Catalysis dioxiranes

In the metal-free epoxidation of enones and enoates, practically useful yields and enantioselectivity have been achieved by using catalysts based on chiral electrophilic ketones, peptides, and chiral phase-transfer agents. (E)-configured acyclic enones are comparatively easy substrates that can be converted to enantiomeri-cally highly enriched epoxides by all three methods. Currently, chiral ketones/ dioxiranes constitute the only catalyst system that enables asymmetric and metal-free epoxidation of (E)-enoates. There seems to be no metal-free method for efficient asymmetric epoxidation of achiral (Z)-enones. Exocyclic (E)-enones have been epoxidized with excellent ee using either phase-transfer catalysis or polyamino acids. In contrast, generation of enantiopure epoxides from normal endocyclic... 

The research reviewed here reflects the intense activity of the preceding 30 years in the field of oxidation catalyst immobilization. Obviously, the literature contains many erroneous and unreliable results, particularly with respect to leaching of active metal components. Examples are the reactions with silica- or alumina-supported Mo, W, or Cr and organic peroxides as the oxidants. The majority of these reports simply deal with homogeneous catalysis. Nevertheless, many concepts have been proposed in which truly heterogeneous catalysis has been obtained. Examples include the Mo-polybenzimidazole epoxidation catalyst (243), the Os-tetrasubstituted dio-late catalyst for dx-dihydroxylation (391), the heteronuclear P-W epoxidation catalysts (359, 377), and the dioxirane systems (406, 407). 

The conversion of alkenes to oxiranes using ketone catalysts in the presence of a terminal oxidant such as Oxone has proved to be an important advance in the past decade, especially for the formation of chiral oxiranes (see Section 1.03.4.3.3(ii)). The conversion of alkenes to oxiranes has been comprehensively reviewed <20020R219>. The formation of the dioxirane species usually proceeds in situ, whether it be the formation of methyl(trifluoromethyl)-dioxirane in an academic setting <1995JOC3887>, or under conditions amenable to large-scale conversion of aromatic alkenes to oxiranes (oxone, acetone, ethyl acetate, no phase-transfer catalysis) <20020PD405>. 

In this section we will consider the asymmetric epoxidation of unactivated olefins. Three membered-rings containing two heteroatoms (dioxiranes, oxazidines, and oxaziridinium salts) are usually employed for the epoxidation of this type of olefin. Especially goods results in the field of the asymmetric epoxidation have been obtained with chiral ketone catalysis and iminium salts. In this section, the landmarks of these two significant areas and some applications of these methodologies will be presented. 

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