Stereoselectivity epoxidation of alkenes

Stereoselective epoxidation of alkenes, desymmetrization of maso-TV-sulfonylaziri-dines, Baeyer-Villiger oxidation of cyclobutanones, Diels-Alder reactions of 1,2-dihydropyridines, and polymerization of lactides using metal complexes of chiral binaphthyl Schiff-base ligands 03CCR(242)97. 

The first example of the immobilization of a chiral ketone to promote the enan-tioselective epoxidation of alkenes with Oxone has been reported by Sartori and coworkers [322]. They anchored a-fhiorotropinone on KG-60 silica, MCM-41 and a Merrifield resin. The catalysts were tested for the epoxidation of 1-phenylcyclo-hexene but the polymer-supported fhiorotropinone 121 showed a low activity and selectivity. The catalyst immobilized on inorganic supports promoted the stereoselective epoxidation of alkenes with ee values up to 80% and could be reused with the same performance for three runs. 

These reagents may be considered to be one of the elusive aza-analogues of peroxyacids, and there are significant mechanistic similarities between the Rees-Atkinson reaction and the Bartlett epoxidation. Chiral Q-reagents have been used to effect highly stereoselective aziridination of alkenes (Scheme 4.13) [1],... 

Preparation of nonracemic epoxides has been extensively studied in recent years since these compounds represent useful building blocks in stereoselective synthesis, and the epoxide functionality constitutes the essential framework of various namrally occurring and biologically active compounds. The enantiomericaUy enriched a-fluorotropinone was anchored onto amorphous KG-60 silica (Figure 6.6) this supported chiral catalyst (KG-60-FT ) promoted the stereoselective epoxidation of several trans- and trisubstituted alkenes with ees up to 80% and was perfectly reusable with the same performance for at least three catalytic cycles. 

The epoxidation of alkenes is completely stereoselective, i.e. cis-alkenes are transformed into cis-epoxides and frans-alkenes into trans-epoxides. 

Electron withdrawing groups on the alkene considerably retard the epoxidation. For example, acrylic esters and acrylonitrile are not reactive, while allyl chloride is about one tenth as reactive as propylene towards TBHP/MoVI.240 The epoxidation of alkenes is completely stereoselective eri-alkenes are exclusively transformed into ds-epoxides and (runs-alkenes into trans-epoxides. Oxygen addition to the double bond preferentially occurs from the less shielded face of the substrate, e.g. in the selective epoxidation of terpenes by r-pentyl hydroperoxide (t-amyl hydroperoxide, TAHP) (equations 67 and 68).225... 

Addition of pyridine bases to the catalytic system caused a considerable increase in the rate and selectivity of the reaction, reaching 80% yield of styrene oxide for 100% styrene conversion (r.t., 30 min). In the presence of this pyridine-modified system, the reactivity of alkenes is in the order styrene > trisubstituted > cis-disubstituted > trans-disubstituted> monosubstituted. The epoxidation of alkenes is not stereoselective. In the absence of pyridine, cis-stilbene was converted into a 1.8 1 trans cis epoxide mixture, whereas the cis isomer prevails in the presence of excess pyridine ligands. Neither chlorohydrins nor pyridine N-oxides are involved in this catalytic system. Attempts to isolate the reactive intermediate led to the characterization of a relatively stable... 

Molecular-orbital calculations indicate that the stereoselective epoxidation of the alkene 192 by peroxy acids arises from stereoelectronic control exerted by a CF3— C bond orientated anti to the alkene plane, in contrast to the previously proposed model for epoxidation of allylic fluoride in which the F—C bond and alkene bonds are in a syn arrangement305. 

Thallium triacetate, TI(0C0CH3)3 1.5H20 (mp 182 C), like the monoacetate, is used for the stereoselective acetoxylation of alkenes [411] and for oxidations of alkenes to epoxides [412]. 

The development of solid-supported chiral oxidants is a challenging area that has yielded interesting results in the development of a chiral supported dioxiran precursor. The preparation of non-racemic epoxides has been extensively studied in recent years since they are important building blocks in stereoselective synthesis. A supported dioxirane precursor based on a-fluorotropinones was shown to promote the epoxidation of alkenes [28, 29]. The reactant was anchored on meso-porous MCM-41 and amorphous silicas. It has shown comparable activity to its homogenous counterpart and good stability on recycling. The enantiomerically enriched version efficiently promotes the enanhoselective epoxidation of alkenes, with ee values up to 80% (Scheme 4.4). 

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