Homogeneous epoxidation

SCHEME 68. Comparison of the transition states of heterogeneous and homogeneous epoxidations of aUyUc alcohols... 

On the other hand, in homogeneous epoxidation catalysis, De Vos et al. [6] have explored the properties of manganese (II) salts in the presence of triazacyclononane derivatives and... 

Dialkylzinc reagents combine with BINOL to generate, in situ, a catalyst for homogeneous epoxidation of (/y)-o /3-enoncs to the corresponding f raws-epoxy ketones. TBHP and cumene hydroperoxide (CHP) are effective terminal oxidants for this process ees of up to 96% have been achieved. Mechanistic investigations point towards an electrophilic activation (Scheme 11) of the substrates by the chiral BINOL-zinc catalyst and a subsequent nucleophilic attack of the oxidant227... 

Adapted with permission from Herrmann, W. A. Fischer, R. W. Marz, D. W. Angew. Chem., Int. Ed. Engl. 1991, 30, 1638-1640 also adapted from Herrmann, W. A. Fischer, R. W. Rauch, M. U, Scherer, W. Alkylrhenium Oxides as Homogeneous Epoxidation Catalysts Activity, Selectivity, Stability, Deactivation-, J. Mol. Catal. 1994,86,243-266 (copyright 1994) with kind permission of Elsevier Science— NL, Sara Burgerhertstraat 25, 1055 KV Amsterdam, The Netherlands. 

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... 

HOMOGENEOUS EPOXIDATION BY EARLY TRANSITION METALS (LEWIS ACID MECHANISM)... 

HOMOGENEOUS EPOXIDATION BY LATE TRANSITION METALS (REDOX MECHANISM)... 

W. A. Herrmann, R. W. Fischer, M. U. Rauch, W. Scherer, Multiple bonds between main-group elements transition metals. 125. Alkylrhenium oxides as homogeneous epoxidation catalysts Activity, selectivity, stability, deactivation, J. Mol. Catal. 86 (1994) 243. 

Heferogeneous olefin epoxidation over solid titania-silica catalysts has been the subject of numerous publications in the open literature. The general picture that emerges is that isolated titanium (IV) species on a silica surface or in a zeolife mafrix are responsible for the high epoxidation activity [2]. This picture is supported by model catalyst work on titanium silasesquioxane complexes [3,4] that form active homogeneous epoxidation catalysts [5] and by various successful atfempfs fo prepare well-defined, site-isolated titanium complexes by grafting molecular precursors on mesoporous silica [6-9]. These site-isolated titanium complexes have been shown to possess catalytic activity in olefin epoxidation. 

In fact, the small pore opening of the MFI and MEL structures (= 5.5 A) makes that hydrogen peroxide is the single stable peroxide capable of entering the zeolite channels and, therefore, permitting the reaction. Yet, tert-butyl hydroperoxide (TBHP) has proved to be an efficient oxidant in the homogeneous epoxidation of olefins catalyzed by titanium alkoxides [4]. 

Configurations of conformationally homogeneous epoxides in trans-bicyclo-[3,3,0]octanes and bicyclo[4,3,0]nonanes have been established from the n.m.r. spectra in the presence of Eufdpmlj. ... 

The epoxide is used as a monomer for polymer production. The byproduct ethylbenzene alcohol can be dehydrated to styrene, also a monomer for the production of polymers. If isobutane is used, iso-butylhydroperoxide replaces ethylbenzene-hydroperoxide as the oxidant. The byproduct tert-butanol can be converted with methanol to an ether that is an important additive in new environmental friendly gasolines. Complexes of Mo, V, or Ti are used in homogeneous epoxidation catalysis, while heterogeneous Ti02/Si02 catalysts can be used also. The active sites consist of a titanium ion with a fourfold coordination of oxygen in a tetrahedral geometry. Titanium acts essentially as a Lewis acid to activate the 0-0 bond in the hydroperoxide. 

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