Olefin Epoxidation Catalysis

Experiments with the isolated bis(peroxo) complex (CH3)Re(02)20 H2O have shown that it is an active species in olefin epoxidation catalysis and several other catalytic reactions [19, 20]. In-situ experiments show that the reaction of MTO with... 

The reaction of olefin epoxidation by peracids was discovered by Prilezhaev [235]. The first observation concerning catalytic olefin epoxidation was made in 1950 by Hawkins [236]. He discovered oxide formation from cyclohexene and 1-octane during the decomposition of cumyl hydroperoxide in the medium of these hydrocarbons in the presence of vanadium pentaoxide. From 1963 to 1965, the Halcon Co. developed and patented the process of preparation of propylene oxide and styrene from propylene and ethylbenzene in which the key stage is the catalytic epoxidation of propylene by ethylbenzene hydroperoxide [237,238]. In 1965, Indictor and Brill [239] published studies on the epoxidation of several olefins by 1,1-dimethylethyl hydroperoxide catalyzed by acetylacetonates of several metals. They observed the high yield of oxide (close to 100% with respect to hydroperoxide) for catalysis by molybdenum, vanadium, and chromium acetylacetonates. The low yield of oxide (15-28%) was observed in the case of catalysis by manganese, cobalt, iron, and copper acetylacetonates. The further studies showed that molybdenum, vanadium, and... 

An important improvement in the catalysis of olefin epoxidation arose with the discovery of methyltrioxorhenium (MTO) and its derivatives as efficient catalysts for olefin epoxidation by Herrmann and coworkers [16-18]. Since then a broad variety of substituted olefins has been successfully used as substrates [103] and the reaction mechanism was studied theoretically [67, 68, 80]. 

Niobia-supported MTO has been prepared either by the deposition of sublimed MTO onto the support, or by the impregnation of the support by a solution of MTO, and has been well characterised [54]. A large variety of oxidation reactions were efficiently performed with niobia-supported MTO, such as olefin metathesis catalysis [53,54], reactions of ethyl diazoacetate, heteroatom oxidation (amine and phosphine oxidations) and olefin epoxidation with hydrogen peroxide [55] (Scheme 13). 

A direct comparison of catalysis of olefin epoxidation with a homogeneous chemical catalyst (Mn salen), an enzyme (CPO), and an antibody resulted in sufficiently high enantioselectivity for all three catalysts, a higher turnover number for the enzyme, and a slightly higher substrate/catalyst ratio for the homogenous catalyst. Criteria for comparison should be quantitative and include catalyst lifetime as well as volumetric productivities, but have been found to depend on the different needs of laboratory synthetic chemists, who need a broadly specific catalyst quickly, versus those of process chemists, who often control catalyst availability and can allow narrow specificity (provided their substrate is accepted) but need high productivity. 

A summary of typical homogeneous catalysts, oxidants used, conditions of use, conversions and yields based on the olefin reactant (imless specified), and TOF is provided at the end of this section (Table 1.6). It is lamented that in general, in the homogeneous epoxidation catalysis field, TOFs are not often reported, as more emphasis is placed on selectivity than on rate. Many times, the reactions are run with hydrogen peroxide as oxidant, in excess because of its tendency to decompose, and the addition is not controlled carefully. The reported TOFs are... 

Fe =0 species have also been implicated in one recent study [65] to explain the dramatic effect of acetic acid in enhancing the epoxide yield and selectivity of olefin oxidations mediated by 6 and 9 (see Section 3) [40]. NMR evidence was obtained by Talsi and coworkers for the formation of Fe =0 species from the reaction of 6 or 9 with H2O2 in the presence of acetic acid at 50 °C. The Fe =0 species may be formed as a consequence of the iron-catalyzed in situ formation of peracetic acid as proposed by Fujita et al. [42], which has been shovm to react with 9 efficiently to form [(TPA)Fe O] [66]. It remains to be established whether such species indeed participate in epoxidation catalysis at higher temperature. 

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