroxidic spedes. 

Chemicals and solvents are purchased from commercial sources and used as received. The rhenium products are stable toward O2 and moisture, and so the synthetic procedures can be carried out in air. Methylrhenium trioxide is prepared by a literature route. Abbreviations used MTO = methyltrioxorhenium(VII) H2EDT = 2-ethanedithiol H(HQ) = 8-hydroxyquinoline. 

Saladino, R., Neri, V., Pelliccia, A., et al. (2002). Preparation and Structural Characterization of Polymer-Supported Methylrhenium Trioxide Systems as Efficient and Selective Catalysts for the Epoxidation of Olefins, J. Org. Chem., 67, pp. 1323-1332 Saladino, R., Neri, V., Pelhccia,... 

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