Rhenium-catalyzed Epoxidations

The use of rhenium-based systems for the epoxidation of olefins has increased considerably during the last ten years [87]. In 1989, Jgrgensen stated, the catalytic 

The 14e compound MTO readily forms coordination complexes of the type MTO-L and MTO-L2 with anionic and uncharged Lewis bases [96], These yellow adducts are typically five- or six-coordinate complexes, and the Re-L system is highly labile. Apart from their fast hydrolysis in wet solvents, MTO-L adducts are much less thermally stable then MTO itself. The pyridine adduct of MTO, for instance, decomposes even at room temperature. In solution, methyltrioxorhenium displays high stability in acidic aqueous media, although its decomposition is strongly accelerated at increased hydroxide concentrations [97, 98], Thus, under basic aqueous conditions MTO decomposes as shown in Equation (4). 

In this oxidative degradation, MTO decomposes into catalytically inert perrhenate and methanol. The decomposition reaction is accelerated at higher pH, presumably through the reaction between the more potent nucleophile H02- and MTO. The decomposition of MTO under basic conditions is rather problematic, since the selectivity for epoxide formation certainly profits from the use of nonacidic conditions. 

This decomposition is, however, rather slow and does not influence the use of MTO in catalysis to any greater extent. 

MTO reacts with hydrogen peroxide to form a mono-peroxo complex (A) which undergoes further reaction to yield a bis-peroxorhenium complex (B). The formation of the peroxo complexes is evident from the appearance of an intensive yellow color of the solution. Both peroxo complexes (A and B) have been detected by their methyl resonances using and NMR spectroscopy. Furthermore, the structure of the bis-peroxo complex B has been determined by crystallography [58]. In solution, B is the most abundant species in the equilibrium, suggesting that this is the thermodynamically most stable peroxo complex. The coordination of a water molecule to B has been established by NMR spectroscopy, however no such coordination has been observed for A, indicating either no coordinated water or high lability of such a 

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 

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The use of various heterocyclic additives in the MTO-catalyzed epoxidation has been demonstrated to be of great importance for substrate conversion, as well as for the product selectivity. With regard to selectivity, the role of the additive is obviously to protect the product epoxides from deleterious, acid-catalyzed (Brons-ted or Lewis acid) ring-opening reactions. This can be achieved by direct coordination of the heterocyclic additive to the rhenium metal, thereby significantly decreasing its Lewis acidity. In addition, the basic nature of the additives will increase the pH of the reaction media. 

In the case of the rhenium-catalyzed oxidation of methoxy- and hydroxy-substituted substrates, there is some complementary work concerning the general mechanism of the arene oxidation [10b, 11]. Since the major products in the oxidation of such arenes or phenols are the quinones, the formation of intermediary epoxides seems to be a predominant reaction step. When p-substituted phenols such as 2,6-di( -butyl)-4-methylphenol are treated with the MTO/H2O2 oxidant and acetic acid as solvent, the formation of hydroxydienones is observed. This is also reported for the oxidation using dimethyldioxirane as oxidant [20]. Since an arene oxide intermediate was postulated for the dioxirane oxidation, a similar mechanism is plausible here [11], e. g., for the oxidation of l,2,3-trimethoxy-5-methylbenzene (Scheme 3) or 2,6-di(f-butyl)-4-methyl-phenol. 

Regio- and Diastereoselectivity. Unlike many other transition metal-catalyzed epoxidation reactions, Uttle-to-no diastereoselectivity is usually detected in the MTO-catalyzed epoxidation of aUylic alcohols. An extensive comparative study has been undertaken showing that metal alcoholate binding does not apply in MTO-catalyzed epoxidation reactions. This observation tends to suggest that a rhenium peroxo complex is the active oxidant. Corr5)utational experiments have demonstrated that the rhenium bis peroxo) complex (probably the hydrated form) is the active species in the MTO-catalyzed epoxidation of propenol. ... 

Supported oxo-rhenium catalysts in heterogeneous systems have also been reported, for example, the polystyrene-supported (catecholato)oxo-rhenium(VII) complexes (38), obtained from the reaction of polystyrene-supported catechol with [ReOCl3(PPh3)2], which catalyze alcohol oxidation to ketones or aldehydes with dimethylsulfoxide and epoxide... 

Catalytic oxidation of 239 to the quinone 240 was also effected with H2O2 catalyzed by methyltrioxorhenium(VII) (McRcOb) (Scheme 60)", where a small amount of hydroxy-substituted quinone 280 was produced in addition to 240 (70%). In this reaction, MeRe03 is stepwise converted by H2O2 into the mono- and bis(peroxo)rhenium complex MeRe(02)20-H20 (281). This active oxidant then reacts with the phenol to give the epoxide 282, which is further converted to the two quinones (240 and 280). 

Another TM-catalyzed reaction that has been studied at the DFT and ab initio levels of theory is the epoxidation of ethylene with rhenium peroxo complexes. Table 17 shows calculated reaction energies at the MP2, B3LYP, and CCSD(T) levels of theory using basis set II (84). 

The two seminal contributions of Mimoun and Sharpless laboratories led to a controversy on the reaction mechanism that was lasting longer than for two decades [82] and expanded to the olefin epoxidation with other metal peroxo complexes, in particular those of rhenium. Kinetic studies of Al-Ajlouni and Espenson [83,84] on the MTO-catalyzed olefin epoxidation with H2O2 revealed the importance of both mono- and diperoxo species in the catalytic process as well as substituent effects on reaction rates, but the molecular mechanism remained uncertain. 

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