Stereoselectivity alkene epoxidation

Stereoselective alkene epoxidations have been performed using the ionic liquid bmim PI (, in a biphase with dichloromethane, using a Mn-salen catalyst [10], as shown in Scheme 9.5. This gave yields in excess of 70% and enantiomeric... 

Cr Vsalen) complexes arc widely employed as catalysts of stereoselective alkene epoxidations [3,85,33,15.86], kinetic resolution of epoxides [87-89], alcohol oxidations [90],... 

The radical mechanism has also been proposed as a general mechanism for oxidation of alkenes and aromatics, but several objections have been raised because of the absence of products typically associated with radical reactions. In classical radical reactions, alkenes should react also at the allylic position and give rise to allyl-substituted products, not exclusively epoxides methyl-substituted aromatics should react at the benzylic position. The products expected from such reactions are absent. Another argument was made against the radical mechanism based on the stereoselectivity of epoxidation. Radical intermediates are free to rotate around the C C bond, with the consequence that both cis- and /rani-epoxides are formed from a single alkene isomer, contrary to the evidence obtained with titanium silicates (Clerici et al., 1993). 

Epoxidation. Oxone decomposes in the presence of a ketone (such as acetone) to form a species, possibly a dioxirane (a), which can epoxidize alkenes in high yield in reactions generally conducted in CH2C12-H20 with a phase-transfer catalyst. An added ketone is not necessary for efficient epoxidation of an unsaturated ketone. The method is particularly useful for preparation of epoxides that are unstable to heat or acids and bases.3 The acetone-Oxone system is comparable to m-chloroperbenzoic acid in the stereoselectivity of epoxidation of allylic alcohols. It is also similar to the peracid in preferential attack of the double bond in geraniol (dienol) that is further removed from the hydroxyl group.4... 

The Jacobsen-Katsuki-catalysts (Fig. 13) have recently received much attention as the most widely used alkene epoxidation catalysts. An example of Jacobsen s manganese-salen catalyst is shown in Fig. 13. They promote the stereoselective conversion of prochiral olefins to chiral epoxides with enantiomeric excesses regularly better than 90% and sometimes exceeding 98%.82,89,92,93,128 The oxidation state of the metal changes during the catalytic cycle as shown in Scheme 8. 

Epoxide opening. A stereoselective alkene synthesis starts from reaction of triethylsilyloxirane with an organocuprate reagent, and it is concluded by oxidation of the j8-silyl alcohol to the aldehyde, Grignard reaction and elimination of [Et Si/OH]. The elimination leads to either the ( )- or the (Z)-alkene by using different reagents. 

The epoxidation of an alkene is stereoselective. The epoxidation of m-2-butene, for example, yields only CM-2-butene oxide ... 

Ph3Si ethylene oxide catalytically adds PhSH to give a- and -adducts in 6 1 ratio, and serves as a vinyl dication equivalent with RCu to give alkenes stereoselectively, while epoxides rearrange in the presence of silylated nucleophiles Diphenylmethylsilylation of cyclopropane carboxylates gives the C-silylated ester which then form C-silylated cyclopropyl ketones, P-silyl cyclopropyl carbinols, and alkylidene cyclopropancs, which ring expand to the... 

Mansuy et al. discovered the nitrene transfer reactions from iminoiodinanes to C=C double bond afrbrding aziridines in 1984 [22]. The reaction of styrene with PhI=NTs in the presence of a catalytic amount of Fe(TPP)(Cl) or Mn(TPP) (Cl) complex in dichloromethane at room temperature provided the corresponding aziridine products as shown in Scheme 2.14. TheiV-tosylaziridination proceeded in a stereoselective manner, where the more stable trans products were obtained from either cis- or tranr-stilbene. Those characteristics are totally different from the Fe(TPP)Cl-catalyzed alkene epoxidation using Phl=0, which proceeds in a stereospecific manner. 

The Pd-catalyzed hydrogenolysis of vinyloxiranes with formate affords homoallyl alcohols, rather than allylic alcohols regioselectively. The reaction is stereospecific and proceeds by inversion of the stereochemistry of the C—O bond[394,395]. The stereochemistry of the products is controlled by the geometry of the alkene group in vinyloxiranes. The stereoselective formation of stereoisomers of the syn hydroxy group in 630 and the ami in 632 from the ( )-epoxide 629 and the (Z)-epoxide 631 respectively is an example. 

Epoxides are regio- and stereoselectively transformed into fluorohydrins by silicon tetrafluoride m the presence of a Lewis base, such as diisopropyleth-ylamme and, m certain instances, water or tetrabutylammonium fluoride The reactions proceed under very mild conditions (0 to 20 C in 1,2-diohloroethane or diethyl ether) and are highly chemoselective alkenes, ethers, long-chain internal oxiranes, and carbon-silicon bonds remain intact The stereochemical outcome of the epoxide ring opening with silicon tetrafluoride depends on an additive used, without addition of water or a quaternary ammonium fluoride, as fluorohydrins are formed, whereas m the presence of these additives, only anti opening leading to trans isomers is observed [17, 18] (Table 2)... 

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],... 

Styrene was successfully oxidized to the S-product both by xylene monooxygenase from P. putida mt-2 [113] and styrene monooxygenase from Pseudomonas sp.VLB120 [114] (Scheme 9.13), with the latter enzyme displaying a particularly large substrate tolerance with excellent stereoselectivity (>99% ee). In this context it is interesting to note that both xylene monooxygenase as well as chloroperoxidase are very selective for mono-epoxidation in case of presence of multiple alkene functionalities [115]. 

The microsomal epoxide hydrolases (microsomal EH, mEH), predominantly found in the endoplasmic reticulum, regio- and stereoselectively catalyze the hydration of both alkene and arene oxides, including oxides of polycyclic aromatic hydrocarbons. These enzymes have been purified to homogeneity from various species and tissues [22] [41 - 46], The human microsomal EH contains 455 amino acids (Mr 52.5 kDa) and is the product of the EPHX1 gene [47] (also known as HYL1 [48]). 

---